From fa7499b001f2325623c36abad1b4a39ff5160be9 Mon Sep 17 00:00:00 2001
From: TQ Hirsch <thequux@thequux.com>
Date: Sun, 26 Jan 2025 03:02:54 +0100
Subject: [PATCH] Initial commit

---
 DroidSansMonoSlashed.ttf |   Bin 0 -> 117744 bytes
 README.adoc              |    26 +
 docproc.py               |   332 +
 prelude.ps               |   126 +
 sources/mscp.txt         | 29145 +++++++++++++++++++++++++++++++++++++
 sources/tmscp.txt        |  9676 ++++++++++++
 6 files changed, 39305 insertions(+)
 create mode 100644 DroidSansMonoSlashed.ttf
 create mode 100644 README.adoc
 create mode 100644 docproc.py
 create mode 100644 prelude.ps
 create mode 100644 sources/mscp.txt
 create mode 100644 sources/tmscp.txt

diff --git a/DroidSansMonoSlashed.ttf b/DroidSansMonoSlashed.ttf
new file mode 100644
index 0000000000000000000000000000000000000000..8c44b47edc5882630da72658926f5d295717ad62
GIT binary patch
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diff --git a/README.adoc b/README.adoc
new file mode 100644
index 0000000..325f76f
--- /dev/null
+++ b/README.adoc
@@ -0,0 +1,26 @@
+= DEC documentation processor
+
+== Intro
+This converts DEC-derived .txt files from whatever tool they used to generate documents to PDF including 
+a table of contents (which is handy with 400-odd page documents)
+
+The code is terrible, and a non-trivial amount of the heavy lifting (including
+all the page layout) is done in PostScript, but it works for the Mass
+Storage Control Protocol documents from bitsavers, so I'm happy.
+
+== Usage
+
+....
+python3 docproc.py -o mscp.ps mscp.txt
+ps2pdf mscp.ps mscp.pdf
+....
+
+In order to change the font (which you probably will need to do), edit `prelude.ps` and look for the line that has `findfont` on it.
+
+If you want the line numbers included, change `/zDN false def` to `/zDN true def` in `prelude.ps`. Depending on the font you're using, you may or may not also need to change the font size (`/zCH`)
+
+== Disclaimer
+This was a weekend hack, so don't expect much readability.
+If it breaks, good luck with that. If you fix it, send a patch.
+
+It also doesn't do quite as much as you might want: inline section references are ignored, tables aren't linked (though that)
\ No newline at end of file
diff --git a/docproc.py b/docproc.py
new file mode 100644
index 0000000..336cca8
--- /dev/null
+++ b/docproc.py
@@ -0,0 +1,332 @@
+#!/usr/bin/env python3
+import sys
+from dataclasses import dataclass
+import re
+import io
+from typing import TextIO, Union
+
+import click
+
+
+@dataclass
+class Line:
+    line_no: str
+    is_change: bool
+    content: str
+
+
+@dataclass
+class Outline:
+    dest_name: str
+    label: str
+    subitems: list["Outline"]
+
+roman_lookup = []
+roman_lookup_rev = {}
+roman_lookup_re = re.compile("")
+
+def build_roman():
+    roman_lookup_raw = [
+        "/i/ii/iii/iv/v/vi/vii/ix",
+        "/x/xx/xxx/xl/l/lx/lxx/xc",
+        "/c/cc/ccc/cd/d/dc/dcc/cm",
+        "/m/mm/mmm"
+    ]
+    re_parts = []
+    for i,pats in enumerate(roman_lookup_raw):
+        pats = pats.split('/')
+        base = 10**i
+        roman_lookup.append((base, pats))
+        re_parts.append("(" + "|".join(pats) + ")")
+        for j, pat in enumerate(pats):
+            roman_lookup_rev[pat] = base * j
+    roman_lookup_re = re.compile("^" + "".join(roman_lookup) + "$", re.IGNORECASE)
+
+def romanize(n):
+    res = []
+    for base, pats in roman_lookup:
+        res.append(pats[(n // base) % 10])
+    return "".join(reversed(res))
+
+def unromanize(s):
+    m = roman_lookup_re.match(s)
+    if m:
+        res = 0
+        for item in m.groups()[1:]:
+            res += int(roman_lookup_rev[item])
+        return res
+    raise ValueError(f"{s!r} is not a valid roman numeral")
+
+ps_str_escapes = re.compile(r'([()\\])')
+def ps_string(s):
+    s = ps_str_escapes.sub(r'\\\1', s)
+    return f"({s})"
+
+
+
+class Processor:
+    line1pat = re.compile(r'^\s*Digital Equipment Corporation\s\s+Confidential And Proprietary\s*$')
+    ch_pat = re.compile(r'^\s*((?:\S|\s\S)+)\s\s+Page (\S+)')
+    toc_chapter_re = re.compile(r'^\s*(CHAPTER \d+|APPENDIX [A-Z]+|FIGURES|TABLES)(?:\s+((?:\S| \S)+))?\s*$')
+    toc_section_re = re.compile(r'^\s*((?:\d+|[A-Z])(?:(?:\.\d+)+|-\d+))\s+((?:\S|\s[^. ])+)\s+\s*(?:\. )*\s*([A-Z]-\d+|\d+-\d+)$')
+    bdy_chapter_re = re.compile(r'^\s*(?:CHAPTER|APENDIX)\s*(\d+|[A-Z]+)\s*$')
+    bdy_section_re = re.compile(r'^\s*((?:[A-Z]|\d+)(?:\.\d+)+)\s\s((?:\S|\s\S)+)\s*$')
+
+    # Config settings
+    out: TextIO = sys.stdout
+    guess_page = True
+    ln_cols = 6
+    change_col = 8
+    include_num = False
+    number_color = 0.5
+    header_color = 0.5
+    marginalia = 11
+    header_lines = 4
+    trailer_lines = 1
+
+    fontsize = 10
+    linespc = fontsize * 1.2
+
+    lines: list[Line] = []
+
+    # running state
+    last_chapter = None
+    is_toc = False
+    last_page = None
+    page_n: int = 1
+    input_line: int = 0
+    pg_start: int = 0
+    toc_pfx: str = "sec."
+
+    chapters: dict[str, str]
+    outline: list[Outline]
+
+    def __init__(self, file: Union[TextIO, str]):
+        if isinstance(file, str):
+            self.out = open(file, "wt")
+        else:
+            self.out = file
+
+        # Emit front-matter
+        with open("prelude.ps", "rt") as prelude:
+            for line in prelude:
+                self.emit(line.strip("\n"))
+
+        self.outline = []
+        self.chapters = {}
+
+    def do_line(self, line):
+        if '\f' in line:
+            self.guess_page = False
+            segments = line.split('\f')
+            for segment in segments[:-1]:
+                if segment != "":
+                    self.real_do_line(segment)
+                self.flush_page()
+            self.real_do_line(segments[-1])
+        else:
+            self.real_do_line(line)
+        self.input_line += 1
+
+    def real_do_line(self, line):
+        # Expand tabs
+        if '\t' in line:
+            pad = " " * 8
+            segs = []
+            for seg in line.split('\t'):
+                segs.append(seg)
+                segs.append(pad[:8-(len(seg)%8)])
+            line = "".join(segs)
+        if self.guess_page and self.line1pat.match(line):
+            self.flush_page()
+        is_change = len(line) > self.change_col and line[self.change_col] != ' '
+        if len(line) >= self.marginalia:
+            line_no = line[:self.ln_cols]
+            content = line[self.marginalia:]
+        else:
+            line_no = (line + " " * self.ln_cols)[:self.ln_cols]
+            content = ""
+        self.lines.append(Line(line_no = line_no, content=content, is_change=is_change))
+
+    def flush_page(self):
+        height = len(self.lines)
+        width = max(len(line.content) for line in self.lines)
+
+        last_chapter = self.last_chapter
+        chapter_m = self.ch_pat.match(self.lines[2].content)
+        if chapter_m is not None:
+            self.last_chapter = chapter = chapter_m.group(1)
+            page = chapter_m.group(2)
+        else:
+            print(f"Warning: no page number found on pg {self.pg_start}", file=sys.stderr)
+            chapter = last_chapter
+            page = self.guess_next_page()
+
+        self.last_page = page
+
+        if chapter != last_chapter:
+            if chapter == "CONTENTS":
+                self.is_toc = True
+            else:
+                self.is_toc = False
+
+        lines = list(enumerate(self.lines))
+        links = []
+        dests = []
+
+        if self.is_toc:
+            # Process TOC entries
+            partial_toc = ""
+            partial_toc_start = None
+            for i, line in lines[self.header_lines:-self.trailer_lines]:
+                if line.content.strip() == "":
+                    partial_toc_start = None
+                    partial_toc = ""
+                    continue
+                elif line.content.strip() == "CONTENTS":
+                    dests.append((i, "sec.CONTENTS"))
+                    self.outline.append(Outline("sec.CONTENTS", "CONTENTS", []))
+                    continue
+                elif m := self.toc_chapter_re.match(line.content):
+                    label = m.group(1)
+                    title = m.group(2)
+                    if ' ' in label:
+                        num = label.split(' ')[1]
+                    else:
+                        num = label
+                        if label == "FIGURES":
+                            self.toc_pfx = "sec.FIGURES."
+                            dests.append((i, "sec.FIGURES"))
+                        elif label == "TABLES":
+                            self.toc_pfx = "sec.TABLES."
+                            dests.append((i, "sec.TABLES"))
+                    partial_toc_start = None
+                    partial_toc = ""
+                    left = len(line.content) - len(line.content.lstrip())
+                    right = len(line.content.rstrip())
+                    links.append((left, i, right, i, f"sec.{num}"))
+                    self.outline.append(Outline(f"sec.{num}", f"{label} - {title}" if title else label, []))
+                    continue
+
+                # TODO: This won't work if an entry is split across pages.
+                # To fix, partial_toc and partial_toc_start must be saved after/restored before
+                # this function
+                if partial_toc_start is not None:
+                    partial_toc += " " + line.content.strip()
+                else:
+                    partial_toc = line.content.rstrip()
+                    partial_toc_start = i
+
+                if m := self.toc_section_re.match(partial_toc):
+                    num = m.group(1)
+                    title = m.group(2)
+                    left = partial_toc.index(num[0])
+                    right = len(line.content.rstrip())
+                    links.append((left, partial_toc_start, right, i, f"{self.toc_pfx}{num}"))
+                    self.add_outline(self.toc_pfx + num, title)
+                    partial_toc = ""
+                    partial_toc_start = None
+                elif partial_toc:
+                    print(f"No match: {partial_toc!r}", file=sys.stderr)
+        else:
+            # Process body lines
+            partial_toc = ""
+            partial_toc_start = None
+            for i, line in lines[self.header_lines:-self.trailer_lines]:
+                if line.content.strip() == "":
+                    partial_toc_start = None
+                    partial_toc = ""
+                    continue
+                elif m := self.bdy_chapter_re.match(line.content):
+                    num = "sec." + m.group(1)
+                    dests.append((i, num))
+
+                if partial_toc_start is not None:
+                    partial_toc += " " + line.content.strip()
+                else:
+                    partial_toc = line.content.rstrip()
+                    partial_toc_start = i
+
+                if m := self.bdy_section_re.match(partial_toc):
+                    num = m.group(1)
+                    dests.append((partial_toc_start, "sec." + num))
+
+
+
+
+        # Begin emitting page
+        self.emit(f"%%Page: {ps_string(page)} {self.page_n}")
+        self.emit(f"{height} {width} {ps_string(page)} bP")
+
+
+        for i, line in lines[:self.header_lines]:
+            self.emit(f"{i} gL {ps_string(line.content)} tH")
+
+        for i, line in lines[self.header_lines:-self.trailer_lines]:
+            chg = " mC" if  line.is_change else ""
+            self.emit(f"{i} gL {ps_string(line.line_no)} tN {ps_string(line.content)} tB{chg}")
+
+        for i, line in lines[-self.trailer_lines:]:
+            self.emit(f"{i} gL {ps_string(line.content)} tH")
+
+        for link in links:
+            self.emit(f"{link[0]} {link[1]} {link[2]} {link[3]} {ps_string(link[4])} mL")
+        for dest in dests:
+            self.emit(f"{dest[0]} {ps_string(dest[1])} mD")
+
+        self.emit("sP")
+
+        self.lines = []
+        self.pg_start = self.input_line
+
+    def guess_next_page(self):
+        if self.last_page is not None:
+            if m := re.match(r"^(\d*-)(\d*)$", self.last_page):
+                return f"{m.group(1)}{1+int(m.group(2))}"
+            else:
+                return romanize(1 + unromanize(self.last_page))
+        else:
+            return "i"
+
+    def emit(self, s):
+        print(s, file=self.out)
+
+    def emit_trailer(self):
+        self.emit("%%Trailer")
+        self.emit_outline(self.outline)
+        self.emit("%%EOF")
+
+    def emit_outline(self, outline: list[Outline]):
+        for item in outline:
+            self.emit(f"{ps_string(item.dest_name)} {ps_string(item.label)} {len(item.subitems)} mO")
+            self.emit_outline(item.subitems)
+
+    def add_outline(self, dest, title):
+        entry = Outline(dest, title, [])
+        typ = dest.split('.', 1)[0]
+
+        def recur(pfx, elist: list[Outline]):
+            for item in elist:
+                if dest.startswith(item.dest_name + "."):
+                    recur(item.dest_name, item.subitems)
+                    break
+            else:
+                if typ == "sec":
+                    assert dest.rsplit(".", 1)[0] == pfx
+                elist.append(entry)
+        recur(typ, self.outline)
+
+
+@click.command()
+@click.option("-o", "--output", type=click.File("w"))
+@click.argument("input", type=click.File("r"), default="-")
+def main(output, input):
+    proc = Processor(output or sys.stdout)
+    for line in input:
+        proc.do_line(line.strip("\n"))
+    proc.flush_page()
+    proc.emit_trailer()
+
+
+if __name__ == '__main__':
+    main()
\ No newline at end of file
diff --git a/prelude.ps b/prelude.ps
new file mode 100644
index 0000000..38f7bbe
--- /dev/null
+++ b/prelude.ps
@@ -0,0 +1,126 @@
+%!PS-Adobe-3.0
+%%Creator: Docproc.py
+%%Orientation: Portrait
+%%DocumentMedia: Letter 612 792 90 white ( )
+%%BeginDefaults
+%%PageMedia: Letter
+%%EndDefaults
+
+%%BeginProlog
+/zDN false def
+/zNW zDN { 6 } { 0 } ifelse def
+/bP { % begin Page
+	% height width pageName --
+	% height and width are in characters
+	
+	mark exch /Label exch /PAGELABEL pdfmark
+	dup
+	zPW exch zNW add zCW mul sub 2 div zNW zCW mul add /zX0 exch def
+	1 add zCW mul zX0 add /zXC exch def
+	zPH exch 
+	% -- zPH nlines 
+	  1 sub zLS mul zCH add 
+	% -- zPH zTH
+	add 2 div zCH sub /zY0 exch def
+} bind def
+/gL {
+	zLS mul neg zY0 add /zY1 exch def
+} bind def % go line
+/tH {
+	zX0 zY1 moveto
+	0.5 setgray
+	show
+} bind def % text Header
+/tN zDN {{
+	zX0 zNW 1 add zCW mul sub zY1 moveto
+	0.7 setgray
+	show
+}} {{ }} ifelse bind def % text lineNo
+/tB {
+	zX0 zY1 moveto
+	0 setgray
+	show
+} bind def % text body
+/mC {
+
+	0.75 0 0 setrgbcolor
+	newpath
+	1 setlinewidth
+	zXC zY1 -0.2 zLS mul add moveto
+	0 zLS rlineto
+	stroke
+} bind def % mark Change
+/fC {
+	0.2 add zLS mul neg zY0 add exch
+	zCW mul zX0 add exch
+} bind def % from Charpos
+/mL { % -- left top right bottom name
+	5 dict begin
+		/_N exch def
+		/_B exch def
+		/_R exch def
+		/_T exch def
+		/_L exch def
+		mark
+		/Rect [
+			_L _B fC
+			_R _T 1 sub fC
+		]
+		/Border [ 0 0 0.5 ]
+		/C [ 0 0 1 ]
+		/Subtype /Link
+		/Dest _N cvn
+		/ANN
+		pdfmark
+
+%		0 1 0 setrgbcolor
+%		1 setlinewidth
+%		newpath
+%
+%		_L _T 1 sub fC moveto
+%		_R _T 1 sub fC lineto
+%		_R _B fC lineto
+%		_L _B fC lineto
+%		closepath
+%		stroke
+	end
+} bind def % mark Link
+/mD { % -- top name
+	2 dict begin
+		/_N exch def
+		/_T exch 0 exch 1 sub fC exch pop def
+		mark 
+		/Dest _N cvn
+		/View [ /XYZ null _T null ]
+		/DEST pdfmark
+	end
+} bind def % mark Destination
+/mO { % -- dest title subitems
+	3 dict begin
+		/_S exch def
+		/_T exch def
+		/_D exch def
+		mark
+		/Title _T 
+		_S 0 gt { 
+			/Count _S
+		} if
+		/Dest _D cvn
+		/OUT pdfmark
+	end
+} bind def
+/sP { showpage } bind def
+
+%%EndProlog
+
+%%BeginSetup
+/zCH 10 def % char height
+/zLS zCH 1.2 mul def % line spacing
+(DroidSansMonoSlashed) findfont zCH scalefont setfont
+
+% Globals
+/zCW (M) stringwidth pop def % char width
+/zPH { currentpagedevice /PageSize get 1 get } def % page height
+/zPW { currentpagedevice /PageSize get 0 get } def % page width
+%%EndSetup
+
diff --git a/sources/mscp.txt b/sources/mscp.txt
new file mode 100644
index 0000000..a26f106
--- /dev/null
+++ b/sources/mscp.txt
@@ -0,0 +1,29145 @@
+
+
+
+
+                  
+
+
+
+
+
+
+
+
+
+
+                             Mass Storage Control Protocol
+
+                             ECO Controlled Version 2.4.0
+
+                                     11 June 1992
+
+
+
+
+
+
+                       Send inquiries and comments to SSAG::SSAG
+
+
+
+
+
+               This document defines the Mass Storage Control  Protocol
+               (MSCP),   an  applications  protocol  intended  for  use
+               between hosts and intelligent mass storage subsystems.
+
+
+
+
+
+
+
+              DIGITAL EQUIPMENT CORPORATION CONFIDENTIAL AND PROPRIETARY
+
+               This  document  is  an  unpublished  work  and  contains
+               valuable   trade  secrets  which  are  confidential  and
+               proprietary to Digital Equipment  Corporation,  and  may
+               only be disclosed to individuals who have entered into a
+               confidentiality agreement with Digital, and may  not  be
+               copied or reproduced in whole or in part.
+
+               Copyright (c) Digital Equipment Corporation  1992.   All
+               Rights Reserved.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                  Page ii
+                                                                11 June 1992
+
+
+                                           CONTENTS
+
+
+
+           CHAPTER 1       INTRODUCTION
+
+                   1.1     Overview Of MSCP Subsystem   . . . . . . . .  1-1
+                   1.2     Purpose  . . . . . . . . . . . . . . . . . .  1-3
+                   1.3     Method Of Presentation . . . . . . . . . . .  1-3
+                   1.4     Scope  . . . . . . . . . . . . . . . . . . .  1-3
+
+
+           CHAPTER 2       TERMINOLOGY
+
+
+           CHAPTER 3       CLASS DRIVER / MSCP SERVER COMMUNICATIONS
+
+                   3.1     Connection . . . . . . . . . . . . . . . . .  3-1
+                   3.2     Flow Control . . . . . . . . . . . . . . . .  3-2
+                   3.2.1     Class Driver Responsibilities  . . . . . .  3-5
+                   3.2.2     MSCP Server Responsibilities . . . . . . .  3-7
+                   3.3     Multihost Communication  . . . . . . . . . . 3-10
+
+
+           CHAPTER 4       ALGORITHMS AND USAGE RULES
+
+                   4.1     Controller States  . . . . . . . . . . . . .  4-1
+                   4.2     Controls And Indicators  . . . . . . . . . .  4-4
+                   4.3     Unit States  . . . . . . . . . . . . . . . .  4-6
+                   4.4     Unit Numbers . . . . . . . . . . . . . . . . 4-14
+                   4.5     Command Categories And Execution Order . . . 4-18
+                   4.6     Class Driver / MSCP Server Synchronization . 4-22
+                   4.7     Class Driver Error Recovery  . . . . . . . . 4-24
+                   4.8     Serious Exceptions . . . . . . . . . . . . . 4-25
+                   4.9     Host Access Timeouts . . . . . . . . . . . . 4-25
+                   4.10    Command Timeouts . . . . . . . . . . . . . . 4-28
+                   4.11    Host Buffer Access . . . . . . . . . . . . . 4-31
+                   4.12    Disk Geometry And Format . . . . . . . . . . 4-31
+                   4.12.1    Replacement Control Table (RCT) Overview . 4-32
+                   4.12.2    Logical Block Contents . . . . . . . . . . 4-33
+                   4.12.3    Performance Implications . . . . . . . . . 4-34
+                   4.13    Bad Block Replacement  . . . . . . . . . . . 4-37
+                   4.14    Write Protection . . . . . . . . . . . . . . 4-37
+                   4.15    Compare Operations . . . . . . . . . . . . . 4-39
+                   4.16    Special Drive Topologies . . . . . . . . . . 4-41
+                   4.16.1    Multiaccess Drives . . . . . . . . . . . . 4-41
+                   4.16.2    Multiunit Drives And Formatters  . . . . . 4-43
+                   4.17    Controller And Unit Identifiers  . . . . . . 4-45
+                   4.18    Media Type Identifiers . . . . . . . . . . . 4-47
+                   4.19    Controller-based Cache . . . . . . . . . . . 4-49
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                 Page iii
+                                                                11 June 1992
+
+
+           CHAPTER 5       MSCP CONTROL MESSAGES
+
+                   5.1     Generic Control Message Format . . . . . . .  5-1
+                   5.2     Reserved And Undefined Fields  . . . . . . .  5-2
+                   5.3     Command Messages . . . . . . . . . . . . . .  5-4
+                   5.3.1     Transfer Command Message Format  . . . . .  5-7
+                   5.3.2     Command Modifiers  . . . . . . . . . . . . 5-13
+                   5.4     End Messages . . . . . . . . . . . . . . . . 5-15
+                   5.4.1     Transfer Command End Message Format  . . . 5-19
+                   5.5     Status And Event Codes . . . . . . . . . . . 5-21
+                   5.6     Controller Flags . . . . . . . . . . . . . . 5-32
+                   5.7     Unit Flags . . . . . . . . . . . . . . . . . 5-37
+                   5.8     Attention Messages . . . . . . . . . . . . . 5-41
+                   5.8.1     ACCESS PATH Attention Message  . . . . . . 5-43
+                   5.8.1.1     Attention Message Format . . . . . . . . 5-43
+                   5.8.1.2     Description  . . . . . . . . . . . . . . 5-43
+                   5.8.2     AVAILABLE Attention Message  . . . . . . . 5-44
+                   5.8.2.1     Attention Message Format . . . . . . . . 5-44
+                   5.8.2.2     Description  . . . . . . . . . . . . . . 5-45
+                   5.8.3     DUPLICATE UNIT NUMBER Attention Message  . 5-47
+                   5.8.3.1     Attention Message Format . . . . . . . . 5-47
+                   5.8.3.2     Description  . . . . . . . . . . . . . . 5-47
+                   5.9     Error Log Messages . . . . . . . . . . . . . 5-48
+                   5.9.1     Maximum Error Log Message Size . . . . . . 5-53
+                   5.9.2     Generic Error Log Message Format . . . . . 5-57
+                   5.9.3     CONTROLLER ERRORS Error Log Format . . . . 5-63
+                   5.9.4     MEMORY ERRORS Error Log Format . . . . . . 5-64
+                   5.9.5     DISK TRANSFER ERRORS Error Log Format  . . 5-67
+                   5.9.6     SDI ERRORS Error Log Format  . . . . . . . 5-69
+                   5.9.7     SMALL DISK ERRORS Error Log Format . . . . 5-71
+                   5.9.8     BAD BLOCK REPLACEMENT ATTEMPT Error Log 
+                             Format . . . . . . . . . . . . . . . . . . 5-73
+                   5.9.9     DISK COPY DATA CORRELATION Error Log 
+                             Format . . . . . . . . . . . . . . . . . . 5-75
+
+
+           CHAPTER 6       MINIMAL MSCP COMMAND SET
+
+                   6.1     Overview . . . . . . . . . . . . . . . . . .  6-1
+                   6.1.1     No-Operation Commands  . . . . . . . . . .  6-2
+                   6.2     ABORT Command  . . . . . . . . . . . . . . .  6-4
+                   6.2.1     Command Message Format . . . . . . . . . .  6-4
+                   6.2.2     End Message Format . . . . . . . . . . . .  6-5
+                   6.2.3     Description  . . . . . . . . . . . . . . .  6-6
+                   6.3     ACCESS Command . . . . . . . . . . . . . . .  6-7
+                   6.3.1     Command Message Format . . . . . . . . . .  6-7
+                   6.3.2     End Message Format . . . . . . . . . . . .  6-8
+                   6.3.3     Description  . . . . . . . . . . . . . . .  6-9
+                   6.4     AVAILABLE Command  . . . . . . . . . . . . . 6-10
+                   6.4.1     Command Message Format . . . . . . . . . . 6-10
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                  Page iv
+                                                                11 June 1992
+
+
+                   6.4.2     End Message Format . . . . . . . . . . . . 6-11
+                   6.4.3     Description  . . . . . . . . . . . . . . . 6-12
+                   6.5     COMPARE HOST DATA Command  . . . . . . . . . 6-13
+                   6.5.1     Command Message Format . . . . . . . . . . 6-13
+                   6.5.2     End Message Format . . . . . . . . . . . . 6-14
+                   6.5.3     Description  . . . . . . . . . . . . . . . 6-15
+                   6.6     DETERMINE ACCESS PATHS Command . . . . . . . 6-16
+                   6.6.1     Command Message Format . . . . . . . . . . 6-16
+                   6.6.2     End Message Format . . . . . . . . . . . . 6-17
+                   6.6.3     Description  . . . . . . . . . . . . . . . 6-18
+                   6.7     DISK COPY DATA Command . . . . . . . . . . . 6-19
+                   6.7.1     Command Message Format . . . . . . . . . . 6-19
+                   6.7.2     End Message Format . . . . . . . . . . . . 6-28
+                   6.7.3     Description  . . . . . . . . . . . . . . . 6-43
+                   6.7.3.1     Operation  . . . . . . . . . . . . . . . 6-44
+                   6.7.3.2     Error Logs And Correlation Logs  . . . . 6-54
+                   6.8     DISPLAY Command  . . . . . . . . . . . . . . 6-57
+                   6.8.1     Command Message Format . . . . . . . . . . 6-57
+                   6.8.2     End Message Format . . . . . . . . . . . . 6-59
+                   6.8.3     Description  . . . . . . . . . . . . . . . 6-61
+                   6.9     ERASE Command  . . . . . . . . . . . . . . . 6-67
+                   6.9.1     Command Message Format . . . . . . . . . . 6-67
+                   6.9.2     End Message Format . . . . . . . . . . . . 6-70
+                   6.9.3     Description  . . . . . . . . . . . . . . . 6-73
+                   6.10    FORMAT Command . . . . . . . . . . . . . . . 6-74
+                   6.10.1    Command Message Format . . . . . . . . . . 6-74
+                   6.10.2    End Message Format . . . . . . . . . . . . 6-75
+                   6.10.3    Description  . . . . . . . . . . . . . . . 6-76
+                   6.11    GET COMMAND STATUS Command . . . . . . . . . 6-79
+                   6.11.1    Command Message Format . . . . . . . . . . 6-79
+                   6.11.2    End Message Format . . . . . . . . . . . . 6-80
+                   6.11.3    Description  . . . . . . . . . . . . . . . 6-81
+                   6.12    GET UNIT STATUS Command  . . . . . . . . . . 6-82
+                   6.12.1    Command Message Format . . . . . . . . . . 6-82
+                   6.12.2    End Message Format . . . . . . . . . . . . 6-83
+                   6.12.3    Description  . . . . . . . . . . . . . . . 6-87
+                   6.13    ONLINE Command . . . . . . . . . . . . . . . 6-88
+                   6.13.1    Command Message Format . . . . . . . . . . 6-88
+                   6.13.2    End Message Format . . . . . . . . . . . . 6-91
+                   6.13.3    Description  . . . . . . . . . . . . . . . 6-94
+                   6.14    READ Command . . . . . . . . . . . . . . . . 6-95
+                   6.14.1    Command Message Format . . . . . . . . . . 6-95
+                   6.14.2    End Message Format . . . . . . . . . . . . 6-96
+                   6.14.3    Description  . . . . . . . . . . . . . . . 6-97
+                   6.15    REPLACE Command  . . . . . . . . . . . . . . 6-98
+                   6.15.1    Command Message Format . . . . . . . . . . 6-98
+                   6.15.2    End Message Format . . . . . . . . . . .  6-100
+                   6.15.3    Description  . . . . . . . . . . . . . .  6-101
+                   6.16    SET CONTROLLER CHARACTERISTICS Command . .  6-102
+                   6.16.1    Command Message Format . . . . . . . . .  6-102
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                   Page v
+                                                                11 June 1992
+
+
+                   6.16.2    End Message Format . . . . . . . . . . .  6-105
+                   6.16.3    Description  . . . . . . . . . . . . . .  6-107
+                   6.17    SET UNIT CHARACTERISTICS Command . . . . .  6-108
+                   6.17.1    Command Message Format . . . . . . . . .  6-108
+                   6.17.2    End Message Format . . . . . . . . . . .  6-110
+                   6.17.3    Description  . . . . . . . . . . . . . .  6-113
+                   6.18    WRITE Command  . . . . . . . . . . . . . .  6-114
+                   6.18.1    Command Message Format . . . . . . . . .  6-114
+                   6.18.2    End Message Format . . . . . . . . . . .  6-118
+                   6.18.3    Description  . . . . . . . . . . . . . .  6-121
+                   6.19    WRITE HISTORY MANAGEMENT Command . . . . .  6-122
+                   6.19.1    Command Message Format . . . . . . . . .  6-122
+                   6.19.2    End Message Format . . . . . . . . . . .  6-128
+                   6.19.3    Description  . . . . . . . . . . . . . .  6-132
+
+
+           CHAPTER 7       MULTIHOST SUPPORT 
+
+                   7.1     Overview . . . . . . . . . . . . . . . . . .  7-1
+                   7.2     Multihost ABORT Command  . . . . . . . . . .  7-3
+                   7.2.1     Command Message Format . . . . . . . . . .  7-3
+                   7.2.2     End Message Format . . . . . . . . . . . .  7-4
+                   7.2.3     Description  . . . . . . . . . . . . . . .  7-5
+                   7.3     Multihost ACCESS NONVOLATILE MEMORY Command   7-6
+                   7.3.1     Command Message Format . . . . . . . . . .  7-6
+                   7.3.2     End Message Format . . . . . . . . . . . .  7-8
+                   7.3.3     Description  . . . . . . . . . . . . . . .  7-9
+                   7.4     Multihost AVAILABLE Command  . . . . . . . . 7-12
+                   7.4.1     Command Message Format . . . . . . . . . . 7-12
+                   7.4.2     End Message Format . . . . . . . . . . . . 7-15
+                   7.4.3     Description  . . . . . . . . . . . . . . . 7-18
+                   7.5     Multihost GET COMMAND STATUS Command . . . . 7-19
+                   7.5.1     Command Message Format . . . . . . . . . . 7-19
+                   7.5.2     End Message Format . . . . . . . . . . . . 7-20
+                   7.5.3     Description  . . . . . . . . . . . . . . . 7-24
+                   7.6     Multihost GET UNIT STATUS Command  . . . . . 7-25
+                   7.6.1     Command Message Format . . . . . . . . . . 7-25
+                   7.6.2     End Message Format . . . . . . . . . . . . 7-26
+                   7.6.3     Description  . . . . . . . . . . . . . . . 7-28
+                   7.7     Multihost ONLINE Command . . . . . . . . . . 7-29
+                   7.7.1     Command Message Format . . . . . . . . . . 7-29
+                   7.7.2     End Message Format . . . . . . . . . . . . 7-32
+                   7.7.3     Description  . . . . . . . . . . . . . . . 7-36
+                   7.8     Multihost SET CONTROLLER CHARACTERISTICS 
+                           Command  . . . . . . . . . . . . . . . . . . 7-37
+                   7.8.1     Command Message Format . . . . . . . . . . 7-37
+                   7.8.2     End Message Format . . . . . . . . . . . . 7-39
+                   7.8.3     Description  . . . . . . . . . . . . . . . 7-41
+                   7.9     Multihost SET UNIT CHARACTERISTICS Command . 7-42
+                   7.9.1     Command Message Format . . . . . . . . . . 7-42
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                  Page vi
+                                                                11 June 1992
+
+
+                   7.9.2     End Message Format . . . . . . . . . . . . 7-45
+                   7.9.3     Description  . . . . . . . . . . . . . . . 7-48
+                   7.10    Multihost TERMINATE CLASS DRIVER/SERVER 
+                           CONNECTION Command . . . . . . . . . . . . . 7-49
+                   7.10.1    Command Message Format . . . . . . . . . . 7-49
+                   7.10.2    End Message Format . . . . . . . . . . . . 7-51
+                   7.10.3    Description  . . . . . . . . . . . . . . . 7-52
+
+
+           CHAPTER 8       CONTROLLER INITIATED BAD BLOCK REPLACEMENT
+
+                   8.1     Controller Initiated Bad Block Replacement 
+                           Overview . . . . . . . . . . . . . . . . . .  8-1
+                   8.2     Data Safety Write Protection . . . . . . . .  8-2
+                   8.3     Bad Block Replacement  . . . . . . . . . . .  8-3
+                   8.4     Replacement Control Table Access . . . . . .  8-6
+                   8.5     Atomic Bad Block Replacement . . . . . . . .  8-8
+                   8.6     Error Log Messages . . . . . . . . . . . . .  8-8
+                   8.7     Unit Context Information . . . . . . . . . .  8-9
+                   8.8     Actions During ONLINE  . . . . . . . . . . . 8-10
+                   8.9     Actions During SET UNIT CHARACTERISTICS  . . 8-12
+                   8.10    Actions Before First Modification  . . . . . 8-14
+                   8.11    MSCP Control Message Format Changes  . . . . 8-16
+                   8.11.1    Controller Flags . . . . . . . . . . . . . 8-16
+                   8.11.2    Unit Flags . . . . . . . . . . . . . . . . 8-16
+                   8.11.3    Transfer Commands  . . . . . . . . . . . . 8-17
+                   8.11.4    ONLINE and SET UNIT CHARACTERISTICS 
+                             Commands . . . . . . . . . . . . . . . . . 8-17
+                   8.11.5    GET UNIT STATUS Command  . . . . . . . . . 8-19
+                   8.11.6    REPLACE Command  . . . . . . . . . . . . . 8-19
+                   8.12    Host Support For Controller Initiated Bad 
+                           Block Replacement  . . . . . . . . . . . . . 8-20
+
+
+           APPENDIX A      OPCODE, FLAG, AND OFFSET DEFINITIONS
+
+
+           APPENDIX B      STATUS AND EVENT CODE DEFINITIONS
+
+
+           APPENDIX C      CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS
+
+
+           APPENDIX D      BUFFER DESCRIPTOR FORMATS
+
+
+           APPENDIX E      WAIVERS AND EXCEPTIONS
+
+                   E.1     UDA50  . . . . . . . . . . . . . . . . . . .  E-1
+                   E.1.1     Unit And Controller Revision Numbers . . .  E-1
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                 Page vii
+                                                                11 June 1992
+
+
+                   E.2     RC25 . . . . . . . . . . . . . . . . . . . .  E-1
+                   E.2.1     Error Log Message Ordering . . . . . . . .  E-1
+                   E.2.2     Data Error (subcode "Forced Error") 
+                             Reporting  . . . . . . . . . . . . . . . .  E-2
+                   E.2.3     Improper Duplicate Unit Number Processing   E-3
+                   E.3     HSC And VAX/VMS  . . . . . . . . . . . . . .  E-6
+                   E.3.1     "Bundled" Shadowing  . . . . . . . . . . .  E-6
+                   E.3.1.1     Shadowing Concepts . . . . . . . . . . .  E-6
+                   E.3.1.1.1     Shadowing Functionality  . . . . . . .  E-6
+                   E.3.1.1.2     Invalid Commands . . . . . . . . . . .  E-7
+                   E.3.1.1.3     Virtual And Member Units . . . . . . .  E-9
+                   E.3.1.1.4     Shadow Member Compatibility  . . . . .  E-9
+                   E.3.1.1.4.1     Hardware Write Protected Shadow Sets E-10
+                   E.3.1.1.5     Shadow Set Management  . . . . . . . . E-11
+                   E.3.1.1.6     MSCP Command Changes . . . . . . . . . E-13
+                   E.3.1.1.6.1     ONLINE Command Changes . . . . . . . E-13
+                   E.3.1.1.6.1.1     Changes in the ONLINE Command 
+                                     Syntax . . . . . . . . . . . . . . E-13
+                   E.3.1.1.6.1.2     Changes Which Enforce Shadow Set 
+                                     Membership Rules . . . . . . . . . E-17
+                   E.3.1.1.6.1.3     Changes in Sequential Gatekeeping  E-18
+                   E.3.1.1.6.1.4     Performing Shadow Copy Operations  E-19
+                   E.3.1.1.6.1.5     Restrictions and Miscellaneous 
+                                     Notes  . . . . . . . . . . . . . . E-21
+                   E.3.1.1.6.2     ONLINE End Message Changes . . . . . E-21
+                   E.3.1.1.6.3     SET UNIT CHARACTERISTICS Command 
+                                   Changes  . . . . . . . . . . . . . . E-23
+                   E.3.1.1.6.4     SET UNIT CHARACTERISTICS End Message 
+                                   Changes  . . . . . . . . . . . . . . E-24
+                   E.3.1.1.6.5     AVAILABLE Command Changes  . . . . . E-24
+                   E.3.1.1.6.6     GET UNIT STATUS Command End Message 
+                                   Changes  . . . . . . . . . . . . . . E-25
+                   E.3.1.1.6.7     DETERMINE ACCESS PATHS Command 
+                                   Changes  . . . . . . . . . . . . . . E-25
+                   E.3.1.1.6.8     COMPARE CONTROLLER DATA Command 
+                                   Changes  . . . . . . . . . . . . . . E-26
+                   E.3.1.1.6.9     Transfer Commands  . . . . . . . . . E-26
+                   E.3.1.1.6.9.1     READ Operations  . . . . . . . . . E-27
+                   E.3.1.1.6.9.2     WRITE Operations . . . . . . . . . E-30
+                   E.3.1.1.6.9.3     Host Write Failure Recovery  . . . E-32
+                   E.3.1.1.6.9.4     Read/Write Concurrency . . . . . . E-34
+                   E.3.1.1.6.10    Shadow Set Membership Changes  . . . E-35
+                   E.3.1.1.6.11    Multihost Access . . . . . . . . . . E-35
+                   E.3.1.1.6.11.1    Member Unit Actions  . . . . . . . E-35
+                   E.3.1.1.6.11.2    Shadow Set Virtual Unit Actions  . E-37
+                   E.3.1.1.6.11.3    Shadow Copy Actions  . . . . . . . E-38
+                   E.3.1.1.6.11.4    Get Unit Status Actions  . . . . . E-38
+                   E.3.1.1.6.12    Error Logs . . . . . . . . . . . . . E-38
+                   E.3.1.1.7     "Bundled Shadowing" Definitions  . . . E-39
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                Page viii
+                                                                11 June 1992
+
+
+                   E.3.1.1.8     "Bundled Shadowing" Specific Status 
+                                 Code Definitions . . . . . . . . . . . E-42
+                   E.4     TU81 . . . . . . . . . . . . . . . . . . . . E-45
+                   E.4.1     "Enable Miscellaneous Error Log Messages" 
+                             Modifier Not Fully Supported . . . . . . . E-45
+                   E.5     VAX/VMS  . . . . . . . . . . . . . . . . . . E-45
+                   E.5.1     Bad Block Replacement Attempt Error Log 
+                             Usage  . . . . . . . . . . . . . . . . . . E-45
+                   E.6     RV80 . . . . . . . . . . . . . . . . . . . . E-45
+                   E.6.1     Media Loader Error Logs  . . . . . . . . . E-45
+                   E.6.1.1     Controller And Unit Identifiers  . . . . E-46
+                   E.6.1.2     Media Loader Error Log Format  . . . . . E-48
+                   E.6.1.3     Error Log Message Offsets  . . . . . . . E-51
+                   E.6.1.4     Error Log Message Format Codes . . . . . E-52
+                   E.6.1.5     Status And Event Codes . . . . . . . . . E-52
+                   E.6.1.6     Media Loader Error Status Or Event 
+                               Subcodes . . . . . . . . . . . . . . . . E-53
+                   E.6.1.7     Controller And Unit Identifier Class 
+                               Values . . . . . . . . . . . . . . . . . E-53
+                   E.6.1.8     Media Loader Identifier Values . . . . . E-54
+
+
+           APPENDIX F      PORT ADDRESS AND SYSTEM ADDRESS FORMATS
+
+                   F.1     Port Address Formats . . . . . . . . . . . .  F-1
+                   F.1.1     CI Port Address  . . . . . . . . . . . . .  F-1
+                   F.2     System Address Formats . . . . . . . . . . .  F-1
+                   F.2.1     SCA System Address . . . . . . . . . . . .  F-2
+
+
+           APPENDIX G      REVISION HISTORY
+
+
+           APPENDIX H      UNANNOUNCED PRODUCTS
+
+
+           FIGURES
+
+                   1-1     Example System . . . . . . . . . . . . . . .  1-2
+                   8-1     Controller Initiated Bad Block Replacement 
+                           Layers . . . . . . . . . . . . . . . . . . .  8-2
+
+
+           TABLES
+
+                   A-1     Control Message Opcodes  . . . . . . . . . .  A-1
+                   A-2     Command Modifiers  . . . . . . . . . . . . .  A-3
+                   A-3     End Message Flags  . . . . . . . . . . . . .  A-5
+                   A-4     Controller Flags . . . . . . . . . . . . . .  A-6
+                   A-5     Unit Flags . . . . . . . . . . . . . . . . .  A-7
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTENTS                                                  Page ix
+                                                                11 June 1992
+
+
+                   A-6     Command Message Offsets  . . . . . . . . . .  A-8
+                   A-7     End and Attention Message Offsets  . . . . . A-11
+                   A-8     Error Log Message Offsets  . . . . . . . . . A-14
+                   A-9     Error Log Message Format Codes . . . . . . . A-17
+                   A-10    Error Log Message Flags  . . . . . . . . . . A-17
+                   A-11    Bad Block Replacement Attempt "Replace 
+                           Flags" . . . . . . . . . . . . . . . . . . . A-18
+                   A-12    Access Nonvolatile Memory Command Operation 
+                           Codes  . . . . . . . . . . . . . . . . . . . A-19
+                   A-13    Format Function Codes  . . . . . . . . . . . A-19
+                   A-14    WRITE HISTORY MANAGEMENT Command Operation 
+                           Codes  . . . . . . . . . . . . . . . . . . . A-20
+                   A-15    Write History Entry Entry Flags  . . . . . . A-21
+                   A-16    Write History Entry Offsets  . . . . . . . . A-21
+                   A-17    Cache Access Attribute Values  . . . . . . . A-21
+                   B-1     Status and Event Codes . . . . . . . . . . .  B-2
+                   B-2     Status Only Subcode Values . . . . . . . . .  B-3
+                   B-3     "Media Format Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . .  B-5
+                   B-4     "Compare Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . .  B-6
+                   B-5     "Data Error" Status or Event Subcode Values   B-6
+                   B-6     "Host Buffer Access Error" Status or Event 
+                           Subcode Values . . . . . . . . . . . . . . .  B-9
+                   B-7     "Controller Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . . B-10
+                   B-8     "Drive Error" Status or Event Subcode Values B-14
+                   B-9     "Bad Block Replacement Completion" Event 
+                           Only Subcode Values  . . . . . . . . . . . . B-16
+                   B-10    "Media Loader Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . . B-16
+                   B-11    "Informational" Event Only Subcode Values  . B-17
+                   B-12    "Subcommand Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . . B-17
+                   B-13    "Cache Error" Status or Event Subcode Values B-18
+                   C-1     Controller and Unit "Class" Values . . . . .  C-1
+                   C-2     Mass Storage Controller "Model" Values . . .  C-2
+                   C-3     Disk Drive/Media Identifier Values . . . . .  C-5
+                   C-4     Tape Drive/Media Identifier Values . . . . .  C-8
+                   C-5     Media Loader Identifier Values . . . . . . . C-10
+                   C-6     SCSI Storage Device/Media Identifier Values  C-11
+                   H-1     Unannounced Controller "Model" Values  . . .  H-1
+                   H-2     Unannounced Disk Drive/Media Identifier 
+                           Values . . . . . . . . . . . . . . . . . . .  H-2
+                   H-3     Unannounced Tape Drive/Media Identifier 
+                           Values . . . . . . . . . . . . . . . . . . .  H-3
+                   H-4     Unannounced SCSI Storage Device/Media 
+                           Identifier Values  . . . . . . . . . . . . .  H-4
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           INTRODUCTION                                             Page 1-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 1
+
+     2                               INTRODUCTION
+
+
+     3     1.1  Overview Of MSCP Subsystem
+
+     4     Mass Storage Control Protocol (MSCP) is the protocol  used  by  a
+     5     family of mass storage controllers and devices designed and built
+     6     by Digital Equipment Corporation.  In a system that uses an  MSCP
+     7     storage   subsystem,  the  controller  contains  intelligence  to
+     8     perform the detailed I/O handling tasks.  This arrangement allows
+     9     the  host  to simply send command messages (requests for reads or
+    10     writes) to the controller and receive response messages back from
+    11     the  controller.   The  host need not concern itself with details
+    12     such as device type,  media  geometry,  media  format,  or  error
+    13     recovery.
+
+    14     The host uses two levels of software  to  accomplish  its  tasks.
+    15     The  higher level is called a "class driver."  The class driver's
+    16     knowledge of devices is limited to  the  device  class  (such  as
+    17     disks)  and  their  capacity.   The class driver does not have to
+    18     know the detailed nature of the communications  link  (I/O  bus),
+    19     controller, or devices that are being used.
+
+    20     The second level of host software is called a "port driver."  The
+    21     port  driver  passes  messages to/from the communications link or
+    22     bus.  It is not aware of the messages' meaning.  The port  driver
+    23     does  have to know the exact nature of the communications link or
+    24     bus (communications mechanism).
+
+    25     In the controller architecture, there  are  also  two  levels  of
+    26     software.   The  lower  of these two is also a "port driver" and,
+    27     like the port driver in the host, is concerned only with  passing
+    28     messages  on  and off of the bus.  The higher level of controller
+    29     software is the "MSCP Server."  It constitutes  the  intelligence
+    30     of  the controller and therefore defines the functionality of the
+    31     controller.
+
+    32     The MSCP server concerns itself with determining  the  number  of
+    33     devices, their type, geometry, unit number, availability, status,
+    34     etc.  The MSCP server receives requests from the host  and  sends
+    35     responses  to  the host.  It optimizes the requests, performs the
+    36     operations, transfers the data to/from the  host,  transfers  the
+    37     data  to/from the device, and buffers the data as necessary.  The
+    38     MSCP server performs error detection and  recovery,  and  reports
+    39     any significant errors to the host.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           INTRODUCTION                                             Page 1-2
+           Overview Of MSCP Subsystem                           11 June 1992
+
+
+     1     Because the MSCP server handles the error detection and  recovery
+     2     by   itself,   the   host  sees  "perfect  media,"  an  important
+     3     characteristic of an MSCP subsystem.  That is, the host need only
+     4     report errors to higher level (user) software, as the MSCP server
+     5     performs  all  error  recovery  and  media  defect  (bad   block)
+     6     handling.
+
+     7     The host's class driver and the controller's  MSCP  server  route
+     8     their  messages  through  the  path of the two port drivers and a
+     9     hardware  interconnect.   This  is  their  physical   connection.
+    10     However,  their  logical  communication  is  a  direct connection
+    11     because the port driver details are below their level of concern.
+    12     Therefore,  there  are two paths to consider:  a physical message
+    13     path and a logical  MSCP  connection.   This  is  illustrated  in
+    14     Figure 1-1.
+
+
+    15                 Host                      Mass Storage Controller
+    16          + - - - - - - - - +                + - - - - - - - - +  
+    17          |                 |                |                 |  
+    18             +-----------+                      +-----------+     
+    19          |  |   Class   |  |      MSCP      |  |   MSCP    |  |  
+    20             |   Driver  | <------------------> |   Server  |     
+    21          |  +-----------+  |                |  +-----------+  |  
+    22                   A                                  A           
+    23          |        |        |                |        |        |  
+    24                   V                                  V           
+    25          |  +-----------+  | Communications |  +-----------+  |  
+    26             |   Port    | <------------------> |   Port    |     
+    27          |  |   Driver  |  |   Protocols    |  |   Driver  |  |  
+    28             +-----------+                      +-----------+     
+    29          |        A        |                |        A        |  
+    30          + - - - -|- - - - +                + - - - -|- - - - +  
+    31                   |                                  |           
+    32                   V                                  V           
+    33               +------------------------------------------+       
+    34               |  Port  |                        |  Port  |       
+    35               |  -  -  +                        +  -  -  |       
+    36               |         Communications Mechanism         |       
+    37               |                                          |       
+    38               +------------------------------------------+       
+
+
+    39                       Figure 1-1:  Example System
+
+
+    40     In summary, an MSCP subsystem is characterized by an  intelligent
+    41     controller that provides the host with the view of perfect media.
+    42     It is further characterized by host independence from a  specific
+    43     bus, controller, or device type.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           INTRODUCTION                                             Page 1-3
+           Purpose                                              11 June 1992
+
+
+     1     1.2  Purpose
+
+     2     The purpose of this manual is to provide information on the rules
+     3     of  MSCP  to  the  detail necessary for both writing a host class
+     4     driver and implementing an MSCP server.
+
+     5     1.3  Method Of Presentation
+
+     6     The method of presentation used in this manual is:
+
+     7           o  to define new terms and concepts.
+
+     8           o  list the responsibilities of the class driver.
+
+     9           o  list the responsibilities of the MSCP  server  that  are
+    10              applicable to the class driver.
+
+    11           o  list the responsibilities of the storage unit  that  are
+    12              applicable to the class driver.
+
+    13           o  explain each command and response message.
+
+    14           o  explain each error message.
+
+    15           o  provide appropriate tables of consolidated information.
+
+
+    16     1.4  Scope
+
+    17     The scope of this manual is limited to the details  of  the  MSCP
+    18     itself  and  does not provide information on any specific type of
+    19     host processor or any specific operating  system.   It  does  not
+    20     assume any particular bus, controller, device type, host, or port
+    21     driver implementation.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           TERMINOLOGY                                              Page 2-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 2
+
+     2                                TERMINOLOGY
+
+
+
+     3     Aborted Command
+
+     4         From the viewpoint of a class driver, any command  for  which
+     5         it  has  issued  an  ABORT command.  From the viewpoint of an
+     6         MSCP server, a command for which it  has  received  an  ABORT
+     7         command,  located  the  specified command, and taken explicit
+     8         action to abort  it.   An  aborted  command  will  be  either
+     9         rejected or terminated.  See Section 4.5, "Command Categories
+    10         and Execution Order." 
+
+
+    11     Backing-store
+
+    12         In a cached memory system, the memory  being  served  by  the
+    13         cache.   This  memory  is usually much larger and much slower
+    14         than the cache memory and its performance is enhanced by  the
+    15         cache  between  it  and the user.  In the case of cached mass
+    16         storage, the backing store is the disk or tape media.
+
+    17     Cache
+
+    18         A fast memory placed between a larger slow  memory  known  as
+    19         the  backing-store  and  the  user.   By retaining frequently
+    20         accessed data in the cache, the apparent performance  of  the
+    21         backing-store  is improved.  A number of different algorithms
+    22         may be used for the selection of data retained in the cache.
+
+    23     Cache Consistency
+
+    24         The property that ensures that cached data behaves exactly as
+    25         if  no  cache  existed.   That is, only the last written data
+    26         will be returned to users  and  in  the  case  of  write-back
+    27         cache,  the last written data will be properly written to the
+    28         backing-store even in the event of power failure.
+
+
+    29     Command Categories
+
+    30         All MSCP commands fall into one of four  command  categories:
+    31         Immediate   commands,   Non-Sequential  commands,  Sequential
+    32         commands, and Special commands.  Each  command  category  has
+    33         certain  constraints  on when those commands may be executed,
+    34         thus limiting the scope  of  controller  optimizations.   See
+    35         Section 4.5, "Command Categories and Execution Order." 
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           TERMINOLOGY                                              Page 2-2
+                                                                11 June 1992
+
+
+     1     Completed Command
+
+     2         A command that the MSCP server has executed  through  to  its
+     3         normal  completion, which may be either successful completion
+     4         or  an  unrecoverable  error.   See  Section  4.5,   "Command
+     5         Categories and Execution Order." 
+
+
+     6     Controller Timeout Interval
+
+     7         A  time  interval,  measured  in  seconds,  supplied  by  the
+     8         controller   or   MSCP   server   in   the   SET   CONTROLLER
+     9         CHARACTERISTICS command's end message.  Controllers  or  MSCP
+    10         servers  guarantee  that  they  will  complete  all Immediate
+    11         commands plus some measurable amount of useful work on  their
+    12         oldest   outstanding   non-Immediate   command   within   the
+    13         controller timeout  interval.   See  Section  4.10,  "Command
+    14         Timeouts." 
+
+
+    15     Forced Error
+
+    16         A data error (in a disk block)  that  has  been  deliberately
+    17         caused by use of the "Force Error" command modifier.  Used to
+    18         indicate that the  data  in  the  block  is  of  questionable
+    19         validity.   For example, an unrecoverable error occurred when
+    20         the data was copied from some other block.
+
+
+    21     Forced Error Indicator
+
+    22         The logical flag, present in each disk block, used to  record
+    23         the presence of a Forced Error.  Depending upon the detailed,
+    24         low level format of the disk device, this may be  implemented
+    25         either as an actual bit flag or as a special pattern (such as
+    26         the complement of  the  normal  value)  of  error  correcting
+    27         and/or error detecting codes.
+
+
+    28     Immediate Commands
+
+    29         Commands that MSCP servers shall execute immediately, without
+    30         waiting  for  any  other  commands  to  complete.   Immediate
+    31         commands  are  typically  status  inquiries,  and  shall   be
+    32         completed within the controller timeout interval.
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           TERMINOLOGY                                              Page 2-3
+                                                                11 June 1992
+
+
+     1     May
+
+     2         Denotes a lack of prohibition when used as a  directive.   It
+     3         is  used  to  indicate  an  option or options architecturally
+     4         permissible for an implementation.  No bias  for  or  against
+     5         the  option is implied.  "May not" denotes strict prohibition
+     6         when used as a directive.
+
+
+     7     Non-Sequential Commands
+
+     8         Commands whose execution order MSCP servers may rearrange  in
+     9         order  to  optimize  performance.  The optimization shall not
+    10         move a Non-Sequential command past the barrier imposed  by  a
+    11         Sequential command.
+
+
+    12     Nugatory
+
+    13         Of little or no consequence:  Trifling, Inconsequential.  See
+    14         Section 6.4, "AVAILABLE Command." 
+
+
+    15     Prefetch
+
+    16         The act of reading data logically  following  requested  data
+    17         during  a data transfer command.  If the user access patterns
+    18         frequently access the  data  following  previously  requested
+    19         data, performance improvement will result.
+
+
+    20     Rejected Command
+
+    21         A command that the MSCP server rejects, discards, aborts,  or
+    22         otherwise  finishes  before  it  begins  to  execute it.  See
+    23         Section 4.5, "Command Categories and Execution Order." 
+
+
+    24     Sequential Commands
+
+    25         Commands that MSCP servers shall execute in the  exact  order
+    26         that  they  were  received  from  class  drivers.  Sequential
+    27         commands typically change a unit's state or context.
+
+
+    28     Shall
+
+    29         Denotes an  architectural  requirement.   It  is  used  as  a
+    30         directive to express what is mandatory for an implementation.
+    31         "Shall not" denotes strict prohibition.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           TERMINOLOGY                                              Page 2-4
+                                                                11 June 1992
+
+
+     1     Should
+
+     2         Denotes an emphatic recommendation when used as a  directive.
+     3         It  is  used where "shall" might be too strong because of the
+     4         possibility of technical, implementation  specific  obstacles
+     5         or other overriding considerations.
+
+
+     6     Special Commands
+
+     7         Commands that have both the execution  order  constraints  of
+     8         Non-Sequential   commands   plus   certain  special,  command
+     9         dependent execution order constraints.
+
+
+    10     Terminated Command
+
+    11         A command that the MSCP server terminates after it  has  been
+    12         partially executed.  See Section 4.5, "Command Categories and
+    13         Execution Order." 
+
+
+    14     Write-back Cache
+
+    15         A cache in which a write operation places  the  data  in  the
+    16         cache and the completion response is returned before the data
+    17         is written to the backing-store.
+
+    18     Write-through Cache
+
+    19         A cache in which a write operation places  the  data  in  the
+    20         cache   and   writes  it  to  the  backing-store  before  the
+    21         completion response is returned.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 3
+
+     2                 CLASS DRIVER / MSCP SERVER COMMUNICATIONS
+
+
+     3     3.1  Connection
+
+     4     Host class drivers use the host port driver to  communicate  with
+     5     MSCP  servers  in  controllers.   MSCP  servers similarly use the
+     6     controller port driver  to  communicate  with  class  drivers  in
+     7     hosts.   This  communication  takes  place across a link called a
+     8     connection.
+
+     9     The state of the connection is directly equivalent to  the  state
+    10     of  the  controller  or  MSCP  server  with  respect to the class
+    11     driver.  The controller is "Controller-Online" if and only if the
+    12     connection  is  established  and  functioning.  The controller is
+    13     "Controller-Available" if the connection is not established,  but
+    14     it  is  believed that it could be established.  The controller is
+    15     "Controller-Offline" if the connection is not established and  it
+    16     is believed that it cannot be established.
+
+    17     Three types  of  communications  services  are  used  across  the
+    18     connection between a class driver and an MSCP server:
+
+    19           o  A sequential message communications  service,  used  for
+    20              MSCP   control  messages  --  i.e.,  command,  end,  and
+    21              attention messages.  This service guarantees sequential,
+    22              duplicate-free delivery for all messages sent across the
+    23              same connection.  This service supports messages  of  at
+    24              least 48 bytes in length.
+
+    25           o  A datagram communication service, used  for  MSCP  error
+    26              log messages.  This service delivers messages sent on it
+    27              with very high probability.  Messages may  be  delivered
+    28              out   of   sequence,   lost,   or  duplicated,  but  the
+    29              probability of any of these occurring shall be very low.
+    30              This service supports messages of no more than 128 bytes
+    31              in length.
+
+    32           o  A block data communication service, used  to  move  data
+    33              between   hosts  and  mass  storage  controllers.   This
+    34              service  provides  a  reliable,  efficient   method   of
+    35              transferring  the  contents  of  a  named  buffer in one
+    36              subsystem  to  a  named  buffer  in  another  subsystem.
+    37              Buffers  are  identified  by  buffer  descriptors, which
+    38              identify both the buffer and  the  subsystem  (host)  in
+    39              which the buffer resides.
+
+    40     The communications mechanism or port drivers discard all messages
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-2
+           Connection                                           11 June 1992
+
+
+     1     that,  at  the time a connection is terminated, have been sent or
+     2     queued to  be  sent  via  the  sequential  message  and  datagram
+     3     services  but  have not yet been delivered.  Block data transfers
+     4     may or may not be terminated when a connection is terminated.  If
+     5     terminated,  they  may  have  already  been  partially completed.
+     6     Block data transfers from a previous incarnation of a  connection
+     7     are  guaranteed  to  have  been terminated when the connection is
+     8     re-established.
+
+     9     Besides using these three communications services directly,  MSCP
+    10     uses  the  establishment  of the connection itself to synchronize
+    11     class drivers and MSCP servers.  Either the class driver  or  the
+    12     MSCP  server  will  terminate the connection between them (become
+    13     "Controller-Available")  if  it   determines   that   they   must
+    14     resynchronize  with each other.  Events that require class driver
+    15     / MSCP server resynchronization include certain errors or loss of
+    16     context by either process.  The connection is also terminated, by
+    17     a port driver, if an unrecoverable communications  error  occurs.
+    18     Termination   of   the  connection  signals  the  processes  that
+    19     resynchronization  is  necessary.    The   resynchronization   is
+    20     accomplished  by  each  process  discarding all context regarding
+    21     outstanding  commands  or  transactions,  after   which   a   new
+    22     connection is established.
+
+    23     Following  resynchronization,  commands  which  were  outstanding
+    24     before  the resynchronization was performed may have completed to
+    25     an indeterminate extent.  Such commands may  have  been  rejected
+    26     (never  started), may have been terminated (partially completed),
+    27     or may have been fully completed.  The  only  guarantee  is  that
+    28     they  are  no longer outstanding, implying that the controller is
+    29     no longer performing work for them and that the class driver will
+    30     not  receive  an  end  message  for  them.   The  fact  that  the
+    31     controller is no longer performing work for them implies that  no
+    32     state changes or modification of data will take place as a result
+    33     of such commands.
+
+    34     3.2  Flow Control
+
+    35     Especially critical to MSCP is the concept of  flow  control  and
+    36     the  flow  control  based requirements that MSCP imposes on class
+    37     drivers and MSCP servers.  These items are discussed below.
+
+    38     Flow control arises from the need to avoid the congestion  and/or
+    39     deadlock  which  can  occur  if  one  process  sends messages too
+    40     quickly to another process.   The  receiving  process  must  have
+    41     buffers  in  which to place the incoming messages.  When all such
+    42     buffers are full, additional messages cannot be handled.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-3
+           Flow Control                                         11 June 1992
+
+
+     1     The datagram communications service does not  use  flow  control.
+     2     If   no   buffers  are  available,  incoming  datagrams  will  be
+     3     discarded.  Thus the characteristic  that  the  datagram  service
+     4     does  not  guarantee  delivery,  instead  only  assuring  a  high
+     5     probability of delivery.  This high probability is dependent upon
+     6     the  receiver  (i.e.,  the  class  driver)  always having buffers
+     7     queued for incoming datagrams.
+
+     8     The sequential  message  communications  service  does  use  flow
+     9     control.   When a potential receiving process queues a buffer for
+    10     receiving messages on a connection, the presence of  this  buffer
+    11     is  communicated  (via  the underlying communications service) to
+    12     the potential sending process at the other end of the connection.
+    13     This  message  notifying  the  potential  sending  process of the
+    14     queued buffer grants the sending process a credit, which  is  the
+    15     privilege  to  send  a  message.  Therefore messages will only be
+    16     sent when the sending process knows that  the  receiving  process
+    17     has  queued  a  buffer  into  which  the message can be received,
+    18     ensuring that the receiving process will be able  to  handle  the
+    19     message.
+
+    20     A typical implementation of flow  control  will  be  somewhat  as
+    21     follows.   The  port driver maintains a counter on behalf of each
+    22     process participating in a connection.  That  counter  holds  the
+    23     process's  current  credit balance -- i.e., the number of receive
+    24     buffers that its partner has queued less the number  of  messages
+    25     that  it  has  sent.   Every  time the process's partner queues a
+    26     receive buffer, a message is  sent  causing  the  counter  to  be
+    27     incremented.  Every time the process sends a message, the counter
+    28     is decremented.  Messages may  only  be  sent  when  the  counter
+    29     (credit balance) is greater than zero, thus guaranteeing that the
+    30     counter will never be negative.  Indeed, we will see  later  that
+    31     some  messages  require  that  the  counter  be  greater  than  a
+    32     threshold larger than zero.
+
+    33     Due to the inherent asynchrony of communications between multiple
+    34     processes  or  subsystems,  revoking  or  canceling  a previously
+    35     queued receive buffer is not  straightforward.   The  problem  is
+    36     that  the  buffer cannot be revoked and returned to the receiving
+    37     process until after the  sending  process  has  acknowledged  its
+    38     revocation,  as otherwise the sending process may attempt to send
+    39     a message  that  requires  the  revoked  buffer.   Therefore  the
+    40     algorithm for revoking receive buffers is as follows:
+
+    41          1.  The  revoking  process  (the  process  which  originally
+    42              queued the receive buffers) requests that some number of
+    43              buffers be revoked.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-4
+           Flow Control                                         11 June 1992
+
+
+     1          2.  The revocation request is communicated to  the  revoking
+     2              process's partner (actually to its port driver).
+
+     3          3.  The  revoking  process's  partner  (actually  its   port
+     4              driver)  compares  the  number  of buffers to be revoked
+     5              against its current credit balance and a threshold.   If
+     6              the requested number of buffers / credits can be revoked
+     7              (i.e.,  subtracted  from  the  credit  balance)  without
+     8              lowering  the  credit  balance below the threshold, then
+     9              all of them will be revoked.  Otherwise, if  the  credit
+    10              balance  is  above  the  threshold,  it  is  set  to the
+    11              threshold and the difference between  its  former  value
+    12              and  the  threshold  is  the number of buffers / credits
+    13              actually revoked.  If the credit balance is  already  at
+    14              or  below  the  threshold, then it stays the same and no
+    15              buffers / credits are revoked.
+
+    16          4.  The actual  number  of  buffers  /  credits  revoked  is
+    17              communicated back to the revoking process's port driver.
+
+    18          5.  The revoking process's port driver returns  the  buffers
+    19              actually revoked to the revoking process.
+
+    20     If a threshold of zero is used, the revoking or receiving process
+    21     can  always  get  back  all  of  its  buffers.   The fact that an
+    22     attempted revocation failed implies that the buffers have already
+    23     been  returned  to the process, since messages have been received
+    24     into them.
+
+    25     The above algorithm uses a threshold to prevent revocation  below
+    26     some  lower  limit.   The  mechanism  by  which this threshold is
+    27     obtained  is  not  critical  to  either  MSCP  or  to  the  above
+    28     algorithm.   The rules below are phrased as if the threshold were
+    29     supplied by the  revoking  process  as  part  of  the  revocation
+    30     request.   This is purely to simplify the wording of those rules;
+    31     often  the  thresholds  will  be  constants  determined  when   a
+    32     connection is established.  In such an implementation a threshold
+    33     of zero should be used when the class driver is revoking  credits
+    34     it  has  granted to the MSCP server and a threshold of one should
+    35     be used when the MSCP server is revoking credits it  has  granted
+    36     to the class driver.
+
+    37     MSCP is only concerned with credits  required  by  the  sequenced
+    38     message   service.   Some  communications  services  may  require
+    39     credits for the block data communication service  as  well.   Any
+    40     such  credits are invisible to MSCP, being communications service
+    41     dependent, and shall be  provided  in  addition  to  the  credits
+    42     required by the rules below.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-5
+           Flow Control                                         11 June 1992
+
+
+     1     Note that the above  discussion  merely  describes  a  conceptual
+     2     model   for   flow   control   within   the   sequential  message
+     3     communications  service.   There  is  no  requirement  that  flow
+     4     control  actually  be  implemented  this  way,  provided that the
+     5     results are the same.  For example,  almost  all  implementations
+     6     will  carry  credit information in a header added to messages and
+     7     processed by the receiving process's  port  driver,  rather  than
+     8     communicating  credits  with  separate  messages.  Some extremely
+     9     well behaved communications mechanisms may not need to  implement
+    10     explicit flow control at all, since the underlying communications
+    11     mechanism may provide it implicitly.
+
+    12     3.2.1  Class Driver Responsibilities
+
+    13     Given the  above  model  for  flow  control,  we  can  state  the
+    14     requirements  MSCP  places  on  class  drivers  and MSCP servers.
+    15     Class drivers shall obey the following rules:
+
+    16          1.  All MSCP commands fall into one of  several  categories;
+    17              for  this  discussion  we  distinguish between Immediate
+    18              commands   (one   specific   command    category)    and
+    19              non-Immediate  commands  (the union of all other command
+    20              categories).  When the class driver's credit balance  is
+    21              zero, the class driver may not issue any commands.  When
+    22              it is one, the class driver  may  only  issue  Immediate
+    23              commands.   When  it  is two or larger, the class driver
+    24              may issue both Immediate and non-Immediate commands.  If
+    25              the  class driver's credit balance is one and there is a
+    26              GET COMMAND STATUS command waiting to be issued for  the
+    27              command  timeout  algorithm, then the class driver shall
+    28              issue that  GET  COMMAND  STATUS  command  as  the  next
+    29              command.
+
+    30              In essence, this rule means that the class driver  shall
+    31              reserve  one credit for the exclusive use of the command
+    32              timeout algorithm.  This credit may  be  "borrowed"  for
+    33              issuing  Immediate  commands, since such commands always
+    34              complete quickly.  The goal is  to  guarantee  that  the
+    35              command   timeout  algorithm  will  always  be  able  to
+    36              promptly issue a GET COMMAND STATUS command.
+
+    37          2.  The class driver shall queue a receive buffer  for  each
+    38              command  that  it  sends to an MSCP server.  The receive
+    39              buffer will be used to hold the command's  end  message.
+    40              The  receive  buffer  shall  be queued either before the
+    41              command is sent or as part of  an  atomic  (indivisible)
+    42              action that includes sending the command.  The important
+    43              point is that the MSCP server must  receive  the  credit
+    44              for  the  receive  buffer  either before or concurrently
+    45              with receiving the command.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-6
+           Flow Control                                         11 June 1992
+
+
+     1          3.  In addition to queuing receive buffers for end messages,
+     2              class drivers that enable attention messages shall queue
+     3              at  least  one  receive  buffer  in  which  to   receive
+     4              attention  messages.   Such  a  receive  buffer shall be
+     5              queued  before  the  class  driver   enables   attention
+     6              messages  --  i.e.,  before the class driver sends a SET
+     7              CONTROLLER   CHARACTERISTICS   command   that    enables
+     8              attention  messages.   Additional receive buffers may be
+     9              queued at any time.  Note that, with  most  controllers,
+    10              there  is  no benefit to queuing more than one attention
+    11              message receive buffer.
+
+    12          4.  Upon receiving an attention message,  the  class  driver
+    13              shall  immediately  queue  another (or the same) receive
+    14              buffer back  for  more  attention  messages.   The  only
+    15              resource  that  the  class  driver  may  require between
+    16              receiving  an  attention  message  and  queuing  another
+    17              receive  buffer  is  host  CPU cycles.  The class driver
+    18              shall not require that it be able to send or receive any
+    19              other  messages  or  wait for any I/O to complete before
+    20              queuing  another  receive  buffer.    This   effectively
+    21              requires  that  all  code  and data structures needed to
+    22              process attention messages be  permanently  resident  in
+    23              physical  memory,  so that an incoming attention message
+    24              can be immediately processed and the buffer in which  it
+    25              was received immediately requeued.
+
+    26          5.  With one exception, the class driver shall never  revoke
+    27              receive  buffers.   The one exception is after disabling
+    28              attention messages --  i.e.,  after  receiving  the  end
+    29              message  for  the SET CONTROLLER CHARACTERISTICS command
+    30              that disabled attention  messages.   At  that  time  the
+    31              class driver may revoke as many buffers as it has queued
+    32              for attention messages (i.e., total  number  of  buffers
+    33              queued  less  number  of  outstanding  commands).   This
+    34              revocation should specify a  threshold  of  zero.   Note
+    35              that  this  revocation  is  guaranteed to succeed if the
+    36              MSCP server is operating correctly.
+
+    37              Note that the class driver can accomplish the effect  of
+    38              revoking  end  message  receive buffers by aborting MSCP
+    39              commands.  The receive buffers will be returned  to  the
+    40              class driver as soon as the (aborted) commands complete.
+
+    41     Failure of a class driver to follow the above rules may  lead  to
+    42     controller  deadlock,  command  timeouts,  or  the controller not
+    43     obeying its rules given below.  Any connection or process in  the
+    44     controller's subsystem may be affected, rather than just the MSCP
+    45     server with which the class driver is communicating.   Note  that
+    46     class  drivers  that enable error logging must also keep datagram
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-7
+           Flow Control                                         11 June 1992
+
+
+     1     buffers queued so that they can receive error log messages.   Not
+     2     keeping  datagram  buffers queued may result in loss of error log
+     3     messages.
+
+     4     3.2.2  MSCP Server Responsibilities
+
+     5     The  rules  for  MSCP  servers  vary   depending   upon   certain
+     6     characteristics  of  the  server.   There  is  one general set of
+     7     rules, plus a simplification that certain classes of servers  may
+     8     follow.   In  all cases, an MSCP server's implementation of these
+     9     rules may require, for its correct operation (and thus  adherence
+    10     to  these  rules),  that class drivers correctly follow the rules
+    11     given for them above.  The general set of rules, which  shall  be
+    12     followed  by  all MSCP servers that are not in any of the special
+    13     cases identified later, are as follows:
+
+    14          1.  So long as it is "Controller-Online" to a class  driver,
+    15              an  MSCP  server shall ensure that the sum of the number
+    16              of commands that are outstanding from that class  driver
+    17              plus the number of unused receive buffers / credits that
+    18              it has granted to that class driver is never lower  than
+    19              the  values given below.  That is, the sum of the actual
+    20              and potential outstanding commands shall be at least the
+    21              values  below.   Between  the  time  that the controller
+    22              becomes "Controller-Online" and the  completion  of  the
+    23              first  SET  CONTROLLER CHARACTERISTICS command, this sum
+    24              shall be at least one.  Following the completion of  the
+    25              first  SET  CONTROLLER  CHARACTERISTICS  command, and so
+    26              long as the controller remains "Controller-Online," this
+    27              sum  shall  be at least two.  The first unit of this sum
+    28              allows the class driver  to  issue  Immediate  commands.
+    29              Any  excess,  beyond the value one, may be used to issue
+    30              "real" commands such as data transfers.  Note that these
+    31              requirements  imply  that,  following  a  SET CONTROLLER
+    32              CHARACTERISTICS command, the MSCP  server  shall  either
+    33              grant  a minimum of two receive buffers / credits to the
+    34              class driver or else terminate  the  connection  (become
+    35              "Controller-Available") with the class driver.
+
+    36          2.  So long as it is "Controller-Online" to a class  driver,
+    37              an  MSCP  server shall ensure that the sum of the number
+    38              of Immediate commands that  are  outstanding  from  that
+    39              class driver plus the number of unused receive buffers /
+    40              credits that it has granted  to  that  class  driver  is
+    41              always at least one.  That is, the sum of the actual and
+    42              potential outstanding Immediate  commands  shall  be  at
+    43              least one.  This is in addition to requirement 1 above.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-8
+           Flow Control                                         11 June 1992
+
+
+     1          3.  An MSCP server may revoke receive buffers or credits  at
+     2              any  time so long as it continues to meet requirements 1
+     3              and 2 above.  Note that, in order to meet requirement 2,
+     4              the  sum  of  the  threshold used for the revoke request
+     5              plus the number of outstanding Immediate commands  (from
+     6              that   class   driver)  must  be  at  least  one.   This
+     7              restriction will typically be  met  by  always  using  a
+     8              threshold of one.
+
+     9          4.  Notwithstanding all  that  has  been  said  above,  MSCP
+    10              servers  shall  allow hosts to perform transfers without
+    11              the   host   having   to   issue   a   SET    CONTROLLER
+    12              CHARACTERISTICS  command.  This is necessary to simplify
+    13              the implementation of host bootstraps.  The commands  to
+    14              perform  such  transfers  shall  be issued one at a time
+    15              (rule 1 above ensures that the host has at least the one
+    16              credit  it  needs  to  issue  these commands).  The MSCP
+    17              server shall either grant the  host  a  minimum  of  two
+    18              credits  when  it  becomes  "Controller-Online"  or else
+    19              perform commands regardless of the fact that the host is
+    20              nominally   violating   the  requirements  described  in
+    21              Section 3.2.1,  "Class  Driver  Responsibilities."   The
+    22              specific  requirement  violated is that the host will be
+    23              issuing non-Immediate commands in spite  of  its  credit
+    24              balance being only one.
+
+    25          5.  An MSCP server shall keep track of the excess,  if  any,
+    26              of  its  credit  balance  over the number of outstanding
+    27              commands.  The server may only issue attention  messages
+    28              when  this  excess  is  greater  than  zero.   Attention
+    29              messages that cannot be  issued  immediately  should  be
+    30              saved  until  credits  are available with which to issue
+    31              them.  The  controller  shall  continue  to  accept  new
+    32              commands,  process  outstanding  commands, and issue end
+    33              messages for completed commands while attention messages
+    34              are  being  saved.  If the conditions that triggered the
+    35              generation of an attention message disappear before that
+    36              attention   message  can  be  issued  (sent),  then  the
+    37              attention message may or may not still have to be  sent.
+    38              See   the   individual  attention  message  descriptions
+    39              contained in Section 5.8, "Attention Messages." 
+
+    40     With many communications  mechanisms  there  is  an  inherent  or
+    41     unavoidable  race  that MSCP servers must be prepared to resolve.
+    42     It can occur whenever the credit for an end message might be sent
+    43     as  a  distinct message, rather than always being embedded in the
+    44     communications mechanism header  for  the  corresponding  command
+    45     message.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS                Page 3-9
+           Flow Control                                         11 June 1992
+
+
+     1     Suppose the host class driver grants  one  credit  for  attention
+     2     messages,  and  that  some  combination of events causes the MSCP
+     3     server to want to send two  attention  messages.   The  attention
+     4     message  credit  is  used to send the first attention message and
+     5     the second is queued  for  later  transmission.   Meanwhile,  the
+     6     class  driver  sends  a  command  and  its associated end message
+     7     credit to the MSCP server as  two  distinct  messages.   Per  the
+     8     class  driver  requirements  described  in  Section 3.2.1, "Class
+     9     Driver Responsibilities," the notification  of  the  end  message
+    10     credit must be sent first.  When this notification arrives at the
+    11     controller, the MSCP server thinks it is  the  attention  message
+    12     credit being returned (since credits are anonymous) and sends the
+    13     second attention message.   Immediately  thereafter  the  command
+    14     arrives  and the MSCP server discovers an apparent rule violation
+    15     by the class driver, namely  that  it  appears  to  have  sent  a
+    16     command without an accompanying credit for the end message.
+
+    17     This credit imbalance problem will ultimately  be  resolved  when
+    18     the  class  driver  returns the attention message credit.  As the
+    19     class driver is not allowed to perform any I/O  before  returning
+    20     attention message credits, we are assured that the credit will be
+    21     returned both promptly and without any possibility  of  deadlock.
+    22     Therefore,  whenever  this  race occurs, the MSCP server may hang
+    23     until the attention message credit is returned.  Although  it  is
+    24     permissible  for  all  MSCP  servers  for  all device classes and
+    25     connections to hang, it is  highly  desirable  if  only  the  one
+    26     connection is affected.
+
+    27     Typically the command(s) that  is(are)  lacking  an  end  message
+    28     credit  would be held in a queue, with no work done on them until
+    29     their end message credits arrive.  Note that if the host did  not
+    30     return  attention  message credits promptly, this would result in
+    31     command timeouts.  The MSCP  server  shall  service  the  waiting
+    32     commands  in  their  order  of  arrival.  Thus if another command
+    33     arrives with a credit embedded in  its  communications  mechanism
+    34     header,  the MSCP server shall transfer that credit to the oldest
+    35     command waiting for an end  message  credit  and  place  the  new
+    36     command on the waiting queue.
+
+    37     MSCP servers that limit the rate at which they generate attention
+    38     messages  can  replace  rule  5  above with a simpler rule.  This
+    39     alternate rule may be used, at the server's option, by  any  MSCP
+    40     server  that  will  generate  no  more  than  an  average  of two
+    41     attention messages  per  second,  averaged  over  the  controller
+    42     timeout  interval  (see  Section 4.10, "Command Timeouts").  That
+    43     is, if the controller timeout interval is  N  seconds,  then  the
+    44     server  will  generate no more than N*2 attention messages within
+    45     any N second period.  The MSCP server may assume, in  determining
+    46     its  maximum  attention message rate, that human operators do not
+    47     engage in pathological activity.  I.e., it may assume that  cases
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS               Page 3-10
+           Flow Control                                         11 June 1992
+
+
+     1     such  as an operator continuously actuating the Run/Stop switches
+     2     on one or more drives will never occur.  Any MSCP server that can
+     3     meet  this  attention message rate restriction may substitute the
+     4     following alternate rule for rule 5 above:
+
+     5          5'. An MSCP server may  issue  attention  messages  and  end
+     6              messages  whenever  its  credit  balance is greater than
+     7              zero.  Attention messages and end messages  that  cannot
+     8              be  issued immediately should be saved until credits are
+     9              available with which to issue them.  If  the  conditions
+    10              that  triggered  the  generation of an attention message
+    11              disappear before the attention  message  can  be  issued
+    12              (sent), then that attention message may or may not still
+    13              have to be sent.  See the individual  attention  message
+    14              descriptions   contained   in  Section  5.8,  "Attention
+    15              Messages."  Saved end messages  shall  always  be  sent,
+    16              unless     the     MSCP     server     first     becomes
+    17              "Controller-Available" (i.e., terminates its  connection
+    18              with  the  class  driver),  in  which case the saved end
+    19              messages shall not be  sent.   Note  that  end  messages
+    20              shall   always   be   sent   in  the  order  that  their
+    21              corresponding commands completed.
+
+    22              An MSCP server may deadlock  or  cease  operating  on  a
+    23              connection  whenever  it has saved attention messages or
+    24              end messages waiting to be issued  on  that  connection.
+    25              The  only operations that it is required to perform when
+    26              it has such messages  waiting  are  to  accept  incoming
+    27              credit  notifications,  send  the  waiting messages when
+    28              credits become available, and  resume  normal  operation
+    29              when  all  waiting  messages  have been sent.  Note that
+    30              accepting  incoming  credit  notifications  will   often
+    31              require  that  the MSCP server also accept new commands,
+    32              although it need  not  begin  processing  of  those  new
+    33              commands  until  all  waiting  messages  have been sent.
+    34              Note also that only the one MSCP server may deadlock  or
+    35              cease  operating;  other  processes  (i.e.,  other  MSCP
+    36              servers) in the same subsystem or controller  and  other
+    37              connections  to  the  same  MSCP  server  shall  not  be
+    38              affected.
+
+    39     The effect of this modified rule is  to  allow  MSCP  servers  to
+    40     "borrow" end message credits for the purpose of sending attention
+    41     message credits.
+
+    42     3.3  Multihost Communication
+
+    43     Chapter 1, "Introduction," Figure 1-1, "Example System," shows  a
+    44     "Single  Host"  system,  where  a  single  host is connected to a
+    45     single  controller.   Certain  communications  mechanisms  permit
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CLASS DRIVER / MSCP SERVER COMMUNICATIONS               Page 3-11
+           Multihost Communication                              11 June 1992
+
+
+     1     communications  between  multiple entities (nodes) interconnected
+     2     along  the  same  physical  path.   With  such  a  communications
+     3     mechanism  it  is  possible to create a "Multihost" system, where
+     4     multiple hosts are connected to a single controller.
+
+     5     Controllers which  support  multinode  communications  mechanisms
+     6     will  usually (not an absolute requirement of MSCP) implement the
+     7     Multihost Support option (see Section 5.6,  "Controller  Flags").
+     8     Multihost   Support  allows  simultaneous  communication  between
+     9     multiple  host  class  drivers   and   an   MSCP   server.    The
+    10     communication  takes  place  across  separate logical connections
+    11     established between the class drivers and the MSCP  server.   The
+    12     MSCP  server shall treat each connection with a class driver as a
+    13     separate and unique entity, with each connection obeying all  the
+    14     communication  rules  detailed  in the preceding sections of this
+    15     chapter.  New connections may be  accepted  or  rejected  without
+    16     regard to the number of existing connections (other than resource
+    17     limitations), and no  notification  will  be  given  to  the  new
+    18     connection  as  to  the presence or absence of other connections.
+    19     Class driver/server synchronization is on a per connection basis,
+    20     and  the  loss  of synchronization between a class driver and the
+    21     controller on a specific connection should not affect  any  other
+    22     connections  which  may  be  open,  unless  all  connections  are
+    23     affected and a hard initialization of the controller is required.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 4
+
+     2                        ALGORITHMS AND USAGE RULES
+
+
+     3     4.1  Controller States
+
+     4     The controller may be in any of three states relative to  a  host
+     5     class  driver.   The  controller  may  be  in  a  different state
+     6     relative to each host  or  each  class  driver.   The  controller
+     7     states are:
+
+     8     Controller-Offline
+
+     9         A  controller  is  "Controller-Offline"  to  a  class  driver
+    10         whenever  it is not available to that class driver and cannot
+    11         perform  any  operations  on  its  behalf.   Possible  causes
+    12         include  inoperative  hardware  or  an operator disabling the
+    13         controller.  A  controller  is  "Controller-Offline"  exactly
+    14         when it is not possible to establish a connection between the
+    15         class driver and the MSCP server within the controller.  Note
+    16         that  a  controller  may be "Controller-Offline" to some of a
+    17         host's  class  drivers  yet  be   "Controller-Available"   or
+    18         "Controller-Online" to others.
+
+    19     Controller-Available
+
+    20         A controller is  "Controller-Available"  to  a  class  driver
+    21         whenever  it  could perform operations for that class driver,
+    22         but the driver has not yet synchronized with the  controller.
+    23         A  controller is "Controller-Available" exactly when it would
+    24         be possible to  establish  a  connection  between  the  class
+    25         driver  and  the  MSCP  server  within the controller, but no
+    26         connection has yet been established.
+
+    27     Controller-Online
+
+    28         A  controller  is  "Controller-Online"  to  a  class   driver
+    29         whenever it can both perform operations for that class driver
+    30         and the driver  has  synchronized  with  the  controller.   A
+    31         controller  is  "Controller-Online" exactly when a connection
+    32         exists between the class driver and the  MSCP  server  within
+    33         the controller.  This is the state used for normal operation.
+
+    34     Strictly speaking, the term "controller  state"  is  a  misnomer.
+    35     The  states  described above actually exist between an individual
+    36     class driver and an individual MSCP  server.   A  host  may  have
+    37     several class drivers and a controller subsystem may have several
+    38     MSCP servers (usually  one  for  each  device  class  supported).
+    39     Furthermore,  controllers  that  provide  Multihost Support allow
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-2
+           Controller States                                    11 June 1992
+
+
+     1     multiple connections between its  server(s)  and  multiple  class
+     2     drivers resident in one or more hosts.
+
+     3     Note also that the  controller  state  (MSCP  server  state?)  is
+     4     distinct from the state of any unit connected to the controller.
+
+     5     An MSCP server (controller) enters the "Controller-Offline" state
+     6     relative to a host whenever the MSCP server ceases to function or
+     7     otherwise becomes unable to  perform  operations  for  the  host.
+     8     Possible causes include:
+
+     9          1.  Controller hardware, software, or power failure.
+
+    10          2.  Controller   initialization,   either    requested    or
+    11              spontaneous.
+
+    12          3.  An operator (typically Field Service)  disables  all  or
+    13              part of the controller.
+
+    14          4.  Communications mechanism failures.
+
+    15          5.  The controller has not been built yet, the controller is
+    16              still in its shipping crate, or it has otherwise not yet
+    17              been installed.
+
+
+    18     An MSCP server enters the "Controller-Available"  state  relative
+    19     to a host class driver when:
+
+    20          1.  The controller or MSCP server  is  "Controller-Offline,"
+    21              and  all  causes  of  it  being "Controller-Offline" are
+    22              removed.
+
+    23          2.  The MSCP server is  "Controller-Online,"  and  the  MSCP
+    24              server cannot successfully send a control message (i.e.,
+    25              an MSCP end or attention  message)  to  the  host  class
+    26              driver.
+
+    27          3.  The MSCP server is  "Controller-Online,"  and  the  host
+    28              access  timeout  expires.  See Section 4.9, "Host Access
+    29              Timeouts." 
+
+    30          4.  The MSCP server is  "Controller-Online,"  and  the  MSCP
+    31              server  receives  an  "Invalid  Command"  (see  "Invalid
+    32              Command" status in Section 5.5, "Status Codes") from the
+    33              host.       Note     that     this     transition     to
+    34              "Controller-Available"  is   optional,   and   therefore
+    35              controller dependent.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-3
+           Controller States                                    11 June 1992
+
+
+     1          5.  The host class driver terminates the connection  between
+     2              the class driver and the MSCP server.
+
+     3          6.  A port driver or the communications mechanism terminates
+     4              the  connection  between  the  class driver and the MSCP
+     5              server, generally due to a communications error.
+
+     6          7.  An MSCP server terminates the connection between  itself
+     7              and  a host class driver as a result of the execution of
+     8              a TERMINATE CLASS DRIVER/SERVER  CONNECTION  command  as
+     9              described  in  Section  7.10, "Multihost TERMINATE CLASS
+    10              DRIVER/SERVER CONNECTION Command." 
+
+
+    11     The port driver should inform the class driver whenever the  MSCP
+    12     server  enters  the  "Controller-Available"  state.  How the port
+    13     driver  obtains  this  information  is  communications  mechanism
+    14     dependent.   Note  that  the notification that the controller has
+    15     become "Controller-Available"  is  not  necessarily  prompt.   In
+    16     particular,  with some communications mechanisms the notification
+    17     may not occur until the next  time  the  class  driver  issues  a
+    18     command to the controller.  Furthermore, the port driver need not
+    19     notify the class driver at all if a compound (multiple) error  is
+    20     associated with the MSCP server becoming "Controller-Available." 
+    21     In such a case the class driver will ultimately become  aware  of
+    22     the  state  change  when its Command Timeout expires (see Section
+    23     4.10, "Command Timeouts").
+
+    24     Since  no  connection  exists  to  an   MSCP   server   that   is
+    25     "Controller-Offline"      or      "Controller-Available,"     the
+    26     communications  mechanism  will  either  reject  or  discard  any
+    27     messages  (commands)  that a class driver attempts to send to it.
+    28     An   MSCP   server   that   becomes    "Controller-Offline"    or
+    29     "Controller-Available"  may either terminate commands in progress
+    30     or else continue processing the  commands  that  it  has  already
+    31     received.    However,  if  a  Sequential  command  from  a  given
+    32     connection is  terminated  or  rejected,  then  all  subsequently
+    33     received  non-Immediate  commands  from the same connection shall
+    34     also be rejected (i.e., shall never begin processing).
+
+    35     Typically, the MSCP server will continue  processing  outstanding
+    36     commands  until  it  "notices"  that  the connection to the class
+    37     driver has been terminated, at which point it will terminate  any
+    38     commands  still  outstanding and reject any commands that haven't
+    39     begun processing yet.  Note that the MSCP server shall  guarantee
+    40     that  all  outstanding  commands  have  been  completed, aborted,
+    41     rejected, or terminated (i.e.,  that  there  are  no  outstanding
+    42     commands)    before    it    completes    a    transition    from
+    43     "Controller-Available" to "Controller-Online." 
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-4
+           Controller States                                    11 June 1992
+
+
+     1     The MSCP server enters the "Controller-Online" state relative  to
+     2     a  host  class  driver  upon  successful synchronization with the
+     3     class driver.  The class driver synchronizes with the MSCP server
+     4     by establishing a connection with the MSCP server.  Note that the
+     5     MSCP  server  shall  guarantee  that  there  are  no  outstanding
+     6     commands "leftover" from a previous incarnation of the connection
+     7     before it allows the new incarnation  of  the  connection  to  be
+     8     established and enters the "Controller-Online" state.
+
+     9     4.2  Controls And Indicators
+
+    10     All storage units used with MSCP must have the following controls
+    11     and indicators:
+
+    12           o  Unit number select mechanism.
+
+    13           o  Unit number display mechanism.
+
+    14           o  Run/Stop switch (on disk units) or a Load/Unload  switch
+    15              (on tape units).
+
+    16           o  Write protect switch or mechanism.
+
+    17           o  Write protect status indicator.
+
+
+    18     The unit number select mechanism on an MSCP storage unit shall be
+    19     capable  of specifying any unit number in the range 0 through 251
+    20     inclusive.  Larger unit number ranges are acceptable, so long  as
+    21     they  include  the  range  0 through 251.  The unit number select
+    22     mechanism shall  operate  without  host  intervention  and  shall
+    23     preserve  unit  numbers across power failures and other losses of
+    24     context.
+
+    25     Alteration of the unit number must  be  possible  in  the  field.
+    26     That  is,  the  unit  number shall be alterable by Field Service,
+    27     both when a device is first installed  and  subsequently  when  a
+    28     system is reconfigured.  The preferred unit select mechanism is a
+    29     removable unit number  plug,  allowing  the  unit  number  to  be
+    30     altered  by  users  as well as by Field Service.  Alternatives to
+    31     unit number plugs, however, are acceptable so long as they can be
+    32     altered  by Field Service and they provide the full 0 through 251
+    33     unit number range.
+
+    34     A single unit number select mechanism may be shared by the  units
+    35     of a multiunit drive or formatter (see Section 4.16.2, "Multiunit
+    36     Drives and Formatters").  Units that share a unit  number  select
+    37     mechanism shall always have consecutive unit numbers.  If exactly
+    38     two units share a unit number select mechanism, it is  acceptable
+    39     for  the  first  unit  to always have an even unit number and the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-5
+           Controls And Indicators                              11 June 1992
+
+
+     1     last unit to always have an odd unit number.  If exactly N  units
+     2     share  a  unit  number select mechanism, it is acceptable for the
+     3     first unit's unit number to always be a multiple of N,  the  next
+     4     unit's unit number to always be a multiple of N plus 1, etc.
+
+     5     The unit number display mechanism on an MSCP storage  unit  shall
+     6     display  the  unit  number(s) specified by the unit number select
+     7     mechanism.  The display shall be  visible  to  normal  (non-Field
+     8     Service) human operators.  If the unit number select mechanism is
+     9     a removable unit plug, then the unit number display mechanism may
+    10     merely be a number printed on the plug.  If several units share a
+    11     unit number select mechanism, then they may  also  share  a  unit
+    12     number display mechanism.
+
+    13     The Run/Stop switch or Load/Unload switch shall be  alterable  by
+    14     normal  (non-Field  Service) human operators to allow or disallow
+    15     host access to the unit(s).  When in the Run  or  Load  position,
+    16     this  switch indicates that hosts should be allowed to access the
+    17     unit(s).  When in  the  Stop  or  Unload  position,  this  switch
+    18     indicates that hosts should not be allowed to access the unit(s).
+    19     If the unit(s) have removable media, the  switch  also  indicates
+    20     that human operators should be allowed to remove the units' media
+    21     (i.e., that the unit should be  spun-down).   A  single  Run/Stop
+    22     switch may be shared by any units of a multiunit drive that share
+    23     a spindle (i.e., that must be spun-up and spun-down together).
+
+    24     The write protect switch or mechanism shall either be an operator
+    25     accessible  switch or else some kind of mechanical deformation of
+    26     the media, such as a tape write-ring or the write-lockout tab  on
+    27     a  cassette.   In  either  case,  it shall be alterable by normal
+    28     (non-Field Service) human operators.  A  separate  write  protect
+    29     switch  or  mechanism  shall  be  provided  for  each  unit  of a
+    30     multiunit drive.  When actuated,  the  write  protect  switch  or
+    31     mechanism  prevents  write  access  to  the  unit  by  hosts  and
+    32     controllers.   In  the  case  of  a  write  protect  switch,  the
+    33     transition  to the write protected state shall be "smooth" rather
+    34     than immediate.  See Section 4.14, "Write Protection." 
+
+    35     The write protect status  display  mechanism  shall  display  the
+    36     write  protect  status  of  the  unit.   A separate write protect
+    37     status display mechanism shall be provided for  each  unit  of  a
+    38     multiunit  drive.  The write protect status display shall be user
+    39     visible while a volume is mounted in the unit.  That is, the user
+    40     shall  not  be  required  to  remove  the volume from the unit to
+    41     determine whether or not it is write protected.
+
+    42     The preferred write protect status display mechanism is a  light,
+    43     located  within the write protect switch, that is on whenever the
+    44     unit is write protected.  The light shall be  lit  regardless  of
+    45     the  reason  for the unit being write protected -- i.e., it shall
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-6
+           Controls And Indicators                              11 June 1992
+
+
+     1     be lit when the unit is Software Write Protected or  Data  Safety
+     2     Write  Protected  as  well  as  when  the  unit is Hardware Write
+     3     Protected due to its write  protect  mechanism  being  activated.
+     4     See Section 4.14, "Write Protection." 
+
+     5     4.3  Unit States
+
+     6     Each unit may be in one of three states relative  to  each  class
+     7     driver that is "Controller-Online" to an MSCP server.  (Actually,
+     8     it is really each unit number that these states apply to,  rather
+     9     than  to  a  unit  proper.) Each unit may be in a different state
+    10     relative to each "Controller-Online" class  driver.   Controllers
+    11     shall  therefore  maintain  the  unit  state  separately for each
+    12     connection.  Note that a unit's state is meaningless with respect
+    13     to  a  class  driver  that is not "Controller-Online" to the MSCP
+    14     server or when no class driver is "Controller-Online" to the MSCP
+    15     server.
+
+    16     The three unit states are:
+
+    17     Unit-Offline
+
+    18         A unit is "Unit-Offline" whenever it  is  unable  to  satisfy
+    19         normal  host  requests.   Except  for  status  queries,  MSCP
+    20         commands addressed to a unit that is "Unit-Offline" shall  be
+    21         rejected.   Furthermore,  many device characteristics may not
+    22         be available to status queries.
+
+    23     Unit-Available
+
+    24         A unit is "Unit-Available"  whenever  it  would  be  able  to
+    25         satisfy  normal  host  requests, except that the host has not
+    26         yet  issued   an   ONLINE   command   to   bring   the   unit
+    27         "Unit-Online." 
+
+    28     Unit-Online
+
+    29         A unit is "Unit-Online" whenever it is able to satisfy normal
+    30         host  requests  and  the  host has issued a successful ONLINE
+    31         command.
+
+    32     A  fourth  unit  state,  actually  a  pseudo-state,  exists  with
+    33     controllers  that  provide  Multihost  Support.  Each MSCP server
+    34     contained in such a controller  shall  implement  the  "Exclusive
+    35     Access"  feature,  which  allows  a  single  host class driver to
+    36     reserve a unit for its exclusive use.  The host that has obtained
+    37     "Exclusive  Access"  of  a unit has complete control of that unit
+    38     and may perform all normal unit operations.  All other hosts  are
+    39     prevented  from  performing  any  operation  that would alter the
+    40     state or context of the unit in any manner.  The other hosts are,
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-7
+           Unit States                                          11 June 1992
+
+
+     1     however,  permitted  to  obtain  the  unit's  characteristics  as
+     2     maintained by the controller via all commands that  return  those
+     3     characteristics.  While a unit is under the "Exclusive Access" of
+     4     a host the controller shall prevent any other controller to which
+     5     the  unit  may be connected from bringing that unit or some other
+     6     unit with which it shares an access path online with  respect  to
+     7     that  controller  (see  Sections 4.16.1, "Multiaccess Drives" and
+     8     4.16.2, "Multiunit Drives and Formatters").   "Exclusive  Access"
+     9     is  termed  a  pseudo-state  because  that  context is maintained
+    10     solely  by  the  controller  without  the  knowledge  nor  direct
+    11     participation of the unit.  The state of the unit with respect to
+    12     the host that has "Exclusive Access" is totally dependent on  the
+    13     operations  that  host  has  requested:  it may be "Unit-Online,"
+    14     "Unit-Available," or "Unit-Offline" (provided  the  unit  remains
+    15     operative   and   known   to   the   controller).   The  unit  is
+    16     "Unit-Offline" with respect to all other hosts.  Host control  of
+    17     "Exclusive  Access"  is  provided  through  the  operation of the
+    18     AVAILABLE, ONLINE or SET UNIT CHARACTERISTICS commands.  See  the
+    19     description  of those commands contained in Chapter 7, "Multihost
+    20     Support" for more details.
+
+    21     The "Unit-Offline" state has  seven  substates,  related  to  the
+    22     exact  reason for the unit being "Unit-Offline."  These substates
+    23     are:
+
+    24          1.  Unit inoperative.  Some fatal  error  condition  in  the
+    25              drive  prevents  the unit from becoming "Unit-Available"
+    26              or "Unit-Online." 
+
+    27          2.  Unit  disabled.   Field  Service  or  a  diagnostic  has
+    28              decided  that  continued  operation  of the unit's drive
+    29              will lead  to  progressive  deterioration  and  eventual
+    30              destruction  of some portion of the drive or media.  The
+    31              unit  has  been  disabled  to  prevent  its   use,   and
+    32              consequent  destruction,  until Field Service can repair
+    33              the   problem.    This   cause   of   the   unit   being
+    34              "Unit-Offline"  can  be overridden, and the unit brought
+    35              "Unit-Online," by use of the  "Allow  Self  Destruction"
+    36              modifier  to the ONLINE command.  Note that some devices
+    37              may have no way of detecting progressive  deterioration,
+    38              and consequently will never enter this substate.
+
+    39          3.  Unit known.  The controller  knows  that  the  specified
+    40              unit  (i.e.,  a  unit  with  the  specified unit number)
+    41              exists, but the unit is "Unit-Offline" for  some  normal
+    42              (nonerror)  condition.   Typical causes of this substate
+    43              include the unit's Run/Stop or Load/Unload switch  being
+    44              in  the  Stop  or  Unload  position  or  no volume being
+    45              mounted in the unit.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-8
+           Unit States                                          11 June 1992
+
+
+     1          4.  Under exclusive use of another host.  The specified unit
+     2              exists   and  would  be  "Unit-Available,"  except  that
+     3              another host connected via the same controller has  been
+     4              granted "Exclusive Access" of the unit.
+
+     5          5.  Online to another controller.  The specified unit exists
+     6              and  would  be  "Unit-Available," except that it or some
+     7              other unit with which it shares an  access  path  (i.e.,
+     8              some   other   unit  on  the  same  multiunit  drive  or
+     9              formatter) is "Unit-Online" to one or more hosts  or  is
+    10              under  the  "Exclusive  Access"  of  a  host via another
+    11              controller.  The unit will become  "Unit-Available"  via
+    12              this controller if it and all units with which it shares
+    13              an access path cease being "Unit-Online" to any hosts or
+    14              under  the  "Exclusive  Access" of a particular host via
+    15              that other controller.  In terms of  the  "Unit-Offline"
+    16              substate   that   is   visible  to  and  reported  by  a
+    17              controller, a unit that is actually  online  to  another
+    18              controller  will typically oscillate between the "Online
+    19              to another controller" substate and the  "Unit  unknown"
+    20              substate.    The   frequency   of   the  oscillation  is
+    21              determined  by  the  frequency  at  which  hosts   issue
+    22              DETERMINE  ACCESS PATHS commands to the other controller
+    23              for this unit or any unit with which it shares an access
+    24              path.   See  Sections  4.16.1,  "Multiaccess Drives" and
+    25              4.16.2,  "Multiunit  Drives  and  Formatters"  for  more
+    26              information.
+
+    27          6.  Duplicate unit numbers.  That is, two or  more  distinct
+    28              units  have  the  same  unit  number  assigned  to them.
+    29              Controllers  shall  check  for  duplicate  unit  numbers
+    30              across  all  units  of  the  same  device class that are
+    31              "Unit-Online," that are "Unit-Available," or that are in
+    32              one  of  the  following  "Unit-Offline" substates:  unit
+    33              disabled, unit known, under  exclusive  use  of  another
+    34              host,  duplicate  unit  numbers,  or  online  to another
+    35              controller.  A controller may or may not, at its option,
+    36              include  inoperative  units  when checking for duplicate
+    37              unit numbers.  Controllers may not check units that  are
+    38              unknown  (as  described  below)  nor  may they check for
+    39              duplicate unit numbers across different device  classes.
+    40              Note that a duplicate unit number does not affect a unit
+    41              that is already "Unit-Online" to or under the "Exclusive
+    42              Access" of a host.
+
+    43          7.  Unit unknown.  As far as the controller  can  determine,
+    44              no unit exists with the specified unit number.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                               Page 4-9
+           Unit States                                          11 June 1992
+
+
+     1     It is possible for  a  unit  to  be  "Unit-Offline"  for  several
+     2     reasons at the same time (unless the unit is unknown).  If a unit
+     3     has a duplicate unit number and  is  not  inoperative,  then  the
+     4     controller  shall  report the duplicate unit number; other causes
+     5     of the unit being "Unit-Offline" may  also  be  reported  at  the
+     6     controller's  option.   In  all  other cases the controller shall
+     7     report at least one cause of the unit being  "Unit-Offline,"  but
+     8     which  one it reports and whether or not it reports more than one
+     9     is optional with the controller.
+
+    10     The fact that a unit is inoperative may not be detectable until a
+    11     host  attempts  to bring the unit "Unit-Online."  Such units will
+    12     be treated as and appear to  be  "Unit-Available"  until  a  host
+    13     issues   an  ONLINE  command.   The  ONLINE  command  will  fail,
+    14     typically with a "Drive Error" status code.  At this time  either
+    15     the  fact  that  the unit is inoperative shall be recorded in the
+    16     unit  itself  or  else  AVAILABLE  attention  messages  shall  be
+    17     suppressed  for the unit, exactly as if an AVAILABLE command with
+    18     the "Spin-down" modifier set had been  issued  for  the  unit  as
+    19     described  later  in  this  section.   In  either case, AVAILABLE
+    20     attention messages shall not be generated for that  unit  by  any
+    21     controller  until  a  human interacts with the unit or some other
+    22     event occurs that may clear the error, such as a power failure.
+
+    23     Controllers  and/or  drives  should  keep  all  units  that   are
+    24     "Unit-Offline"  due  to  being  inoperative,  disabled, or having
+    25     duplicate unit numbers spun-down,  except  when  such  units  are
+    26     under  the  control  of a diagnostic.  The handling of units that
+    27     are in fact  inoperative,  but  that  the  controller  and  drive
+    28     believe to be operative, is described in the preceding paragraph.
+
+    29     Controllers may spontaneously release a host's "Exclusive Access"
+    30     of  a  unit  only  if  the unit becomes inoperative, disabled, or
+    31     unknown.
+
+    32     For the purpose of automatic configuration  (i.e.,  for  the  GET
+    33     UNIT  STATUS  command  with the "Next Unit" modifier) controllers
+    34     shall  acknowledge  the  existence  of   all   units   that   are
+    35     "Unit-Online"  or  "Unit-Available,"  or  that  are in one of the
+    36     following "Unit-Offline" substates:  unit disabled,  unit  known,
+    37     under  exclusive  use of another host, or duplicate unit numbers.
+    38     Unknown  units  shall  not  be  acknowledged.   Units  that   are
+    39     inoperative  or  online  to  another controller may or may not be
+    40     acknowledged at the controller's option.
+
+    41     Controllers shall report duplicate unit numbers with a  DUPLICATE
+    42     UNIT  NUMBER  attention  message, then monitor the affected units
+    43     for the cessation of the duplicate unit number  condition.   When
+    44     all  units  except one have had their unit number changed or have
+    45     become unknown, the remaining unit becomes "Unit-Available." 
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-10
+           Unit States                                          11 June 1992
+
+
+     1     Controllers shall report units that they know  to  be  online  to
+     2     other  controllers  with  an  ACCESS PATH attention message.  See
+     3     Section 4.16.1, "Multiaccess Drives," for more information.
+
+     4     The "Unit-Offline" substates  are  all  reported  with  a  single
+     5     status  code;  they  are  partially  distinguished  via different
+     6     subcodes.  In addition, the substates are  distinguished  by  the
+     7     functioning  of  the  "Allow  Self  Destruction"  modifier to the
+     8     ONLINE command, the "Next Unit" modifier to the GET  UNIT  STATUS
+     9     command,  what  unit characteristics are returned by the GET UNIT
+    10     STATUS, ONLINE, and SET UNIT CHARACTERISTICS commands, and by the
+    11     DUPLICATE UNIT NUMBER and ACCESS PATH attention messages.
+
+    12     Possible causes of a unit being "Unit-Offline," and the resulting
+    13     "Unit-Offline" substate, include:
+
+    14          1.  The unit is "Unit-Online" or is under "Exclusive Access"
+    15              via  another  controller.  The unit is either unknown or
+    16              else known to be online to another controller, depending
+    17              upon  how  recently the other controller has processed a
+    18              DETERMINE ACCESS PATHS command for this unit or  a  unit
+    19              with  which  it  shares  an  access  path.  See Sections
+    20              4.16.1,  "Multiaccess  Drives"  and  4.16.2,  "Multiunit
+    21              Drives and Formatters" for more information.
+
+    22          2.  A power failure  that  affects  the  unit  but  not  the
+    23              controller.  The unit is unknown.
+
+    24          3.  Hardware failure  in  the  unit  or  in  the  connection
+    25              between the unit and the controller.  The unit is either
+    26              unknown or inoperative.  Note that the unit  may  appear
+    27              to be "Unit-Available" until an ONLINE command is issued
+    28              for  it,  at  which  time  it  will  be  recognized   as
+    29              inoperative.
+
+    30          4.  Disconnecting the unit from the controller.  The unit is
+    31              unknown.
+
+    32          5.  Disabling  the  unit  number  select  mechanism   (e.g.,
+    33              removing  the  unit  number  select  plug).  The unit is
+    34              unknown.
+
+    35          6.  Duplicate unit numbers.   This  condition  has  its  own
+    36              substate.   Note  that  this condition will not affect a
+    37              unit  that  is  already  "Unit-Online"   or   is   under
+    38              "Exclusive Access." 
+
+    39          7.  Duplicate unit identifiers.  The  unit  is  inoperative.
+    40              Note  that  the  controller need not check for duplicate
+    41              unit identifiers.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-11
+           Unit States                                          11 June 1992
+
+
+     1          8.  Disabling the unit  with  the  Run/Stop  or  Load/Unload
+     2              switch.  The unit is known.
+
+     3          9.  Disabling the unit with port  selection  switches.   The
+     4              unit is unknown.
+
+     5         10.  Removal of the unit from service  by  operator  command,
+     6              typically  for  diagnostics,  formatting, maintenance or
+     7              repair.  The unit is inoperative.
+
+     8         11.  An internal controller diagnostic decides  the  unit  is
+     9              sick and removes it from service.  The unit is disabled.
+
+    10         12.  No volume is present in the drive.  The unit is known.
+
+    11         13.  The unit has not been built yet, the unit  is  still  in
+    12              its  shipping  crate,  or  it  has  otherwise  not  been
+    13              installed yet.  The unit is unknown.
+
+    14     In general, a unit that  is  "Unit-Offline"  to  one  host  class
+    15     driver  via a specific MSCP server is "Unit-Offline" for the same
+    16     reason (same substate) to all host class drivers  via  that  same
+    17     MSCP server.  There are two exceptions:
+
+    18          1.  A duplicate unit number condition that  arises  while  a
+    19              unit is "Unit-Online" to one or more class drivers.  The
+    20              unit remains "Unit-Online" to  those  class  drivers  to
+    21              which  it is already "Unit-Online," yet is "Unit-Offline
+    22              (substate 'Duplicate Unit')" to all other class drivers.
+
+    23          2.  A host has been granted "Exclusive Access" of the  unit.
+    24              The  state  of  the  unit  relative to the host that was
+    25              granted "Exclusive Access" remains unchanged.  The  unit
+    26              is  "Unit-Offline  (substate  'Exclusive  Use')"  to all
+    27              other class drivers.
+
+
+    28     All normal operator initiated transitions to  the  "Unit-Offline"
+    29     state shall be smooth, rather than abrupt.  Attempts to disable a
+    30     unit with its Run/Stop or Load/Unload switch and/or  attempts  to
+    31     dismount  (remove)  a  volume  from  a  unit  are, by definition,
+    32     "normal operator initiated transitions," and shall  therefore  be
+    33     smooth.   Any  other action that a human operator would typically
+    34     use to disable a drive in a nonemergency situation shall also  be
+    35     smooth.   Note  that this does not preclude the existence of some
+    36     special mechanism for immediately disabling a unit or drive in an
+    37     emergency,  provided that it is not the normal way of disabling a
+    38     unit or drive.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-12
+           Unit States                                          11 June 1992
+
+
+     1     A "smooth" transition to the "Unit-Offline"  state  implies  that
+     2     all  write  operations,  including  multiblock  write operations,
+     3     shall either be completed in  their  entirety  or  else  rejected
+     4     (never  begun).  To accomplish a smooth transition the controller
+     5     shall complete all write operations (commands) that have  already
+     6     been  initiated.   Outstanding write operations that haven't been
+     7     initiated  yet  may  either  be  completed  or  rejected.   Other
+     8     outstanding   commands,   including   read   operations,  may  be
+     9     completed, rejected, or terminated, regardless of whether or  not
+    10     they  have been initiated.  However, if a Sequential command from
+    11     a  given  connection  is  rejected  or   terminated,   then   all
+    12     subsequently   received  non-Immediate  commands  from  the  same
+    13     connection shall be rejected.  Any new commands issued by a  host
+    14     should be rejected.
+
+    15     Note that establishing write protection shall also  be  a  smooth
+    16     transition.
+
+    17     A unit enters the "Unit-Available" state when:
+
+    18          1.  The unit is "Unit-Offline" and all causes  of  the  unit
+    19              being  "Unit-Offline"  are  removed.   The  unit becomes
+    20              "Unit-Available" with respect to all "Controller-Online"
+    21              class  drivers  unless  the unit is under the "Exclusive
+    22              Access" of a  host.   In  that  case  the  unit  becomes
+    23              "Unit-Available"  only  to  the  class  driver  that has
+    24              "Exclusive Access" of the unit.
+
+    25          2.  The unit is "Unit-Online" and a host class driver issues
+    26              an  AVAILABLE  command  for  the unit.  The unit becomes
+    27              "Unit-Available" with respect to that class driver.   If
+    28              the  "All Class Drivers" modifier was set, the unit also
+    29              becomes "Unit-Available" with respect to all other class
+    30              drivers  to  which  it  is  "Unit-Online"  via that MSCP
+    31              server, unless the "Exclusive Access" modifier was  also
+    32              set.   If  the  "Exclusive  Access" modifier was set the
+    33              unit becomes "Unit-Available" to as well  as  under  the
+    34              "Exclusive  Access"  of  the  issuing  class  driver and
+    35              "Unit-Offline (substate 'Exclusive Use')" to  all  other
+    36              class drivers.
+
+    37          3.  The unit is "Unit-Online"  or  "Unit-Available"  to  and
+    38              under  the  "Exclusive Access" of a host and that host's
+    39              class driver issues an AVAILABLE, ONLINE,  or  SET  UNIT
+    40              CHARACTERISTICS  command  with  the  "Exclusive  Access"
+    41              modifier clear, thereby releasing "Exclusive Access"  of
+    42              the  unit.   The  state  of the unit with respect to the
+    43              issuing class driver depends on which of those  commands
+    44              was issued as well as the state of the unit prior to the
+    45              command being issued.  (See the  descriptions  of  those
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-13
+           Unit States                                          11 June 1992
+
+
+     1              commands  in Chapter 7, "Multihost Support" for complete
+     2              details.) The unit becomes "Unit-Available" to all other
+     3              class drivers, regardless of the unit state with respect
+     4              to the issuing class driver.
+
+
+     5     If a class driver has enabled attention messages, the MSCP server
+     6     uses AVAILABLE attention messages to notify the class driver that
+     7     a unit has asynchronously become "Unit-Available."   If  a  class
+     8     driver  itself  sends  the MSCP server an AVAILABLE command, then
+     9     the transition to "Unit-Available" is synchronous to  that  class
+    10     driver and an AVAILABLE attention message need not be sent to it.
+    11     Other class drivers, however, are notified.
+
+    12     The  possible   causes   of   an   asynchronous   transition   to
+    13     "Unit-Available" are as follows:
+
+    14          1.  Any transition from "Unit-Offline" to "Unit-Available." 
+    15              This specifically includes the following cases:
+
+    16              a.  The unit was online to another controller and ceases
+    17                  to  be  online  to that other controller.  Note that
+    18                  this includes the  possibility  that  the  unit  was
+    19                  "Unit-Online"  to or under the "Exclusive Access" of
+    20                  the same class driver via that other controller.
+
+    21              b.  The unit was under the "Exclusive Access" of a  host
+    22                  and that host has released the "Exclusive Access" of
+    23                  the unit.
+
+
+    24          2.  A transition  from  "Unit-Online"  to  "Unit-Available,"
+    25              with  respect  to  this  class driver, that is caused by
+    26              some other class driver  issuing  an  AVAILABLE  command
+    27              with the "All Class Drivers" modifier set.
+
+    28          3.  Any  spontaneous  transition   from   "Unit-Online"   to
+    29              "Unit-Available."     I.e.,    any    transition    from
+    30              "Unit-Online" to "Unit-Available" that is not caused  by
+    31              an AVAILABLE command.
+
+    32     The one  exception  to  this  applies  to  a  unit  that  becomes
+    33     "Unit-Available"  due  to  an  AVAILABLE  command  that  has  the
+    34     "Spin-down" modifier set or as a side effect  of  certain  errors
+    35     that  also  spin-down the unit.  Such commands or errors indicate
+    36     that all class drivers are disinterested in the volume mounted on
+    37     the  unit,  so  that  no  class  driver should be notified of the
+    38     transition until an operator mounts a  new  volume  or  otherwise
+    39     interacts  with the unit.  Therefore the request to spin-down the
+    40     unit suppresses AVAILABLE attention messages for that unit  until
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-14
+           Unit States                                          11 June 1992
+
+
+     1     an  operator  interacts  with  the  unit.  Note that the messages
+     2     shall be suppressed for all  class  drivers,  MSCP  servers,  and
+     3     controllers  that  may  connect  to the unit, regardless of which
+     4     individual MSCP server and controller actually requested that the
+     5     unit  spin-down.   This  effectively  means that, for multiaccess
+     6     drives, the fact that AVAILABLE attention messages are suppressed
+     7     must  be  recorded  in  the  drive  itself,  rather  than  in the
+     8     controller.
+
+     9     AVAILABLE attention messages shall be suppressed at  least  until
+    10     an  operator  interacts  with  the unit's drive, the unit becomes
+    11     "Unit-Online" to any class driver via any  MSCP  server,  or  the
+    12     unit's  drive loses context.  It is not acceptable for the unit's
+    13     drive to "lose context" solely to forget that AVAILABLE attention
+    14     messages  have  been  suppressed.  Loss of context must be due to
+    15     some external reason, such as a power failure.   The  suppression
+    16     of AVAILABLE attention messages shall be canceled or forgotten if
+    17     the unit becomes "Unit-Online" to any class driver via  any  MSCP
+    18     server  or  if  an  operator  actuates  the  unit's  Run/Stop  or
+    19     Load/Unload switch, changes the unit number selected by the  Unit
+    20     Number  Select  Mechanism,  or  mounts  a different volume in the
+    21     unit.  Note that AVAILABLE attention messages are not  suppressed
+    22     (i.e.,  their suppression is instantly canceled or forgotten) if,
+    23     after a  class  driver  issues  an  AVAILABLE  command  with  the
+    24     "Spin-down"  modifier  set,  the unit is still "Unit-Online" with
+    25     respect to one or more other class drivers.
+
+    26     MSCP  servers  may  send  redundant  or   unnecessary   AVAILABLE
+    27     attention  messages at any time, provided that attention messages
+    28     have been enabled, the  messages  have  not  been  suppressed  as
+    29     described  above, and the extra messages are infrequent enough to
+    30     avoid  causing   any   significant   overhead   in   either   the
+    31     communications mechanism or a host.  It is specifically allowable
+    32     for an  MSCP  server  to  precede  every  DUPLICATE  UNIT  NUMBER
+    33     attention  message  with  an  AVAILABLE attention message for the
+    34     same unit number.
+
+    35     A unit enters the "Unit-Online" state with  respect  to  a  class
+    36     driver upon the successful completion of an ONLINE command issued
+    37     by that class driver.
+
+    38     4.4  Unit Numbers
+
+    39     As described in Section 4.2, "Controls and Indicators," all  disk
+    40     and  tape drives accessible via MSCP must have a user visible and
+    41     Field Service alterable unit number.  The characters displayed as
+    42     the unit number, interpreted as a decimal number, shall match the
+    43     binary value passed in the "unit number" field  of  MSCP  control
+    44     messages.   Multiunit drives and formatters may use a single unit
+    45     number select mechanism and display for the range of  consecutive
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-15
+           Unit Numbers                                         11 June 1992
+
+
+     1     unit numbers assigned to their units.
+
+     2     MSCP supports unit numbers in the range  0  through  65535.   All
+     3     controllers  and  units shall support unit numbers in the range 0
+     4     through 251.  Unit numbers between 252 and 65535 may be supported
+     5     or  not  at  the controller's and/or unit's option.  This implies
+     6     that all units accessible via  MSCP  shall  have  a  unit  number
+     7     select  mechanism capable of specifying and a unit number display
+     8     mechanism capable of displaying any unit number in  the  range  0
+     9     through 251.  Controllers shall treat unsupported unit numbers as
+    10     if the specified unit is "Unit-Offline" due to being unknown (see
+    11     Section 4.3, "Unit States").
+
+    12     The intent of this broad range of unit numbers is that, within  a
+    13     device class, all units that are accessible to a single host have
+    14     unique  unit  numbers.   In  pursuit  of  this  goal  units  with
+    15     duplicate  unit  numbers  are  considered  to be "Unit-Offline." 
+    16     However, this must be balanced against another goal, namely  that
+    17     transient actions performed on another unit should not be allowed
+    18     to affect continued  host  access  to  a  unit  that  is  already
+    19     "Unit-Online"  or  under  "Exclusive  Access."  Controllers shall
+    20     balance these two goals with the following algorithms:
+
+    21          1.  Controllers detect and respond to duplicate unit numbers
+    22              across  all  units  that  are  "Unit-Online,"  that  are
+    23              "Unit-Available," or that are in one  of  the  following
+    24              "Unit-Offline"  substates:   unit  disabled, unit known,
+    25              under exclusive use  of  another  host,  duplicate  unit
+    26              numbers,  or  online  to another controller.  Units that
+    27              are "Unit-Offline" due to being  unknown  shall  not  be
+    28              considered   for   duplicate   unit   number  detection.
+    29              Controllers may or may not, at the controller's  option,
+    30              consider  units  that  are  inoperative.  Detection of a
+    31              duplicate  unit  number  condition  on  one  unit  of  a
+    32              multiunit  drive  is  treated as a duplicate unit number
+    33              condition on all other units that share one or  more  of
+    34              the  following  components  with  the  unit  having  the
+    35              duplicate unit number:
+
+    36              a.  A unit number select mechanism.
+
+    37              b.  A Run/Stop or Load/Unload switch.
+
+    38              c.  A spindle or other mechanical components.
+
+
+    39          2.  Discovery of a duplicate unit number condition does  not
+    40              affect  any  unit that is already "Unit-Online" or under
+    41              "Exclusive Access."  The state of such  a  unit  remains
+    42              unchanged  until  some other event (such as an AVAILABLE
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-16
+           Unit Numbers                                         11 June 1992
+
+
+     1              command or loss of controller context) occurs that would
+     2              otherwise  cause  it  to  become "Unit-Available" to all
+     3              hosts, at which time the unit becomes "Unit-Offline" and
+     4              is  spun-down  as  described  below.   Note  that in the
+     5              already   "Unit-Online"   case    the    unit    becomes
+     6              "Unit-Offline"  to all other hosts (and therefore cannot
+     7              be brought "Unit-Online" by them) even while it  remains
+     8              "Unit-Online" to its current hosts.
+
+     9          3.  When a controller becomes  aware  of  a  duplicate  unit
+    10              number  condition  on two or more units connected to it,
+    11              it  immediately  spins-down  all  such  units  that  are
+    12              "Unit-Available"  or  "Unit-Offline" with respect to all
+    13              hosts.  When a unit  that  was  "Unit-Online"  or  under
+    14              "Exclusive   Access"   with   a  duplicate  unit  number
+    15              condition becomes "Unit-Available" to all hosts for  any
+    16              reason,  the  controller  immediately spins it down.  In
+    17              both cases, units that belong to a multiunit  drive  and
+    18              share a spindle or other mechanical components are to be
+    19              spun down only if all of the  units  of  the  drive  are
+    20              either "Unit-Offline" or "Unit-Available" to all hosts.
+
+    21          4.  The controller reports "Unit-Offline" due  to  having  a
+    22              duplicate  unit  number  as the state for all units with
+    23              duplicate unit number conditions that  are  not  already
+    24              "Unit-Online" or under "Exclusive Access."  Other causes
+    25              of the units being "Unit-Offline"  may  or  may  not  be
+    26              reported at the controller's option.
+
+    27          5.  The controller recognizes when a duplicate  unit  number
+    28              condition goes away.  That is, the controller recognizes
+    29              when all units except one with the same  duplicate  unit
+    30              number  have  their  unit  numbers changed and/or become
+    31              unknown.   When  this  occurs,  the   controller   sends
+    32              AVAILABLE  attention  messages for the remaining unit if
+    33              there is no other reason for  it  being  "Unit-Offline,"
+    34              since it has just become "Unit-Available." 
+
+    35     In addition to the above algorithms, controllers report duplicate
+    36     unit  number conditions to class drivers using the DUPLICATE UNIT
+    37     NUMBER attention message.  This allows hosts  to  complain  to  a
+    38     human  operator,  who will presumably remedy the condition.  This
+    39     message is sent to all  "Controller-Online"  class  drivers  that
+    40     have  attention  messages  enabled  when  a duplicate unit number
+    41     condition is first detected.  It is permissible for a  controller
+    42     to  send  redundant  or  extra  DUPLICATE  UNIT  NUMBER attention
+    43     messages,  provided  that  the  reported  duplicate  unit  number
+    44     condition  actually  exists.   See Section 5.8.3, "Duplicate Unit
+    45     Number Attention Message," for more details.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-17
+           Unit Numbers                                         11 June 1992
+
+
+     1     The  above  algorithms  effectively  enforce  nonduplicate   unit
+     2     numbers  across  the  drives  connected  to  the same controller.
+     3     Although not required by  MSCP,  it  is  recommended  that  class
+     4     drivers  use the following algorithm to enforce nonduplicate unit
+     5     numbers across the drives accessible to that class driver:
+
+     6          1.  Upon becoming aware of a unit (i.e., upon  receiving  an
+     7              AVAILABLE  attention  message),  the class driver should
+     8              check if it is already aware of a  unit  with  the  same
+     9              unit  number  on  a different MSCP server.  If it is not
+    10              aware of such a unit, it should exit.  If it is aware of
+    11              such  a  unit, it should check if that unit has the same
+    12              unit identifier.
+
+    13          2.  If the unit identifiers are the same,  it  is  the  same
+    14              unit  multiaccessed to several controllers, so exit.  If
+    15              the unit identifiers are  different,  the  class  driver
+    16              should issue a GET UNIT STATUS command to see if the old
+    17              unit still exists.
+
+    18          3.  If  the  old  unit  no   longer   exists   (i.e.,   it's
+    19              "Unit-Offline"),  exit.   If  the old unit still exists,
+    20              the class driver should check that the  unit  identifier
+    21              returned  by  GET UNIT STATUS is also different from the
+    22              unit identifier that  was  in  the  AVAILABLE  attention
+    23              message.   If  the  unit identifiers are the same, exit.
+    24              If the unit identifiers are different, the class  driver
+    25              should  issue  an AVAILABLE command with the "Spin-down"
+    26              modifier set for the new unit.  The class driver  should
+    27              also  issue  an  AVAILABLE  command with the "Spin-down"
+    28              modifier set for the old unit if the class driver is not
+    29              already "Unit-Online" to that unit.
+
+    30          4.  Whenever  a  class  driver   brings   an   MSCP   server
+    31              "Controller-Online,"  the  class  driver  should  issue,
+    32              through that MSCP server, AVAILABLE  commands  with  the
+    33              "Spin-down"  modifier  set  for  all  units  that it has
+    34              "Unit-Online" via another MSCP server.
+
+    35          5.  Whenever a class driver brings a unit "Unit-Online," the
+    36              class  driver should issue ONLINE commands for that same
+    37              unit number to all  MSCP  servers.   If  more  than  one
+    38              ONLINE  command  succeeds, the class driver should check
+    39              that the unit identifiers returned by all the successful
+    40              ONLINE  commands  are  the  same.   If  any  of the unit
+    41              identifiers are different, then the class driver  should
+    42              treat all the ONLINE commands as having failed and issue
+    43              AVAILABLE commands with the "Spin-down" modifier set  to
+    44              all MSCP servers on which the ONLINE command succeeded.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-18
+           Unit Numbers                                         11 June 1992
+
+
+     1     Note that this algorithm uses the fact that an AVAILABLE  command
+     2     with  the "Spin-down" modifier set suppresses AVAILABLE attention
+     3     messages until a human operator interacts with the unit.
+
+     4     4.5  Command Categories And Execution Order
+
+     5     MSCP commands fall into one of  four  command  categories.   Each
+     6     category  has  a  set  of  execution order restrictions that MSCP
+     7     servers must satisfy.  The four categories and their restrictions
+     8     are described below.
+
+     9     Immediate commands are those commands, such as status  inquiries,
+    10     that  both  require very little time to complete and do not cause
+    11     any unit context changes.  MSCP servers shall  process  Immediate
+    12     commands  immediately,  without waiting for any other commands to
+    13     complete.  MSCP servers  shall  guarantee  that  all  outstanding
+    14     Immediate  commands plus an additional GET COMMAND STATUS command
+    15     issued by each "Controller-Online"  class  driver  will  complete
+    16     within the controller timeout interval.
+
+    17     A class driver may issue Immediate commands whenever  its  credit
+    18     balance is greater than zero, whereas all other commands may only
+    19     be issued when the  class  driver's  credit  balance  is  two  or
+    20     larger.   This is discussed further in Chapter 3, "Class Driver /
+    21     MSCP Server Communications."  Class drivers are  thus  guaranteed
+    22     to  be  able to issue at least one Immediate command, and have it
+    23     executed,  regardless  of  what  other  commands  they  may  have
+    24     outstanding.  In particular, the class driver is guaranteed to be
+    25     able to issue a GET COMMAND STATUS command and have  it  complete
+    26     within the controller timeout interval.
+
+    27     Sequential commands are those commands that, for the  same  unit,
+    28     shall   be   executed  in  precise  order.   Sequential  commands
+    29     typically alter a unit's context, such  as  by  changing  a  unit
+    30     characteristic  or by altering the position of a tape drive.  All
+    31     Sequential commands for a particular unit shall  be  executed  in
+    32     the  exact  order  that  the  MSCP  server  receives  them.   The
+    33     execution of a Sequential command may not be interleaved with the
+    34     execution  of any other Sequential or Non-Sequential commands for
+    35     the same unit.  Furthermore, any Non-Sequential commands received
+    36     before  a particular Sequential command shall be completed before
+    37     execution  of   that   Sequential   command   begins,   and   any
+    38     Non-Sequential  commands  received  after a particular Sequential
+    39     command shall not begin execution  until  after  that  Sequential
+    40     command  is  completed.   Sequential  commands  are, in effect, a
+    41     barrier that Non-Sequential commands may not pass  or  penetrate.
+    42     Note  that  Sequential  command  execution is performed on a unit
+    43     basis rather than a single connection  basis.   Controllers  that
+    44     provide  Multihost Support shall therefore enforce the Sequential
+    45     command  execution  guarantees  across  all   "Controller-Online"
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-19
+           Command Categories And Execution Order               11 June 1992
+
+
+     1     connections.
+
+     2     Non-Sequential commands are those commands that  controllers  may
+     3     reorder   so   as   to  optimize  performance.   Controllers  may
+     4     furthermore interleave the execution  of  several  Non-Sequential
+     5     commands  among themselves, performing each command a fragment at
+     6     a time.  The only restrictions are:
+
+     7          1.  Execution of a Non-Sequential command  shall  not  cross
+     8              the barrier created by a Sequential command for the same
+     9              unit.
+
+    10          2.  The controller shall complete useful work on its  oldest
+    11              outstanding    command   on   each   "Controller-Online"
+    12              connection within the controller timeout interval, so as
+    13              not  to  cause  a  command  timeout.   See Section 4.10,
+    14              "Command Timeouts." 
+
+
+    15     Note that controller performance optimization  of  Non-Sequential
+    16     commands,  although  not  a strict requirement of MSCP, is highly
+    17     recommended.  The optimization  algorithms  used  are  controller
+    18     implementation   dependent   and  will  undoubtedly  differ  from
+    19     controller to controller.   Controllers  that  provide  Multihost
+    20     Support  are  expressly  permitted  to include all Non-Sequential
+    21     commands received from  all  "Controller-Online"  class  drivers,
+    22     directed  to the same unit, in the optimization (algorithm) being
+    23     performed on the unit.  One consequence of such optimization that
+    24     must be noted is that if more than one data transfer request with
+    25     the  same  target   received   from   different   class   drivers
+    26     (connections)  are  outstanding at the same time, the ordering of
+    27     the data written/read will be indeterminate.
+
+    28     Special commands are similar to Non-Sequential commands, but have
+    29     additional,  unique  execution order requirements.  The execution
+    30     order requirements for Special  commands  are  described  in  the
+    31     commands' description.
+
+    32     Note that tape class devices only have Immediate  and  Sequential
+    33     commands.   There is no such thing as a Non-Sequential or Special
+    34     tape class command.
+
+    35     Execution order is based on  the  order  in  which  commands  are
+    36     received by the controller's MSCP server.  For commands sent by a
+    37     single class driver, the order of transmission  is  identical  to
+    38     the  order  of  reception.   For  commands  sent by several class
+    39     drivers, the order of reception is essentially random.  The  only
+    40     way  that a class driver can ensure that one of its commands will
+    41     be received after some other  command  issued  by  another  class
+    42     driver is to wait until the other class driver receives the first
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-20
+           Command Categories And Execution Order               11 June 1992
+
+
+     1     command's  end  message  (i.e.,  wait  until  the  first  command
+     2     completes) before sending the command.
+
+     3     For the purpose of determining  execution  order,  a  command  is
+     4     completed  when the controller's MSCP server queues the command's
+     5     end message for transmission to the host.   The  controller  need
+     6     not  wait  until  the  host  has  confirmed its receipt, although
+     7     sequential message delivery guarantees must  be  preserved.   The
+     8     gist  of this is that after termination of the connection between
+     9     a host class  driver  and  an  MSCP  server,  the  absence  of  a
+    10     Sequential  command's  end  message  implies  nothing  about  the
+    11     execution or nonexecution of the Sequential command.
+
+    12     When an MSCP server finishes processing or executing  a  command,
+    13     the  command  can finish in any of four ways.  Based on which way
+    14     the server finishes a command, the command is said to  have  been
+    15     completed, rejected, terminated, or aborted.
+
+    16     A completed command is  a  command  that  the  server  has  fully
+    17     executed  to  its normal conclusion.  A command that is completed
+    18     has either succeeded or encountered an unrecoverable  error.   In
+    19     normal operation all commands are completed.
+
+    20     A rejected command is a command for which the server  never  even
+    21     starts processing.  Some commands are rejected due to termination
+    22     of the connection between the MSCP server and the  class  driver.
+    23     Such  commands  are discarded by the MSCP server, port driver, or
+    24     communications mechanism before any work is done on them.   Other
+    25     commands  are  rejected due to being illegal commands or due to a
+    26     unit being in an illegal state.  For example,  transfer  commands
+    27     are rejected if the specified unit is not "Unit-Online." 
+
+    28     A terminated command is a command for  which  the  server  begins
+    29     processing,  but  then  terminates  processing  part  way through
+    30     command execution.  Transfer commands  are  terminated  if  their
+    31     unit is disconnected (cable breaks or drive loses power) part way
+    32     through the transfer.  Transfer commands may also  be  terminated
+    33     if the connection between the MSCP server and the class driver is
+    34     terminated  part  way  through  the  transfer.   Generally   only
+    35     Non-Sequential   commands  can  be  terminated.   Termination  of
+    36     Sequential commands is discussed below.
+
+    37     An aborted command is a command that the class driver  has  asked
+    38     to  have  aborted  via an ABORT command, and that furthermore has
+    39     been located and explicitly aborted by the MSCP server.  That is,
+    40     commands  that  the  MSCP  server  just  allows  to  complete are
+    41     completed, not aborted.  An aborted command is effectively either
+    42     rejected  or terminated, depending upon whether or not the server
+    43     has begun execution of the command when it processes  the  ABORT.
+    44     (This  definition  of an aborted command is from the viewpoint of
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-21
+           Command Categories And Execution Order               11 June 1992
+
+
+     1     an MSCP server.  From the viewpoint of a class driver, an aborted
+     2     command  is  any  command  for  which  an  ABORT command has been
+     3     issued.) The end message of an aborted command shall be  returned
+     4     within  one  controller  timeout  interval following rejection or
+     5     termination of the command regardless of  the  aborted  command's
+     6     command category.
+
+     7     Immediate commands are always completed if the connection between
+     8     MSCP  server  and  class  driver  remains intact; they may not be
+     9     rejected,  terminated,  or  aborted.   If   the   connection   is
+    10     terminated before an Immediate command can be completed, then the
+    11     command  may  be  completed,  rejected,  or  terminated.    Since
+    12     Immediate  commands  do not change the state, context, or data of
+    13     the unit, these three command completion modes are equivalent  if
+    14     the connection has been terminated.
+
+    15     Non-Sequential commands may, in general, be completed,  rejected,
+    16     terminated,  or  aborted.   This  is  true both if the connection
+    17     remains intact or if it is terminated.   However,  there  is  one
+    18     restriction  regarding  write operations.  Write operations shall
+    19     not be terminated as a result of any normal operator interactions
+    20     with the unit.  That is, write operations shall not be terminated
+    21     (partially completed) as a result of an  operator  attempting  to
+    22     spin-down,  dismount,  or write protect the unit.  The purpose of
+    23     this restriction is to prevent data  corruption  due  to  partial
+    24     updates.   This  is  further  discussed  under  the term "smooth"
+    25     transitions in Section 4.3,  "Unit  States,"  and  Section  4.14,
+    26     "Write Protection." 
+
+    27     Sequential commands may be completed or rejected, but in  general
+    28     may  not  be  terminated.  If a Sequential command is aborted, it
+    29     shall be aborted before it  begins  execution  (rejected)  rather
+    30     than  in  the  middle  of execution (terminated).  The reason for
+    31     this is that  Sequential  commands  appear  to  hosts  as  atomic
+    32     (indivisible)  operations.  Thus, there is both this requirement,
+    33     that  Sequential  commands  cannot   be   terminated,   therefore
+    34     preventing  subsequent  commands from acting on partially updated
+    35     unit state and context, and the requirement that execution  of  a
+    36     Sequential  command  cannot be interleaved with any other command
+    37     on the same unit, therefore preventing a concurrent command  from
+    38     acting on partially updated unit state and context.
+
+    39     There are two exceptions to the  requirement  that  a  Sequential
+    40     command  may  not  be terminated.  The first is that a server may
+    41     terminate a  Sequential  command  in  the  middle  of  execution,
+    42     provided  that  the termination process undoes all unit state and
+    43     context changes that have  been  made  on  the  unit.   From  the
+    44     viewpoint of the class driver, this case is exactly equivalent to
+    45     the command having been rejected.  The second exception  is  that
+    46     the  controller  and/or  unit  loses  context, generally due to a
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-22
+           Command Categories And Execution Order               11 June 1992
+
+
+     1     power failure or similar  event.   Since  the  unit's  state  and
+     2     context  will  be  completely  reset  when  power  and/or  normal
+     3     operation are restored,  the  fact  that  it  was  left  in  some
+     4     indeterminate state does not matter.  Note that if the controller
+     5     loses context, it must lose context for all  connections  to  the
+     6     affected  unit's device class (implying that all such connections
+     7     have been terminated), as otherwise some other class  driver  may
+     8     see the unit in an indeterminate state.
+
+     9     Note that any command that fails due to an  unrecoverable  device
+    10     or  data  error  is  a completed command.  Commands are rejected,
+    11     terminated, or aborted only in  response  to  inappropriate  unit
+    12     state  or  context, connection termination, and being aborted via
+    13     ABORT commands.
+
+    14     Commands that are rejected, terminated, or  aborted  shall  still
+    15     return  their  proper end message format with valid parameters as
+    16     defined in the individual command descriptions.   The  only  case
+    17     where  a  different  end message format may be returned is if the
+    18     command is rejected as a protocol error type "Invalid  Command." 
+    19     A  special  end message format and end message opcode is used for
+    20     that purpose.  See  "Invalid  Command"  status  in  Section  5.5,
+    21     "Status Codes" for more detail.
+
+    22     4.6  Class Driver / MSCP Server Synchronization
+
+    23     Synchronization  of  a  class  driver  with  an  MSCP  server  is
+    24     accomplished  by  establishing  or re-establishing the connection
+    25     between  the  class  driver  and  the  MSCP  server.   When   the
+    26     connection  is  established  or  re-established,  the MSCP server
+    27     terminates all commands that  are  outstanding  from  that  class
+    28     driver.   This  forces  the dialogue between the class driver and
+    29     MSCP server to a known, synchronized state:  that  of  having  no
+    30     outstanding  commands.   After  establishing  the  connection the
+    31     class  driver  can  issue   commands   without   worrying   about
+    32     duplicating  command  reference numbers or other unfortunate side
+    33     effects.  Note  that  synchronizing  with  the  MSCP  server,  if
+    34     successful,     causes     the     MSCP    server    to    become
+    35     "Controller-Online." 
+
+    36     As stated above,  the  main  purpose  of  synchronization  is  to
+    37     guarantee  that  there  are no outstanding commands, thus forcing
+    38     the dialogue between the class driver and MSCP server to a  known
+    39     state.   MSCP  servers  shall  ensure  that this guarantee is met
+    40     before they allow synchronization to complete (i.e., before  they
+    41     become  "Controller-Online").   In particular, MSCP servers shall
+    42     guarantee that no end messages will be sent  and  that  no  units
+    43     will  have  their  state, context, or (data) contents changed for
+    44     any commands that were issued on an earlier  incarnation  of  the
+    45     connection  between  the class driver and MSCP server.  Note that
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-23
+           Class Driver / MSCP Server Synchronization           11 June 1992
+
+
+     1     an MSCP server may allow outstanding commands to complete, either
+     2     partially  or  entirely,  but  if  it  does  it  must  delay  the
+     3     completion  of   synchronization   (delay   the   transition   to
+     4     "Controller-Online")  until  all  such commands have completed or
+     5     terminated.
+
+     6     Class drivers shall synchronize with the MSCP server whenever the
+     7     host  boots,  recovers from a power failure, loses context, or is
+     8     recovering from certain errors.  After synchronizing with an MSCP
+     9     server, the class driver should do the following:
+
+    10          1.  Issue  a  SET  CONTROLLER  CHARACTERISTICS  command   to
+    11              establish  host-settable  controller characteristics and
+    12              obtain non-host-settable controller characteristics.
+
+    13          2.  Issue ONLINE commands  for  all  units  that  the  class
+    14              driver wishes to be "Unit-Online." 
+
+    15          3.  Reissue all commands,  if  any,  that  were  outstanding
+    16              before  the  class  driver  synchronized  with  the MSCP
+    17              server in the exact  order  that  they  were  originally
+    18              issued.   However,  any  commands that have been aborted
+    19              (i.e., for which an ABORT command has been issued) shall
+    20              not be reissued.  Instead the class driver should assume
+    21              that the command has been successfully  aborted  and  is
+    22              therefore no longer outstanding.
+
+    23     There is one  exception  to  reissuing  all  commands  that  were
+    24     outstanding.   The  class  driver  shall  maintain  a retry limit
+    25     count, to ensure that its oldest  outstanding  command  won't  be
+    26     retried  infinitely  many  times.  The magnitude of this limit is
+    27     host dependent, although the oldest command shall be  retried  at
+    28     least  once.   When the class driver's oldest outstanding command
+    29     exceeds the retry limit count, an error shall be returned  rather
+    30     than retrying the command again.  All other outstanding commands,
+    31     however, should still be retried.   See  Section  4.10,  "Command
+    32     Timeouts," for more information.
+
+    33     It may be possible that a class driver will receive messages from
+    34     an   MSCP   server   after   the   class   driver  has  initiated
+    35     synchronization   with   the   MSCP   server   but   before   the
+    36     synchronization  completes.   Whether  or  not  this  is  in fact
+    37     possible is communications mechanism  dependent,  as  it  depends
+    38     upon the detailed design of the communications mechanism and port
+    39     driver.  Any attention messages that the  class  driver  receives
+    40     during this interval should be discarded.  Any error log messages
+    41     should be logged in the normal fashion.  Any  end  messages  that
+    42     the  class  driver  receives  during  this interval may be either
+    43     handled normally (thus completing the corresponding  command)  or
+    44     else discarded.  If the class driver discards one end message, it
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-24
+           Class Driver / MSCP Server Synchronization           11 June 1992
+
+
+     1     shall   discard   all   subsequent   end   messages   until   the
+     2     synchronization  completes.   It  is  recommended,  although  not
+     3     required,  that  class  drivers  handle  all  such  end  messages
+     4     normally.
+
+     5     4.7  Class Driver Error Recovery
+
+     6     The principal method of error recovery used by class  drivers  is
+     7     to  resynchronize  with  the MSCP server, as described in Section
+     8     4.6,  "Class  Driver  /  MSCP   Server   Synchronization."    All
+     9     communications  mechanism  failures  and many controller failures
+    10     are reported by terminating  the  connection  between  the  class
+    11     driver  and  MSCP  server,  in response to which the class driver
+    12     should attempt to resynchronize with the  MSCP  server.   If  the
+    13     class  driver  decides  that  the  controller  is  insane, either
+    14     because the class driver received an invalid message or because a
+    15     command  timed out, it should recover by resynchronizing with the
+    16     MSCP server.  Similarly, if the  MSCP  server  decides  that  the
+    17     class  driver  is  insane, it may terminate the connection to the
+    18     class driver.  If the class driver is in fact actually  sane,  it
+    19     will  resynchronize  with  the  MSCP server after the port driver
+    20     notifies it that the connection has been terminated.
+
+    21     Aside  from  resynchronization  as  described  in  the  preceding
+    22     paragraph, class drivers need perform very little error recovery,
+    23     since  controllers  handle  all  recoverable  errors.   The  only
+    24     exceptions to this guideline are as follows:
+
+    25          1.  Errors on multiaccess drives  should  be  retried  using
+    26              another controller.
+
+    27          2.  Commands   that   fail   due   to   the    unit    being
+    28              "Unit-Available"  should  typically  be  reissued  after
+    29              bringing the unit "Unit-Online." 
+
+    30          3.  High  availability  systems  may  wish  to  perform  the
+    31              enhanced error recovery described below.
+
+
+    32     High  availability  systems  may  wish  to  use   the   following
+    33     additional  error  recovery  strategy  in  order  to minimize the
+    34     impact of certain types  of  controller  problems.   Rather  than
+    35     reissuing  outstanding commands all at once after resynchronizing
+    36     with a controller, such  a  system  should  instead  reissue  the
+    37     oldest  outstanding command all by itself.  The host class driver
+    38     should reissue all other  outstanding  commands  only  after  the
+    39     oldest  command completes.  The other outstanding commands may be
+    40     reissued either all at once (recommended) or one at a time.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-25
+           Class Driver Error Recovery                          11 June 1992
+
+
+     1     If the oldest command times out or otherwise  fails  again  after
+     2     being  reissued, then it was presumably the source of the problem
+     3     and  normal  operation  will  resume.   If  the  oldest   command
+     4     succeeds,  then  the  problem  is  most  likely  due to some race
+     5     condition within the controller  and  will  not  recur.   If  the
+     6     problem  does recur, then successive iterations of this algorithm
+     7     will execute commands one at a time until the  offending  command
+     8     is found and discarded.
+
+     9     4.8  Serious Exceptions
+
+    10     A serious exception is  an  unrecoverable  error  or  some  other
+    11     unusual   condition  that  requires  host  acknowledgment  before
+    12     additional commands are processed.  Serious exceptions occur only
+    13     on  tape  class  devices.  See Tape Mass Storage Control Protocol
+    14     Specification (TMSCP) for complete details.
+
+    15     4.9  Host Access Timeouts
+
+    16     MSCP servers provide  host  access  timeouts  to  guarantee  that
+    17     multiaccess  drives  (described  in  Section 4.16.1, "Multiaccess
+    18     Drives") will be accessible in spite  of  certain  communications
+    19     mechanism failures.  If the communications path between hosts and
+    20     a controller fails when one or more drives are  "Unit-Online"  or
+    21     under  "Exclusive  Access"  via that controller, the drives would
+    22     normally become inaccessible.  The drives can't be  accessed  via
+    23     the  controller  through  which  they  are "Unit-Online" or under
+    24     "Exclusive Access," since that controller can't be accessed,  and
+    25     they  can't  be  accessed via a second controller, since they are
+    26     already accessed through the first controller.  The  host  access
+    27     timeout, if enabled, eliminates this problem by causing the first
+    28     controller to automatically release  all  drives  if  it  doesn't
+    29     receive a command within a specified time interval.
+
+    30     The exact mechanism used for host access timeouts is to have  the
+    31     controller's  MSCP  server become "Controller-Available" relative
+    32     to any host class driver whose host access timeout expires.   The
+    33     MSCP server automatically resets the timeout whenever it receives
+    34     a command from the class driver or has a command outstanding from
+    35     that  class  driver.   Therefore the timeout will never expire if
+    36     the class driver maintains a reasonably constant dialog with  the
+    37     MSCP  server.   Note  that implementing host access timeouts on a
+    38     per host  basis  has  the  benefit  that  the  MSCP  server  will
+    39     automatically  release  any resources allocated to a host if that
+    40     host goes down.
+
+    41     If communication with all hosts ceases, the host  access  timeout
+    42     of  each  class  driver  will ultimately expire, causing the MSCP
+    43     server to become "Controller-Available" relative  to  all  hosts.
+    44     Each  unit  will  be  released, allowing it to be accessed via an
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-26
+           Host Access Timeouts                                 11 June 1992
+
+
+     1     alternate controller, as soon  as  all  class  drivers  that  are
+     2     "Unit-Online"  or  under  "Exclusive  Access" with respect to the
+     3     unit (or any other unit that shares its access path) cease to  be
+     4     "Unit-Online"  or  under "Exclusive Access" by virtue of becoming
+     5     "Controller-Available."  Ultimately all class drivers will become
+     6     "Controller-Available," thus releasing all units.
+
+     7     Class drivers specify the time interval that an MSCP server  will
+     8     use   for   the   host  access  timeout  in  the  SET  CONTROLLER
+     9     CHARACTERISTICS command.  A host access  timeout  value  of  zero
+    10     specifies  that  host  access  timeouts  should  be  disabled.  A
+    11     default host access timeout of 60 seconds is used from the time a
+    12     class  driver  becomes  "Controller-Online"  until  it issues its
+    13     first SET CONTROLLER CHARACTERISTICS command.
+
+    14     Each class driver may specify a  separate  time  interval.   This
+    15     allows  class drivers to vary the host access timeout interval in
+    16     accordance with host policy and availability requirements.   High
+    17     availability  systems  should  typically  use a fairly short host
+    18     access timeout, on the order of 10 to 30 seconds, together with a
+    19     background  process  that  verifies the continued availability of
+    20     the host  to  controller  communications  path.   The  background
+    21     process would have the desirable side effect of ensuring that the
+    22     host access timeout never expired.  Normal systems should  use  a
+    23     larger  timeout,  on  the  order  of  1  to  5  minutes, to avoid
+    24     excessive resynchronizations, yet still allow  failure  recovery.
+    25     Single user and other specialized systems may disable host access
+    26     timeouts.  Note that host access  timeouts  should  typically  be
+    27     enabled  even  on  systems  that  do not seem to require them, as
+    28     drives may be multiaccessed to a totally separate host used as  a
+    29     backup system.
+
+    30     MSCP servers shall implement host access timeouts subject to  the
+    31     following constraints:
+
+    32          1.  The host access timeout expires, for a particular  class
+    33              driver,  at  the  amount  of  the  host  access  timeout
+    34              interval after the controller or MSCP  server  completes
+    35              all   outstanding   commands  from  that  class  driver,
+    36              provided that no new commands  are  received  from  that
+    37              class driver during this interval.
+
+    38          2.  The MSCP server shall enter  the  "Controller-Available"
+    39              state  (as  a  result of host access timeout expiration)
+    40              relative to a class driver no sooner than the moment  of
+    41              host access timeout expiration defined in item 1 above.
+
+    42          3.  The MSCP server should enter the  "Controller-Available"
+    43              state (as a result of host access timeout expiration) as
+    44              soon as possible after the moment of host access timeout
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-27
+           Host Access Timeouts                                 11 June 1992
+
+
+     1              expiration  defined  in  item  1 above.  Except when the
+     2              controller  is  saturated  with  work  for  other  class
+     3              drivers, this shall be no later than one second plus the
+     4              amount of the host access  timeout  interval  after  the
+     5              moment of host access timeout expiration.
+
+     6     In other words, the host access timeout is measured from the time
+     7     that  the  last  command is completed.  The MSCP server may defer
+     8     recognition of host access  timeout  expiration  for  up  to  one
+     9     second  plus  the  amount  of  the  host access timeout after the
+    10     formal expiration of the  timeout.   That  is,  the  host  access
+    11     timeout  shall  be  implemented  with  an  accuracy  range of -0%
+    12     through +100%+1 second.  Furthermore, this accuracy range may  be
+    13     extended  on  the  high  side if the controller is saturated with
+    14     work for other class drivers.
+
+    15     This definition of host access timeouts has  been  structured  to
+    16     allow  at  least  two  alternative implementations.  In the first
+    17     implementation, the controller or MSCP server starts a timer when
+    18     it goes "idle" -- that is, when it completes the last outstanding
+    19     command.  The duration of the timer is the  host  access  timeout
+    20     interval.   The timer is designed to not err on the low side (too
+    21     short an interval) and to err by no more than a factor of two  on
+    22     the  high  side  (too  long  an  interval).  If the timer expires
+    23     before the MSCP server receives another command, declare  a  host
+    24     access  timeout  and  become  "Controller-Available."  The second
+    25     implementation uses a continuously running timer that "ticks"  at
+    26     the  host  access  timeout  interval.  If there is no outstanding
+    27     command and the timer "ticks" twice  in  succession  without  the
+    28     MSCP server having received a command from the host, then declare
+    29     a host access timeout and become "Controller-Available."
+
+    30     Each controller or MSCP server has a minimum and a  maximum  host
+    31     access  timeout  interval  that it implements.  If a class driver
+    32     specifies a host access timeout interval that is  less  than  the
+    33     minimum,  then  the  MSCP  server  uses  its minimum.  If a class
+    34     driver specifies a host access timeout interval that  is  greater
+    35     than  the  controller's  maximum,  then  the  controller uses its
+    36     maximum.  A controller's or MSCP  server's  minimum  host  access
+    37     timeout  interval shall be 10 seconds or less.  A controller's or
+    38     MSCP server's maximum host access timeout interval  shall  be  at
+    39     least   255  seconds.   That  is,  all  controllers  shall  fully
+    40     implement host access timeout intervals in the range  10  through
+    41     255   seconds.   Support  of  intervals  outside  this  range  is
+    42     controller dependent.  The range of host access timeout intervals
+    43     that a controller supports shall be described in the controller's
+    44     Functional Specification; it cannot be obtained via MSCP.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-28
+           Host Access Timeouts                                 11 June 1992
+
+
+     1     Note that all controllers and MSCP servers shall  also  implement
+     2     the  host  access  timeout value zero, which disables host access
+     3     timeouts.
+
+     4     4.10  Command Timeouts
+
+     5     Host class drivers use command timeouts  to  guarantee  that  all
+     6     controller or communications mechanism failures will be detected.
+     7     The failures detected by command timeouts include partially  sane
+     8     or  deadlocked  controllers,  which  may  continue to process new
+     9     commands even though one or more old commands have been lost  and
+    10     will  never complete.  The use of command timeouts centers around
+    11     the GET COMMAND STATUS command.  Indeed, the primary  purpose  of
+    12     the GET COMMAND STATUS command is for command timeouts.
+
+    13     A controller is sane if and only if it will  ultimately  complete
+    14     all  commands  submitted to it.  For practical purposes, the term
+    15     "ultimately" must be replaced with the phrase "within  reasonable
+    16     time."   What  constitutes  a  "reasonable  time" varies with the
+    17     complexity of the command and the performance of  the  controller
+    18     and  drives.   We  can eliminate the difficulty of the host class
+    19     driver having to derive this "reasonable time" by  restating  the
+    20     definition of a sane controller as follows:  A controller is sane
+    21     if and only if it will always complete useful work on its  oldest
+    22     outstanding request within reasonable time.  This definition lets
+    23     us set the "reasonable time" to some fixed value,  and  vary  the
+    24     units  in  which  we  measure  "useful  work"  according  to  the
+    25     complexity of the command.  Command timeouts are  based  on  this
+    26     second definition.
+
+    27     A class  driver  implements  the  command  timeout  mechanism  as
+    28     follows.     For    each    MSCP    server   to   which   it   is
+    29     "Controller-Online,"  the  class  driver  keeps  track  of  which
+    30     command  is  its  oldest  outstanding  command  plus the previous
+    31     "command status" value for its oldest outstanding  command.   The
+    32     previous  "command  status"  value  should  be  set  to  all ones
+    33     whenever the oldest command completes and a new  command  becomes
+    34     the  oldest.   The  class  driver should issue GET COMMAND STATUS
+    35     commands for its oldest outstanding command at intervals  of  the
+    36     controller  timeout  interval  or  longer.  When each GET COMMAND
+    37     STATUS command completes, the class driver checks if the  command
+    38     for  which  it  was  issued  is  still outstanding and, if it is,
+    39     verifies that the "command status" returned by  the  GET  COMMAND
+    40     STATUS command is lower than the previous "command status."  This
+    41     value measures the amount of work remaining before completion  of
+    42     the  command.   If the value ever increases or stays the same, or
+    43     if a GET COMMAND  STATUS  command  ever  takes  longer  than  the
+    44     controller  timeout  interval  to complete, then the class driver
+    45     should assume that the controller has  failed  and  resynchronize
+    46     with the controller.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-29
+           Command Timeouts                                     11 June 1992
+
+
+     1     In addition to guaranteeing that they will complete  useful  work
+     2     on  their  oldest  outstanding  command,  controllers  shall also
+     3     guarantee that they will complete all aborted commands within the
+     4     controller  timeout  interval.  That is, an aborted command's end
+     5     message shall be sent no later than the amount of the  controller
+     6     timeout  interval  after  the  ABORT command's end message.  Host
+     7     class drivers can check this by  setting  the  previous  "command
+     8     status"  value to one whenever the oldest outstanding command has
+     9     been aborted (i.e., when the  class  driver  receives  the  ABORT
+    10     command's  end  message  indicating  that  its oldest outstanding
+    11     command has been aborted).
+
+    12     Single host controllers (i.e., controllers that  do  not  provide
+    13     Multihost  Support)  need not guarantee that all aborted commands
+    14     will  complete  within  the  controller   timeout   interval   if
+    15     excessively  pathological  situations  arise  (e.g.,  compound or
+    16     multiple errors, causing error recovery sequences to stretch  out
+    17     to unrealistic lengths).
+
+    18     If the command timeout for an aborted command expires due to such
+    19     a  situation,  the  class driver will resynchronize with the MSCP
+    20     server and reissue all outstanding commands except for those that
+    21     had  been  aborted.   Since  the  aborted  commands  will  not be
+    22     reissued, the timeout will not recur.   Thus  this  exception  is
+    23     transparent to host class drivers.
+
+    24     The above algorithm applies to non-Immediate commands.  With  one
+    25     exception  the  same  algorithm  may  also  be used for Immediate
+    26     commands, although  a  simpler  one  (using  the  fact  that  all
+    27     Immediate   commands   complete  within  the  controller  timeout
+    28     interval) is also appropriate.  The  one  exception  is  the  GET
+    29     COMMAND  STATUS  command  used  by  the timeout algorithm itself.
+    30     This command shall  be  timed  out  as  follows.   If  the  class
+    31     driver's credit balance is zero when it attempts to issue the GET
+    32     COMMAND STATUS command, it shall queue  the  GET  COMMAND  STATUS
+    33     command  for  immediate  transmission  (before any other commands
+    34     that may be outstanding) when its credit balance becomes nonzero.
+    35     If  the  controller timeout interval expires again before the GET
+    36     COMMAND STATUS command has both been transmitted  and  completed,
+    37     then  the  class  driver  should  assume  that the controller has
+    38     failed.   Note  that  the  class  driver's  credit   balance   is
+    39     guaranteed  to be nonzero when all outstanding Immediate commands
+    40     have completed.  Note  also  that  controllers  or  MSCP  servers
+    41     guarantee  that  all  outstanding  Immediate  commands  plus  one
+    42     additional GET COMMAND STATUS command will  complete  within  the
+    43     controller timeout interval.
+
+    44     Upon concluding that a controller has failed,  the  class  driver
+    45     shall resynchronize with the controller's MSCP server and reissue
+    46     all commands (except commands that it has tried  to  abort)  that
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-30
+           Command Timeouts                                     11 June 1992
+
+
+     1     were  outstanding to that MSCP server in the same order that they
+     2     were originally issued.  In particular,  the  oldest  outstanding
+     3     command  (the  one  that  timed out) shall be issued first (after
+     4     initial SET CONTROLLER CHARACTERISTICS and ONLINE commands).  The
+     5     only  exception  to this is if the command's retry count expires.
+     6     The class driver should maintain a retry count of the  number  of
+     7     times  the oldest command has timed out and been retried.  If the
+     8     retry count exceeds a  host  dependent  retry  limit,  the  class
+     9     driver  should  return  an error rather than retrying the command
+    10     again.  The size of the retry limit is determined by host policy,
+    11     except  that  all  commands shall be retried at least once.  Note
+    12     that subcode zero of the "Controller Error" status code has  been
+    13     reserved  for  commands  that  exceed  their  retry  count.  This
+    14     subcode shall never be generated by MSCP servers; it is generated
+    15     by  class drivers or non-MSCP entities (e.g., a controller's port
+    16     driver) as a standard means of reporting command timeout errors.
+
+    17     In order to implement command timeouts,  the  host  class  driver
+    18     shall  first  obtain  the controller timeout interval via the SET
+    19     CONTROLLER CHARACTERISTICS command.  Therefore the  class  driver
+    20     should  issue  a  SET  CONTROLLER  CHARACTERISTICS command as the
+    21     first command  after  becoming  "Controller-Online."   The  class
+    22     driver should use a controller timeout interval of 10 seconds for
+    23     this initial command.  The class driver shall never  use  a  time
+    24     interval that is shorter than the controller specified controller
+    25     timeout interval for its command timeout determination,  although
+    26     the  class driver may use a time interval that is longer than the
+    27     one specified by the controller.  The controller timeout interval
+    28     specified  by  the  controller shall not be larger than 4 minutes
+    29     and 15 seconds (255 seconds).
+
+    30     One characteristic of this command timeout  algorithm  that  MSCP
+    31     servers  need  not implement, and indeed most will not implement,
+    32     is the GET COMMAND STATUS command for any command that  the  MSCP
+    33     server  can guarantee will complete within the controller timeout
+    34     interval.  The GET COMMAND STATUS command  should  always  return
+    35     the  value zero as the "command status" of such a command.  It is
+    36     acceptable  if,  due  to  vagaries  of  controller   optimization
+    37     algorithms,  such  a  command will occasionally timeout.  This is
+    38     acceptable,  so  long  as  the  frequency  of  such  timeouts  is
+    39     extremely  small and the controller immediately begins processing
+    40     the first command it receives after resynchronization.  Since the
+    41     class  driver  reissues commands in the same order that they were
+    42     originally issued, the oldest or timed out  command  is  reissued
+    43     first, effectively guaranteeing that it will not be delayed again
+    44     by the controller's optimization algorithms.  (The  simplest  way
+    45     for  most  controllers  to  implement  this is to treat the first
+    46     Non-Sequential  command  received  across  a  newly   established
+    47     connection  as  if  the  Express  Request modifier were set.  See
+    48     Section 5.3.2, "Command  Modifiers"  for  a  description  of  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-31
+           Command Timeouts                                     11 June 1992
+
+
+     1     affects of that modifier.)
+
+     2     4.11  Host Buffer Access
+
+
+     3     The  block  data  communications  service   (see   Section   3.1,
+     4     "Connection")  transfers  the  contents  of  one  named buffer to
+     5     another.  Hosts describe buffers (name and size)  with  a  buffer
+     6     descriptor  and  byte  count in the command message of a transfer
+     7     command.
+
+     8     In a read operation, the host provides a buffer  to  receive  the
+     9     data.   To  the  host,  that  buffer  (the portion of host memory
+    10     described by the buffer descriptor and specified byte  count)  is
+    11     exactly  equivalent  to  an  undefined  field  in an MSCP control
+    12     message from the time that  the  class  driver  issues  the  READ
+    13     command  to  its port driver to the time that it receives the end
+    14     message for that READ command from its port driver.  That is, the
+    15     contents  are  controller  implementation dependent and cannot be
+    16     used in any meaningful way by the class driver.  The class driver
+    17     shall  ignore  the  contents  of the buffer and may not alter the
+    18     contents in any way.  The server may alter the contents  of  that
+    19     memory in any way it chooses during that time.
+
+    20     Note that this holds true  regardless  of  any  sequencing  rules
+    21     imposed  by  command categories on execution order.  For example,
+    22     the host queues an ONLINE command followed  by  a  READ  command.
+    23     The contents of the READ buffer are undefined from the moment the
+    24     READ command is  queued,  despite  the  ONLINE  command  being  a
+    25     Sequential  command.  The contents of the buffer remain undefined
+    26     until the end message for the READ command  is  received  by  the
+    27     class  driver,  even before receipt of the ONLINE end message and
+    28     regardless of the success or failure of the ONLINE command.
+
+    29     Note also that once the end message  has  been  received  by  the
+    30     class  driver,  only  that portion of the buffer described by the
+    31     "byte count" in the end message is  defined  as  containing  data
+    32     from  the  unit.  The contents of the remainder of the buffer are
+    33     undefined.
+
+    34     In a write type operation, the host loads the data to be  written
+    35     into  a  buffer in host memory.  The server shall not access that
+    36     buffer before the server receives the command  message  or  after
+    37     the server queues the end message for transmission to the host.
+
+    38     4.12  Disk Geometry And Format
+
+    39     The host accessible portion of a disk unit consists of  a  vector
+    40     of  fixed  length  blocks, called logical blocks.  Logical blocks
+    41     are identified by logical block numbers (LBNs) which  range  from
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-32
+           Disk Geometry And Format                             11 June 1992
+
+
+     1     zero through N-1, where "N" is the total number of logical blocks
+     2     on the unit.  The logical blocks on a unit are divided  into  two
+     3     mutually exclusive regions:
+
+     4          1.  The host area consists of those logical blocks available
+     5              for  host data storage.  Logical blocks in the host area
+     6              have LBNs in the range zero through US-1, where "US"  is
+     7              the "unit size," or number of logical blocks in the host
+     8              area.  The host obtains "US" or the "unit size" from the
+     9              ONLINE or SET UNIT CHARACTERISTICS command end messages.
+
+    10          2.  The unit's Replacement  Control  Table  (RCT),  used  to
+    11              record   bad   block   replacement   information.   This
+    12              information  is  further  described  below.    The   RCT
+    13              (actually,  multiple  copies  of  the  RCT) occupies the
+    14              logical blocks numbered US through N-1.
+
+    15     The presence of the RCT is optional; see the description below.
+
+    16     All of the logical blocks in the host area are guaranteed  to  be
+    17     "good"  -- that is, to be free of permanent or hard errors (media
+    18     defects).   Most  controllers  implement  this  via   bad   block
+    19     replacement.    A  pool  of  replacement  blocks,  identified  by
+    20     replacement block  numbers  (RBNs),  is  provided  on  the  disk.
+    21     Replacement  blocks  are  not  directly accessible to hosts.  All
+    22     host area logical blocks that are bad (i.e., that  contain  media
+    23     defects  leading  to  hard errors or large numbers of correctable
+    24     errors) are replaced by a replacement block.  The mechanism  used
+    25     to  perform  this  replacement,  and  to revector accesses to bad
+    26     logical blocks to the proper replacement blocks, is described  in
+    27     the Digital Storage Architecture Disk Format Specification (DSDF)
+    28     and/or the controller's functional specification.   The  pool  of
+    29     replacement  blocks is typically distributed throughout the disk,
+    30     so that revectoring of  logical  block  accesses  to  replacement
+    31     blocks  has  negligible impact on performance.  See Section 4.13,
+    32     "Bad Block Replacement," and DSDF for  more  information  on  bad
+    33     block replacement.
+
+    34     4.12.1  Replacement Control Table (RCT) Overview
+
+    35     The logical blocks that contain the RCT are not guaranteed to  be
+    36     "good."   Since  the  RCT  describes  the  bad  logical  block to
+    37     replacement block mapping, mapping bad RCT blocks to  replacement
+    38     blocks  would present an insoluble recursion problem.  Instead of
+    39     using replacement blocks for  bad  RCT  blocks,  the  RCT  region
+    40     actually  contains  multiple copies of the RCT.  Hosts obtain the
+    41     size of each RCT copy and the number of RCT copies  via  the  GET
+    42     UNIT  STATUS command.  The last copy of the RCT may be truncated.
+    43     See the Digital Storage Architecture  Disk  Format  Specification
+    44     (DSDF).   Note  that  the  disk  is unusable if the corresponding
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-33
+           Disk Geometry And Format                             11 June 1992
+
+
+     1     block is bad in every copy of the RCT.
+
+     2     The detailed  format  and  access  algorithms  for  the  RCT  are
+     3     described  in  DSDF.   In  general, however, each copy of the RCT
+     4     contains the following information:
+
+     5          1.  One entry per replacement block, identifying the logical
+     6              block, if any, that has been replaced by the replacement
+     7              block.
+
+     8          2.  Context   information   identifying   the   bad    block
+     9              replacement  operation,  if  any,  that  is currently in
+    10              progress on this unit.   This  information  is  used  to
+    11              complete the bad block replacement operation if the host
+    12              and/or controller should crash  in  the  middle  of  bad
+    13              block replacement.
+
+
+    14     Alternatively, a controller may  use  a  nonstandard  replacement
+    15     scheme  or  some  scheme  other  than  bad  block  replacement to
+    16     guarantee that the logical blocks in the host area  are  "good." 
+    17     The  mechanisms  used  by such a controller to guarantee that the
+    18     host area logical blocks are "good" shall be totally invisible to
+    19     hosts,   and   shall  not  require  host  cooperation  for  their
+    20     initialization, use, or maintenance.  Such a controller  or  unit
+    21     will not provide a host accessible RCT.
+
+    22     4.12.2  Logical Block Contents
+
+    23     The host visible portion of each  logical  block  consists  of  a
+    24     fixed  number  of data bytes.  The number of data bytes is either
+    25     512 or 576, determined when the disk volume  is  formatted.   All
+    26     logical  blocks  on  a  disk  volume have the same number of data
+    27     bytes.
+
+    28     The data bytes are the only portion of logical  blocks  that  are
+    29     directly  host visible.  Certain other items, however, must be in
+    30     each logical block as their presence is implied by  various  MSCP
+    31     functions:
+
+    32          1.  Each block shall contain a forced error indicator, so as
+    33              to  properly  implement  and recognize the "Force Error"
+    34              command modifier.
+
+    35          2.  Each block shall contain a bad block indicator, so  that
+    36              references  to  bad blocks can be efficiently revectored
+    37              to the proper replacement blocks.  This  is  unnecessary
+    38              if  the controller does not use bad block replacement to
+    39              provide a "perfect" host area.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-34
+           Disk Geometry And Format                             11 June 1992
+
+
+     1     Note that the forced error indicator is fully distinct  from  the
+     2     512  or 576 bytes of data in each block.  Blocks written with the
+     3     "Force Error" command modifier shall, when read back, return both
+     4     correct  data and the presence of a forced error.  Blocks written
+     5     with forced errors shall have similar error rate  characteristics
+     6     as  blocks  written  normally,  without  forced errors.  A forced
+     7     error indicates that the host or controller process  writing  the
+     8     block  did  not  have  fully valid data to write into that block.
+     9     The disk subsystem (controller and unit) shall, however, preserve
+    10     the data so as to avoid further data corruption.
+
+    11     4.12.3  Performance Implications
+
+    12     As stated above, the host area of  a  disk  is  structured  as  a
+    13     vector of logical blocks.  From a performance viewpoint, however,
+    14     it  is  more  appropriate  to  view  the  host  area  as  a  four
+    15     dimensional hyper-cube.  The four dimensions are cylinder, group,
+    16     track, and sector.  Thus, it is possible to decompose  a  logical
+    17     block  number  into  a  unique  quadruple  of numbers; namely the
+    18     block's cylinder number, group number, track number,  and  sector
+    19     number.  Cylinder number is most significant and sector number is
+    20     least significant.
+
+    21     Alternatively, we can define a track as  consisting  of  a  fixed
+    22     number  of  blocks,  a  group  as consisting of a fixed number of
+    23     tracks, and a cylinder as consisting of a fixed number of groups.
+    24     The position of a block within a track is the block's sector.
+
+    25     "Spindles" are physical  entities  across  which  both  seek  and
+    26     rotational  latencies  are unpredictable.  A unit consists of one
+    27     or more spindles or spindle "groups."
+
+    28     The terms sector, track, cylinder, and spindle all come from  the
+    29     geometry  of  classical  disk drives.  Groups can be viewed as an
+    30     optimization  for  short  seeks  whose  seek   time   is   easily
+    31     predictable.
+
+    32     At any particular instant, the set of  logical  blocks  that  are
+    33     instantaneously accessible is those blocks in all tracks that are
+    34     in a particular sector, group, and cylinder.  In the  absence  of
+    35     transfers   to  a  different  group  or  cylinder,  this  set  of
+    36     instantaneously accessible blocks changes over  time  by  keeping
+    37     the  group  and  cylinder  constant while incrementing the sector
+    38     modulo the number of blocks/sectors in a track.  Referring to our
+    39     hyper-cube  analogy, the set of instantaneously accessible blocks
+    40     form a line parallel to the track axis.  This line moves parallel
+    41     to  the  sector axis, wrapping around when it reaches the edge of
+    42     the hyper-cube.  A disk therefore provides cyclic access  to  the
+    43     blocks in a particular group and cylinder.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-35
+           Disk Geometry And Format                             11 June 1992
+
+
+     1     This access structure to logical blocks implies that, to a  close
+     2     approximation,  the  track to which a block belongs has no effect
+     3     upon performance.  That is,  switching  tracks  within  the  same
+     4     group  and cylinder effectively requires zero time.  Two separate
+     5     transfers  in  the  same  group   and   cylinder   have   similar
+     6     performance,  regardless  of what tracks they are on.  To a first
+     7     order approximation, two separate transfers on successive sectors
+     8     of  different tracks in the same group and cylinder have the same
+     9     performance as a single, two sector (block) transfer.
+
+    10     Changing cylinders, however, does require  a  certain  amount  of
+    11     time.   The  amount  of  time  required  to  switch  between  two
+    12     cylinders is approximated by a monotonically increasing  function
+    13     of  the difference between the two cylinder numbers.  The time to
+    14     switch between cylinders is typically not affected by whether  or
+    15     not  groups  are  also being changed.  After switching cylinders,
+    16     the sector position of the disk (i.e., which sector's blocks  are
+    17     instantaneously accessible) is unpredictable.
+
+    18     Changing  groups  also  requires  a  certain  amount   of   time.
+    19     Generally, the time to switch between groups in the same cylinder
+    20     is no more than, and often less than, the time to switch  between
+    21     one  cylinder  and  the  next.   If cylinders and groups are both
+    22     changed  at  the  same  time,  the  time  to  switch  groups   is
+    23     effectively  zero,  as  it  is  included  in  the  time to switch
+    24     cylinders.
+
+    25     When changing from one  group  to  the  next  (successive)  group
+    26     within  the  same cylinder, the time required to switch groups is
+    27     predictable.  Therefore the sector position following  completion
+    28     of  the  group  switch  is also predictable, and transfers can be
+    29     optimized across group boundaries.  If a transfer to sector S  in
+    30     group  G  is  followed  by  a  transfer  in group G+1 of the same
+    31     cylinder, then sector S+1 (modulo the number of sectors/blocks in
+    32     a  track) is the optimal sector for the new transfer and sector S
+    33     is the least desirable.  The main effect of this is  to  optimize
+    34     continuous (spiral) transfers that cross group boundaries.
+
+    35     Note that what has been described herein is the  model  for  disk
+    36     logical  geometry,  which  may  have  a tenuous relationship to a
+    37     disk's actual physical geometry.  Disk designers should devise  a
+    38     logical to physical geometry mapping which optimizes the accuracy
+    39     of the model herein described.  This will generally  be  done  as
+    40     follows.   Head  or  track switches that effectively require zero
+    41     time (i.e., that require less than the intersector time) will  be
+    42     reported  as  logical  MSCP  tracks.  Head or track switches that
+    43     require significant amounts of time  will  be  divided  into  two
+    44     classes:   those  that require only a little time (typically less
+    45     than one rotation) and whose time is predictable, and those  that
+    46     require  somewhat  more  time and/or a lot of time.  The switches
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-36
+           Disk Geometry And Format                             11 June 1992
+
+
+     1     that require only a little time will be reported as logical  MSCP
+     2     groups.   The switches that require more time will be reported as
+     3     logical MSCP cylinders, where the switches should  be  mapped  to
+     4     cylinders  in  such  a  manner  as to minimize the amount of time
+     5     required to switch between cylinders that are  numerically  close
+     6     together.
+
+     7     The effect  of  this  geometry  on  multiblock  transfers  is  as
+     8     follows.   A multiblock transfer requires a certain minimum time,
+     9     which is the time to transfer  one  block  or  sector  times  the
+    10     number  of  blocks  in  the  transfer.  Crossing track boundaries
+    11     requires no additional time.  Crossing a group boundary  requires
+    12     a  small,  relatively fixed additional amount of time.  This time
+    13     is typically less than the  time  to  transfer  an  entire  track
+    14     (i.e.,  less  than  one  rotation).  Crossing a cylinder boundary
+    15     requires a somewhat larger additional amount of time.  This  time
+    16     is typically at least the time to transfer an entire track (i.e.,
+    17     at least one rotation).
+
+    18     The effect of this  geometry  on  host  allocation  policies  for
+    19     random-access   files   is  as  follows.   Whenever  possible,  a
+    20     random-access file should be allocated within a single group.  If
+    21     this is not possible, the host should try to allocate it within a
+    22     single cylinder.  If this is also not possible, the  host  should
+    23     allocate it in the minimum number of adjacent cylinders.
+
+    24     When a block has a high probability of being accessed immediately
+    25     after another block, hosts should attempt to allocate both blocks
+    26     in the same group or, if  that  is  not  possible,  in  the  same
+    27     cylinder.   If  both  blocks  cannot be allocated within the same
+    28     cylinder, then they should be in  cylinders  that  are  as  close
+    29     together as possible.
+
+    30     Hosts obtain the size of a unit's tracks, groups,  and  cylinders
+    31     from  the  GET  UNIT STATUS command's end message.  Some of these
+    32     disk geometry concepts may not apply to all  disk  units.   Units
+    33     report  the  fact  that a concept doesn't apply by specifying its
+    34     size to be the same as the next larger concept.  For  example,  a
+    35     disk  unit  that  doesn't have groups would specify one group per
+    36     cylinder, implying that groups and cylinders are the  same  size.
+    37     Note  that  the  value  of zero is reserved and illegal for track
+    38     size, group size, and cylinder size.
+
+    39     Track  size,  in  addition  to  being  useful   for   performance
+    40     prediction,  is  also used in the bad block replacement algorithm
+    41     specified  in  the  Digital  Storage  Architecture  Disk   Format
+    42     Specification  (DSDF).   It  is  possible  that  the  track  size
+    43     appropriate for performance considerations will be different from
+    44     the  track  size  required  by  bad  block  replacement.  If this
+    45     occurs, the track size required by bad block replacement shall be
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-37
+           Disk Geometry And Format                             11 June 1992
+
+
+     1     specified,   as   the   performance   effects   are  a  secondary
+     2     consideration.
+
+     3     Note that all of this performance  oriented  disk  geometry  only
+     4     applies  to  the host area of a disk unit.  The concepts need not
+     5     apply to the Replacement Control Table (RCT) and any other  areas
+     6     of  a disk, as access to those areas is not performance sensitive
+     7     and/or not host visible.
+
+     8     4.13  Bad Block Replacement
+
+     9     Bad block replacement is a technique used with disk class devices
+    10     to  present  each  unit  as  a single logically contiguous set of
+    11     usable blocks.
+
+    12     A unit may operate in one of two modes:  Controller Initiated Bad
+    13     Block  Replacement  or  Host  Initiated Bad Block Replacement.  A
+    14     unit's mode is  determined  by  whether  or  not  the  controller
+    15     requires   host   assistance  in  performing  dynamic  bad  block
+    16     replacement for that unit.  Controllers report this to hosts  via
+    17     the  "Controller  Initiated Bad Block Replacement" unit flag (see
+    18     Section 5.7, "Unit Flags" for more detail).
+
+    19     For a complete description  of  Controller  Initiated  Bad  Block
+    20     Replacement,  see  Chapter  8,  "Controller  Initiated  Bad Block
+    21     Replacement."
+
+    22     For  a  complete  description  of  Host   Initiated   Bad   Block
+    23     Replacement,  see  the  Digital  Storage Architecture Disk Format
+    24     (DSDF)  specification  section  on  "Host  Initiated  Bad   Block
+    25     Replacement."
+
+    26     4.14  Write Protection
+
+    27     There are three kinds of write protection that can  apply  to  an
+    28     MSCP   device:    Hardware   Write   Protection,  Software  Write
+    29     Protection, and Data Safety Write Protection.  Each kind of write
+    30     protection is controlled from a separate source as follows:
+
+    31              Hardware Write Protection
+
+    32                Hardware Write Protection is controlled by the  user
+    33                or  operator  manipulating  the unit's write protect
+    34                mechanism.  The status of Hardware Write  Protection
+    35                may  change  at  unpredictable  times in response to
+    36                human intervention.
+
+    37                When Hardware Write Protection is established (i.e.,
+    38                when  a  user  activates  the  unit's  write protect
+    39                mechanism), the controller shall  provide  a  smooth
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-38
+           Write Protection                                     11 June 1992
+
+
+     1                transition to the write protect state.  That is, the
+     2                controller  shall  complete  all  write   operations
+     3                (commands) that it has already initiated on the unit
+     4                before actually prohibiting writes.  Note,  however,
+     5                that  the  controller  should immediately reject any
+     6                new write operations that it receives after the user
+     7                activates the unit's write protect mechanism.  Write
+     8                operations that the controller received  before  the
+     9                write  protect  mechanism  was  activated,  but that
+    10                haven't been initiated yet, may either  be  rejected
+    11                or  completed  at  the controller's option.  The end
+    12                result of this is that each individual write command
+    13                or  operation is either completed in its entirety or
+    14                else rejected before any of its data is  written  to
+    15                the  unit.   Note that it is not possible for either
+    16                the controller or the  host  to  perform  bad  block
+    17                replacement when a unit is Hardware Write Protected,
+    18                since it requires writing to the unit's  Replacement
+    19                Control Table.
+
+    20              Software Write Protection
+
+    21                Software Write Protection is controlled by the  host
+    22                or  hosts.   Host(s)  set  or  change  the status of
+    23                Software Write Protection via the ONLINE or SET UNIT
+    24                CHARACTERISTICS commands ("Enable Set Write Protect"
+    25                modifier and "Software Write  Protect"  unit  flag).
+    26                In  general,  the  detailed causes of Software Write
+    27                Protection are matters  of  host  policy.   However,
+    28                Host  Initiated  Bad Block Replacement, as described
+    29                in the  Digital  Storage  Architecture  Disk  Format
+    30                Specification   (DSDF),   requires  that  hosts  use
+    31                Software Write Protection to emulate certain  causes
+    32                of Data Safety Write Protection.
+
+    33                Unlike the transition to Hardware Write Protect  the
+    34                transition  to Software Write Protect is a naturally
+    35                smooth process since it is established by Sequential
+    36                commands     (i.e.,     ONLINE    and    SET    UNIT
+    37                CHARACTERISTICS),  which  implies  that  all   write
+    38                operations   will   have  been  completed  prior  to
+    39                Software Write Protection being established.
+
+    40              Data Safety Write Protection
+
+    41                Data Safety Write Protection is  controlled  by  the
+    42                MSCP  server.   It is set whenever some condition in
+    43                the unit or volume prevents reliable modification of
+    44                data on the volume.  Possible causes include:
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-39
+           Write Protection                                     11 June 1992
+
+
+     1                   o  An incomplete bad block replacement.
+
+     2                   o  An invalid RCT.
+
+     3                   o  The unit is  only  capable  of  reading  the
+     4                      volume's   format  (e.g.,  a  single-density
+     5                      volume in a double-density drive).
+
+     6                  Note that there can be  multiple  causes  for  a
+     7                  unit   being   Data   Safety   Write  Protected.
+     8                  Furthermore, some causes represent errors  while
+     9                  others represent normal occurrences.
+
+    10                  Each individual cause can  only  be  established
+    11                  when  a  unit  is brought "Unit-Online."  Causes
+    12                  can be eliminated -- possibly allowing the  unit
+    13                  to  cease  being  Data Safety Write Protected --
+    14                  while the unit remains "Unit-Online."   However,
+    15                  the  server  can  only establish a new cause for
+    16                  the unit to be Data Safety  Write  Protected  by
+    17                  forcing  the  unit to be "Unit-Available" to all
+    18                  class   drivers.    Thus   class   drivers   are
+    19                  guaranteed  that  a  unit will not spontaneously
+    20                  become Data Safety  Write  Protected  while  the
+    21                  unit is "Unit-Online." 
+
+
+    22     A unit's Write Protect Status Display  Mechanism  shall  indicate
+    23     the  "inclusive  or"  of all forms of write protection.  That is,
+    24     the display shall indicate that the unit is write protected  when
+    25     it is Hardware or Software or Data Safety Write Protected.
+
+    26     4.15  Compare Operations
+
+    27     MSCP includes the following kinds of compare operations:
+
+    28          1.  The COMPARE HOST DATA command.
+
+    29          2.  Read-compare operations, invoked by the "Compare  Reads"
+    30              unit  flag  or  by  the  "compare"  modifier  on  a READ
+    31              command.
+
+    32          3.  Write-compare  operations,  invoked  by   the   "Compare
+    33              Writes"  unit  flag  or  by  the "compare" modifier on a
+    34              WRITE command.
+
+    35     The operation of these different types of compare  operations  is
+    36     described  below.  Note that all of the compare operations report
+    37     the first difference or other error starting from  the  beginning
+    38     of  the  transfer.  Therefore the compare operation at the end of
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-40
+           Compare Operations                                   11 June 1992
+
+
+     1     the transfer may be aborted if a difference is discovered at  the
+     2     beginning.
+
+     3     The COMPARE HOST DATA command is used to verify that data in host
+     4     memory  matches  data  on  a unit.  The data is obtained from the
+     5     unit in the manner that is most convenient or efficient  for  the
+     6     controller.   In  this  respect  the  COMPARE  HOST  DATA command
+     7     operates identically to a READ command.  Unlike the READ command,
+     8     the  data  is  not  transferred to host memory.  Instead, data is
+     9     obtained from host memory and compared against the data  obtained
+    10     from  the  unit.   Upon  completion of the command the controller
+    11     reports whether the data was identical or  different.   The  data
+    12     being different is reported as a "Compare Error" in the command's
+    13     end message.  No error log message is generated as  this  is  not
+    14     considered   to  be  a  "significant"  error  (since  it  can  be
+    15     deliberately  caused  by  user  programs).   The  controller  may
+    16     attempt retries or not at its option.  As the retries will always
+    17     fail (assuming the host is not modifying  the  buffer  while  the
+    18     transfer  is in progress), retries will have a deleterious effect
+    19     on performance but will in no way effect the final outcome of the
+    20     operation.
+
+    21     Read-compare and write-compare operations are  performed  at  the
+    22     conclusion  of  the  appropriate transfer commands to verify that
+    23     the data was correctly transferred and that the data can  now  be
+    24     obtained  from its destination.  The general algorithm used is to
+    25     obtain the data from its destination and compare it  against  the
+    26     data reobtained from its source.
+
+    27     In particular, during a  read-compare  operation  the  controller
+    28     reobtains the data from the unit and compares it against the data
+    29     obtained from host memory.   Conversely,  for  the  write-compare
+    30     operation  the  controller  obtains  the  data  from the unit and
+    31     compares it against the data reobtained from host memory.
+
+    32     An  additional  requirement  for  performing   read-compare   and
+    33     write-compare  operations  is  that the controller should, to the
+    34     maximum extent allowed by  its  architecture,  operate  all  data
+    35     transfer  paths  in  the  opposite  direction  from  that  of the
+    36     original transfer,  in  order  to  maximize  the  probability  of
+    37     detecting a transfer error.
+
+    38     Ideally, to operate all transfer paths in the opposite direction,
+    39     during  a read-compare operation, the controller would obtain the
+    40     data from the host and send it to the drive, and the drive  would
+    41     actually  compare  the  data  against  the original.  Conversely,
+    42     during a write-compare operation, the data would be sent  to  the
+    43     host port, which would actually perform the comparison.  However,
+    44     neither implementation is currently feasible since no  drive  nor
+    45     communications  mechanism  provides such comparison capabilities.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-41
+           Compare Operations                                   11 June 1992
+
+
+     1     So for now controllers must perform the actual compare functions.
+
+     2     If  a  read-compare  or  write-compare   operation   fails,   the
+     3     controller  shall  interpret  this  as implying that the original
+     4     transfer failed and retry the original transfer  if  appropriate.
+     5     If  the  controller successfully obtains the data from its source
+     6     and destination, but the data is different, then  the  controller
+     7     shall retry the original transfer and report the compare error in
+     8     an error log message.   If  the  controller  cannot  successfully
+     9     obtain  the  data from its destination, but the error is one that
+    10     may be eliminated by rewriting the data to the destination,  then
+    11     the  controller shall also retry the original transfer and report
+    12     the  error  (from  the  attempt  to  obtain  the  data  from  its
+    13     destination)  in an error log message.  All other errors need not
+    14     be retried, but shall be reported in an error log  message.   The
+    15     only  exception  to the above is commands that have the "Suppress
+    16     Error Recovery" modifier set.  The controller may or may not,  at
+    17     the controller's option, retry the original transfer if a compare
+    18     error occurs in such a command.
+
+    19     For example, "Data Errors," such as an uncorrectable  ECC  error,
+    20     shall  be  retried on write-compare operations.  They need not be
+    21     retried on read-compare operations, since an unrecoverable  "Data
+    22     Error"  implies that the READ itself will fail.  "Compare Errors"
+    23     shall always be retried.   Note  that  the  controller  need  not
+    24     discriminate  among  types  of  errors -- it may always retry all
+    25     errors   during   read-compare   or   write-compare   operations,
+    26     regardless  of whether or not the error will inhibit the original
+    27     transfer.
+
+    28     The number of retries required for read-compare and write-compare
+    29     operations  is  controller  dependent.   However, all controllers
+    30     shall retry such operations at least once.  The exact  number  of
+    31     retries  that  a  controller implements should be chosen based on
+    32     undetected error rate characteristics.  The controller may either
+    33     retry  the  entire  transfer, or else only retry the portion that
+    34     includes the error.
+
+    35     4.16  Special Drive Topologies
+
+    36     4.16.1  Multiaccess Drives
+
+    37     A multiaccess drive is a drive that may be  physically  connected
+    38     to  more than one controller simultaneously via separate physical
+    39     paths (ports).  The primary  purpose  of  the  multiaccess  drive
+    40     capability  is  to  enhance the availability of data by providing
+    41     alternate access paths to the data, thereby safeguarding  against
+    42     individual host, controller, or communications mechanism failure.
+    43     High availability systems typically  utilize  multiaccess  drives
+    44     and  the  replication  of  some  or all of the other major system
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-42
+           Special Drive Topologies                             11 June 1992
+
+
+     1     components in order to recover from such failures  without  human
+     2     intervention.
+
+     3     Although a multiaccess drive may be physically connected to  more
+     4     than  one  controller,  only  one  controller at any one time may
+     5     establish  the  logical  connection  (i.e.,  bring   it   online)
+     6     necessary  for  any  unit of the drive to become "Unit-Online" or
+     7     under the "Exclusive Access" of a host via that  controller  (see
+     8     Section  4.3,  "Unit States").  Note that the detailed definition
+     9     of the logical connection between a controller  and  a  drive  is
+    10     device   dependent,   and  therefore  not  the  concern  of  this
+    11     specification.
+
+    12     Once the logical connection between a  multiaccess  drive  and  a
+    13     controller  is established, the unit of a drive or the units of a
+    14     multiunit drive or  formatter  (see  Section  4.16.2,  "Multiunit
+    15     Drives  and  Formatters")  can  only  be  accessed  via that same
+    16     controller and MSCP server.  The unit or units  therefore  become
+    17     inaccessible  (i.e., "Unit-Offline") via all other controllers to
+    18     which the multiaccess drive is physically connected.
+
+    19     A controller may only retain the logical connection with a  drive
+    20     as  long  as one or more units of that drive are "Unit-Online" or
+    21     under  "Exclusive  Access"  via  that  controller.   That  is,  a
+    22     controller  shall  release its logical connection with a drive as
+    23     soon as all units of the drive cease to be "Unit-Online"  to  any
+    24     host  or under the "Exclusive Access" of a particular host.  Host
+    25     class drivers may therefore switch access paths to a  multiaccess
+    26     drive  from  one  controller  to another under program control by
+    27     making all  units  of  the  drive  "Unit-Available"  through  the
+    28     controller  to  which the drive is currently logically connected,
+    29     then bringing one of the units "Unit-Online" or under  "Exclusive
+    30     Access" via the other controller.
+
+    31     A controller shall also release the logical  connections  to  all
+    32     units  of  a  multiaccess  drive whenever the host access timeout
+    33     (specified  via  the  SET  CONTROLLER  CHARACTERISTICS   command)
+    34     expires,  in  order  to  permit  access  to the drive via another
+    35     access  path.   See  Section  4.9,  "Host  Access  Timeouts"  for
+    36     complete details.
+
+    37     The logical connection between a  controller  and  a  multiaccess
+    38     drive  must  exist  in  order  for  any  unit  of the drive to be
+    39     "Unit-Online" or under "Exclusive Access"  with  respect  to  any
+    40     host.   However,  certain status information must be available to
+    41     all controllers without the existence of  a  logical  connection.
+    42     The specific cases are:
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-43
+           Special Drive Topologies                             11 June 1992
+
+
+     1          1.  When no units of the drive are "Unit-Online" to or under
+     2              the  "Exclusive  Access"  of  any class driver (that is,
+     3              when they are all "Unit-Available"  or  "Unit-Offline"),
+     4              the  multiaccess  drive shall report its existence, unit
+     5              number(s),   and   certain   characteristics   to    all
+     6              controllers   to  which  it  is  attached.   The  actual
+     7              mechanism by  which  this  information  is  reported  is
+     8              device  dependent, and therefore may use such schemes as
+     9              time-multiplexing or polling of a  single  communication
+    10              path.  However, controllers shall present the conceptual
+    11              view  to  hosts  that  this  information   is   reported
+    12              continuously and simultaneously to all controllers.  The
+    13              drive characteristics that shall be reported  are  those
+    14              returned  in  the AVAILABLE attention message and in the
+    15              GET UNIT STATUS and SET UNIT CHARACTERISTICS command end
+    16              messages  for  the  current  state  of the drive's units
+    17              ("Unit-Available" or "Unit-Offline").
+
+    18          2.  When a drive is logically connected to some  controller,
+    19              and  that  controller  receives a DETERMINE ACCESS PATHS
+    20              command for one of the units that is "Unit-Online,"  the
+    21              drive  shall  report  its  existence,  unit  number, and
+    22              certain characteristics  to  all  other  controllers  to
+    23              which it is attached.  The characteristics that it shall
+    24              report are those returned in the ACCESS  PATH  attention
+    25              message.   As  detailed  in  Section  6.6.3,  "DETERMINE
+    26              ACCESS   PATHS    Command,    Description"    successful
+    27              communication  of  the ACCESS PATH attention message may
+    28              be probabilistic rather than guaranteed.  Note that  the
+    29              drive  need  only make this report when instructed to do
+    30              so by the controller.
+
+    31     Note that those two cases are only of  concern  with  multiaccess
+    32     drives,  as  a  single  access  drive  cannot have the problem of
+    33     trying to communicate with  two  or  more  controllers  at  once.
+    34     Also,  if  a  multiaccess  drive  is  logically  connected  to  a
+    35     controller, and that controller never receives a DETERMINE ACCESS
+    36     PATHS  command  for a "Unit-Online" unit of that drive, then that
+    37     drive need not communicate in any way with any other  controller.
+    38     The  drive  need  not even report its existence until the logical
+    39     connection is broken (i.e., it  ceases  to  be  "Unit-Online"  or
+    40     under  "Exclusive Access") or the controller receives a DETERMINE
+    41     ACCESS PATHS command.
+
+    42     4.16.2  Multiunit Drives And Formatters
+
+    43     A multiunit drive is a single physical disk drive that appears as
+    44     several   independent  units  to  hosts.   So-called  fixed  plus
+    45     removable disk drives, providing one removable disk unit and  one
+    46     nonremovable  disk unit, are the most common example of multiunit
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-44
+           Special Drive Topologies                             11 June 1992
+
+
+     1     drives.  A multiunit formatter is a single set  of  interface  or
+     2     read/write    electronics   that   connects   several   otherwise
+     3     independent units to controllers.  Multiunit formatters are  most
+     4     commonly  used  with  tape drives, although there is nothing that
+     5     prevents their use with disk drives as well.
+
+     6     All the units of a multiunit drive or formatter  share  a  single
+     7     access  path  to controllers.  This implies that all of the units
+     8     must be "connected" to the same controller.   In  particular,  if
+     9     one  unit  of  a multiunit drive or formatter is "Unit-Online" or
+    10     under "Exclusive Access" via a controller,  then  all  the  other
+    11     units of the multiunit drive or formatter may only be accessed by
+    12     that  same  controller.    That   is,   the   other   units   are
+    13     "Unit-Offline"  to  all  other  controllers.   Awareness  of this
+    14     characteristic is critical for high availability systems -- if  a
+    15     failed  operation  on  a  multiaccess  unit  is to be retried via
+    16     another controller, and the unit is part of a multiunit drive  or
+    17     formatter,  then  all  units  of  the  drive or formatter must be
+    18     switched to the other controller.
+
+    19     Some units  of  a  multiunit  disk  drive  may  share  mechanical
+    20     components as well as interface electronics.  Such units are said
+    21     to share a spindle.  That  is,  the  units  must  either  be  all
+    22     spinning or all not spinning, just as if the units shared a drive
+    23     motor or spindle (which they typically will).   Such  units  must
+    24     also  share  a  single  Run/Stop  switch,  since  they are always
+    25     spun-up and spun-down together.  Hosts  must  also  be  aware  of
+    26     units  which  share  a  spindle,  as  dismounting  one  such unit
+    27     requires that all units sharing the same spindle be spun-down.
+
+    28     The units of a multiunit drive or formatter are identified by the
+    29     "multiunit  code"  unit  characteristic field.  Hosts obtain this
+    30     two byte field via the AVAILABLE attention message or in the  end
+    31     message of a GET UNIT STATUS, ONLINE, or SET UNIT CHARACTERISTICS
+    32     command.  The low  byte  of  this  field  contains  a  controller
+    33     dependent  encoding of the access path between the controller and
+    34     the drive.  The high byte of this  field  contains  a  controller
+    35     dependent  encoding  of the spindle, on a particular access path,
+    36     that the unit uses.  Controllers may use any encoding whatsoever,
+    37     provided that each access path and each spindle (within an access
+    38     path) has a unique value.  Note that  the  access  path  byte  is
+    39     implicitly   qualified  by  the  controller's  or  MSCP  server's
+    40     identity, and that the spindle byte is  implicitly  qualified  by
+    41     the access path.
+
+    42     Hosts use the "multiunit code" field as  follows.   When  a  host
+    43     decides  to  spin-down  a unit, it scans all other units that are
+    44     "Unit-Online" via the same MSCP  server  for  those  units  whose
+    45     entire "multiunit code" field (both bytes) matches the unit being
+    46     spun-down.   Such  units,  if  any,  share  a  spindle  or  other
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-45
+           Special Drive Topologies                             11 June 1992
+
+
+     1     mechanical components with the unit being spun-down, so that they
+     2     must be spun-down together.  When a host decides to access a unit
+     3     via  a  different  controller,  it scans all other units that are
+     4     "Unit-Online" via the same MSCP server for those units whose  low
+     5     byte  of  the  "multiunit  code"  field  matches  the  unit being
+     6     switched.  Such units, if any, share an access path with the unit
+     7     being  switched,  so  that  they must also be switched to the new
+     8     controller.
+
+     9     Note that the low byte of the "multiunit code" field (the  access
+    10     path)  is meaningless for units that are inherently restricted to
+    11     a single controller.  Controllers may return any fixed  value  as
+    12     the access path encoding for such units, provided that it doesn't
+    13     duplicate the value returned for any units on the same controller
+    14     that are not inherently restricted to a single controller.
+
+    15     This use and format of the "multiunit  code"  field  implies  the
+    16     following architectural restrictions on all controllers:
+
+    17          1.  All units that  share  a  spindle  or  other  mechanical
+    18              components  shall  also share an access path.  Note that
+    19              this is an essential restriction for  multiunit  drives,
+    20              regardless  of  how  shared components are communicated.
+    21              If units that share a spindle did not  share  an  access
+    22              path,  then  they  could be simultaneously "Unit-Online"
+    23              via  different  controllers,  making  it  impossible  to
+    24              coordinate a simultaneous spin-down.
+
+    25          2.  There is a maximum of 256 access paths  per  controller.
+    26              In  the  absence of multiunit drives or formatters, this
+    27              implies a maximum of 256 units per controller.
+
+    28          3.  There is a maximum of 256 spindles per access path.   In
+    29              the  absence  of shared spindles, this implies a maximum
+    30              of 256 units per access path or formatter.
+
+
+    31     4.17  Controller And Unit Identifiers
+
+    32     MSCP  requires  that  all  controllers  and  drives  have  unique
+    33     identifiers,  called controller identifiers and unit identifiers.
+    34     The structure of these identifiers is as follows:
+
+    35                     31                             0 
+    36                     +-------------------------------+
+    37                     |      unique device number     |
+    38                     +-------+-------+               +
+    39                     | class | model |               |
+    40                     +-------+-------+---------------+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-46
+           Controller And Unit Identifiers                      11 June 1992
+
+
+     1     The fields in a controller or unit identifier are as follows:
+
+     2         unique device number
+
+     3             Uniquely identifies the device among all devices of  that
+     4             same  class and model.  The device serial number could be
+     5             used as the "unique device  number,"  though  that  isn't
+     6             required.
+
+     7         model
+
+     8             Identifies the exact model of the  subsystem  within  its
+     9             class.   All  valid  class  and  model codes are nonzero,
+    10             implying that all valid identifiers are  nonzero.   Table
+    11             C-2, "Mass Storage Controller 'Model' Values"contains the
+    12             "model"  values  for  both  disk  and  tape  controllers.
+    13             Tables C-3, "Disk Drive/Media Identifier Values" and C-4,
+    14             "Tape Drive/Media Identifier Values" contain the  "model"
+    15             values for disk and tape drives, respectively.
+
+    16         class
+
+    17             Identifies the type of the subsystem -- controller,  disk
+    18             drive,  etc.   Table  C-1,  "Controller  and Unit 'Class'
+    19             Values" contains the "class" values for both  controllers
+    20             and drives.
+
+
+    21     The  Storage  Systems  Architecture  Group  is  responsible   for
+    22     assigning  new  values.   Requests  for  additions  to the tables
+    23     should be addressed to SSAG::SSAG.  Note that different units  of
+    24     multiunit  drives  are  distinguished by having different "model"
+    25     bytes;  the  "class"  and  "unique  device  number"  fields   are
+    26     typically  identical.   Note  also  that all MSCP servers for the
+    27     same device class within the same  controller  shall  return  the
+    28     same controller identifier.
+
+    29     As previously stated, MSCP  requires  that  controller  and  unit
+    30     identifiers  be  unique  across  all devices accessible via MSCP.
+    31     This clearly cannot be checked by controllers.  Controllers  can,
+    32     however,  enforce  unique  unit identifiers across the units that
+    33     are attached to themselves.  This is  done  using  the  following
+    34     algorithms:
+
+    35          1.  Controllers should detect and respond to duplicate  unit
+    36              identifiers  across all units whose unit identifiers the
+    37              controller can obtain, including all  units  that  would
+    38              otherwise   be   "Unit-Online"   or   "Unit-Available." 
+    39              Detection of a duplicate unit identifier on one unit  of
+    40              a  multiunit  drive  is  treated  as  a  duplicate  unit
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-47
+           Controller And Unit Identifiers                      11 June 1992
+
+
+     1              identifier condition on all other units that  share  one
+     2              or more of the following components with the unit having
+     3              the duplicate unit identifier:
+
+     4              a.  A unit number select mechanism.
+
+     5              b.  A Run/Stop or Load/Unload switch.
+
+     6              c.  A spindle or other mechanical components.
+
+     7              Note  that  duplicate  unit  identifiers  are   detected
+     8              regardless   of  the  state  of  a  unit's  Run/Stop  or
+     9              Load/Unload switch.
+
+    10          2.  Whenever a controller becomes aware of a duplicate  unit
+    11              identifier, it immediately spins-down all units with the
+    12              duplicate  identifier  and   forces   them   to   remain
+    13              spun-down.    The   controller   spins-down   the  units
+    14              regardless  of  their  current  state.   The  controller
+    15              typically  forces  them  to remain spun-down by spinning
+    16              them down again whenever an operator spins them up.
+
+    17          3.  The   controller    reports    "Unit-Offline    (subcode
+    18              'Inoperative')" status for all units with duplicate unit
+    19              identifiers.  In addition, if the unit might potentially
+    20              be  connected  to  another  controller,  the  controller
+    21              should  flag  the  presence  of   the   duplicate   unit
+    22              identifier  in  the  drive.   Other controllers, if any,
+    23              should check this  flag  and  also  treat  the  unit  as
+    24              "Unit-Offline" if the flag is set.
+
+    25     Whether or not a controller does in fact check for duplicate unit
+    26     identifiers  is controller dependent.  Note that a duplicate unit
+    27     identifier is a drastic failure,  indicative  of  some  otherwise
+    28     undiagnosed hardware malfunction.
+
+    29     4.18  Media Type Identifiers
+
+    30     Controllers  return  a  media  type  identifier  for  each   unit
+    31     accessible  via  that  controller.   This  identifier encodes two
+    32     pieces of information:
+
+    33          1.  The preferred device type name for use  with  the  unit.
+    34              These  two alphabetic characters are conventionally used
+    35              with the unit number and a controller designator as  the
+    36              fully  qualified  operating  system  identifier  for the
+    37              unit.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-48
+           Media Type Identifiers                               11 June 1992
+
+
+     1          2.  The name (product name) of the media used on  the  unit.
+     2              This  name  should  be printed on the unit's front panel
+     3              and on all removable media that may  be  used  with  the
+     4              unit.
+
+     5     The primary reason for returning this information is to  simplify
+     6     operating system support for generic device allocation.
+
+     7     The media type identifier returned via MSCP is a 32 bit  quantity
+     8     encoded as follows:
+
+     9                     31   26   21   16   11  7 6    0 
+    10                     +----+----+----+----+----+------+
+    11                     | D0 | D1 | A0 | A1 | A2 |   N  |
+    12                     +----+----+----+----+----+------+
+
+    13     where the fields are as follows:
+
+    14     D0
+    15     D1  The preferred device type name for the unit.  D0 and  D1  are
+    16         five bit fields, each encoding one alphabetic character.  "A"
+    17         is encoded with the value 1, "B" with the value 2,  etc.   D0
+    18         encodes  the  left  character of the device type name, D1 the
+    19         right character.
+
+    20     A0
+    21     A1
+    22     A2
+    23     N   The name of the media used on the unit.  A0  through  A2  are
+    24         five  bit  fields,  each  encoding an alphabetic character or
+    25         null.  "A" is encoded with the value 1, "B" with the value 2,
+    26         etc.   Zero  represents a null or the absence of a character.
+    27         One to three characters of the media name are  encoded,  left
+    28         justified,  in  A0  through  A2.   N  is  a  seven  bit field
+    29         containing the value of two decimal digits.
+
+    30     Note that the encoding of the media name assumes  that  the  name
+    31     consists  of one, two, or three alphabetic characters followed by
+    32     exactly two digits (i.e., ann, aann, or  aaann).   MSCP  requires
+    33     that  the  product  names  for all mass storage devices adhere to
+    34     this format.
+
+    35     Currently defined device type and media  names  (i.e.,  currently
+    36     defined  media  type identifier values) are listed in Appendix C,
+    37     "Controller, Unit, and  Media  Type  Identifiers."   Tables  C-3,
+    38     "Disk  Drive/Media  Identifier Values" and C-4, "Tape Drive/Media
+    39     Identifier Values" contain the "media identifier" values for disk
+    40     and  tape drives, respectively.  The Storage Systems Architecture
+    41     Group is responsible for  assigning  new  values.   Requests  for
+    42     additions to the tables should be addressed to SSAG::SSAG.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-49
+           Controller-based Cache                               11 June 1992
+
+
+     1  |  4.19  Controller-based Cache
+
+     2     The purpose of a controller-based cache is to minimize  the  mean
+     3     response  time  for the aggregate set of I/O requests serviced by
+     4     the controller.   Controller-based  caches  are  intended  to  be
+     5     invisible  to  the  user except for this performance improvement.
+     6     Implementations shall ensure the data integrity  with  the  cache
+     7     employed  is  comparable  to the backing-store even through power
+     8     failures, so that the only reason to use or avoid  the  cache  is
+     9     performance.   Implementing controller-based cache does not waive
+    10     any normal storage device responsibilities  as  defined  in  this
+    11     specification.
+
+    12     This implies that a cached device has the same responsibility  as
+    13     any  other device in terms of setting the forced error flag in an
+    14     LBN if the data integrity of that LBN  is  in  question.   If  it
+    15     knows  corrupt  data exists on the device but does not know which
+    16     data, it shall assume  the  Offline/Unit  inoperative  state  and
+    17     maintain this state even through power failure.  In this case, it
+    18     must provide a diagnostic or DUP path for recovery of user  data.
+    19     No single point of failure may allow the unreported return of the
+    20     wrong or corrupt data.
+
+    21     The following are implementation examples  that  demonstrate  the
+    22     implications of these responsibilities.
+
+
+    23           o  If  storage  units  are  dual  ported  to  two  or  more
+    24              controllers, the controllers will write data to the unit
+    25              backing-store as well as the cache before returning  the
+    26              command  complete response so that the failure of one of
+    27              the controllers cannot cause loss of user  data  or  its
+    28              consistency.
+
+    29           o  Controllers whose  cache  may  lose  user  data  or  its
+    30              consistency in the event of power failure will write the
+    31              data to the backing-store as well as  the  cache  before
+    32              returning  the  command complete response.  For commands
+    33              with the compare modifier, the command complete will not
+    34              be sent until the compare has been made with the backing
+    35              store.
+
+    36           o  Controllers whose cache is capable  of  maintaining  the
+    37              user  data  integrity  and  its  consistency  at a level
+    38              comparable to  the  backing-store,  even  through  power
+    39              failure,  can  return  the  command complete response as
+    40              soon as the data is in the cache, before writing to  the
+    41              backing-store.   For commands with the compare modifier,
+    42              the command complete response may also  be  returned  as
+    43              soon as the user data is compared to the cache data.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           ALGORITHMS AND USAGE RULES                              Page 4-50
+           Controller-based Cache                               11 June 1992
+
+
+     1           o  A controller-based cache implementation can degrade  its
+     2              cache  actions in the event of component failure as long
+     3              as the specified  data  integrity  is  maintained.   For
+     4              example,  if  a controller implementing a cache that can
+     5              maintain its integrity through power failure  recognizes
+     6              its  cache  memory  integrity is degrading, it can flush
+     7              the cache and revert to a mode where it  writes  to  the
+     8              backing-store  as well as the cache before returning the
+     9              command complete response.
+
+
+    10     The manner in which users access their data varies greatly.  Some
+    11     access   it   totally   randomly,   others   walk  through  files
+    12     sequentially.  Some access  the  same  data  many  times,  others
+    13     access  each  piece  of  data  only once.  These differing access
+    14     patterns have a great  effect  on  the  performance  of  a  cache
+    15     algorithm.
+
+    16     Transfer commands are provided with a "Cache  Access  Attributes"
+    17     field  so  that  the  cache  action  can  be  optimized for these
+    18     patterns.  This is an optional use field  that  allows  users  to
+    19     provide  "advice"  as  to  the  access  patterns  they expect.  A
+    20     particular cache implementation,  knowing  its  specific  caching
+    21     algorithm  and  its current state, may make use of this advice to
+    22     optimize its performance.  No specific  action  is  mandatory  or
+    23     architecturally  defined.   Each implementation shall specify its
+    24     actions.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 5
+
+     2                           MSCP CONTROL MESSAGES
+
+
+     3     5.1  Generic Control Message Format
+
+     4     All MSCP control messages -- i.e., command, end, error  log,  and
+     5     attention  messages -- consist of a 12 byte header and a variable
+     6     length parameter area.  The generic control message format is  as
+     7     follows:
+
+     8                     31                             0 
+     9                    +--------------------------------+
+    10                    |                                |
+    11                    +------- control message --------+
+    12                    |         type specific          |
+    13                    +-------      header     --------+
+    14                    |                                |
+    15                    +--------------------------------+
+    16                    |                                |
+    17                    /          parameters            /
+    18                    /                                /
+    19                    |                                |
+    20                    +--------------------------------+
+
+    21     The device class (disk or tape) does not appear  in  the  control
+    22     message.   It is implied by the connection or MSCP server to/from
+    23     which the control message is sent/received.
+
+    24     Multibyte numbers are stored least significant byte first  (i.e.,
+    25     using the standard VAX number formats).
+
+    26     The communications mechanism conveys both the text of  a  message
+    27     and  its  length.   The  receiver of a message uses the length to
+    28     verify that all required parameters are in fact present.
+
+    29     The communications mechanism may restrict the  allowable  message
+    30     lengths.   For example, it might require that all messages have a
+    31     fixed length (typically equal to the longest  message  supported)
+    32     or that the length be an even multiple of bytes.  For this reason
+    33     the message lengths defined by MSCP are minimum lengths.  Senders
+    34     may  pad  messages  as necessary to meet communications mechanism
+    35     length restrictions.  The contents of the padding (that  is,  the
+    36     contents of any data past the end of the message formats shown in
+    37     this document) are reserved and follow  the  rules  for  reserved
+    38     fields  defined  in  Section 5.2, "Reserved and Undefined Fields"
+    39     (i.e., such padding contains zeros).
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-2
+           Reserved And Undefined Fields                        11 June 1992
+
+
+     1     5.2  Reserved And Undefined Fields
+
+     2     Reserved fields are those fields that are intended  for  possible
+     3     future  extensions  to MSCP.  The use of such fields shall follow
+     4     certain rules, in order to ensure that such future extensions can
+     5     be  upwards  compatible  with  the  current  version of MSCP.  In
+     6     general, the sender of a message shall supply the value  zero  in
+     7     all  reserved  fields.  The action for a message receiver varies,
+     8     and is discussed below.
+
+     9     An undefined field is just that -- its  contents  are  controller
+    10     implementation  dependent,  and  therefore  cannot be used in any
+    11     meaningful way by class drivers.  Undefined fields  are  provided
+    12     in  order  to  simplify controller implementation.  Class drivers
+    13     shall ignore the contents of undefined fields.
+
+    14     A field, as used in this discussion, may  have  any  length.   In
+    15     particular,  it  may be an individual bit of a flags word or byte
+    16     as well as an entire byte, word, or whatever.
+
+    17     Class drivers shall supply the value zero in the reserved  fields
+    18     of  all  messages  (commands) that they send to a controller, and
+    19     shall also ignore the contents of  reserved  fields  in  all  the
+    20     messages   (end  messages,  attention  messages,  and  error  log
+    21     messages) that they receive from an MSCP  server.   MSCP  servers
+    22     shall  supply  the  value  zero  in  the  reserved  fields of all
+    23     messages  (end  messages,  attention  messages,  and  error   log
+    24     messages)  that  they  send to class drivers.  MSCP servers shall
+    25     either ignore the contents of reserved  fields  in  the  messages
+    26     (commands)  that  they  receive from class drivers or verify that
+    27     the contents are zero.  The command is  treated  as  an  "Invalid
+    28     Command"  ("reserved field" protocol error; see "Invalid Command"
+    29     status  in  Section  5.5,  "Status  and  Event  Codes")  if   the
+    30     controller  chooses  to  verify  and the contents are found to be
+    31     nonzero.  Whether an MSCP server verifies  that  reserved  fields
+    32     are  zero is controller dependent, and need not be consistent for
+    33     all reserved fields.
+
+    34     Many  controllers  generate  command  end  messages   by   simply
+    35     modifying   the   commands'   command  messages.   That  is,  the
+    36     controller copies a command  message  into  an  internal  buffer,
+    37     modifies  it in place during execution of the command, then sends
+    38     the resulting contents of the internal buffer  as  the  command's
+    39     end message.  To simplify such an implementation, controllers may
+    40     merely  "echo"  command  message   reserved   fields   when   the
+    41     corresponding  field  in  the  end  message should be zero.  More
+    42     precisely, if some field in the end message of a  command  should
+    43     be  zero,  and  the  corresponding  (same  position) field in the
+    44     command's command message is a reserved field, the controller may
+    45     copy  the  reserved  field  from  the  command message to the end
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-3
+           Reserved And Undefined Fields                        11 June 1992
+
+
+     1     message rather than explicitly  zeroing  the  field  in  the  end
+     2     message.
+
+     3     The above paragraphs have listed all of the allowable  controller
+     4     actions  when  a  controller  receives  a  command message with a
+     5     nonzero value in a reserved field.  That is,  when  a  controller
+     6     receives  a  command  message  with a nonzero value in a reserved
+     7     field it shall do one of the following:
+
+     8          1.  Reject the command as an  "Invalid  Command"  ("reserved
+     9              field"  protocol  error; see "Invalid Command" status in
+    10              Section 5.5, "Status and Event Codes").
+
+    11          2.  Totally ignore the  nonzero  contents  of  the  reserved
+    12              field.   That  is, the command's execution, results, and
+    13              end message  contents  are  totally  unaffected  by  the
+    14              nonzero value.
+
+    15          3.  If the corresponding field in  the  end  message  should
+    16              have  a  zero  value,  echo the contents of the reserved
+    17              field in the command message as the value of  the  field
+    18              in  the  end  message.  In all other ways totally ignore
+    19              the nonzero contents of the reserved  field.   That  is,
+    20              the  command's  execution,  results, and the contents of
+    21              all  other  fields  in  the  end  message  are   totally
+    22              unaffected by the nonzero value.
+
+    23     Note that option 3 is primarily of use when a field  is  reserved
+    24     in both command and end messages.
+
+    25     It is recommended, although not required,  that  which  of  these
+    26     options   a   controller  implements  be  based  on  whether  the
+    27     controller's  code  is  hard  (ROM  based)  or   soft   (loadable
+    28     microcode,  implying  it is easily altered).  The purpose of this
+    29     is to provide the maximum degree  of  error  checking  consistent
+    30     with  ease  of future extensions to MSCP.  Controllers whose code
+    31     is soft or relatively easy to alter should  implement  option  1,
+    32     verifying  that  reserved  fields  are in fact zero.  Controllers
+    33     whose code is hard or difficult to alter should implement  option
+    34     3  when  the corresponding fields in command and end messages are
+    35     both reserved and option 2 in all other cases.
+
+    36     Note that controllers  shall  implement  option  1  whenever  the
+    37     internal  functioning  of  the  controller  may  be  altered if a
+    38     reserved field contains a nonzero value.
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-4
+           Command Messages                                     11 June 1992
+
+
+     1     5.3  Command Messages
+
+     2     A host class driver sends a command message to an MSCP server  to
+     3     have  a  certain operation performed.  Note that command messages
+     4     are flow controlled as described in Section 3.2,  "Flow  Control"
+     5     and associated subsections.
+
+     6     The generic command message format is as follows:
+
+     7                     31                             0 
+     8                    +--------------------------------+
+     9                    |   command reference number     |
+    10                    +---------------+----------------+
+    11                    |   reserved    |  unit number   |
+    12                    +---------------+-------+--------+
+    13                    |   modifiers   |  caa  | opcode |
+    14                    +---------------+-------+--------+
+    15                    |                                |
+    16                    /          parameters            /
+    17                    /                                /
+    18                    |                                |
+    19                    +--------------------------------+
+
+    20     The fields in the generic command message are as follows:
+
+    21         command reference number
+
+    22             A 32 bit, unique, nonzero number used  to  identify  host
+    23             commands.  Class drivers should supply a unique reference
+    24             number in each command that they send to an MSCP  server.
+    25             Command  reference numbers are not interpreted in any way
+    26             by MSCP servers.  Their purpose is to  provide  a  unique
+    27             identifier  by  which  class  drivers  can name commands.
+    28             They are used by class drivers to match end messages  and
+    29             error log messages with the corresponding command message
+    30             and to identify the object of an  ABORT  or  GET  COMMAND
+    31             STATUS  command.   A  class  driver  may  supply  a  zero
+    32             reference number if it  does  not  need  to  associate  a
+    33             command with its end message.
+
+    34             Command reference numbers  shall  be  unique  across  all
+    35             commands  that  are  outstanding  on the same connection.
+    36             I.e.,  they  shall  be  unique  across  all   outstanding
+    37             commands  issued  by  a  single  class driver (host) to a
+    38             single  MSCP  server.   The  class  driver  may  reuse  a
+    39             command's  reference number when the command is no longer
+    40             outstanding -- i.e., after receiving  the  command's  end
+    41             message  or  after  resynchronizing with the MSCP server.
+    42             Command reference numbers need not be unique for commands
+    43             issued  by  different  class  drivers  --  i.e., commands
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-5
+           Command Messages                                     11 June 1992
+
+
+     1             issued by different hosts or commands for different  MSCP
+     2             servers  from the same host.  Therefore controllers shall
+     3             internally use the combination  of  a  command  reference
+     4             number  and  the  connection  on  which  the  command was
+     5             received as  the  unique  identifier  of  an  outstanding
+     6             command.
+
+     7         unit number
+
+     8             Identifies the specific unit within the device  class  to
+     9             which  the  message  applies.   This  value is the binary
+    10             equivalent of the decimal unit number  displayed  by  the
+    11             unit  select  mechanism.  Note that the unit number field
+    12             appears in all command messages with the exception of the
+    13             ACCESS    NONVOLATILE    MEMORY    and   SET   CONTROLLER
+    14             CHARACTERISTICS  commands.   It  is  reserved  in   those
+    15             messages.
+
+    16         opcode
+
+    17             Identifies  the  operation  to  be   performed   by   the
+    18             controller.   The  opcode implicitly specifies the length
+    19             and format of the message, including  the  interpretation
+    20             of any parameters that are present.
+
+    21             Appendix A, "Opcode, Flag, and Offset Definitions," Table
+    22             A-1,  "Control  Message Opcodes," lists the valid opcodes
+    23             and their meanings.
+
+    24             A command message is rejected  as  an  "Invalid  Command"
+    25             ("invalid  opcode"  protocol error; see "Invalid Command"
+    26             status in Section 5.5, "Status and Event Codes")  if  its
+    27             opcode is:  zero, any value not listed as a valid command
+    28             message opcode in the "Standard Command Messages" portion
+    29             of  Table  A-1,  or  an  optional  command  listed in the
+    30             "Optional Command Messages" portion of Table A-1 that  is
+    31             not supported by the controller.
+
+    32             The "Controller Implementation Specific Command Messages"
+    33             portion  of  Table  A-1 shows a range of opcodes reserved
+    34             for implementation of controller specific special purpose
+    35             functions.  Implementation of any or all of those opcodes
+    36             is a controller dependent  option.   Those  opcodes  will
+    37             typically  be  used  by  control  layers  internal  to  a
+    38             controller  to  invoke  diagnostic  or  maintenance  type
+    39             functions.  Controllers may also honor those opcodes from
+    40             the host for  use  by  manufacturing  or  other  external
+    41             diagnostics.   In cases where such an opcode is available
+    42             for host use, the command message  format  and  operation
+    43             associated  with  the  opcode  shall  be described in the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-6
+           Command Messages                                     11 June 1992
+
+
+     1             controller's functional specification.  Although  not  an
+     2             absolute  requirement  of  MSCP,  it  is recommended that
+     3             controllers  implement  a   manually   actuated   control
+     4             mechanism  (e.g.,  the  installation  of  a  jumper) that
+     5             explicitly enables and disables those  opcodes  available
+     6             for  host use in order to prevent unintentional execution
+     7             of a special purpose function during normal operations.
+
+     8             A command message received from  the  host  containing  a
+     9             "Controller   Implementation  Specific  Command  Message"
+    10             opcode is rejected  as  an  "Invalid  Command"  ("invalid
+    11             opcode"  protocol  error; see "Invalid Command" status in
+    12             Section 5.5, "Status and Event Codes") if the controller:
+
+    13             1.  does not implement the opcode at all.
+
+    14             2.  implements the opcode for internal use only.
+
+    15             3.  implements a manually actuated control mechanism that
+    16                 enables  and disables host use of the opcode and host
+    17                 use  has  not  been  explicitly  enabled   via   that
+    18                 mechanism.
+
+
+
+    19         caa (cache  access  attributes)    -- for  transfer  commands
+    20             only
+
+    21             The cache access attributes field  may  contain  a  value
+    22             that  advises  the  controller-based cache manager of the
+    23             user's expected  future  access  of  data.   See  Section
+    24             5.3.1, "Transfer Command Message Format."
+
+    25         modifiers
+
+    26             The modifiers field contains bit flags  that  modify  the
+    27             operation  identified by the command's opcode, or zero if
+    28             no modifiers are specified.  See Section 5.3.2,  "Command
+    29             Modifiers," for more details.
+
+    30         parameters
+
+    31             The parameter area is  typically  divided  into  distinct
+    32             fields  that  convey the information necessary to perform
+    33             the desired operation in a defined  manner.   The  length
+    34             and  content  of the parameter area varies depending upon
+    35             the command's opcode.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-7
+           Command Messages                                     11 June 1992
+
+
+     1     Chapter 6, "Minimal MSCP Command Set" describes the operation and
+     2     command  message  format  of  the various commands required to be
+     3     supported by an MSCP disk controller.
+
+     4     The following section describes the command message format common
+     5     to many data transfer commands.
+
+     6     5.3.1  Transfer Command Message Format
+
+     7     Although the parameters and their  layout  are  command  (opcode)
+     8     dependent,  many  commands  perform  data  transfers and thus use
+     9     similar sets of  parameters.   Therefore  data  transfer  related
+    10     command messages are typically laid out as follows:
+
+    11                     31                             0 
+    12                    +--------------------------------+
+    13                    |   command reference number     |
+    14                    +---------------+----------------+
+    15                    |   reserved    |  unit number   |
+    16                    +---------------+-------+--------+
+    17                    |   modifiers   |  caa  | opcode |
+    18                    +---------------+-------+--------+
+    19                    |          byte count            |
+    20                    +--------------------------------+
+    21                    |                                |
+    22                    +---         buffer           ---+
+    23                    |                                |
+    24                    +---       descriptor         ---+
+    25                    |                                |
+    26                    +--------------------------------+
+    27                    |      logical block number      |
+    28                    +--------------------------------+
+
+    29     The fields in the transfer command message are as follows:
+
+    30         command reference number
+    31         unit number
+    32         opcode
+
+
+    33         caa (cache access attributes)
+
+    34             The Cache Access Attributes (caa) field is only used with
+    35             specified  transfer  commands.   The  individual  command
+    36             descriptions  contained  in  Chapter  6,  "Minimal   MSCP
+    37             Command  Set"  show  the  commands for which Cache Access
+    38             Attributes are applicable.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-8
+           Command Messages                                     11 June 1992
+
+
+     1             The Cache Access Attributes field contains a  value  that
+     2             describes the access attributes of the data.  The content
+     3             of this field is not intended to  be  a  command  to  the
+     4             cache,  but  rather  it  is advice to the cache as to the
+     5             access attributes of the data which may affect the manner
+     6             in  which  the cache manages itself.  Since this field is
+     7             only advisory in nature, any device may ignore  it.   The
+     8             implementor  of the controller-based cache is responsible
+     9             for  defining  the  action  an  implementation  takes  in
+    10             response to each Cache Access Attribute value.
+
+    11             Two generic attributes are defined and  four  values  are
+    12             assigned  to  the  four  combinations  of  these  generic
+    13             attributes.   The  assigned  values  are   described   in
+    14             Appendix  A,  "Opcodes,  Flags,  and Offset Definitions,"
+    15             Table  A-17,  "Cache  Access   Attribute   Values."   The
+    16             attributes are as follows:
+
+
+    17                 Random - A storage element implements  prefetch  with
+    18                 the  assumption that it is worthwhile prefetching the
+    19                 data logically following user requested data.
+
+    20                  -  Random should be specified if the  user  believes
+    21                     this will not be true for his request.
+
+    22                  -  Otherwise, Random should not be specified.
+
+
+    23                 Single - A storage element implements  a  cache  with
+    24                 the  assumption  that  it  is worthwhile caching data
+    25                 that has been previously accessed.
+
+    26                  -  Single should be specified if the  user  believes
+    27                     this will not be true for his request.
+
+    28                  -  Otherwise, Single should not be specified.
+
+
+
+    29             The zero value is defined as  the  default  value.   Each
+    30             implementation  shall  define  its action in terms of the
+    31             generic  attributes  in  response  to  this  value.   All
+    32             unassigned values shall be acted upon as though they were
+    33             the default value, unless modified by the System Manager.
+
+    34             Some users may have unique access patterns  that  require
+    35             unique  definition.   In  these  cases,  a  user  may  be
+    36             assigned one or more Cache Access  Attribute  values  for
+    37             the  application.   These values shall be assigned by the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                    Page 5-9
+           Command Messages                                     11 June 1992
+
+
+     1             Responsible Architect.   Its  access  pattern  definition
+     2             will  be  negotiated  between  the  user  and  the  cache
+     3             implementors.
+
+     4             A system manager may modify a device action  to  a  Cache
+     5             Access    Attribute    value    at    configuration    or
+     6             reconfiguration.  This modification consists of  changing
+     7             the  action  of  a given value to that defined by another
+     8             value.  For example, the system manager  could  make  the
+     9             action  in  response  to  value 85 hex be the same as the
+    10             action in response to value 80 hex.  Implementations that
+    11             allow such modification should allow all 256 Cache Access
+    12             Attribute Values to be  modified,  including  those  that
+    13             have not yet been assigned in this specification.
+
+    14         modifiers
+
+    15  |          See Section 5.3.2, "Command Modifiers." 
+
+    16         byte count
+
+    17             The total requested length of the data transfer in bytes.
+    18             This length shall meet both architectural constraints and
+    19             a server specified maximum.
+
+    20             An MSCP server may, at its  option,  return  the  maximum
+    21             byte  count  size  that it supports in the SET CONTROLLER
+    22             CHARACTERISTICS end message  (see  Section  6.16.2,  "SET
+    23             CONTROLLER   CHARACTERISTICS,  End  Message  Format"  for
+    24             default values used if the size is not  provided  by  the
+    25             server).   Class  drivers  should not issue commands with
+    26             larger byte counts.  The server  response  to  a  command
+    27             whose  byte  count is larger than the server's maximum is
+    28             undefined.  If the server does not reject the command, it
+    29             is subject to the following constraints:
+
+
+    30               1.  The server may not destroy  hardware  or  do  any
+    31                   other  action  that  would  require field service
+    32                   attention.
+
+    33               2.  The server may alter host memory only in response
+    34                   to  a READ command, and then only that portion of
+    35                   host  memory  implied  by  the  command's  buffer
+    36                   descriptor and specified byte count.  See Section
+    37                   4.11, "Host Buffer Access."
+
+    38               3.  The server may alter data on  the  unit  only  in
+    39                   response  to  a  WRITE or ERASE command, and then
+    40                   only that portion of  the  unit  implied  by  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-10
+           Command Messages                                     11 June 1992
+
+
+     1                   logical block number and specified byte count.
+
+     2               4.  Any error reported in the command's  end  message
+     3                   or  in  an  error  log message must have actually
+     4                   occurred at the position reported.
+
+     5               5.  With one exception, any other commands  that  may
+     6                   be  outstanding  at  the same time as the command
+     7                   whose  byte  count  is  too  large  may  not   be
+     8                   affected.   The  one exception is that the server
+     9                   need not properly implement the  command  timeout
+    10                   algorithm  for  any  such  command,  not just the
+    11                   command whose byte count is too large.
+
+
+    12             Byte counts shall also meet the  following  architectural
+    13             constraints, which vary depending upon the device class.
+
+    14  |          For tape class devices, if the "byte count" specified  is
+    15  |          larger  than a format dependent maximum block size or the
+    16  |          byte count  is  zero,  the  command  is  rejected  as  an
+    17  |          "Invalid  Command" subcode "invalid byte count" parameter
+    18  |          error; (see "Invalid Command" status in TMSCP, Chapter 4,
+    19  |          Section "Status Codes").
+
+    20             For disk class devices, the requirements  depend  on  the
+    21             contents of the "logical block number" field.
+
+    22             If the "logical block number" field identifies a  logical
+    23             block  in  the  host  area  of the disk volume (i.e., the
+    24             "logical block number"  is  less  than  the  "unit  size"
+    25             returned  in  the ONLINE and SET UNIT CHARACTERISTICS end
+    26             messages), then the "byte count" shall be  less  than  or
+    27             equal to the following maximum byte count:
+
+    28                 ( unit size - logical block number ) * block size
+
+    29             where "unit size" is the unit's host area size  (returned
+    30             in the ONLINE and SET UNIT CHARACTERISTICS end messages),
+    31             "logical block number" is the contents  of  the  "logical
+    32             block  number"  field  in the command message, and "block
+    33             size" is the volume's  block  size,  either  512  or  576
+    34             bytes.   The  controller  or MSCP server shall check that
+    35             the "byte count" is less  than  or  equal  to  the  above
+    36             maximum.   That  is,  the controller or MSCP server shall
+    37             reject or terminate any transfer command that  begins  in
+    38             the  host  area of a disk volume and attempts to continue
+    39             into the volume's Replacement Control Table  (RCT).   The
+    40             command is rejected as an "Invalid Command" ("byte count"
+    41             parameter error) if this restriction is violated.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-11
+           Command Messages                                     11 June 1992
+
+
+     1             If the "logical block number" field identifies a  logical
+     2             block  in the disk volume's RCT (i.e., the "logical block
+     3             number" is greater than  or  equal  to  the  "unit  size"
+     4             returned  in  the ONLINE and SET UNIT CHARACTERISTICS end
+     5             messages), then the "byte count"  shall  be  exactly  the
+     6             block size (either 512 or 576 bytes).  The treatment of a
+     7             "byte count" that does not match the block  size  differs
+     8             depending  on  whether or not the controller performs bad
+     9             block replacement.   Section  8.4,  "Replacement  Control
+    10             Table  Access"  describes  the  action  a controller that
+    11             performs  bad  block  replacement  will  take.   If   the
+    12             controller  does  not perform bad block replacement and a
+    13             different  (than  block  size)  "byte  count"  value   is
+    14             provided,  the controller may either attempt the transfer
+    15             with the  specified  "byte  count"  or  else  reject  the
+    16             command  as  an "Invalid Command" ("byte count" parameter
+    17             error).  If the controller attempts the transfer, and the
+    18             "byte  count"  causes  the  transfer  to extend past some
+    19             boundary within the RCT, then the controller performs the
+    20             transfer  up  to that boundary and terminates the command
+    21             as an "Invalid Command" ("byte count"  parameter  error).
+    22             The  location  of the boundary or boundaries at which the
+    23             controller  terminates  the   transfer   are   controller
+    24             dependent.
+
+    25             For all disk and  tape  transfer  commands  that  contain
+    26             "buffer  descriptors" (i.e., all transfer commands except
+    27             ACCESS and ERASE), the "byte count" shall  also  be  less
+    28             than  or  equal  to  the size of the buffer identified by
+    29             "buffer descriptor."  Note that "buffer descriptor,"  and
+    30             thus the size of the buffer, is inherently communications
+    31             mechanism  dependent.   The  size  of  a  buffer  is  not
+    32             necessarily   available  to  the  MSCP  server  until  it
+    33             attempts to transfer past the end of the buffer.  A "Host
+    34             Buffer Access Error" status code is returned if the "byte
+    35             count" exceeds the length of the buffer.  Note that  such
+    36             errors  are  not  necessarily  distinguishable from other
+    37             causes of "Host Buffer Access Errors." 
+
+    38             For disk  transfer  commands  only,  some  communications
+    39             mechanisms  may  prohibit  odd  "byte count" values.  Odd
+    40             "byte count" values shall be  allowed  in  tape  transfer
+    41             commands.   A  "Host  Buffer Access Error" status code is
+    42             returned if the "byte  count"  is  an  illegal  odd  byte
+    43             count.  For disk transfer commands that do not use a host
+    44             buffer (ACCESS and ERASE), a controller may  return  this
+    45             status  code,  or  it  may round the byte count up to the
+    46             nearest whole sector size and perform the operation.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-12
+           Command Messages                                     11 June 1992
+
+
+     1             "Byte count" values that exceed any of the  architectural
+     2             maximum  values  described  above  may be detected either
+     3             before the transfer is initiated  or  when  the  transfer
+     4             attempts to cross the boundary from legal to invalid byte
+     5             counts.  If detected before  the  transfer  is  initiated
+     6             (the  transfer  is  rejected),  the MSCP server shall not
+     7             transfer any data and shall  return  zero  in  the  "byte
+     8             count"  field  of  the end message.  If detected when the
+     9             transfer attempts to cross the boundary (the transfer  is
+    10             terminated),  the  MSCP server shall transfer all data up
+    11             to the maximum legal byte count and  return  the  maximum
+    12             legal  byte  count  in  the "byte count" field of the end
+    13             message.  Data shall not be transferred past the  maximum
+    14             legal  byte count.  In either case the command is treated
+    15             as an "Invalid Command" ("byte count"  parameter  error).
+    16             Which  algorithm  an  MSCP server uses for detecting byte
+    17             counts that are too large is controller dependent.
+
+    18         buffer descriptor
+
+    19             Communication mechanism dependent identification  of  the
+    20             host   buffer   to   use  for  the  data  transfer.   The
+    21             information encoded  in  this  12  byte  (96  bit)  field
+    22             includes:
+
+    23              o  A host identifier (port or node identification).
+
+    24              o  The name of a buffer on the host.
+
+    25             Note that the inclusion of a host identifier  allows  for
+    26             third  party  transfers.   The  buffer descriptor formats
+    27             used by various communication mechanisms  are  listed  in
+    28             Appendix D, "Buffer Descriptor Formats." 
+
+    29         logical block number
+
+    30             Not present in tape transfer commands.  In disk  transfer
+    31             commands, the logical block number (position) on the disk
+    32             volume at which to start the data transfer.   This  value
+    33             shall  not  identify a block past the end of the volume's
+    34             Replacement Control Table  (RCT).   Section  4.12,  "Disk
+    35             Geometry  and  Format,"  describes the mapping of logical
+    36             block numbers to disk volume regions.  If the command  is
+    37             a  write  operation  and the controller is performing bad
+    38             block replacement, this value also shall not  identify  a
+    39             block   within   the   volume's  RCT  (see  Section  8.4,
+    40             "Replacement Control Table Access").  Any of these errors
+    41             cause  the command to be rejected as an "Invalid Command"
+    42             "logical block number" parameter error.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-13
+           Command Messages                                     11 June 1992
+
+
+     1     5.3.2  Command Modifiers
+
+     2     The  allowable  modifiers  on  a  command  are  command  (opcode)
+     3     dependent.   The  individual  command  descriptions  contained in
+     4     Chapters 6, "Minimal MSCP Command Set" and 7, "Multihost Support"
+     5     list  the  allowable  modifiers for each command.  Modifiers that
+     6     are only allowed on one command are described in  that  command's
+     7     description.   Modifiers  that  are  common  to many commands are
+     8     described below:
+
+     9         Clear Serious Exception
+
+    10             Clears the "serious exception" condition on the specified
+    11             unit.  Ignored if there is no serious exception condition
+    12             in effect.  Serious exceptions only occur on  tape  class
+    13             devices.    See   Tape   Mass  Storage  Control  Protocol
+    14             Specification (TMSCP) for complete details.
+
+    15         Compare
+
+    16             Applicable  to  data  transfer   commands.    After   the
+    17             transfer,  the  data  will be read back from the transfer
+    18             destination  and  verified  against  the  original   data
+    19             reobtained  from the source.  Specifying this modifier is
+    20             similar, but not identical,  to  following  the  transfer
+    21             command with a COMPARE HOST DATA command.  In particular,
+    22             if the compare operation fails, an error log  message  is
+    23             generated   (if   enabled)   and  the  original  transfer
+    24             operation retried.  (With the COMPARE HOST DATA  command,
+    25             an  error log message is not generated on compare errors.
+    26             Retries are unnecessary, although innocuous and therefore
+    27             allowable.) See Section 4.15, "Compare Operations," for a
+    28             more detailed description of this modifier's effects.
+
+    29         Express Request
+
+    30             Applicable to  Non-Sequential  commands.   This  modifier
+    31             requests   that   the   controller   ignore   its  normal
+    32             optimization policies in order to complete  this  command
+    33             as  quickly  as  possible.   The  exact implementation of
+    34             express requests is controller dependent  --  in  general
+    35             the   controller   will  complete  some  or  all  of  its
+    36             outstanding  commands  before   completing   an   express
+    37             request.
+
+    38             Use of express requests disables  the  normal  controller
+    39             guarantees  that ensure that all commands are serviced in
+    40             a timely manner.   If  express  requests  are  repeatedly
+    41             issued,  some  or all other outstanding commands may time
+    42             out (i.e., never be completed).
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-14
+           Command Messages                                     11 June 1992
+
+
+     1             Express  requests  do  NOT  override  sequential  command
+     2             execution  guarantees.   Some  controllers may completely
+     3             ignore the express request modifier.  The exact treatment
+     4             of express requests should be described in a controller's
+     5             functional specification.
+
+     6         Force Error
+
+     7             Applicable only to write commands directed to disk  class
+     8             devices.   Causes  the data to be written with the forced
+     9             error indicator set, so that all  attempts  to  read  the
+    10             data  will  fail.   The error will be preserved until the
+    11             next time  the  block  is  written.   The  forced  errors
+    12             produced  with  this  modifier shall be recognized by the
+    13             controller as deliberate and never reported to the  error
+    14             log.  Note that the data shall be written to the block so
+    15             as to be obtainable by subsequent READ commands,  exactly
+    16             as  if  this  modifier  were  not  specified.   The  only
+    17             difference  is  whether  subsequent  READ  commands  will
+    18             return success or the forced error status code.
+
+    19         Suppress Error Correction
+
+    20             Suppresses  error  correction  mechanisms,  such  as  ECC
+    21             correction.   Error recovery mechanisms, such as retries,
+    22             are not affected by this  modifier.   This  modifier,  in
+    23             effect,  lowers  the  threshold  at  which  an  error  is
+    24             considered to be uncorrectable.  It typically lowers  the
+    25             threshold  to zero, although that is not required.  Since
+    26             error correction is drive  type  dependent,  the  lowered
+    27             error  correction threshold is also drive type dependent.
+    28             If a drive has several error correction mechanisms, it is
+    29             permissible  for  this  modifier to suppress some and not
+    30             affect others.
+
+    31         Suppress Error Recovery
+
+    32             Suppresses most error recovery mechanisms, such  as  read
+    33             retries.    Error  correction  mechanisms,  such  as  ECC
+    34             correction, and some error recovery mechanisms,  such  as
+    35             seek retry, are not affected by this modifier.  The exact
+    36             definition  of  which  error  recovery   mechanisms   are
+    37             suppressed  and  which  are  not  affected  is drive type
+    38             dependent.
+
+    39     Refer to Appendix A,  "Opcode,  Flag,  and  Offset  Definitions,"
+    40     Table  A-2,  "Command  Modifiers"  for the values assigned to all
+    41     command modifiers.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-15
+           Command Messages                                     11 June 1992
+
+
+     1     All modifiers that are not explicitly allowed for a  command  are
+     2     reserved,   and   shall   be   treated  in  accordance  with  the
+     3     requirements  for  reserved  fields  described  in  Section  5.2,
+     4     "Reserved  and  Undefined  Fields."   The  only exception is that
+     5     certain modifiers (allowable  on  certain  commands)  are  either
+     6     ignored  or  treated  as  reserved  depending on whether or not a
+     7     controller supports certain optional MSCP functions.   See  Table
+     8     A-2  for  complete  details.   Note  that those modifiers are not
+     9     listed in the individual command descriptions.
+
+    10     5.4  End Messages
+
+    11     An MSCP server sends an end message to a class driver  to  report
+    12     completion  of  a  command.   Note  that  end  messages  are flow
+    13     controlled as  described  in  Section  3.2,  "Flow  Control"  and
+    14     associated subsections.
+
+    15     The generic end message format is as follows:
+
+    16                     31                             0 
+    17                    +--------------------------------+
+    18                    |   command reference number     |
+    19                    +---------------+----------------+
+    20                    |sequence number|  unit number   |
+    21                    +---------------+-------+--------+
+    22                    |    status     | flags | endcode|
+    23                    +---------------+-------+--------+
+    24                    |                                |
+    25                    /          parameters            /
+    26                    /                                /
+    27                    |                                |
+    28                    +--------------------------------+
+
+    29     The fields in the generic end message are as follows:
+
+    30         command reference number
+    31         unit number
+
+    32             Both of these fields are copied from the command message.
+    33             See Section 5.3, "Command Messages." 
+
+    34         sequence number
+
+    35             This field  depends  on  the  state  of  the  "error  log
+    36             generated"  flag  in  the "flags" field.  If that flag is
+    37             set, this field contains the  "sequence  number"  of  the
+    38             LAST  error  log  message  that refers to this command --
+    39             i.e., that  contains  this  command's  command  reference
+    40             number.   If that flag is clear, this field will be zero.
+    41             (Note that MSCP servers meeting certain requirements need
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-16
+           End Messages                                         11 June 1992
+
+
+     1             not  implement  error  log sequence numbers.  See Section
+     2             5.9, "Error  Log  Messages."   Servers  that  meet  those
+     3             requirements  shall  always return a zero in the sequence
+     4             number field.) This field allows the  host  to  determine
+     5             when all error log messages for a particular command have
+     6             been received and when the command's  context  saved  for
+     7             inclusion  in  the  error  log,  if any, can be released.
+     8             Section  5.9,  "Error  Log  Messages"  provides  complete
+     9             details  regarding  host  and  MSCP  server error logging
+    10             operation.
+
+    11         endcode
+
+    12             Identifies this message as an end message and the type of
+    13             command  (opcode)  that this is an end message for.  This
+    14             field  implicitly  specifies  the  length,  format,   and
+    15             interpretation of the parameters.
+
+    16             Normally the endcode is formed  by  adding  the  constant
+    17             (OP.END)  to  the command opcode (see Table A-1, "Control
+    18             Message Opcodes").  An exception is the  Invalid  Command
+    19             End Message (see "Invalid Command" status in Section 5.5,
+    20             "Status and Event Codes").
+
+    21         flags
+
+    22             Bit flags, collectively called end flags, used to  report
+    23             various  conditions  detected due to this command but not
+    24             directly related to success or  failure.   The  following
+    25             flags are defined:
+
+    26                 Allocation Failure
+
+    27                     Set if the server was unable to  allocate  a  new
+    28                     "write  history  entry" in a History Log modified
+    29                     write type command.   Indicates  that  no  "write
+    30                     history  entry"  was  generated for this command.
+    31                     This condition  is  recorded  in  the  Allocation
+    32                     Failure   Table  in  the  controller.   For  more
+    33                     details,  see  Section  6.19.3,  "WRITE   HISTORY
+    34                     MANAGEMENT   Command,   Description,"   and   the
+    35                     descriptions of the History Log and  Reuse  Entry
+    36                     modifiers  in  any  write  type  command  (WRITE,
+    37                     ERASE, and DISK COPY DATA).
+
+    38                 Bad Block Reported
+
+    39                     Set if one or more bad blocks were  detected  and
+    40                     the  host  is  performing  bad block replacement.
+    41                     Indicates that the host should  replace  the  bad
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-17
+           End Messages                                         11 June 1992
+
+
+     1                     block  identified in the "first bad block" field.
+     2                     Note that this end flag  is  applicable  only  to
+     3                     disk class devices.
+
+     4                 Bad Blocks Unreported
+
+     5                     Set if one or more bad blocks were  detected  and
+     6                     not  reported in the "first bad block" field.  If
+     7                     the "Bad Block Reported" flag is also  set,  this
+     8                     indicates  that  two  or  more  bad  blocks  were
+     9                     detected,  the  host  is  performing  bad   block
+    10                     replacement, and the "first bad block" field only
+    11                     reports the first bad block in the transfer.   If
+    12                     the  "Bad  Block  Reported"  flag  is clear, this
+    13                     indicates  that  one  or  more  bad  blocks  were
+    14                     detected  and  that the controller either already
+    15                     has or soon will replace them.   Note  that  this
+    16                     end   flag  is  applicable  only  to  disk  class
+    17                     devices.
+
+    18                 Error Log Generated
+
+    19                     Set if  one  or  more  error  log  messages  were
+    20                     generated  that  refer  to  this command -- i.e.,
+    21                     that contain  this  command's  command  reference
+    22                     number.   When  set this flag signals the host to
+    23                     save  any  command  context  that  it  wishes  to
+    24                     include  in  its  error  log  until all error log
+    25                     messages associated with the command have arrived
+    26                     (see   also  "sequence  number"  field  described
+    27                     above).
+
+    28                 History Logged
+
+    29                     Set if the "write history log" contains  a  valid
+    30                     "write  history  entry"  for  this  command.  See
+    31                     Section   6.19.3,   "WRITE   HISTORY   MANAGEMENT
+    32                     Command,   Description,"  for  details  of  write
+    33                     history logging.
+
+    34                 Serious Exception
+
+    35                     Set if  an  unrecoverable  error  or  some  other
+    36                     unusual  exception  condition  requires  that the
+    37                     host recognize the  exception  before  additional
+    38                     commands are processed.  The host shall clear the
+    39                     error before subsequent commands can be executed.
+    40                     Note  that  this  end  flag is applicable only to
+    41                     tape  class  devices.   See  Tape  Mass   Storage
+    42                     Control   Protocol   Specification   (TMSCP)  for
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-18
+           End Messages                                         11 June 1992
+
+
+     1                     complete details.
+
+     2             Refer  to  Appendix  A,   "Opcode,   Flag,   and   Offset
+     3             Definitions,"  Table  A-3,  "End  Message  Flags" for the
+     4             values assigned to the defined flags.  All other bits  in
+     5             the  "flags"  field are reserved, and shall be treated in
+     6             accordance with  the  requirements  for  reserved  fields
+     7             described   in   Section  5.2,  "Reserved  and  Undefined
+     8             Fields." 
+
+     9         status
+
+    10             The status field contains a status  code  that  indicates
+    11             whether  the  operation was successfully completed or, if
+    12             it wasn't successful, what type of  error  occurred  (see
+    13             also  Section  5.5, "Status and Event Codes").  Note that
+    14             recoverable errors are reported as successful  completion
+    15             of  the command.  All errors, whether recoverable or not,
+    16             are reported in a separate  error  log  message  if  they
+    17             should  be  logged (see Section 5.9, "Error Log Messages"
+    18             for more detail).
+
+    19             Note that  if  several  errors  occur  in  a  nontransfer
+    20             operation,  the  error  that  is  reported  is controller
+    21             dependent  unless  the  individual  command   description
+    22             states  otherwise.  See the "status" field description in
+    23             Section 5.4.1, "Transfer Command End Message Format"  for
+    24             specifics   of   multiple  error  handling  for  transfer
+    25             commands.
+
+    26     Chapter 6, "Minimal MSCP Command Set"  describes  the  operation,
+    27     command  message  format,  and  end message format of the various
+    28     commands required to be supported by  an  MSCP  disk  controller.
+    29     The  following section describes the end message format common to
+    30     many data transfer commands.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-19
+           End Messages                                         11 June 1992
+
+
+     1     5.4.1  Transfer Command End Message Format
+
+     2     Although the parameters and their layout  are  command  (endcode)
+     3     dependent,  many  commands perform data transfers and thus return
+     4     similar sets of parameters.  Therefore data transfer related  end
+     5     messages are typically laid out as follows:
+
+     6                     31                             0 
+     7                    +--------------------------------+
+     8                    |   command reference number     |
+     9                    +---------------+----------------+
+    10                    |sequence number|  unit number   |
+    11                    +---------------+-------+--------+
+    12                    |    status     | flags |endcode |
+    13                    +---------------+-------+--------+
+    14                    |          byte count            |
+    15                    +--------------------------------+
+    16                    |                                |
+    17                    +---                          ---+
+    18                    |           undefined            |
+    19                    +---                          ---+
+    20                    |                                |
+    21                    +--------------------------------+
+    22                    |        first bad block         |
+    23                    +--------------------------------+
+
+    24     The fields in the transfer command end message are as follows:
+
+    25         command reference number
+    26         unit number
+    27         sequence number
+    28         endcode
+    29         flags
+
+    30             See Section 5.4, "End Messages." 
+
+    31         status
+
+    32             If several errors occur  in  a  transfer  operation,  the
+    33             status  code  reports  the  first error starting from the
+    34             beginning of the transfer (i.e., the lowest byte count or
+    35             lowest  logical  block  number).   The  only exception is
+    36             transfer commands that include a compare operation (i.e.,
+    37             read-compare and write-compare operations); errors in the
+    38             original transfer may take precedence over errors  during
+    39             the compare operation.
+
+    40             If a "Forced Error" and some other  error  occur  at  the
+    41             same  point  (same  byte  count  or logical block number)
+    42             within a transfer operation, the  other  error  shall  be
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-20
+           End Messages                                         11 June 1992
+
+
+     1             reported.  If a "Compare Error" and some other error that
+     2             is not a "Forced Error" occur at the same point within  a
+     3             transfer  operation,  the  other error shall be reported.
+     4             If a "Forced Error" and a "Compare Error" both  occur  at
+     5             the  same point, and no other error occurs at that point,
+     6             the "Compare Error" shall be reported.  Otherwise,  which
+     7             error  of  multiple  errors  occurring  at the same point
+     8             should  be  reported   is   controller   dependent.    An
+     9             alternative  way  of stating this is that "Forced Errors"
+    10             are the error of last resort and  that  "Compare  Errors"
+    11  |          are  the  error  of  second  to  last  resort.   See also
+    12  |          "Section 5.5, "Status and Event Codes." 
+
+    13         byte count
+
+    14             In transfer command end messages,  the  number  of  bytes
+    15             successfully  transferred, counting from the start of the
+    16             transfer to the first error (i.e., the lowest byte  count
+    17             or lowest logical block number with an error).  Data that
+    18             follows  the  first  error  is  not  counted,   even   if
+    19             transferred successfully.  The only exception is transfer
+    20             commands  that  include  a   compare   operation   (i.e.,
+    21             read-compare and write-compare operations); errors in the
+    22             original transfer may take precedence over errors  during
+    23             the compare operation.
+
+    24             The error  identified  by  "status"  must  have  actually
+    25             occurred at the position identified by "byte count."  The
+    26             controller must have successfully transferred all data up
+    27             to  that  point.   If Data Error (subcode "Forced Error")
+    28             status is returned for a READ  command  (addressed  to  a
+    29             disk   class  device),  the  controller  must  have  also
+    30             transferred all the data contained in  the  block  marked
+    31             with  forced  error.   Beyond this point the state of the
+    32             transfer is undefined.  None of  the  transfer  may  have
+    33             been  performed  or  attempted,  all  of it may have been
+    34             attempted (with unknown success), or some parts may  have
+    35             been attempted and others not.
+
+    36             Again, the only exception to this  is  transfer  commands
+    37             that  include  a compare operation.  Such a command makes
+    38             two passes over the data; one for the  original  transfer
+    39             and  another for the compare operation.  For most errors,
+    40             there is no way to determine in which pass the error  was
+    41             detected.   Therefore  the  only  guarantee  is  that the
+    42             original  transfer  was  performed  up   to   the   point
+    43             identified  by "byte count" without detecting any errors.
+    44             The compare operation may or may not have been  performed
+    45             up to that point.  If a "Compare Error" is reported, then
+    46             both the original transfer and the compare operation have
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-21
+           End Messages                                         11 June 1992
+
+
+     1             been successfully performed up to the point identified by
+     2             "byte count."  The state of both  the  original  transfer
+     3             and  the  compare operation after that point, however, is
+     4             undefined.  This implies  that  the  compare  pass  of  a
+     5             transfer command that includes a compare operation may be
+     6             done for the entire transfer as a unit, block  by  block,
+     7             or anywhere in between.
+
+     8             For disk class devices, the granularity of the byte count
+     9             on  errors  (i.e., the resolution with which the point of
+    10             error is identified) need not be  any  smaller  than  the
+    11             volume's  block  size.  That is, the byte count need only
+    12             identify the block in which the  error  occurred,  rather
+    13             than  the  exact  word  or byte.  In particular, the byte
+    14             count returned with such errors as "Compare  Errors"  and
+    15             "Host  Buffer Access Errors" need only identify the block
+    16             in which the error occurred, rather than the  exact  word
+    17             or  byte.   Note that a block is identified by the number
+    18             of  the  first  byte  in  the  block.   Controllers   may
+    19             optionally  provide  finer granularity for the byte count
+    20             field  on  errors.   If  an  error   is   not   reported,
+    21             controllers shall return the exact byte count that was in
+    22             the command message.
+
+    23             For tape class devices, the granularity of the byte count
+    24             on  errors  shall  be  either one word (two bytes) or one
+    25             byte.  If an error is  not  reported,  controllers  shall
+    26             return  the  exact  byte count that was transferred.  For
+    27             host buffer access details see Section 4.11, "Host Buffer
+    28             Access."
+
+    29         first bad block
+
+    30             In disk transfer command end messages, the logical  block
+    31             number  of  the first bad block (i.e., the bad block with
+    32             the lowest logical  block  number)  detected  during  the
+    33             transfer that the host should replace.  Only valid if the
+    34             "Bad Block Reported" flag is set.  Undefined (garbage) if
+    35             the "Bad Block Reported" flag is clear.
+
+    36     5.5  Status And Event Codes
+
+    37  |  Status and Event codes are essentially identical except  for  the
+    38  |  manner  in  which  they are returned.  The "status code" field in
+    39  |  returned in end  messages  and  is  used  to  report  whether  an
+    40  |  operation   was   successfully   completed  or,  if  it  was  not
+    41  |  successfully completed, what type of error occurred.  The  "event
+    42  |  code"  field  is  returned  in  error  log  messages  and has the
+    43  |  identical structure and encoding of a status code, but identifies
+    44  |  the  specific  error  or  event  being  reported by the error log
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-22
+           Status And Event Codes                               11 June 1992
+
+
+     1  |  message.  Errors that are reported in both an end message and  an
+     2  |  error  log message use identical values for the "status code" and
+     3  |  "event code" fields.  The same value may not be  used  to  report
+     4  |  one  type  of  event  as  a "status code" and a different type of
+     5  |  event as an "event code".
+
+     6  |  There are, however, certain codes that are used as "status  only"
+     7  |  codes  and  others used as "event only" codes.  For example, if a
+     8  |  command is issued to a unit that is Unit-Offline, the  result  is
+     9  |  reported  as a "status only" code.  It is not necessary to report
+    10  |  this information in an error log; it is up to  the  program  that
+    11  |  initiated the command to take the appropriate action.  If a WRITE
+    12  |  command  is  issued  and  a  bad  block  is  encountered  and  is
+    13  |  successfully  replaced,  the  end  message  for  the command will
+    14  |  return the appropriate status outcome, in addition to setting the
+    15  |  flags  for  bad  block  reported and error log message generated.
+    16  |  The Bad Block Replacement Attempted  event  code  will  never  be
+    17  |  returned  as  a status code; it will only be returned as an event
+    18  |  code in an error log message.
+
+    19     The "status code" and the "event code" fields are divided into  a
+    20     5-bit major status code and an 11-bit status and/or event subcode
+    21     arranged as follows:
+
+    22                       15                         0 
+    23                       +-----------------+---------+
+    24                       |     subcode     |  code   |
+    25                       +-----------------+---------+
+
+    26     The 5-bit major status code conveys the status  information  that
+    27     hosts  need  for  normal  operation.   Therefore the major status
+    28     codes are a formal part of MSCP.  All  controllers  shall  return
+    29     the same major status codes for similar situations.
+
+    30     The 11-bit subcode exists to specify the exact error  or  unusual
+    31     situation  encountered  with  very  fine  detail.   As such it is
+    32     primarily used for diagnostic purposes, and hosts should not need
+    33     to examine it during normal operation.
+
+    34     Subcodes related to protocol or state errors are a formal part of
+    35     MSCP.   All  controllers  shall  return  the  same  subcodes  for
+    36     protocol or state  errors.   These  subcodes  are  generally  bit
+    37     flags,  allowing  several  causes  of the major status code to be
+    38     reported.
+
+    39     Subcodes related to  controller  and/or  drive  errors,  however,
+    40     shall be allowed to vary from one controller or drive to another.
+    41     There is no requirement that the same subcodes  be  returned  for
+    42     similar drive or controller errors.  These subcodes are generally
+    43     specific values, corresponding to one specific  event  or  error.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-23
+           Status And Event Codes                               11 June 1992
+
+
+     1     Each subcode shall have the same meaning whenever it is used.  It
+     2     is the use of a subcode that may vary (i.e.,  whether  or  not  a
+     3     specific  controller returns that subcode), not its meaning.  The
+     4     defined subcodes are listed in Appendix B, "Status and Event Code
+     5     Definitions."  This list may expand (via an ECO to MSCP) whenever
+     6     a new drive or controller type is introduced.
+
+     7     The major status codes  that  may  be  returned  in  end  message
+     8     "status  code" fields are listed below along with the general use
+     9     made of  subcodes.   The  actual  subcodes  used  are  listed  in
+    10     Appendix  B, "Status and Event Code Definitions."  Those subcodes
+    11     that  are  a  formal  part  of  MSCP  are  also  listed  in   the
+    12     descriptions of the commands that may return them.
+
+    13         Success
+
+    14             The command was successfully completed.  This status code
+    15             may  also be returned, for some commands, if the intended
+    16             effect of  the  command  has  already  been  accomplished
+    17             (e.g.,   requesting   a  drive  that  isn't  spinning  to
+    18             spin-down).  The subcode consists of bit  flags  used  to
+    19             report  various  "alternate"  forms  of success.  See the
+    20             individual command descriptions for details.
+
+    21             The status code value associated with  "Success"  is,  by
+    22             definition,  zero.   Subcode value zero (i.e., no subcode
+    23             bits  set)  is  "Normal"  success,  and  implies   normal
+    24             completion of a command.  One subcode bit, the "Duplicate
+    25             Unit Number" bit, is common to many commands.  This  bit,
+    26             when  set, implies that the unit is "Unit-Online" and the
+    27             command succeeded, but that the unit has a duplicate unit
+    28             number.   The unit will become "Unit-Offline," due to the
+    29             duplicate unit  number,  as  soon  as  it  ceases  to  be
+    30             "Unit-Online."   Other  "Success" subcode bits are unique
+    31             to a particular command,  and  are  described  under  the
+    32             individual command descriptions.
+
+    33         Invalid Command
+
+    34             Controllers detect and  report  two  different  types  of
+    35             invalid command errors.
+
+    36             The  first  type  is  called  a  protocol   error.    The
+    37             controller  reports  a  protocol  error if during initial
+    38             command message validation it finds:  1) a message format
+    39             violation, 2) an MSCP version number mismatch, or 3) that
+    40             a field contains a  value  that  specifies  an  undefined
+    41             operation.
+
+    42             The following are MSCP protocol errors:
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-24
+           Status And Event Codes                               11 June 1992
+
+
+     1                 Invalid Message Length
+
+     2                     The command message is too short to  contain  the
+     3                     parameters required by the command's opcode.
+
+     4                 Invalid Modifiers
+
+     5                     An undefined modifier bit is set  or  a  modifier
+     6                     that   is  defined  but  not  supported  by  that
+     7                     controller is set.  See the descriptions  of  the
+     8                     History Log and Reuse Entry modifiers in the DISK
+     9                     COPY DATA, ERASE, and  WRITE  Commands,  Sections
+    10                     6.7.1, 6.9.1, and 6.18.1.
+
+    11                 Invalid MSCP Version
+
+    12                     The "MSCP version"  field  of  a  SET  CONTROLLER
+    13                     CHARACTERISTICS  command  is nonzero (see Section
+    14                     6.16.1, "SET CONTROLLER CHARACTERISTICS  Command,
+    15                     Command Message Format").
+
+    16                 Invalid Opcode
+
+    17                     See  "opcode"  field  in  Section  5.3,  "Command
+    18                     Messages" for possible causes.
+
+    19                 Invalid Operation
+
+    20                     The "operation" field of  an  ACCESS  NONVOLATILE
+    21                     MEMORY   command   or  WRITE  HISTORY  MANAGEMENT
+    22                     command contains an illegal  value  (see  Section
+    23                     7.3.1,   "Multihost   ACCESS  NONVOLATILE  MEMORY
+    24                     Command,  Command  Message  Format"  and  Section
+    25                     6.19.1,   "WRITE   HISTORY   MANAGEMENT  Command,
+    26                     Command Message Format").
+
+    27                 {reserved field violations}
+
+    28                     A  "reserved"  field  in  the   command   message
+    29                     contains  a  nonzero value.  Note that "reserved"
+    30                     field checking  is  optional  (see  Section  5.2,
+    31                     "Reserved and Undefined Fields").
+
+    32                     Note also that modifiers not  explicitly  allowed
+    33                     for  a  command  received  by  the server and any
+    34                     undefined controller flag and unit flag bits  are
+    35                     considered to be "reserved" fields.
+
+    36             The second type is a parameter error.  After  determining
+    37             through  initial  command  message  validation  that  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-25
+           Status And Event Codes                               11 June 1992
+
+
+     1             command  message  has  a  well   defined   meaning,   the
+     2             controller reports a parameter error if some parameter is
+     3             found to be outside the range allowed.   Note  that  some
+     4             parameter  errors  are  detected during follow-on command
+     5             message validation before command execution begins, while
+     6             others may be detected afterwards.
+
+     7             The following are parameter errors:
+
+     8                 Invalid Byte Count
+
+     9                     See  "byte  count"  field   in   Section   5.3.1,
+    10                     "Transfer  Command  Message  Format" for possible
+    11                     causes.  Log modifier  in  the  ERASE  and  WRITE
+    12                     Commands, Sections 6.9.1 and 6.18.1.
+
+    13                 Invalid CRN
+
+    14                     A  value  of  zero  was  supplied  in   the   crn
+    15                     (connection  reference  number)  field  of  a SET
+    16                     CONTROLLER  CHARACTERISTICS  or  TERMINATE  CLASS
+    17                     DRIVER/SERVER CONNECTION command message.
+
+    18                 Invalid Display Text
+
+    19                     The specified display  text  is  invalid  or  not
+    20                     implemented  for  the  specified display item and
+    21                     mode.  For more information, see  Section  6.8.3,
+    22                     "DISPLAY Command, Description."
+
+    23                 Invalid Entry Locator
+
+    24                     The  value  specified  in  the   "entloc   (entry
+    25                     locator)"  field  of  a WRITE HISTORY MANAGEMENT,
+    26                     ERASE, WRITE, or DISK COPY DATA  command  is  not
+    27                     within  the limits of the server's "write history
+    28                     entry" set.  See Section 6.19.3,  "WRITE  HISTORY
+    29                     MANAGEMENT Command, Description" for more detail.
+
+    30                 Invalid Entry Id
+
+    31                     The value specified in the "entry id" field of  a
+    32                     WRITE  HISTORY  MANAGEMENT, ERASE, WRITE, or DISK
+    33                     COPY DATA command does not match the value in the
+    34                     "write  history  entry"  identified  by the entry
+    35                     locator or the host  supplied  a  value  of  FFFF
+    36                     (hex)  in  the  "entry id" field of a History Log
+    37                     modified write type command.  See Section 6.19.3,
+    38                     "WRITE  HISTORY  MANAGEMENT Command, Description"
+    39                     for more detail.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-26
+           Status And Event Codes                               11 June 1992
+
+
+     1                 Invalid Item
+
+     2                     The  specified   display   item   code   is   not
+     3                     implemented   and   the   "Must  Be  Implemented"
+     4                     modifier was  set.   For  more  information,  see
+     5                     Section 6.8.3, "DISPLAY Command, Description."
+
+     6                 Invalid Length
+
+     7                     The  "length"  field  of  an  ACCESS  NONVOLATILE
+     8                     MEMORY  command  contains  an  odd  value and the
+     9                     "operation" field  requested  a  "Test  and  Set"
+    10                     operation  (see  Section 7.3.1, "Multihost ACCESS
+    11                     NONVOLATILE  MEMORY  Command,   Command   Message
+    12                     Format").
+
+    13                 Invalid Logical Block Count
+    14                 Invalid Src Unit Number
+    15                 Invalid Source Unit Identifier
+    16                 Invalid Destination LBN
+    17                 Invalid Source LBN
+    18                 Invalid Source Unit's Storage Subsystem Port Address
+    19                 Invalid Source Unit's Storage Subsystem System
+    20                         Address
+
+    21                     See  Section  6.7.1,  "DISK  COPY  DATA  Command,
+    22                     Command Message Format" for descriptions of these
+    23                     command message fields.
+
+    24                 Invalid Logical Block Number
+
+    25                     See  "logical  block  number"  field  in  Section
+    26                     5.3.1,  "Transfer  Command  Message  Format"  for
+    27                     possible causes.
+
+    28                 Invalid Mode
+
+    29                     The specified display mode is not implemented for
+    30                     the  specified  display  item  and  the  "Must Be
+    31                     Implemented"  modifier   was   set.    For   more
+    32                     information, see Section 6.8.3, "DISPLAY Command,
+    33                     Description."
+
+    34                 Invalid Replacement Block Number
+
+    35                     See Section 6.15.2, "REPLACE Command, End Message
+    36                     Format" for possible causes.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-27
+           Status And Event Codes                               11 June 1992
+
+
+     1                 Invalid Unit Flags
+
+     2                     The the host-settable unit flags specified in the
+     3                     "unit  flags"  field of an ONLINE command message
+     4                     issued to a unit that  is  already  "Unit-Online"
+     5                     are different from those already in effect on the
+     6                     unit (see Sections  6.13,  "ONLINE  Command"  and
+     7                     7.7, "Multihost ONLINE Command").
+
+     8             Note that an unknown unit number does not  constitute  an
+     9             invalid  parameter  or command.  Unknown unit numbers are
+    10             treated as if the unit were "Unit-Offline." 
+
+    11             In order for hosts to clearly distinguish the  two  types
+    12             of  invalid  commands  controllers  use two different end
+    13             message formats.
+
+    14             Protocol errors are reported via the Invalid Command  End
+    15             Message.   The  Invalid Command End Message is largely an
+    16             image of the invalid command message  that  was  received
+    17             from the host.  In order to construct the Invalid Command
+    18             End Message, the controller makes the  following  changes
+    19             to the invalid command message that it received:
+
+    20               1.  The  controller  stores  the  constant  (OP.END),
+    21                   defined  in Appendix A, "Opcode, Flag, and Offset
+    22                   Definitions,"   Table   A-1,   "Control   Message
+    23                   Opcodes,"  in  the  "endcode"  field, in order to
+    24                   flag this as  an  Invalid  Command  End  Message.
+    25                   (Note  that  the  command  opcode is not added to
+    26                   OP.END.)
+
+    27               2.  The controller may, at its  option,  either  copy
+    28                   the   end   message   "flags"   field   from  the
+    29                   corresponding field in  the  invalid  command  or
+    30                   else clear the end message "flags" field.
+
+    31               3.  The  controller  stores  the  "Invalid   Command"
+    32                   status  code  with  an appropriate subcode in the
+    33                   end message "status" field.  The subcode  is  set
+    34                   equal  to  the byte offset, from the start of the
+    35                   command message, of the field in error  (any  one
+    36                   field, at the controller's option, if several are
+    37                   in error).  Multibyte fields  are  identified  by
+    38                   the  offset  to  their  lowest byte.  Single byte
+    39                   fields  positioned  at  an  odd  offset  may   be
+    40                   identified, at the controller's option, by either
+    41                   their actual offset or their  offset  minus  one.
+    42                   That  is,  offsets  may  be  truncated to an even
+    43                   value.  Subcode zero is used to report  that  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-28
+           Status And Event Codes                               11 June 1992
+
+
+     1                   command  message  was  too  short  to contain the
+     2                   parameters  required  by  the  command's  opcode.
+     3                   Note  that  any  value  is valid for the field at
+     4                   offset zero, the "command reference number." 
+
+     5               4.  The  controller  may  optionally   truncate   the
+     6                   parameter fields ("echoed command parameters") to
+     7                   some  length  convenient  for   the   controller.
+     8                   Controllers  should  try  to  echo as much of the
+     9                   parameters as possible.
+
+
+    10             The contents of any field copied from the invalid command
+    11             are  undefined  if  the  command message was too short to
+    12             contain  that  field.   This  primarily  applies  to  the
+    13             "command reference number" and "unit number" fields.
+
+    14             Note that the controller may, at its  option,  enter  the
+    15             "Controller-Available" state relative to the issuing host
+    16             class driver after it returns  the  Invalid  Command  End
+    17             Message (see Section 4.1, "Controller States").
+
+    18             Parameter errors are reported via the  invalid  command's
+    19             normal  (i.e.,  opcode  dependent, see "endcode" field in
+    20             Section 5.4, "End Messages") end message.   The  "status"
+    21             field  is set equal to the "Invalid Command" status code,
+    22             with the subcode set equal to the byte  offset,  relative
+    23             to  the  start  of  the command message, of the parameter
+    24             field in error.  Multibyte fields are identified  by  the
+    25             offset  to  their  lowest  byte.   All other fields (byte
+    26             count, unit characteristics, etc.) reflect the  state  of
+    27             the  execution of the command up to the point the invalid
+    28             parameter was detected.
+
+    29         Command Aborted
+
+    30             The command was aborted by an  ABORT  command.   The  end
+    31             message  for  the  aborted command (i.e., the end message
+    32             containing the "Command Aborted"  status  code)  has  the
+    33             normal  format  for the command and all fields are valid.
+    34             In particular, the "byte count" field identifies how  far
+    35             a  transfer  command was completed before it was aborted.
+    36             The status of the transfer beyond the returned byte count
+    37             is undefined.  Subcodes are not used.
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-29
+           Status And Event Codes                               11 June 1992
+
+
+     1         Unit-Offline
+
+     2             The unit identified by the "unit number" field of the end
+     3             message  is  in  the  "Unit-Offline"  state.  The subcode
+     4             consists of bit flags  that  indicate  why  the  unit  is
+     5             "Unit-Offline."   Note  that there may be several reasons
+     6             for the unit being "Unit-Offline."   If  the  subcode  is
+     7             zero,  it  implies  that the unit is unknown -- i.e., the
+     8             controller knows of  no  unit  with  the  specified  unit
+     9             number.
+
+    10         Unit-Available
+
+    11             The unit identified by the "unit number" field of the end
+    12             message is in the "Unit-Available" state.  The subcode is
+    13             zero, except when "Exclusive  Access"  of  the  unit  was
+    14             denied.   In  that  case  the "Already In Use" subcode is
+    15             returned (see the descriptions of the AVAILABLE,  ONLINE,
+    16             and  SET  UNIT  CHARACTERISTICS  commands  in  Chapter 7,
+    17             "Multihost Support" for more detail).
+
+    18         Media Format Error
+
+    19             Only returned  by  the  ONLINE  command  for  disk  class
+    20             devices.   The  volume  mounted on the unit appears to be
+    21             formatted incorrectly, so that  it  must  be  reformatted
+    22             (and all data lost) before it may be used.  This error is
+    23             also returned if the volume is formatted  with  576  byte
+    24             sectors   and  the  controller  only  supports  512  byte
+    25             sectors.  Note that the volume may only  "appear"  to  be
+    26             formatted incorrectly; the typical cause of this error is
+    27             a fault in the drive's read/write electronics.   If  this
+    28             is  the  case,  the  volume  can  usually be successfully
+    29             accessed on another drive.
+
+    30             Whenever they return this error  code  controllers  shall
+    31             treat  the  unit  as  if  an  AVAILABLE  command with the
+    32             "Spin-down" modifier set had been  issued  for  it.   The
+    33             unit  is  therefore  always in the "Unit-Available" state
+    34             with AVAILABLE  attention  messages  suppressed  until  a
+    35             human  operator  changes the volume or spins-up the unit.
+    36             The subcode reports  which  integrity  check  the  volume
+    37             failed;  it  is  volume format, and therefore drive type,
+    38             dependent.
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-30
+           Status And Event Codes                               11 June 1992
+
+
+     1         Write Protected
+
+     2             The unit identified by the "unit number" field of the end
+     3             message  is write protected and the command required that
+     4             data be written onto the drive.  The subcode consists  of
+     5             bit  flags  indicating  the reasons why the unit is write
+     6             protected.
+
+     7         Compare Error
+
+     8             A COMPARE HOST DATA command, a read compare operation, or
+     9             a  write  compare  operation  found different data in the
+    10             host buffer and the unit identified by the "unit  number"
+    11             field of the end message.  The subcode is always zero.
+
+    12         Data Error
+
+    13             Invalid or uncorrectable data was obtained from a  drive,
+    14             as  determined  by internal error detecting or correcting
+    15             codes.  The subcode is used to  report  the  exact  error
+    16             detected.  Subcode zero is used for "Forced Errors."  All
+    17             errors caused by the  "Force  Error"  modifier  shall  be
+    18             reported with subcode zero.
+
+    19         Host Buffer Access Error
+
+    20             The controller encountered an error  when  attempting  to
+    21             access  a  buffer in host memory.  The subcode is used to
+    22             report the exact error encountered.  This status code  is
+    23             also returned whenever the command's buffer descriptor or
+    24             byte count violate any communications mechanism dependent
+    25             restrictions.
+
+    26             Note that this status code is NOT used to  report  errors
+    27             encountered when transferring command, end, attention, or
+    28             error log messages between the  controller  and  a  host.
+    29             Such  errors  are  reported by terminating the connection
+    30             between the class driver and MSCP server.  The  mechanism
+    31             for  reporting  such  errors  to  the host's error log is
+    32             communications mechanism dependent.
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-31
+           Status And Event Codes                               11 June 1992
+
+
+     1         Controller Error
+
+     2             The controller encountered an internal controller  error.
+     3             The   subcode   is   used   to  report  the  exact  error
+     4             encountered.  An internal controller error is reported as
+     5             a  "Controller  Error"  if and only if the controller has
+     6             reasonable grounds to trust its  sanity  and  expects  to
+     7             complete,  either  successfully  or  with  an appropriate
+     8             error status code, all of its outstanding commands.   All
+     9             more severe controller errors are reported by terminating
+    10             the connection between the controller's MSCP  server  and
+    11             the  host  class driver.  This is in addition, of course,
+    12             to attempting to generate an error log message.
+
+    13             Subcode zero of this status code is reserved for host  or
+    14             non-MSCP   entity  (e.g.,  a  controller's  port  driver)
+    15             detected command timeouts.  It is never generated  by  an
+    16             MSCP   server.    All   other   subcodes  are  controller
+    17             dependent.
+
+    18             Note that some controller errors may  be  reported  using
+    19             other  error codes if an internal controller error causes
+    20             the controller to misdiagnose the error.
+
+    21         Drive Error
+
+    22             The controller discovered an error within a drive.   Such
+    23             errors  are  typically,  but  not  always,  mechanical in
+    24             nature, since most nonmechanical errors are  reported  as
+    25             "Data  Errors."   The subcode is used to report the exact
+    26             error encountered.
+
+    27             In many cases a "Drive Error" will indicate that the unit
+    28             is  broken  or  inoperative.   If this occurs, the "Drive
+    29             Error" should  be  reported  once  and  the  unit  should
+    30             subsequently  be  reported as being "Unit-Offline" due to
+    31             being inoperative.
+
+    32         Invalid Parameter
+
+    33             The controller encountered an invalid parameter value for
+    34             a  parameter  that  is not passed directly in the command
+    35             message.  Invalid values in fields of a  command  message
+    36             are  reported  with  the  "Invalid  Command" status code.
+    37             "Invalid Parameter" typically reports an invalid value in
+    38             a field in a host-supplied buffer.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-32
+           Status And Event Codes                               11 June 1992
+
+
+     1     The status codes that may be returned for a specific command  are
+     2     command  (opcode)  dependent.   The  status  codes  that  may  be
+     3     returned for each command and any special meaning that they  have
+     4     specific  to  the  command are listed in the command descriptions
+     5     contained in Chapter 6, "Minimal MSCP Command Set" and Chapter 7,
+     6     "Multihost  Support."   Note  that  the format of a command's end
+     7     message is solely determined by  its  opcode.   The  status  code
+     8     returned  in  the  end  message does not affect the end message's
+     9     format.  The only exception is the Invalid  Command  End  Message
+    10     (see  "Invalid  Command"  status above), which contains a special
+    11     endcode.
+
+    12  |  "Event Only" event codes and subcodes shall only be used in error
+    13  |  log  or  informational messages and shall never be used as status
+    14  |  codes reported in end messages.   The  currently  defined  "Event
+    15  |  Only" event codes are:
+
+    16  |      Bad Block Replacement Attempted
+
+    17  |          This event code is  used  after  every  attempt  (by  the
+    18  |          controller or host) to replace a bad block, regardless of
+    19  |          the success or  failure  of  that  attempt  (see  Section
+    20  |          5.8.7, "BAD BLOCK REPLACEMENT ATTEMPT Error Log Format").
+    21  |          For details of bad  block  replacement,  see  Chapter  8,
+    22  |          "Controller  Initiated  Bad  Block  Replacement"  and the
+    23  |          Digital   Storage   Architecture   Disk   Format   (DSDF)
+    24  |          specification.
+
+    25  |      Informational Log
+
+    26  |          Informational Log messages are  used  by  the  server  to
+    27  |          report  such  things  as  statistics or error log summary
+    28  |          information that may have been compiled  in  the  device.
+    29  |          In  all  error  log  messages  with  this event code, the
+    30  |          "Informational"  error  log  message  flag  (see  Section
+    31  |          5.9.2,   "Generic   Error  Log  Message  Format")  should
+    32  |          normally be set.
+
+    33     5.6  Controller Flags
+
+    34     Controllers maintain a set of bit flags,  collectively  known  as
+    35     the   "controller   flags,"   which  represent  the  controller's
+    36     operating characteristics.  Two  different  types  of  controller
+    37     flags  are  used to accommodate two different types of controller
+    38     characteristics.  Host-settable controller flags allow  hosts  to
+    39     enable  or  disable the host selectable operating characteristics
+    40     (e.g., Enable Attention Messages) supported by  all  controllers.
+    41     Controllers  use  non-host-settable  controller flags to indicate
+    42     the presence or  absence  of  implementation  dependent  optional
+    43     functions  (e.g.,  Multihost  Support).   The  currently  defined
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-33
+           Controller Flags                                     11 June 1992
+
+
+     1     controller flags are described later in this section.
+
+     2     The SET CONTROLLER CHARACTERISTICS command is used to modify  the
+     3     host-settable controller flags according to the value supplied in
+     4     the "controller flags"  field  of  the  command  message.   If  a
+     5     host-settable   flag   is   set   the   controller   enables  the
+     6     corresponding host selectable operating characteristic.  A  clear
+     7     flag  causes  the  controller  to  disable it.  Controllers shall
+     8     always ignore  the  values  supplied  for  the  non-host-settable
+     9     controller  flags.   All  host-settable  controller flags are, by
+    10     default,   clear   (i.e.,   all   host    selectable    operating
+    11     characteristics  are  disabled)  whenever  a  class  driver first
+    12     becomes "Controller-Online" to an MSCP server.
+
+    13     The  controller  dependent  values   of   the   non-host-settable
+    14     controller   flags,   as  well  as  the  current  values  of  the
+    15     host-settable controller flags, are returned in  the  "controller
+    16     flags"  field of the SET CONTROLLER CHARACTERISTICS command's end
+    17     message.  A controller shall  set  only  those  non-host-settable
+    18     controller  flags  for  the  corresponding  optional functions it
+    19     supports, if any.
+
+    20     Controllers that provide Multihost Support  shall  maintain,  and
+    21     operate   according   to  the  setting  of,  a  separate  set  of
+    22     host-settable controller flags for each  host  class  driver/MSCP
+    23     server  connection  established.   Each class driver is expressly
+    24     permitted to specify different  settings  for  the  host-settable
+    25     controller  flags  and  as  a result realize different controller
+    26     operation.  The non-host-settable controller flags remain  fixed,
+    27     and  are therefore common to all class drivers of the same device
+    28     class.
+
+    29     The host-settable controller flags are as follows:
+
+    30         Enable Attention Messages
+
+    31             When this flag is set the controller shall send attention
+    32             messages  to  this  host.   If  this  flag  is  clear the
+    33             controller discards attention messages.  Note  that  this
+    34             flag  is applicable to all attention messages, regardless
+    35             of type.
+
+    36         Enable Miscellaneous Error Log Messages
+
+    37             When this flag is set the controller shall send error log
+    38             messages that do not relate to a specific command to this
+    39             host.  If this flag is clear the controller discards  any
+    40             such error logs.
+
+    41         Enable Other Hosts' Error Log Messages
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-34
+           Controller Flags                                     11 June 1992
+
+
+     1             When this flag is set error log messages that  relate  to
+     2             commands  issued  by  other  hosts should be sent to this
+     3             host.  Controllers that provide Multihost  Support  shall
+     4             implement  this  characteristic  separately for each host
+     5             class driver connected.  Even though  this  operation  is
+     6             only  useful in a multihost environment, controllers that
+     7             do not provide Multihost Support shall still  honor  this
+     8             flag.   That  is, the flag may not be treated as reserved
+     9             and the current  state  as  set  by  the  host  shall  be
+    10             returned in the end message field.
+
+    11         Enable This Host's Error Log Messages
+
+    12             When this flag is set error log messages that  relate  to
+    13             commands issued by this host should be sent to this host.
+    14             If this flag is clear the controller discards those error
+    15             logs,  except  in  a  multihost environment where another
+    16             host has set the "Enable Other Hosts' Error Log Messages"
+    17             flag.
+    18     The non-host-settable controller flags are as follows:
+
+    19         Controller Initiated Bad Block Replacement
+
+    20             This flag shall be set if  and  only  if  the  controller
+    21             supports  Controller  Initiated  Bad Block Replacement on
+    22             all disk drives that can be connected to the  controller.
+    23             If  this  flag is set the host will never have to perform
+    24             bad  block  replacement  for  disks  accessed  via   this
+    25             controller.   Note  that  controllers  may  also  support
+    26             Controller Initiated Bad  Block  Replacement  on  certain
+    27             individual disk drives and not others.  In that case this
+    28             flag  shall  be  returned  clear;  the  setting  of   the
+    29             Controller  Initiated  Bad  Block  Replacement  unit flag
+    30             (described  in  Section  5.7,  "Unit  Flags")  determines
+    31             whether   or   not   the  host  must  perform  bad  block
+    32             replacement on a particular disk drive.  Note  also  that
+    33             that  unit flag shall always be returned set if this flag
+    34             is returned set.
+
+    35             Support of Controller Initiated  Bad  Block  Replacement,
+    36             either  on  a  single disk drive or all disk drives, is a
+    37             controller implementation dependent option.  See  Chapter
+    38             8,  "Controller  Initiated  Bad  Block  Replacement"  for
+    39             complete details.
+
+    40         Local Disk Copy Data
+
+    41             This flag shall be  returned  set  if  and  only  if  the
+    42             controller supports interdevice transfer of data, via the
+    43             DISK COPY DATA command (see Section 6.7), between all the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-35
+           Controller Flags                                     11 June 1992
+
+
+     1             disk drives connected to the controller.
+
+     2             Support of Local Disk  Copy  Data  is  an  implementation
+     3             dependent   option.    Local   Disk   Copy  Data  may  be
+     4             implemented by controllers that provide Multihost Support
+     5             as  well  as those that do not.  However, for controllers
+     6             that provide Multihost Support, support  of  Remote  Disk
+     7             Copy  Data  (see the Remote Disk Copy Data flag below) is
+     8             preferred.
+
+     9             Controllers that implement Local  Disk  Copy  Data  shall
+    10             also implement Controller Initiated Bad Block Replacement
+    11             on all disk drives  connected  to  them  (see  Controller
+    12             Initiated Bad Block Replacement flag above).
+
+    13         Multihost Support
+
+    14             This flag shall be  returned  set  if  and  only  if  the
+    15             controller  supports  simultaneous  access,  by  multiple
+    16             hosts, to all drives (disk and  tape)  connected  to  the
+    17             controller.  See Chapter 7, "Multihost Support." 
+
+    18         Remote Disk Copy Data
+
+    19             This flag shall be  returned  set  if  and  only  if  the
+    20             controller supports interdevice transfer of data, via the
+    21             DISK COPY DATA command (see Section  6.7),  between  disk
+    22             drives  connected  to itself and disk drives connected to
+    23             another storage subsystem that is connected to  the  same
+    24             communications mechanism.
+
+    25             Support of Remote Disk Copy  Data  is  an  implementation
+    26             dependent  option.   Remote  Disk  Copy  Data can only be
+    27             implemented by controllers that  also  provide  Multihost
+    28             Support.
+
+    29             Controllers that implement Remote Disk  Copy  Data  shall
+    30             also  implement Local Disk Copy Data (see Local Disk Copy
+    31             Data flag  above)  and  Controller  Initiated  Bad  Block
+    32             Replacement   (see   Controller   Initiated   Bad   Block
+    33             Replacement flag above) on all disk drives  connected  to
+    34             them.
+
+    35         Restricted DISK COPY DATA Operations
+
+    36             This flag shall be  returned  set  if  and  only  if  the
+    37             controller does not support any of:
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-36
+           Controller Flags                                     11 June 1992
+
+
+     1              o  Local  DISK  COPY  DATA  between  units  with  unlike
+     2                 geometries
+
+     3              o  Local DISK COPY DATA where the source and destination
+     4                 LBNs differ
+
+     5              o  DISK COPY DATA where the source and  destination  are
+     6                 the same unit
+
+
+     7             Note that controllers either support  all  of  the  above
+     8             operations  and return this flag clear or support none of
+     9             these options and return the flag set.
+
+    10         Write History Logging Support
+
+    11             This flag shall be  returned  set  if  and  only  if  the
+    12             controller  supports "write history logging" as described
+    13             in Section 6.19.3,  "WRITE  HISTORY  MANAGEMENT  Command,
+    14             Description."   "Write   history   logging"  was  defined
+    15             primarily for the purpose of assisting in  the  operation
+    16             of  host  based  volume  shadowing implementations.  Note
+    17             that Write History Logging support  includes  support  of
+    18             the TERMINATE CLASS DRIVER/SERVER CONNECTION command.
+
+    19         576 Byte Sectors
+
+    20             This flag shall be set if  and  only  if  the  controller
+    21             supports disks formatted with 576 byte sectors as well as
+    22             disks formatted with 512 byte  sectors.   Note  that  all
+    23             controllers  that support disk class devices must support
+    24             disks formatted with 512 byte sectors.
+
+    25     Refer to Appendix A,  "Opcode,  Flag,  and  Offset  Definitions,"
+    26     Table  A-4,  "Controller  Flags"  for  the values assigned to the
+    27     currently defined host-settable and non-host-settable  controller
+    28     flags.   Those  bits in the "controller flags" field that are not
+    29     defined as controller flags are reserved, and shall be treated in
+    30     accordance with the requirements for reserved fields described in
+    31     Section 5.2, "Reserved and Undefined Fields." 
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-37
+           Unit Flags                                           11 June 1992
+
+
+     1     5.7  Unit Flags
+
+     2     For each unit connected to a controller, the controller maintains
+     3     a set of bit flags, collectively known as the "unit flags," which
+     4     represent the unit's operating  characteristics.   Two  different
+     5     types  of  unit flags are used to accommodate two different types
+     6     of unit characteristics.  Host-settable unit flags allow hosts to
+     7     enable    or    disable   certain   host   selectable   operating
+     8     characteristics (e.g.,  Compare  Reads).   The  non-host-settable
+     9     unit  flags  are  used  to  indicate  certain physical or logical
+    10     characteristics (e.g., 576 Byte Sectors, Write Protect)  as  well
+    11     as  the  presence or absence of certain controller implementation
+    12     dependent optional  functions  (e.g.,  Controller  Initiated  Bad
+    13     Block   Replacement).   The  currently  defined  unit  flags  are
+    14     described later in  this  section.   Except  as  noted  in  those
+    15     descriptions    all   controllers   shall   fully   support   all
+    16     non-host-settable unit flags on all units.
+
+    17     The ONLINE and SET UNIT  CHARACTERISTICS  commands  are  used  to
+    18     modify  the  host-settable  unit flags for the unit identified in
+    19     the "unit" field of the command message, according to  the  value
+    20     supplied  in the "unit flags" field of the command message.  If a
+    21     host-settable  unit  flag  is  set  the  controller  enables  the
+    22     corresponding  host  selectable  operating characteristic for the
+    23     unit.  A clear flag causes the controller to  disable  it.   Note
+    24     that  the  state of the unit (Offline, Available, or Online) when
+    25     the command is received affects the setting or  clearing  of  the
+    26     unit  flags.   See  the  descriptions  of the ONLINE and SET UNIT
+    27     CHARACTERISTICS commands in Chapters  6,  "Minimal  MSCP  Command
+    28     Set"  and 7, "Multihost Support" for complete details.  Except as
+    29     noted in the  individual  flag  descriptions  below,  controllers
+    30     shall  ignore  the values supplied for the non-host-settable unit
+    31     flags.
+
+    32     The non-host-settable unit flags, as well as the  current  values
+    33     of the host-settable unit flags, are returned in the "unit flags"
+    34     field  of  the  ONLINE,   GET   UNIT   STATUS,   and   SET   UNIT
+    35     CHARACTERISTICS  command end messages and the AVAILABLE Attention
+    36     message.  Many  unit  flags,  including  all  host-settable  unit
+    37     flags, are only valid when the unit is "Unit-Online."  The values
+    38     returned for such  unit  flags  are  undefined  if  the  unit  is
+    39     "Unit-Offline" or "Unit-Available."  A few non-host-settable unit
+    40     flags are valid when the  unit  is  "Unit-Available"  and  during
+    41     certain  "Unit-Offline" substates.  These flags are identified in
+    42     the individual flag descriptions below.
+
+    43     Controllers that provide Multihost Support maintain only one  set
+    44     of  host-settable  unit flags for a particular unit regardless of
+    45     the number of hosts (class drivers) that have  brought  the  unit
+    46     into  the  "Unit-Online"  state.   The  unit  will  operate in an
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-38
+           Unit Flags                                           11 June 1992
+
+
+     1     identical manner (according to the setting of  the  host-settable
+     2     unit   flags)   for   each   host   that  has  brought  the  unit
+     3     "Unit-Online."   Any  host  may  alter   the   setting   of   the
+     4     host-settable unit flags (for itself and all others) only via the
+     5     SET UNIT CHARACTERISTICS command and only after bringing the unit
+     6     "Unit-Online."   Attempts  to change the host-settable unit flags
+     7     from their current setting via an ONLINE command  (regardless  of
+     8     whether  the  unit was already "Unit-Online" to the issuing host)
+     9     will cause the ONLINE command  to  be  rejected  as  an  "Invalid
+    10     Command"  ("invalid  unit  flag"  parameter  error.  See "Invalid
+    11     Command" status in Section 5.5, "Status and Event Codes").
+
+    12     The unit characteristics flags are as follows:
+
+    13         Compare Reads
+
+    14             A host-settable characteristic; set if all read transfers
+    15             should  be verified with a compare operation.  Equivalent
+    16             to  specifying  the  "Compare"  modifier  on   all   READ
+    17             commands.   See  Section  4.15,  "Compare Operations" for
+    18             complete details.
+
+    19         Compare Writes
+
+    20             A  host-settable  characteristic;  set   if   all   write
+    21             transfers  should  be  verified with a compare operation.
+    22             Equivalent to specifying the "Compare"  modifier  on  all
+    23             WRITE  commands.   See Section 4.15, "Compare Operations"
+    24             for complete details.
+
+    25         Controller Initiated Bad Block Replacement
+
+    26             A non-host-settable characteristic; set if the controller
+    27             will  perform  bad  block replacement for this disk drive
+    28             (unit),  clear  if  the  host  must  perform  bad   block
+    29             replacement  for  this disk drive.  Note that controllers
+    30             may support Controller Initiated Bad Block Replacement on
+    31             all  or  some disk drives connected to it.  If Controller
+    32             Initiated Bad Block Replacement is provided for all  disk
+    33             drives  this  flag and the Controller Initiated Bad Block
+    34             Replacement controller flag (described  in  Section  5.6,
+    35             "Controller Flags") shall both be set.
+
+    36             Note that a host disk class driver  shall  check  if  the
+    37             "Controller Initiated Bad Block Replacement" unit flag is
+    38             set or clear after bringing a unit "Unit-Online."  If  it
+    39             is  clear (implying that the host is performing bad block
+    40             replacement),  then  the  class  driver  shall  invoke  a
+    41             process  that  will access the unit's Replacement Control
+    42             Table to determine if a bad block  replacement  operation
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-39
+           Unit Flags                                           11 June 1992
+
+
+     1             has been partially performed or if the unit must be write
+     2             protected.   The  details   of   this   check   and   its
+     3             consequences   are   described  in  the  Digital  Storage
+     4             Architecture Disk  Format  specification  (DSDF)  in  the
+     5             section   on  "Host  Initiated  Bad  Block  Replacement."
+     6             Controllers that support Controller Initiated  Bad  Block
+     7             Replacement perform these checks themselves.
+
+     8             See  Chapter   8,   "Controller   Initiated   Bad   Block
+     9             Replacement" for complete details.
+
+    10         Exclusive Access
+
+    11             A non-host-settable characteristic; set if this  unit  is
+    12             under  the "Exclusive Access" of a particular host.  That
+    13             is, the unit although  in  a  multihost  environment  and
+    14             normally   accessible  by  multiple  hosts  is  now  only
+    15             accessible by the one host that requested and was granted
+    16             exclusive  access of the unit via an AVAILABLE, ONLINE or
+    17             SET UNIT CHARACTERISTICS command.  See  the  descriptions
+    18             of those commands in Chapter 7, "Multihost Support." 
+
+    19             This  characteristic  shall   be   fully   supported   by
+    20             controllers that provide Multihost Support.
+
+    21         Removable Media
+
+    22             A non-host-settable characteristic; set if this unit  has
+    23             removable media.
+
+    24             This characteristic  shall  be  fully  supported  by  all
+    25             controllers that support units with removable media.
+
+    26             Valid whenever the controller can  determine  the  unit's
+    27             characteristics.   See  the  descriptions of the GET UNIT
+    28             STATUS, ONLINE, and SET UNIT CHARACTERISTICS command  end
+    29             messages  contained  in Chapters 6, "Minimal MSCP Command
+    30             Set" and 7, "Multihost Support" for complete details.
+
+    31         Write History Logging Support
+
+    32             A non-host-settable characteristic; shall be returned set
+    33             if  and  only  if  the controller supports "write history
+    34             logging" for this unit as described  in  Section  6.19.3,
+    35             "WRITE  HISTORY MANAGEMENT Command, Description." See the
+    36             description  of  the  Write   History   Logging   Support
+    37             controller flag in Section 5.6, "Controller Flags."
+
+    38         Write Protect (data safety)
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-40
+           Unit Flags                                           11 June 1992
+
+
+     1             A non-host-settable characteristic; set by the controller
+     2             whenever  some  condition  in the unit or volume prevents
+     3             reliable modification of data on  the  volume.   Possible
+     4             causes include:
+
+
+     5                o  An incomplete bad block replacement.
+
+     6                o  An invalid Replacement Control Table (RCT).
+
+     7                o  The unit is only capable of reading the  volume's
+     8                   format   (e.g.,  a  single-density  volume  in  a
+     9                   double-density drive).
+
+    10             The consequences of Write  Protection  are  described  in
+    11             Section 4.14, "Write Protection." 
+
+    12             This characteristic  shall  be  fully  supported  if  the
+    13             controller   provides   Controller  Initiated  Bad  Block
+    14             Replacement support or supports  units  capable  of  only
+    15             reading  a volume's format (e.g., a single-density volume
+    16             in a double-density drive).
+
+    17         Write Protect (hardware)
+
+    18             A non-host-settable characteristic;  set  if  the  unit's
+    19             write protect mechanism is activated, causing the unit to
+    20             be Hardware Write Protected.  The consequences  of  Write
+    21             Protection   are   described   in  Section  4.14,  "Write
+    22             Protection." 
+
+    23         Write Protect (software)
+
+    24             Normally a non-host-settable characteristic; set  if  the
+    25             unit  is  "Software"  Write  Protected.   A host-settable
+    26             characteristic if the "Enable Set Write Protect"  command
+    27             modifier is set in the ONLINE or SET UNIT CHARACTERISTICS
+    28             command.  If  the  "Enable  Set  Write  Protect"  command
+    29             modifier  is  set  the  controller  enables  or  disables
+    30             "Software" Write Protection according to the  state  (set
+    31             or clear, respectively) of the "Write Protect (software)"
+    32             flag in the "unit flags" field.  If the "Enable Set Write
+    33             Protect" command modifier is clear the controller ignores
+    34             the "Write Protect (software)" flag in the  "unit  flags"
+    35             field;  the  state of "Software" Write Protection remains
+    36             unchanged.  The  consequences  of  Write  Protection  are
+    37             described in Section 4.14, "Write Protection." 
+
+    38         576 Byte Sectors
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-41
+           Unit Flags                                           11 June 1992
+
+
+     1             Normally a non-host-settable characteristic; set  if  the
+     2             disk  volume mounted on the unit has 576 byte sectors.  A
+     3             host-settable characteristic if the  controller  supports
+     4             576  byte  sectors  and  the  "Ignore Media Format Error"
+     5             command modifier  is  set  in  the  ONLINE  command  (see
+     6             Section  6.13.1, "ONLINE Command, Command Message Format"
+     7             for complete details).
+
+     8             This characteristic  shall  be  fully  supported  if  the
+     9             controller  supports  disks  formatted with either 512 or
+    10             576 byte sectors.  If the controller only supports  disks
+    11             formatted  with  512  byte  sectors, the controller shall
+    12             ignore any value the host specifies  for  this  flag  and
+    13             always  return  it clear.  Note that all controllers that
+    14             support disk class devices shall support disks  formatted
+    15             with 512 byte sectors.
+
+    16     Refer to Appendix A,  "Opcode,  Flag,  and  Offset  Definitions,"
+    17     Table  A-5, "Unit Flags" for the values assigned to the currently
+    18     defined host-settable and non-host-settable  unit  flags.   Those
+    19     bits in the "unit flags" field that are not defined as unit flags
+    20     are reserved,  and  shall  be  treated  in  accordance  with  the
+    21     requirements  for  reserved  fields  described  in  Section  5.2,
+    22     "Reserved and Undefined Fields." 
+
+    23     5.8  Attention Messages
+
+    24     MSCP servers report duplicate  unit  number  conditions  and  the
+    25     availability  of  units  spontaneously  to  hosts  via  attention
+    26     messages.  Hosts may enable or disable attention messages via the
+    27     setting   of   the   "Enable  Attention  Messages"  flag  in  the
+    28     "controller flags" field of the  SET  CONTROLLER  CHARACTERISTICS
+    29     command   (see  Section  5.6,  "Controller  Flags").   Note  that
+    30     attention messages are flow controlled as  described  in  Section
+    31     3.2,  "Flow  Control"  and  associated  subsections.  Hosts shall
+    32     discard any attention messages that they do not recognize.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-42
+           Attention Messages                                   11 June 1992
+
+
+     1     The generic attention message format is as follows:
+
+     2                     31                             0 
+     3                    +--------------------------------+
+     4                    |           reserved             |
+     5                    +---------------+----------------+
+     6                    |   reserved    |  unit number   |
+     7                    +---------------+-------+--------+
+     8                    |   reserved    | rsvd  |atncode |
+     9                    +---------------+-------+--------+
+    10                    |                                |
+    11                    /          parameters            /
+    12                    /                                /
+    13                    |                                |
+    14                    +--------------------------------+
+
+    15     The fields in the generic attention message are as follows:
+
+    16         unit number
+
+    17             Identifies the specific unit within the device  class  to
+    18             which  the  attention message applies.  This value is the
+    19             binary equivalent of the decimal unit number displayed by
+    20             the unit select mechanism.
+
+    21         atncode
+
+    22             Identifies  the  attention  message  type.   The  atncode
+    23             implicitly   specifies  the  length  and  format  of  the
+    24             message, including the interpretation of  any  parameters
+    25             that are present.
+
+    26             Appendix A, "Opcode, Flag, and Offset Definitions," Table
+    27             A-1,  "Control Message Opcodes," lists the valid atncodes
+    28             and their meanings.
+
+    29     Appendix A, "Opcode, Flag, and Offset  Definitions,"  Table  A-7,
+    30     "End   and  Attention  Message  Offsets"  defines  the  preferred
+    31     mnemonic and numeric value of the  offset  within  the  attention
+    32     message  of  the  various  attention message parameter fields, as
+    33     well as the size of each field.
+
+    34     The following sections describe the  specific  attention  message
+    35     formats.
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-43
+           Attention Messages                                   11 June 1992
+
+
+     1     5.8.1  ACCESS PATH Attention Message
+
+     2     5.8.1.1  Attention Message Format
+
+     3             31                             0                
+     4             +--------------------------------+              
+     5             |           reserved             |              
+     6             +---------------+----------------+              
+     7             |   reserved    |  unit number   |              
+     8             +---------------+-------+--------+              
+     9             |   reserved    | rsvd  |atncode |              
+    10             +---------------+-------+--------+              
+
+    11  |  The fields in the ACCESS PATH attention message are as follows:
+
+    12         unit number
+
+    13             Identifies the unit for which an alternate access path is
+    14             being reported.
+
+    15         atncode
+
+    16             See Section 5.8, "Attention Messages." 
+
+
+    17     5.8.1.2  Description
+
+    18     MSCP servers use  this  attention  message  to  report  alternate
+    19     access paths to multiaccess units.  This message reports that the
+    20     specified unit is potentially accessible  via  the  sending  MSCP
+    21     server  -- i.e., it would be "Unit-Available" if it and all units
+    22     that share its access path ceased being  "Unit-Online"  or  under
+    23     "Exclusive  Access"  via  another controller.  The specific event
+    24     that causes an MSCP server to send this attention message is  the
+    25     receipt,  by the controller to which the unit is "Unit-Online" or
+    26     is under "Exclusive Access," of a DETERMINE ACCESS PATHS command.
+    27     This  attention  message  is  sent  to all class drivers that are
+    28     "Controller-Online" to the MSCP server and have enabled attention
+    29     messages.   See  Section  4.16.1,  "Multiaccess Drives," for more
+    30     information.
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-44
+           Attention Messages                                   11 June 1992
+
+
+     1     5.8.2  AVAILABLE Attention Message
+
+     2     5.8.2.1  Attention Message Format
+
+     3             31                             0                
+     4             +--------------------------------+              
+     5             |           reserved             |              
+     6             +---------------+----------------+              
+     7             |   reserved    |  unit number   |              
+     8             +---------------+-------+--------+              
+     9             |   reserved    | rsvd  |atncode |              
+    10             +---------------+-------+--------+              
+    11             |  unit flags   | multiunit code |              
+    12             +---------------+----------------+              
+    13             |           undefined            |              
+    14             +--------------------------------+              
+    15             |                                |              
+    16             +---     unit identifier      ---+              
+    17             |                                |              
+    18             +--------------------------------+              
+    19             |     media type identifier      |              
+    20             +---------------+----------------+              
+
+    21  |  The fields in the AVAILABLE attention message are as follows:
+
+    22         unit number
+
+    23             Identifies the unit that just became "Unit-Available." 
+
+    24         atncode
+
+    25             See Section 5.8, "Attention Messages." 
+
+    26         multiunit code
+    27         unit flags
+    28         unit identifier
+    29         media type identifier
+
+    30             Identical to the corresponding fields  in  the  GET  UNIT
+    31             STATUS  or SET UNIT CHARACTERISTICS command end messages.
+    32             These fields shall be valid as defined for the unit being
+    33             "Unit-Available,"  regardless  of the actual state of the
+    34             unit when the message is sent.  That is,  the  "multiunit
+    35             code,"  "unit  identifier,"  and  "media type identifier"
+    36             shall all be valid and the "Removable  Media"  unit  flag
+    37             shall be valid.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-45
+           Attention Messages                                   11 June 1992
+
+
+     1     5.8.2.2  Description
+
+     2     An  MSCP  server  sends  an  AVAILABLE  attention  message  to  a
+     3     "Controller-Online"  class  driver  when  a  unit  asynchronously
+     4     becomes "Unit-Available" to that class driver,  unless  AVAILABLE
+     5     attention  messages  have  been  suppressed  for  that unit by an
+     6     AVAILABLE command with the "Spin-down" modifier or by an error or
+     7     other  condition  with  similar  side  effects.   Changes  to the
+     8     "Unit-Available" state due to the class driver itself issuing  an
+     9     AVAILABLE   command   are  synchronous.   All  other  changes  to
+    10     "Unit-Available"  are  asynchronous.   See  Section  4.3,   "Unit
+    11     States." 
+
+    12     The actual sending of  an  AVAILABLE  attention  message  may  be
+    13     delayed  for  an  arbitrarily  long  time,  due to communications
+    14     mechanism flow control, from the  time  that  the  unit  actually
+    15     becomes  "Unit-Available."   The message shall not be sent if the
+    16     class driver ceases to be "Controller-Online" during this  delay.
+    17     The  message  shall  be sent anyway if the unit, or any unit with
+    18     which it shares an access path, becomes  "Unit-Online"  or  under
+    19     "Exclusive Access" via another controller during this delay.  The
+    20     message may or may not be sent, at the  controller's  option,  if
+    21     the  unit  ceases  to  be  "Unit-Available"  for any other reason
+    22     during this delay.
+
+    23     Note that, due to these delays, it is possible for  an  AVAILABLE
+    24     attention  message  to  be  received  after  the class driver has
+    25     already  brought  the  unit  "Unit-Online"  or  under  "Exclusive
+    26     Access."    Therefore  class  drivers  shall  not  use  AVAILABLE
+    27     attention  messages  to  flag  such  units   as   having   become
+    28     "Unit-Available."   The  proper  procedure is to issue a command,
+    29     such as a GET UNIT STATUS, to a "Unit-Online" unit for  which  an
+    30     AVAILABLE  attention message has been received, and only flag the
+    31     unit as having become "Unit-Available"  if  the  command  returns
+    32     that status code.
+
+    33     AVAILABLE attention messages are not  sent  for  units  that  are
+    34     already  "Unit-Available"  when  a class driver enables attention
+    35     messages.   Class  drivers  that  need  to  be   aware   of   all
+    36     "Unit-Available" units shall enable attention messages, then scan
+    37     all units via the GET UNIT STATUS command with  the  "Next  Unit"
+    38     modifier   set   to   locate   all   units   that   are   already
+    39     "Unit-Available."    All   units   that    subsequently    become
+    40     "Unit-Available"  will  be  reported  with an AVAILABLE attention
+    41     message.
+
+    42     An  MSCP  server  may  send  redundant  or  erroneous   AVAILABLE
+    43     attention  messages  at any time.  The frequency of such messages
+    44     shall be low enough that they  do  not  represent  a  significant
+    45     overhead  for  either hosts or the communications mechanism.  The
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-46
+           Attention Messages                                   11 June 1992
+
+
+     1     information  contained  in  such  messages  (unit  number,   unit
+     2     identifier,  media  type identifier, etc.) shall correspond to an
+     3     actual, physical unit that is  potentially  accessible  via  that
+     4     MSCP  server  (i.e.,  connected  to the controller), although the
+     5     unit need not be "Unit-Available."  Note that hosts must be  able
+     6     to handle seemingly erroneous AVAILABLE attention messages in any
+     7     case, since the unit's state may change before the host  can  act
+     8     on an otherwise correct message.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-47
+           Attention Messages                                   11 June 1992
+
+
+     1     5.8.3  DUPLICATE UNIT NUMBER Attention Message
+
+     2     5.8.3.1  Attention Message Format
+
+     3                     31                             0 
+     4                    +--------------------------------+
+     5                    |           reserved             |
+     6                    +---------------+----------------+
+     7                    |   reserved    |  unit number   |
+     8                    +---------------+-------+--------+
+     9                    |   reserved    | rsvd  |atncode |
+    10                    +---------------+-------+--------+
+
+    11     The fields in the DUPLICATE UNIT NUMBER attention message are  as
+    12     follows:
+
+    13         unit number
+
+    14             Identifies the unit number that is duplicated on  two  or
+    15             more units.
+
+    16         atncode
+
+    17             See Section 5.8, "Attention Messages." 
+
+    18     5.8.3.2  Description
+
+    19     An MSCP server sends DUPLICATE UNIT NUMBER attention messages  to
+    20     notify hosts that two or more units of the same device class have
+    21     the same unit number.  This allows the hosts to  complain  to  an
+    22     operator,  who  can  correct  the  condition.  The DUPLICATE UNIT
+    23     NUMBER  attention  messages  are  sent  to  all  hosts  that  are
+    24     "Controller-Online"  and  have  enabled  attention messages.  See
+    25     Section 4.4, "Unit Numbers," for a  detailed  discussion  of  the
+    26     handling  of  duplicate unit numbers.  Note that a DUPLICATE UNIT
+    27     NUMBER attention message is sent regardless of whether or not one
+    28     of the units is "Unit-Online" or under "Exclusive Access." 
+
+    29     The actual sending of a DUPLICATE UNIT NUMBER  attention  message
+    30     may   be   delayed   for   an   arbitrarily  long  time,  due  to
+    31     communications mechanism flow control, from  the  time  that  the
+    32     controller  first detects the duplicate unit number.  The message
+    33     shall  not  be  sent  if  the   class   driver   ceases   to   be
+    34     "Controller-Online"  during  this  delay.  The message may or may
+    35     not be sent, at the controller's option, if  the  duplicate  unit
+    36     number condition disappears during this delay.
+
+    37     DUPLICATE  UNIT  NUMBER  attention  messages  are  not  sent  for
+    38     duplicate  unit number conditions that already exist when a class
+    39     driver enables attention messages.  Class drivers that need to be
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-48
+           Attention Messages                                   11 June 1992
+
+
+     1     aware  of  all  duplicate  unit  number  conditions  shall enable
+     2     attention messages, then scan all units via the GET  UNIT  STATUS
+     3     command with the "Next Unit" modifier set to locate all duplicate
+     4     unit numbers.  All duplicate unit  numbers  that  the  controller
+     5     subsequently  detects  will  be  reported  with  a DUPLICATE UNIT
+     6     NUMBER attention message.
+
+     7     An MSCP server may send redundant DUPLICATE UNIT NUMBER attention
+     8     messages.   The  frequency  of  such messages shall be low enough
+     9     that they do not represent  a  significant  overhead  for  either
+    10     hosts   or   the   communications  mechanism.   Furthermore,  the
+    11     duplicate unit number  condition  being  reported  must  actually
+    12     exist  at  the  time  the  MSCP  server  decides  to generate the
+    13     DUPLICATE UNIT NUMBER attention message.  Note, however, that the
+    14     duplicate  unit number condition may have disappeared by the time
+    15     the host receives or acts upon the message.
+
+    16     5.9  Error Log Messages
+
+    17     MSCP controllers report errors and  unusual  occurrences  in  two
+    18     ways:  end messages and error log messages.  Unrecoverable errors
+    19     are reported in the end message of the command  that  encountered
+    20     the  error.   Such  errors should be reported back to the program
+    21     that initiated the  command  so  that  it  can  take  appropriate
+    22     action.   Additionally,  all "significant" errors are reported in
+    23     error log messages so that they can be  recorded  in  the  host's
+    24     error  log  for eventual use by Field Service.  The definition of
+    25     what constitutes  a  "significant"  error  is  controller  and/or
+    26     device  specific.  In general, anything that would be of interest
+    27     to Field Service is a "significant" error.  The  errors  reported
+    28     by error log messages may be either recoverable or unrecoverable.
+    29     Note that hosts should not record errors reported in end messages
+    30     in  the  error  log.   If  the error is "significant," a separate
+    31     error log message will be generated.
+
+    32     When a host receives an error log message, the host  port  driver
+    33     passes the class driver the error log message text and the length
+    34     of the error log message.  In addition, the device  class  (i.e.,
+    35     disk  or  tape)  of  the  error  log  message  is implicit in the
+    36     connection on which the message  was  received.   Note  that  the
+    37     order  of  receipt  of  error  log  messages  relative  to end or
+    38     attention messages is expressly undefined.  Therefore,  an  error
+    39     log message may be received either before or after an end message
+    40     that reports the same error or that has the "Error Log Generated"
+    41     flag set.
+
+    42     MSCP servers assign a sequence number to every error log  message
+    43     they  generate.   The sequence number assigned is reported to the
+    44     host in the "Sequence Number" field in each error log message and
+    45     in  each  command  end message that has the "Error Log Generated"
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-49
+           Error Log Messages                                   11 June 1992
+
+
+     1     flag set.  This sequence number serves the following purposes:
+
+     2       1.  It permits the host  to  determine  when  all  error  log
+     3           messages  associated  with a particular command have been
+     4           received.  The host can then release any of the command's
+     5           context it may have saved for inclusion in its error log.
+     6           An example of how a host might use the error log sequence
+     7           number for this purpose appears later in this section.
+
+     8       2.  If the same error log message is sent to multiple  hosts,
+     9           which  then  record  it  in  a  common error log file, it
+    10           allows the multiple copies of  the  same  message  to  be
+    11           recognized as describing the same error.
+
+    12       3.  If a host  requests  that  it  receive  every  error  log
+    13           message  that  an  MSCP  server generates, it allows that
+    14           host to detect missing or  lost  error  log  messages  by
+    15           detecting gaps in the sequence numbers.  In this case the
+    16           host is required to set all three error log enable  flags
+    17           in  the  "controller  flags"  field of the SET CONTROLLER
+    18           CHARACTERISTICS command  (see  Section  5.6,  "Controller
+    19           Flags").
+
+
+    20     Some controllers can achieve these purposes via other means,  and
+    21     therefore  need  not  implement  error log sequence numbers.  The
+    22     requirements such controllers must meet are  described  later  in
+    23     this section.
+
+    24     Each MSCP server implements a single error log  sequence  number,
+    25     which  it  uses for all error log messages for all class drivers.
+    26     The server increments its sequence number each time  it  attempts
+    27     to  generate  an  error log message.  (Note that sequence numbers
+    28     may wrap around, so that sequence number 0 comes  after  sequence
+    29     number 65535.)
+
+    30     Multiple copies of the same error log message shall all have  the
+    31     same  sequence  number.   On each individual connection, the MSCP
+    32     server shall send error log messages in the same order  as  their
+    33     sequence  numbers.  The relative order of error log messages sent
+    34     on different connections is undefined (and therefore a controller
+    35     option).
+
+    36     Whenever an MSCP server has pending  error  log  messages  for  a
+    37     connection,  it shall send at least one error log message on that
+    38     connection within the controller timeout interval.
+
+    39     An MSCP server has pending error log messages  for  a  connection
+    40     whenever it has notified the class driver on that connection that
+    41     one or more error log messages are  forthcoming  (i.e.,  set  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-50
+           Error Log Messages                                   11 June 1992
+
+
+     1     "Error  Log  Generated"  flag in an end message), but has not yet
+     2     sent or dropped all error  log  messages  up  to  the  one  whose
+     3     sequence  number  was  reported  in  the  end  message.  The term
+     4     "dropped" refers to the MSCP server determining that it will  not
+     5     send a message due to lack of some internal resource.
+
+     6     The sequence number is  reset  whenever  the  MSCP  server  loses
+     7     context.   The  first  error  log  message  after  such a loss of
+     8     context has sequence number zero.  The sequence number shall  not
+     9     be  reset  as  a  normal  result  of  the  MSCP  server  becoming
+    10     "Controller-Online" or "Controller-Available" to a class  driver.
+    11     Note  that  each  MSCP  server (i.e., each device class) within a
+    12     controller has its own error log sequence number.
+
+    13     As stated above, the error log sequence number is only reset when
+    14     the  MSCP server loses context.  The MSCP server reports the fact
+    15     that it has lost context by setting the "Sequence  Number  Reset"
+    16     flag  (see  Section 5.9.2, "Generic Error Log Message Format") in
+    17     the first error log message it sends to each class  driver  after
+    18     said  loss  of  context.   This  means,  in effect, that the MSCP
+    19     server must keep track of all class drivers to which it has  sent
+    20     an  error  log message since its last loss of context, regardless
+    21     of whether or not it is currently  "Controller-Online"  to  those
+    22     class drivers.
+
+    23     An example of how a host might use the error log sequence  number
+    24     to  correlate error log messages with outstanding command context
+    25     follows.
+
+    26         The class driver keeps track of the  sequence  number  of
+    27         the  most recent error log message it has received.  This
+    28         sequence number should be initialized to  zero  when  the
+    29         connection  is  established.  Furthermore, in use command
+    30         slots (containing command context) come in two  forms  --
+    31         active  commands  and  commands  waiting  for  error  log
+    32         messages.  The class driver also keeps track  of  whether
+    33         or  not  it has received an error log message within each
+    34         controller  timeout  interval  for  its  command  timeout
+    35         algorithm.
+
+    36         When the class driver receives an end message,  it  first
+    37         checks  the  "Error Log Generated" flag.  If that flag is
+    38         clear, it releases the command slot as no longer in  use.
+    39         If  that flag is set, it calls the RELEASE_SLOT procedure
+    40         (see pseudo-code below) with the "sequence  number"  from
+    41         the  command's end message and the sequence number of the
+    42         most recent  error  log  message  the  class  driver  has
+    43         received.  If RELEASE_SLOT returns TRUE, the class driver
+    44         releases the command slot as no longer in use.   If  that
+    45         procedure  returns FALSE, the class driver marks the slot
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-51
+           Error Log Messages                                   11 June 1992
+
+
+     1         as waiting for error log messages.
+
+     2         When the class driver receives an error log  message,  it
+     3         calls  the  RELEASE_SLOT  procedure,  with  the error log
+     4         sequence number from the command slot  (copied  from  the
+     5         command's end message) and the sequence number of the new
+     6         error log message, once for each  command  slot  that  is
+     7         waiting  for error log messages.  If RELEASE_SLOT returns
+     8         TRUE, the class driver releases the command  slot  as  no
+     9         longer  in  use.  If RELEASE_SLOT returns FALSE, the slot
+    10         remains marked as waiting for error log messages.
+
+    11         Command slots that are waiting  for  error  log  messages
+    12         participate   in   the  class  driver's  command  timeout
+    13         algorithm (see Section 4.10, "Command Timeouts") just the
+    14         same  as  slots  for outstanding commands.  If the oldest
+    15         outstanding  command  slot  is  waiting  for  error   log
+    16         messages,  the  class  driver  checks  if  any  error log
+    17         messages have been received within the controller timeout
+    18         interval.   If  one  or more error log messages have been
+    19         received, the slot remains  in  use.   If  no  error  log
+    20         messages have been received, the command slot is released
+    21         -- the error log message for which the slot  was  waiting
+    22         has been lost.
+
+    23         The RELEASE_SLOT procedure accepts an error log  sequence
+    24         number  from an end message and a sequence number from an
+    25         error log message.  It returns  TRUE  if  the  error  log
+    26         number  comes  after  the  end  message  number and FALSE
+    27         otherwise.  This comparison takes into  account  sequence
+    28         number wrap-around.  The procedure (pseudo-code) is:
+
+    29           function RELEASE_SLOT (
+    30                 end_msg_number : 0..65535 ;
+    31                 err_log_number : 0..65535 ) : boolean ;
+    32           const
+    33                 FUDGE = 1 ;
+    34           var
+    35                 i : -65535..65535 ;
+    36           begin ;
+    37                 { following three lines are just a 16 bit }
+    38                 { signed   subtract  operation,  ignoring }
+    39                 { integer  overflow (i.e., the PDP-11 SUB }
+    40                 { instruction). }
+    41                 i := err_log_number - end_msg_number ;
+    42                 if i < -32768 then i := i + 65536 ;
+    43                 if i > 32767 then i := i - 65536 ;
+    44                 RELEASE_SLOT := (i >= FUDGE-1) ;
+    45           end ;
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-52
+           Error Log Messages                                   11 June 1992
+
+
+     1         The   constant   FUDGE   is   the   maximum   degree   of
+     2         communications  mechanism nonsequentiality for datagrams.
+     3         Assume that datagrams n, n+1, n+2, etc.  are sent in that
+     4         order  across  a connection.  FUDGE is the smallest value
+     5         for which the communications mechanism  guarantees  that,
+     6         if  both  datagram  n  and datagram n+FUDGE are received,
+     7         then datagram n is received before datagram n+FUDGE.  All
+     8         current   and  proposed  communications  mechanisms  have
+     9         sequential datagrams, for which FUDGE is one.   The  only
+    10         type  of  communications  mechanism for which FUDGE might
+    11         have a different value is one  that  has  multiple  paths
+    12         between   nodes,   and  allows  datagrams  for  a  single
+    13         connection to follow different paths.
+
+    14     As mentioned earlier, controllers need not  implement  error  log
+    15     sequence  numbers, since their purposes can be achieved via other
+    16     means.  Controllers that meet all of the  following  requirements
+    17     need not implement error log sequence numbers:
+
+    18          1.  The MSCP server never drops error  log  messages.   That
+    19              is, whenever it has an error log message to generate, it
+    20              blocks or deadlocks until it is  able  to  generate  the
+    21              message.
+
+    22          2.  The communications mechanism between the MSCP server and
+    23              the  class driver guarantees all error log messages will
+    24              be delivered without loss or duplication  in  the  order
+    25              that  they  were generated.  That is, the communications
+    26              mechanism provides the  same  guarantees  for  datagrams
+    27              (error  log  messages)  that  it provides for sequential
+    28              messages (control messages).
+
+    29          3.  The MSCP server and communications  mechanism  guarantee
+    30              that  the  class  driver  will  receive  all  error  log
+    31              messages that reference a particular command's reference
+    32              number before receiving that command's end message.
+
+    33          4.  The  MSCP  server  and  communications   mechanism   are
+    34              inherently incapable of communicating with more than one
+    35              class driver.
+
+    36     MSCP servers that meet the above four requirements  may  generate
+    37     all  error  log  messages with a sequence number of zero and with
+    38     the "Sequence  Number  Reset"  flag  set,  rather  than  actually
+    39     implementing  sequence  numbers.   All  other  MSCP servers shall
+    40     implement an actual error log sequence number.  Such  other  MSCP
+    41     servers  may  not  lose  context  solely  to  reset the error log
+    42     sequence number.  All losses of context must be  caused  by  some
+    43     external event such as a power failure.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-53
+           Error Log Messages                                   11 June 1992
+
+
+     1     Since error log messages are not subject  to  flow  control  (see
+     2     Section  3.2, "Flow Control"), it is possible for a controller to
+     3     generate error log messages faster than a host can record them in
+     4     its  error  log.   In  order  to minimize the probability of this
+     5     happening, and thus minimize the probability of losing error  log
+     6     information,  controllers shall generate no more than three error
+     7     log messages in response  to  a  single  error.   Note  that  the
+     8     definition of what constitutes an error is necessarily controller
+     9     and/or drive dependent.  The three message limit  encompasses  an
+    10     original  error  and  all  error  recovery  /  correction / retry
+    11     sequences associated with that error.  Seemingly unrelated errors
+    12     that  occur in a recovery sequence are generally considered to be
+    13     different errors, and are therefore  not  covered  by  the  three
+    14     message limit.
+
+    15     For example, consider an  uncorrectable  ECC  error  on  a  read.
+    16     Rereads  using  offset  head positioning and the like are part of
+    17     the retry sequence, and thus fall under the three  message  limit
+    18     for   the  original  error.   However,  failure  of  the  command
+    19     directing the drive to use offset  head  positioning  (i.e.,  the
+    20     command  itself  fails,  indicating  that  the heads could not be
+    21     offset) would be considered a separate error, even though both it
+    22     and  the original read error might have a common cause (such as a
+    23     bad cable between the controller and the drive).
+
+    24     In order to achieve this three  message  limit,  most  error  log
+    25     messages summarize the results of an entire retry sequence.  This
+    26     approach generally results in exactly one error log  message  per
+    27     error.   The  other  approach is to generate a separate error log
+    28     message for each attempt or retry that fails.  This approach  can
+    29     only be used with errors for which the number of retries is small
+    30     (i.e., three or fewer attempts total),  as  otherwise  the  three
+    31     message limit will be exceeded.
+
+    32     5.9.1  Maximum Error Log Message Size
+
+    33     MSCP and TMSCP do not place any explicit  limit  on  the  maximum
+    34     size  of  an error log message.  Rather, they simply require that
+    35     in any supported or operational configuration, the maximum  error
+    36     log  message  size  generated  by a device's MSCP or TMSCP server
+    37     must be less than or equal to the maximum supported by  the  host
+    38     class driver and any intervening adapters.  In essence, error log
+    39     message size limits are outside the scope of this specification.
+
+    40     In order to assist in determining if maximum  error  log  message
+    41     sizes  are compatible in a particular configuration, all MSCP and
+    42     TMSCP entities shall register their error log message size limits
+    43     with  the SSAG Architectural Compliance Registry.  Information on
+    44     procedures for using the registry may be obtained by sending mail
+    45     to SSAG::SSAG.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-54
+           Error Log Messages                                   11 June 1992
+
+
+     1     Each MSCP or TMSCP class driver shall register the maximum  error
+     2     log  message  size  they support.  They shall list multiple sizes
+     3     with appropriate qualification if they support different sizes in
+     4     different  circumstances,  such as with different port drivers or
+     5     adapters.  Any limits imposed by adapters shall  be  included  in
+     6     the  class driver's registered limits, since such limitations are
+     7     extremely few in number.
+
+     8     Each MSCP or TMSCP server shall register the maximum size of  any
+     9     error  log  message  it  may  generate.  It is reasonable to list
+    10     multiple sizes if class drivers and/or system configurations  can
+    11     affect what sizes are used.  For example, if the longest messages
+    12     will only be generated if a particular  option  is  used,  it  is
+    13     reasonable to list limits both with and without that option being
+    14     used.  An MSCP or TMSCP server shall not generate  an  error  log
+    15     message  longer  than  what  it  has  registered,  under whatever
+    16     conditions or qualifications are included  in  its  registration.
+    17     Furthermore,  an  MSCP or TMSCP server may not register a maximum
+    18     error log message size unless a  corresponding  class  driver  is
+    19     already registered as supporting at least that maximum.
+
+    20     The purpose of this last restriction is to make it clear that the
+    21     burden  of  obtaining support for a new, longer maximum error log
+    22     message size lies upon the device wishing  to  use  those  longer
+    23     messages.    The  device  development  group  must  first  obtain
+    24     agreement from some host OS group to support the longer messages,
+    25     leading   to   the  host  OS  group  changing  its  Architectural
+    26     Compliance Registry entry to indicate the longer size.  Only then
+    27     may  the  device  group register its device and begin to generate
+    28     the longer messages.
+
+    29     Notwithstanding the above requirements, the following  guidelines
+    30     should be followed by all new or significantly modified products.
+    31     Adherence  to  these  guidelines  will   minimize   support   and
+    32     compatibility problems with regard to error log message sizes.
+
+
+    33          1.  For SSP devices, all class drivers should be designed to
+    34              support a maximum error log message size of 124 bytes or
+    35              larger.  This corresponds to  a  128  byte  SSP  message
+    36              buffer  less  4  bytes overhead.  Note, however, that at
+    37              the time this was written the KFQSA  adapter  imposed  a
+    38              limit of 88 bytes on MSCP or TMSCP error log messages.
+
+    39          2.  For all other  devices,  all  class  drivers  should  be
+    40              designed  to support a maximum error log message size of
+    41              512 bytes or larger.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-55
+           Error Log Messages                                   11 June 1992
+
+
+     1          3.  For SSP devices, servers should be designed to use error
+     2              log  messages  no longer than 60 bytes.  Longer messages
+     3              may  encounter   compatibility   problems   with   older
+     4              software.
+
+     5          4.  For all other devices, servers should be designed to use
+     6              error  log  messages  no  longer than 128 bytes.  Longer
+     7              messages may encounter compatibility problems with older
+     8              software.
+
+
+     9     These are  only  recommended  guidelines,  and  may  be  violated
+    10     without  affecting  MSCP  or  TMSCP compliance.  But special care
+    11     should be taken to consider the risk of incompatibility  whenever
+    12     these recommendations are not followed.
+
+    13     As has already been  stated,  in  any  supported  or  operational
+    14     configuration,  the maximum error log message size generated by a
+    15     device's MSCP or TMSCP server must be less than or equal  to  the
+    16     maximum  supported  by  the host class driver and any intervening
+    17     adapters.  The converse  of  this  is  that  if  a  device  might
+    18     generate  an  error  log  message  longer than an adapter or host
+    19     class driver supports, then their combination is not a  supported
+    20     configuration  and  need  not be operational.  All the same, such
+    21     mis-configured systems  will  occur  in  practice,  so  defensive
+    22     programming  techniques  should be used to simplify diagnosis and
+    23     minimize problems with mis-configurations.   A  system  crash  or
+    24     weird  behavior  that  only  occurred when a particular error log
+    25     message arrived would be extremely difficult to diagnose.
+
+    26     To  this  end  it  is  strongly  recommended  that  all  entities
+    27     participating  in  an  MSCP  or TMSCP connection explicitly check
+    28     error log message sizes, rather than implicitly assuming they are
+    29     valid.   Ideally an explicit message would be displayed to report
+    30     the mis-configuration, although this may be impractical for  most
+    31     devices.   If  the  communications mechanism allows it, error log
+    32     message sizes should be checked when a connection is  established
+    33     rather  than  waiting  for an overly long error log message to be
+    34     generated.  If a mis-configuration  is  present,  the  connection
+    35     should  be rejected, rather than establishing the connection only
+    36     to have a problem at the unpredictable  time  a  long  error  log
+    37     message is generated.
+
+    38     If an error log message longer than that which  is  supported  is
+    39     nonetheless  generated,  it  should be truncated to the supported
+    40     maximum size on that connection.  The actual truncation may occur
+    41     in  the  server,  class  driver,  an  adapter,  or  anywhere else
+    42     convenient.  With most communications mechanisms, any  truncation
+    43     should be performed by the sender before the error log message is
+    44     sent.  For example, truncation to the SCA maximum  datagram  size
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-56
+           Error Log Messages                                   11 June 1992
+
+
+     1     should  occur in the server sending the error log message, before
+     2     it is passed to SCA.  Similarly, truncation to the size of an SSP
+     3     message  buffer  should  occur in the SSP adapter.  Truncation to
+     4     the maximum size supported by the host's error logging  subsystem
+     5     would normally be done in the host.
+
+     6     The truncated size should be recorded with the error log message,
+     7     so that undefined data is not mistakenly interpreted as belonging
+     8     to the message.  Error log analysis tools might subsequently  use
+     9     device dependent knowledge to recognize that the message has been
+    10     truncated and report the mis-configuration.   Connections  should
+    11     not  be  broken if an overly long error log message is generated,
+    12     nor should such messages be discarded due to their size.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-57
+           Error Log Messages                                   11 June 1992
+
+
+     1     5.9.2  Generic Error Log Message Format
+
+
+
+     2                     31                             0 
+     3                    +--------------------------------+
+     4                    |   command reference number     |
+     5                    +---------------+----------------+
+     6                    |sequence number|  unit number   |
+     7                    +---------------+-------+--------+
+     8                    |  event code   | flags | format |
+     9                    +---------------+-------+--------+
+    10                    |                                |
+    11                    +---  controller identifier   ---+
+    12                    |                                |
+    13                    +---------------+-------+--------+
+    14                    | multiunit code|chvrsn | csvrsn |
+    15                    +---------------+-------+--------+
+    16                    |                                |
+    17                    +---     unit identifier      ---+
+    18                    |                                |
+    19                    +---------------+-------+--------+
+    20                    | fmt dependent |uhvrsn | usvrsn |
+    21                    +---------------+-------+--------+
+    22                    |      volume serial number      |
+    23                    +--------------------------------+
+    24                    |                                |
+    25                    /        format dependent        /
+    26                    /           information          /
+    27                    |                                |
+    28                    +--------------------------------+
+
+    29     The fields in the generic error log message are as follows:
+
+    30         command reference number
+
+    31             The command reference number of  the  MSCP  command  that
+    32             caused  the  error reported by this error log message, or
+    33             zero if the error  does  not  correspond  to  a  specific
+    34             outstanding  command.   If  this field contains a command
+    35             reference number, then the  command's  end  message  will
+    36             also have the "error log generated" end message flag set.
+    37             Note that the error log message may  be  received  either
+    38             before or shortly after the command's end message.
+
+    39         unit number
+
+    40             The unit number of  the  unit  to  which  the  error  log
+    41             message  relates,  or zero if the message does not relate
+    42             to a specific unit.  This  field  may  contain  the  unit
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-58
+           Error Log Messages                                   11 June 1992
+
+
+     1             number of any unit of the drive or formatter if the error
+     2             relates to an entire multiunit drive or  formatter.   The
+     3             validity  of this field is determined by the value in the
+     4             "format" field.
+
+     5         sequence number
+
+     6             The sequence number of this error log message  since  the
+     7             last  time  the  MSCP server lost context, or zero if the
+     8             MSCP  server  does  not  implement  error  log   sequence
+     9             numbers.  Note that error log sequence numbers are common
+    10             to all class drivers, and are not reset by  class  driver
+    11             resynchronization.   Note  also that the class driver may
+    12             receive error log messages out of sequence.
+
+    13         format
+
+    14             The value in this field identifies the detailed format of
+    15             the  error  log  message,  as  defined  in  the following
+    16             sections.
+
+    17             Refer  to  Appendix  A,   "Opcode,   Flag,   and   Offset
+    18             Definitions," Table A-9, "Error Log Message Format Codes"
+    19             for the values assigned to the currently defined formats.
+
+    20         flags
+
+    21             Bit flags, collectively called error log  message  flags,
+    22             used  to  report  various  attributes  of the error.  The
+    23             following flags are defined:
+
+    24                 Bad Block Replacement Request
+
+    25                     If set, the block identified  by  the  error  log
+    26                     message  is  a  bad  block,  and  the  Controller
+    27                     Initiated Bad Block Replacement layer  will  soon
+    28                     attempt  to  replace  it.  Note that this flag is
+    29                     reported  only  by   controllers   that   support
+    30                     Controller  Initiated  Bad  Block Replacement for
+    31                     disk class devices.
+
+    32                 Disk Copy Data Correlate
+
+    33                     If set, the reported error  occurred  during  the
+    34                     execution  of  a DISK COPY DATA command where the
+    35                     source unit is located in a remote  subsystem  as
+    36                     described  in  Section  6.7.3,  "DISK  COPY  DATA
+    37                     Command,  Description."   Note  that  the  server
+    38                     sends  a  DISK COPY DATA CORRELATION error log as
+    39                     described in Section 5.9.9, prior to sending  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-59
+           Error Log Messages                                   11 June 1992
+
+
+     1                     end  message  for  a  particular  DISK  COPY DATA
+     2                     command.    See   that   section   for    related
+     3                     information.
+
+     4                 Error During Replacement
+
+     5                     If set, the reported  error  occurred  during  an
+     6                     access  initiated  by  the  bad block replacement
+     7                     process.  Note that this flag is reported only by
+     8                     controllers that support Controller Initiated Bad
+     9                     Block Replacement for disk class devices.
+
+    10                 Operation Successful
+
+    11                     If set, the  operation  causing  this  error  log
+    12                     message  has  been  successfully  completed.  The
+    13                     error log message summarizes the  retry  sequence
+    14                     that  was  necessary to successfully complete the
+    15                     operation.  If clear, the operation has  not  yet
+    16                     been successfully completed.
+
+    17                 Operation Continuing
+
+    18                     If set, the retry  sequence  for  this  operation
+    19                     will   be  continued.   This  error  log  message
+    20                     reports the unsuccessful  completion  of  one  or
+    21                     more  retries.   If clear, the retry sequence for
+    22                     this   operation   has   terminated.     Provided
+    23                     "Operation  Successful"  is also clear, the retry
+    24                     limit for this operation has been reached and  an
+    25                     unrecoverable  error will be reported.  Undefined
+    26                     (meaningless) if "Operation Successful" is set.
+
+    27                 Sequence Number Reset
+
+    28                     If  set,  then  the  error  log  sequence  number
+    29                     ("sequence  number"  field) has been reset by the
+    30                     MSCP server since the last error log message sent
+    31                     to  the  receiving  class  driver.  If clear, the
+    32                     sequence number has not been reset, implying that
+    33                     the "sequence number" field may be used to detect
+    34                     missing error log messages.  Always  set  if  the
+    35                     MSCP server does not implement error log sequence
+    36                     numbers.
+
+    37                 Informational
+
+    38                     If set, the data  contained  in  this  error  log
+    39                     message  is  informational only.  The information
+    40                     is not necessarily related to any single  command
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-60
+           Error Log Messages                                   11 June 1992
+
+
+     1                     or  event.   Note that the error log message does
+     2                     NOT signify the occurrence of an error and  shall
+     3                     not be included in device error counts.
+
+     4             If "Operation Successful" and "Operation Continuing"  are
+     5             both  clear,  then  the  error log message reports a hard
+     6             (unrecoverable)  error.   If  "Operation  Successful"  is
+     7             clear  and  "Operation Continuing" is set, then the error
+     8             log message reports an intermediate step within an  error
+     9             recovery  operation.   It  is not yet certain whether the
+    10             error is hard or soft.  If "Operation Successful" is set,
+    11             then  the error log message summarizes the retry sequence
+    12             used to recover from a soft error.
+
+    13             Refer  to  Appendix  A,   "Opcode,   Flag,   and   Offset
+    14             Definitions,"  Table  A-10, "Error Log Message Flags" for
+    15             the values assigned to the currently defined flags.
+
+    16         event code
+
+    17             Identifies the specific error or event being reported  by
+    18  |          this  error  log  message.   See Section 5.5, "Status and
+    19  |          Event Codes." 
+
+    20             The subcode portion of  the  event  code  is  potentially
+    21             controller  and/or  device  specific.   However, the same
+    22             major code / subcode combination, whenever  it  is  used,
+    23             must  always  have  the  same  meaning.   Therefore,  new
+    24             subcodes may be defined as new  devices  are  introduced,
+    25             but the meaning of old subcodes should not change.  Event
+    26             code values are listed in Appendix B, "Status  and  Event
+    27             Code Definitions." 
+
+    28         controller identifier
+
+    29             Uniquely identifies  the  controller  among  all  devices
+    30             accessible  via  MSCP.  See Section 4.17, "Controller and
+    31             Unit Identifiers." 
+
+    32         csvrsn
+
+    33             The  controller's  software,   firmware,   or   microcode
+    34             revision   number.    Always   zero   if   the  product's
+    35             development and  maintainability  groups  determine  that
+    36             there  is  no  benefit  from  supporting machine readable
+    37             revision numbers.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-61
+           Error Log Messages                                   11 June 1992
+
+
+     1         chvrsn
+
+     2             The controller's hardware revision number.   Always  zero
+     3             if  the  product's development and maintainability groups
+     4             determine  that  there  is  no  benefit  from  supporting
+     5             machine readable revision numbers.
+
+     6         multiunit code
+
+     7             The  multiunit  code,  as  defined  in  Section   4.16.2,
+     8             "Multiunit  Drives  and Formatters," of the unit to which
+     9             the error log message relates.  This  field  may  contain
+    10             the  multiunit code of any unit of the drive or formatter
+    11             if the error relates to  an  entire  multiunit  drive  or
+    12             formatter.
+
+    13         unit identifier
+
+    14             Uniquely identifies the unit among all devices accessible
+    15             via   MSCP.   See  Section  4.17,  "Controller  and  Unit
+    16             Identifiers."  This field is only present for errors that
+    17             relate to a specific unit.
+
+    18         usvrsn
+
+    19             The unit's  software,  firmware,  or  microcode  revision
+    20             number.   This  field  is  only  present  for errors that
+    21             relate to a specific unit.  Always zero if the  product's
+    22             development  and  maintainability  groups  determine that
+    23             there is no  benefit  from  supporting  machine  readable
+    24             revision numbers.
+
+    25         uhvrsn
+
+    26             The unit's hardware revision number.  This field is  only
+    27             present  for  errors  that  relate  to  a  specific unit.
+    28             Always   zero   if   the   product's   development    and
+    29             maintainability groups determine that there is no benefit
+    30             from supporting machine readable revision numbers.
+
+    31         volume serial number
+
+    32             The low order 32 bits of the serial number of the  volume
+    33             that  is  mounted on the unit.  Zero if the unit's format
+    34             does not provide for a volume serial  number.   Undefined
+    35             (garbage)  if there is no volume mounted in the unit, the
+    36             area of the volume that contains the serial number cannot
+    37             be  read  successfully,  the  error  occurred  before the
+    38             volume serial number could be determined  while  bringing
+    39             the  unit "Unit-Online," the unit is not "Unit-Online" to
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-62
+           Error Log Messages                                   11 June 1992
+
+
+     1             any host, or if the "Ignore Media Format Error"  modifier
+     2             was  specified  in  the ONLINE command that first brought
+     3             the unit "Unit-Online."  This field is only  present  for
+     4             errors that relate to a specific disk unit.
+
+     5         fmt dependent
+     6         format dependent information
+
+     7             The format of the remainder  of  the  error  log  message
+     8             depends  upon the value of the "format" field.  Note that
+     9             the fields "unit number"  and  "multiunit  code"  through
+    10             "volume  serial  number"  also depend on the value of the
+    11             "format" field,  as  they  are  only  present  for  those
+    12             formats  used  to report errors that relate to a specific
+    13             unit.
+
+    14     Appendix A, "Opcode, Flag, and Offset  Definitions,"  Table  A-8,
+    15     "Error  Log  Message  Offsets" defines the preferred mnemonic and
+    16     numeric value of the offset within the error log message  of  the
+    17     various  error  log parameter fields, as well as the size of each
+    18     field.
+
+    19     The following sections describe the specific  error  log  message
+    20     formats.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-63
+           Error Log Messages                                   11 June 1992
+
+
+     1     5.9.3  CONTROLLER ERRORS Error Log Format
+
+     2     The  following  error  log  message  format  is  used  to  report
+     3     controller errors:
+
+     4                     31                             0 
+     5                    +--------------------------------+
+     6                    |   command reference number     |
+     7                    +---------------+----------------+
+     8                    |sequence number|    reserved    |
+     9                    +---------------+-------+--------+
+    10                    |  event code   | flags | format |
+    11                    +---------------+-------+--------+
+    12                    |                                |
+    13                    +---  controller identifier   ---+
+    14                    |                                |
+    15                    +---------------+-------+--------+
+    16                    |               |chvrsn | csvrsn |
+    17                    |               +-------+--------+
+    18                    |          controller            |
+    19                    /          dependent             /
+    20                    /          information           /
+    21                    |                                |
+    22                    +--------------------------------+
+
+    23     The fields in the CONTROLLER ERRORS  error  log  message  are  as
+    24     follows:
+
+    25         command reference number
+    26         sequence number
+    27         format
+    28         flags
+    29         event code
+    30         controller identifier
+    31         csvrsn
+    32         chvrsn
+
+    33             See Section 5.9.2, "Generic Error Log Message Format." 
+
+    34         controller dependent information
+
+    35             A variable (controller dependent) amount of  information.
+    36             Often  no  controller  dependent information is provided.
+    37             The length of this information is implied  by  the  total
+    38             length  of  the  error  log  message, passed to the class
+    39             driver  by  the  port  driver.   This  information   will
+    40             typically  not  be  interpreted  by  error log formatting
+    41             programs, instead being printed as a series of values.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-64
+           Error Log Messages                                   11 June 1992
+
+
+     1     5.9.4  MEMORY ERRORS Error Log Format
+
+     2     The following error log message format is used to  report  memory
+     3     errors when the address of the offending location is available to
+     4     the controller.  This format is used for either host or  internal
+     5     controller memory access errors.  The format is:
+
+     6                     31                             0
+     7                    +--------------------------------+
+     8                    |   command reference number     |
+     9                    +---------------+----------------+
+    10                    |sequence number|    reserved    |
+    11                    +---------------+-------+--------+
+    12                    |  event code   | flags | format |
+    13                    +---------------+-------+--------+
+    14                    |                                |
+    15                    +---  controller identifier   ---+
+    16                    |                                |
+    17                    +---------------+-------+--------+
+    18                    |   reserved    |chvrsn | csvrsn |
+    19                    +---------------+-------+--------+
+    20                    |        memory address          |
+    21                    +--------------------------------+
+    22                    |                                |
+    23                    /          controller            /
+    24                    /          dependent             /
+    25                    /          information           /
+    26                    /          (optional)            /
+    27                    |                                |
+    28                    +--------------------------------+
+
+    29     The fields in the MEMORY ERRORS error log message are as follows:
+
+    30         command reference number
+    31         sequence number
+    32         format
+    33         flags
+    34         event code
+    35         controller identifier
+    36         csvrsn
+    37         chvrsn
+
+    38             See Section 5.9.2, "Generic Error Log Message Format." 
+
+    39         event code
+
+    40             The  event  code  identifies  whether  the  error   being
+    41             reported  occurred  in  host  or  controller memory.  For
+    42             errors  in  host  memory  the  event  code  contains   an
+    43             appropriate "Host Buffer Access Error" subcode (see Table
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-65
+           Error Log Messages                                   11 June 1992
+
+
+     1             B-6, "'Host Buffer Error' Status or  Event  Code  Subcode
+     2             Values").  For errors in controller memory the event code
+     3             contains an appropriate "Controller Error"  subcode  (see
+     4             Table  B-7,  "'Controller  Error'  Status  or  Event Code
+     5             Subcode Values").
+
+     6         memory address (host memory access error)
+
+     7             For errors accessing host  memory  the  "memory  address"
+     8             field contains the address in the host's physical address
+     9             space at which the error occurred.
+
+    10             The units of "memory address" are those  natural  to  the
+    11             host  --  bytes for 16 and 32 bit systems or words for 36
+    12             bit  systems.   The  resolution  to  which  a  controller
+    13             identifies  the  address  at  which the error occurred is
+    14             controller dependent,  and  shall  be  described  in  the
+    15             controller's  functional specification.  Disk controllers
+    16             will typically provide a resolution of one disk block.
+
+    17         memory address (controller memory access error)
+
+    18             For  errors  accessing  controller  memory  the   "memory
+    19             address"  field contains the location in the controller's
+    20             memory at which the error occurred.
+
+    21             Only errors that do not affect the  controller's  ability
+    22             to  properly  generate  end  and  error  log messages are
+    23             reported via MSCP and this error log format.  Errors that
+    24             might  affect end and error log messages are reported via
+    25             some mechanism other than MSCP.
+
+    26             The units of "memory address" are those  natural  to  the
+    27             memory  in  which  the  error occurred.  This necessarily
+    28             implies that the units are memory dependent.   Bytes  are
+    29             the preferred units whenever there is any ambiguity as to
+    30             what are the natural units.  The resolution to which  the
+    31             address   of   the  error  is  reported  is  also  memory
+    32             dependent.  It is acceptable for some memories to have no
+    33             address resolution.  That is, errors in such memories are
+    34             reported with a fixed address of zero regardless  of  the
+    35             address at which the error occurred.  This will typically
+    36             be used for memories  in  which  any  error  renders  the
+    37             entire memory unusable.
+
+    38             Controllers  with  multiple  memories  may  identify  the
+    39             particular  memory  in which the error occurred either by
+    40             embedding the  identification  (as  a  subfield)  in  the
+    41             "memory address" field or by providing the identification
+    42             in   the   "controller   dependent   information"   field
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-66
+           Error Log Messages                                   11 June 1992
+
+
+     1             (described   below).   The  units  of  "memory  address,"
+     2             address  resolution,  and  memory  identification  (where
+     3             required) are controller dependent and shall be described
+     4             in the controller's functional specification.
+
+     5         controller dependent information
+
+     6             An optional, variable length field containing  controller
+     7             dependent  information.   The  length  of  this  field is
+     8             implied by the total length of  the  error  log  message,
+     9             passed  to  the  class  driver  by the port driver.  This
+    10             field may be used to provide additional information  when
+    11             reporting  either  controller  or  host  memory errors or
+    12             both.  The use and contents shall  be  described  in  the
+    13             controller's functional specification.
+
+
+
+
+
+
+
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+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-67
+           Error Log Messages                                   11 June 1992
+
+
+     1     5.9.5  DISK TRANSFER ERRORS Error Log Format
+
+     2     The following error log message format is used to  report  errors
+     3     that  occur  during  a  disk  transfer.  Note that this format is
+     4     generally used to report the results of a sequence of retries.
+
+     5                     31                             0 
+     6                    +--------------------------------+
+     7                    |   command reference number     |
+     8                    +---------------+----------------+
+     9                    |sequence number|  unit number   |
+    10                    +---------------+-------+--------+
+    11                    |  event code   | flags | format |
+    12                    +---------------+-------+--------+
+    13                    |                                |
+    14                    +---  controller identifier   ---+
+    15                    |                                |
+    16                    +---------------+-------+--------+
+    17                    |multiunit code |chvrsn | csvrsn |
+    18                    +---------------+-------+--------+
+    19                    |                                |
+    20                    +---     unit identifier      ---+
+    21                    |                                |
+    22                    +-------+-------+-------+--------+
+    23                    | retry | level |uhvrsn | usvrsn |
+    24                    +-------+-------+-------+--------+
+    25                    |      volume serial number      |
+    26                    +--------------------------------+
+    27                    |          header code           |
+    28                    +--------------------------------+
+    29                    |                                |
+    30                    /      controller or disk        /
+    31                    /     dependent information      /
+    32                    |                                |
+    33                    +--------------------------------+
+
+    34     The fields in the DISK TRANSFER ERRORS error log message  are  as
+    35     follows:
+
+    36         command reference number
+    37         unit number
+    38         sequence number
+    39         format
+    40         flags
+    41         event code
+    42         controller identifier
+    43         csvrsn
+    44         chvrsn
+    45         multiunit code
+    46         unit identifier
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-68
+           Error Log Messages                                   11 June 1992
+
+
+     1         usvrsn
+     2         uhvrsn
+     3         volume serial number
+
+     4             See Section 5.9.2, "Generic Error Log Message Format." 
+
+     5         level
+
+     6             The error recovery level used for the most recent attempt
+     7             at  the  transfer.   The error recovery level is a device
+     8             dependent  encoding  of  the   special   error   recovery
+     9             procedures, such as offset head positioning, used for the
+    10             most recent transfer attempt.  The values  zero  and  255
+    11             (all ones) are reserved to indicate that no special error
+    12             recovery procedures were used.
+
+    13         retry
+
+    14             The retry count, within the current error recovery level,
+    15             of  the  most recent attempt at the transfer.  This value
+    16             starts at one for the first attempt  using  a  particular
+    17             error  recovery  level and increments for each subsequent
+    18             attempt at the same level.  This  continues  up  to  some
+    19             drive dependent maximum, at which time the retry count is
+    20             reset to one and the next error recovery level  (if  any)
+    21             is tried.
+
+    22         header code
+
+    23             Identifies the physical disk location at which the  error
+    24             occurred.   If the high four bits are 0000 (binary), then
+    25             the low 28 bits are the logical block number at which the
+    26             error occurred.  If the high four bits are 0110 (binary),
+    27             then the low 28 bits are the replacement block number  at
+    28             which the error occurred.  All other patterns of the high
+    29             four bits of "header code" are reserved, and may  not  be
+    30             returned  without  an ECO to this specification to define
+    31             their interpretation.  The 28 bit logical or  replacement
+    32             block  number  may be decomposed, using the disk geometry
+    33             parameters returned by the GET UNIT  STATUS  command,  to
+    34             obtain the cylinder, group, track, and sector position at
+    35             which the error occurred.
+
+    36         controller or disk dependent information
+
+    37             A variable  (controller  or  disk  dependent)  amount  of
+    38             information.   Often  no  controller  or  disk  dependent
+    39             information is provided.  The length of this  information
+    40             is  implied by the total length of the error log message,
+    41             passed to the class driver  by  the  port  driver.   This
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-69
+           Error Log Messages                                   11 June 1992
+
+
+     1             information  will  typically  not be interpreted by error
+     2             log formatting  programs,  instead  being  printed  as  a
+     3             series of values.
+
+     4     5.9.6  SDI ERRORS Error Log Format
+
+     5     The following error log message format is used by  Standard  Disk
+     6     Interconnect  (SDI)  disk  controllers  to  report drive detected
+     7     errors and SDI communication errors.  Note  that  the  controller
+     8     retries  these errors only once or twice, so a separate error log
+     9     message will be generated for each attempt that fails.
+
+    10                     31                             0 
+    11                    +--------------------------------+
+    12                    |   command reference number     |
+    13                    +---------------+----------------+
+    14                    |sequence number|  unit number   |
+    15                    +---------------+-------+--------+
+    16                    |  event code   | flags | format |
+    17                    +---------------+-------+--------+
+    18                    |                                |
+    19                    +---  controller identifier   ---+
+    20                    |                                |
+    21                    +---------------+-------+--------+
+    22                    |multiunit code |chvrsn | csvrsn |
+    23                    +---------------+-------+--------+
+    24                    |                                |
+    25                    +---     unit identifier      ---+
+    26                    |                                |
+    27                    +---------------+-------+--------+
+    28                    |   reserved    |uhvrsn | usvrsn |
+    29                    +---------------+-------+--------+
+    30                    |      volume serial number      |
+    31                    +--------------------------------+
+    32                    |          header code           |
+    33                    +--------------------------------+
+    34                    |                                |
+    35                    +---       SDI status         ---+
+    36                    |          information           |
+    37                    +---       (12 bytes)         ---+
+    38                    |                                |
+    39                    +--------------------------------+
+
+    40     The fields in the SDI ERRORS error log message are as follows:
+
+    41         command reference number
+    42         unit number
+    43         sequence number
+    44         format
+    45         flags
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-70
+           Error Log Messages                                   11 June 1992
+
+
+     1         event code
+     2         controller identifier
+     3         csvrsn
+     4         chvrsn
+     5         multiunit code
+     6         unit identifier
+     7         usvrsn
+     8         uhvrsn
+     9         volume serial number
+
+    10             See Section 5.9.2, "Generic Error Log Message Format." 
+
+    11         header code
+
+    12             Identifies the physical disk location at which the  error
+    13             occurred.   If the high four bits are 0000 (binary), then
+    14             the low 28 bits are the logical block number at which the
+    15             error occurred.  If the high four bits are 0110 (binary),
+    16             then the low 28 bits are the replacement block number  at
+    17             which the error occurred.  All other patterns of the high
+    18             four bits of "header code" are reserved, and may  not  be
+    19             returned  without  an ECO to this specification to define
+    20             their interpretation.  The 28 bit logical or  replacement
+    21             block  number  may be decomposed, using the disk geometry
+    22             parameters returned by the GET UNIT  STATUS  command,  to
+    23             obtain the cylinder, group, track, and sector position at
+    24             which the error occurred.
+
+    25         SDI status information
+
+    26             Twelve bytes of status information returned  by  the  SDI
+    27             GET  STATUS  command or by the SDI UNSUCCESSFUL response.
+    28             The unit number information returned by the  SDI  command
+    29             or  response  is  not  included,  as  that information is
+    30             provided elsewhere in the error log message.   Otherwise,
+    31             all  of  the SDI status information is included.  See the
+    32             SDI specification for the format of this information.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-71
+           Error Log Messages                                   11 June 1992
+
+
+     1     5.9.7  SMALL DISK ERRORS Error Log Format
+
+     2     The following error log message format is used  by  low-end  disk
+     3     controllers  to report drive detected errors and other errors not
+     4     amenable to the Disk Transfer Error  error  log  message  format.
+     5     Note  that  this  format can only be used with disks that have no
+     6     more than 65535 cylinders.
+
+     7                     31                             0 
+     8                    +--------------------------------+
+     9                    |   command reference number     |
+    10                    +---------------+----------------+
+    11                    |sequence number|  unit number   |
+    12                    +---------------+-------+--------+
+    13                    |  event code   | flags | format |
+    14                    +---------------+-------+------- +
+    15                    |                                |
+    16                    +---  controller identifier   ---+
+    17                    |                                |
+    18                    +---------------+-------+--------+
+    19                    |multiunit code |chvrsn | csvrsn |
+    20                    +---------------+-------+--------+
+    21                    |                                |
+    22                    +---     unit identifier      ---+
+    23                    |                                |
+    24                    +---------------+-------+--------+
+    25                    |   cylinder    |uhvrsn | usvrsn |
+    26                    +---------------+-------+--------+
+    27                    |      volume serial number      |
+    28                    +--------------------------------+
+    29                    |         controller or          |
+    30                    /         drive dependent        /
+    31                    /          information           /
+    32                    |                                |
+    33                    +--------------------------------+
+
+    34     The fields in the SMALL DISK ERRORS  error  log  message  are  as
+    35     follows:
+
+    36         command reference number
+    37         unit number
+    38         sequence number
+    39         format
+    40         flags
+    41         event code
+    42         controller identifier
+    43         csvrsn
+    44         chvrsn
+    45         multiunit code
+    46         unit identifier
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-72
+           Error Log Messages                                   11 June 1992
+
+
+     1         usvrsn
+     2         uhvrsn
+     3         volume serial number
+
+     4             See Section 5.9.2, "Generic Error Log Message Format." 
+
+     5         cylinder
+
+     6             The cylinder to which the current transfer  is  directed.
+     7             This field may or may not be meaningful, depending on the
+     8             value in "event code." 
+
+     9         controller or drive dependent information
+
+    10             A variable (controller  or  drive  dependent)  amount  of
+    11             information.   The  length of this information is implied
+    12             by the total length of the error log message,  passed  to
+    13             the  class  driver  by the port driver.  This information
+    14             will typically not be interpreted by error log formatting
+    15             programs, instead being printed as a series of values.
+
+
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-73
+           Error Log Messages                                   11 June 1992
+
+
+     1     5.9.8  BAD BLOCK REPLACEMENT ATTEMPT Error Log Format
+
+     2     The following error log message format is used by the (Controller
+     3     Initiated)  Bad Block Replacement layer to report completion of a
+     4     bad block replacement attempt.   Note  that  a  message  of  this
+     5     format  is  always generated regardless of the success or failure
+     6     of the replacement attempt.
+
+     7                     31                             0 
+     8                    +--------------------------------+
+     9                    |   command reference number     |
+    10                    +---------------+----------------+
+    11                    |sequence number|  unit number   |
+    12                    +---------------+-------+--------+
+    13                    |  event code   | flags | format |
+    14                    +---------------+-------+--------+
+    15                    |                                |
+    16                    +---  controller identifier   ---+
+    17                    |                                |
+    18                    +---------------+-------+--------+
+    19                    |multiunit code |chvrsn | csvrsn |
+    20                    +---------------+-------+--------+
+    21                    |                                |
+    22                    +---     unit identifier      ---+
+    23                    |                                |
+    24                    +---------------+-------+--------+
+    25                    | replace flags |uhvrsn | usvrsn |
+    26                    +---------------+-------+--------+
+    27                    |      volume serial number      |
+    28                    +--------------------------------+
+    29                    |            Bad LBN             |
+    30                    +--------------------------------+
+    31                    |            Old RBN             |
+    32                    +--------------------------------+
+    33                    |            New RBN             |
+    34                    +---------------+----------------+
+    35                                    |     cause      |
+    36                                    +----------------+
+
+    37     The fields in the BAD BLOCK REPLACEMENT ATTEMPT error log message
+    38     are as follows:
+
+    39         unit number
+    40         sequence number
+    41         format
+    42         flags
+    43         controller identifier
+    44         csvrsn
+    45         chvrsn
+    46         multiunit code
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-74
+           Error Log Messages                                   11 June 1992
+
+
+     1         unit identifier
+     2         usvrsn
+     3         uhvrsn
+     4         volume serial number
+
+     5             See Section 5.9.2, "Generic Error Log Message Format." 
+
+     6         command reference number
+
+     7             Either zero  or  the  command  reference  number  of  the
+     8             command  that  caused  the  bad  block to be detected and
+     9             replaced.  Controllers need not save this value, in which
+    10             case this field will always be zero.
+
+    11         event code
+
+    12             Reports  the  completion  status   of   the   bad   block
+    13             replacement attempt.  The major code is always "Bad Block
+    14             Completion."  Subcodes are defined in  Table  B-9,  "'Bad
+    15             Block   Replacement   Completion'   Event   Only  Subcode
+    16             Values." 
+
+    17         replace flags
+
+    18             Bit flags reporting in detail  the  outcome  of  the  bad
+    19             block  replacement  attempt.   See Table A-11, "Bad Block
+    20             Replacement Attempt 'Replace Flags'" for the  definitions
+    21             of those flags.
+
+    22         Bad LBN
+
+    23             The LBN that was the target of the replacement attempt.
+
+    24         Old RBN
+
+    25             If the "Bad RBN" replace flag is set, the RBN with  which
+    26             the Bad LBN was formerly replaced.  Zero otherwise.
+
+    27         New RBN
+
+    28             If the "Replacement Attempted" replace flag is  set,  the
+    29             RBN  with  which  the  Bad  LBN  is  now  replaced.  Zero
+    30             otherwise.
+
+    31         Cause
+
+    32             The event code from the original error  that  caused  the
+    33             replacement  to be attempted.  Zero if that event code is
+    34             not available.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-75
+           Error Log Messages                                   11 June 1992
+
+
+     1     5.9.9  DISK COPY DATA CORRELATION Error Log Format
+
+
+     2     The following error log message format is used to report specific
+     3     context related to errors detected during the execution of a DISK
+     4     COPY DATA command.  See Section 6.7.3, "DISK COPY  DATA  Command,
+     5     Description."
+
+     6     This error log shall be generated if one or more error  logs  was
+     7     generated  by  a  remote  source.   In  this case, the event code
+     8     returned  will  be  Informational  (subcode   "Disk   Copy   Data
+     9     Correlation").   The  error  logs  generated by the remote source
+    10     will have the Disk Copy Data Correlate error  log  flag  set  and
+    11     will   have   the   same   "command  reference  number"  as  this
+    12     Informational error log.  This error log allows  all  error  logs
+    13     related  to  such  a DISK COPY DATA command to be correlated with
+    14     that command's context.
+
+    15     This error log is also sent for certain classes of  fatal  errors
+    16     detected  by  the  server  during attempted execution of the DISK
+    17     COPY DATA command.  See the descriptions of the status  codes  in
+    18     Section  6.7.2,  "DISK COPY DATA Command, End Message Format" for
+    19     specifics.  In these cases, this  error  log  shall  contain  the
+    20     appropriate  event  code/subcode.   Note  that  for most of these
+    21     fatal errors, this will be the only error  log  related  to  this
+    22     DISK COPY DATA command generated by the server.
+
+    23     Note that in either case this error log is only sent once  for  a
+    24     particular DISK COPY DATA command.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-76
+           Error Log Messages                                   11 June 1992
+
+
+     1                     31                             0 
+     2                    +--------------------------------+
+     3                    |   command reference number     |
+     4                    +---------------+----------------+
+     5                    |sequence number|dst unit number |
+     6                    +---------------+-------+--------+
+     7                    |   event code  | flags | format |
+     8                    +---------------+-------+--------+
+     9                    |                                |
+    10                    +---  controller identifier   ---+
+    11                    |                                |
+    12                    +---------------+-------+--------+
+    13                    |   undefined   |chvrsn | csvrsn |
+    14                    +---------------+-------+--------+
+    15                    |          destination           |
+    16                    +---          unit            ---+
+    17                    |          identifier            |
+    18                    +---------------+----------------+
+    19                    |   undefined   |src unit number |
+    20                    +---------------+----------------+
+    21                    |            source              |
+    22                    +---       controller         ---+
+    23                    |          identifier            |
+    24                    +---------------+-------+--------+
+    25                    |   undefined   |schvrsn|scsvrsn |
+    26                    +---------------+-------+--------+
+    27                    |            source              |
+    28                    +---          unit            ---+
+    29                    |          identifier            |
+    30                    +--------------------------------+
+    31                    |source unit's storage subsystem |
+    32                    +---          port            ---+
+    33                    |            address             |
+    34                    +--------------------------------+
+    35                    |source unit's storage subsystem |
+    36                    +---         system           ---+
+    37                    |            address             |
+    38                    +--------------------------------+
+    39                    |                                |
+    40                    /         event dependent        /
+    41                    /           information          /
+    42                    |                                |
+    43                    +--------------------------------+
+
+    44     The fields in the DISK COPY DATA CORRELATION  error  log  message
+    45     are as follows:
+
+    46         command reference number
+    47         sequence number
+    48         format
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-77
+           Error Log Messages                                   11 June 1992
+
+
+     1         controller identifier
+     2         csvrsn
+     3         chvrsn
+
+     4             See Section 5.9.2, "Generic Error Log Message Format." 
+
+     5             The contents of the "command reference number"  field  in
+     6             this  message  shall  be identical to the contents of the
+     7             "command reference number" field of the  DISK  COPY  DATA
+     8             command for which this message is sent.
+
+     9         dst unit number (destination unit number)
+    10         destination unit identifier
+    11         src unit number
+    12         source unit identifier
+    13         source unit's storage subsystem port address
+    14         source unit's storage subsystem system address
+
+    15             The  values  of   these   fields   are   copied   without
+    16             modification  from  the  corresponding fields of the DISK
+    17             COPY DATA command  message  identified  in  the  "command
+    18             reference  number"  field  of  this message.  See Section
+    19             6.7.1, "DISK COPY DATA Command, Command  Message  Format"
+    20             for descriptions of these fields.
+
+    21         flags
+
+    22             See Section 5.9.2, "Generic Error Log Message Format." 
+
+    23             The Informational error log flag shall  be  set  in  this
+    24             message if the event code is Informational.
+
+    25         event code
+
+    26             For DISK COPY DATA commands that terminate  with  an  end
+    27             message  major  status  code  of  Controller  Error, Host
+    28             Buffer Access Error, or Subcommand Error, the event  code
+    29             shall be set to the corresponding event code/subcode.
+
+    30             Otherwise, the event code is Informational (subcode "Disk
+    31             Copy Data Correlation").
+
+    32         source controller identifier
+    33         scsvrsn (source csvrsn)
+    34         schvrsn (source chvrsn)
+
+    35             The values of these fields are from the  end  message  of
+    36             the  SET  CONTROLLER  CHARACTERISTICS command issued when
+    37             bringing the remote server "Controller-Online."  Zero  if
+    38             that information is not available.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MSCP CONTROL MESSAGES                                   Page 5-78
+           Error Log Messages                                   11 June 1992
+
+
+     1         event dependent information
+
+     2             A variable amount of information dependent on  the  event
+     3             that  caused this error log message to be generated.  The
+     4             length of this information is implied by the total length
+     5             of  the  error log message, passed to the class driver by
+     6             the port driver.  This information will typically not  be
+     7             interpreted  by  error  log  formatting programs, instead
+     8             being printed as a series of values.  See Sections 6.7.2,
+     9             "DISK  COPY DATA Command, End Message Format," and 6.7.3,
+    10             "DISK COPY DATA  Command,  Description"  for  details  on
+    11             what, if anything, should be placed in this field.
+
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 6
+
+     2                         MINIMAL MSCP COMMAND SET
+
+
+     3     6.1  Overview
+
+     4     This chapter describes the minimal MSCP command  set  implemented
+     5     by  an  MSCP  disk  controller.   Refer  to the Tape Mass Storage
+     6     Control Protocol Specification (TMSCP) for details of the minimal
+     7     command  set  implemented  by MSCP tape controllers.  A number of
+     8     commands operate identically on both disk and tape class devices.
+     9     TMSCP  relies  on  the descriptions contained in this chapter for
+    10     those commands.
+
+    11     Controllers that  support  only  the  minimal  MSCP  command  set
+    12     implicitly provide single host operation and rely on the host for
+    13     the replacement of bad blocks.
+
+    14     General purpose disk class drivers  shall  at  the  very  minimum
+    15     implement  both  the  minimal MSCP command set, described in this
+    16     chapter,  and  Controller  Initiated   Bad   Block   Replacement,
+    17     described in Chapter 8.
+
+    18     Note that with the exception of the  commands  noted  in  Section
+    19     6.1.1,  "No-Operation Commands" controllers that support only the
+    20     minimal MSCP command set reject any command not described in this
+    21     chapter as an "Invalid Command" ("invalid opcode" protocol error.
+    22     See "Invalid Command" status in Section 5.5, "Status Codes").
+
+    23     Within this chapter a separate major section is dedicated to each
+    24     minimal  command  set  command.  The major section identifies the
+    25     command by name as well as its  permissible  command  category(s)
+    26     (see  Section  4.5,  "Command  Categories  and Execution Order").
+    27     Separate subordinate  sections  describe  the  command's  command
+    28     message format, end message format, and (operation) description.
+
+    29     The command message format subsection shows  the  layout  of  the
+    30     command  message  fields  and  describes their use.  (Appendix A,
+    31     "Opcode, Flag,  and  Offset  Definitions,"  Table  A-6,  "Command
+    32     Message  Offsets" defines the size and the preferred mnemonic and
+    33     numeric value of the offset of the command message fields.)  Note
+    34     that  the  omission of a parameter field description implies that
+    35     it functions as described elsewhere in  this  specification.   In
+    36     particular, the command message header fields (i.e., the "command
+    37     reference number," "unit number," "opcode," and "modifiers")  are
+    38     described  in  Section  5.3,  "Command  Messages"  and the fields
+    39     common to data transfer commands  (i.e.,  "byte  count,"  "buffer
+    40     descriptor," and "logical block number") are described in Section
+    41     5.3.1, "Transfer Command Message Format." 
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-2
+           Overview                                             11 June 1992
+
+
+     1     The command message format subsection also  lists  the  modifiers
+     2     allowed  for  the  command (modifier use is opcode dependent) and
+     3     describes their use.   Note  that  the  omission  of  a  modifier
+     4     description  implies  that  it  functions as described in Section
+     5     5.3.2, "Command Modifiers."  Note also that any command modifiers
+     6     other than the ones listed for an individual command are reserved
+     7     and treated in accordance  with  the  requirements  for  reserved
+     8     fields described in Section 5.2, "Reserved and Undefined Fields."
+
+     9     The end message format subsection shows the  layout  of  the  end
+    10     message  fields  and  describes their use.  (Appendix A, "Opcode,
+    11     Flag, and Offset Definitions,"  Table  A-7,  "End  and  Attention
+    12     Message  Offsets" defines the size and the preferred mnemonic and
+    13     numeric value of the offset of the end message fields.) Note that
+    14     the  omission  of  a  parameter field description implies that it
+    15     functions as  described  elsewhere  in  this  specification.   In
+    16     particular,  the  end  message  header fields (i.e., the "command
+    17     reference number," "unit number," "sequence  number,"  "endcode,"
+    18     "flags,"  and  "status")  are  described  in  Section  5.4,  "End
+    19     Messages" and the fields common to data transfer commands  (i.e.,
+    20     "status,"  "byte  count," and "first bad block") are described in
+    21     Section 5.4.1, "Transfer Command End Message Format." 
+
+    22     The end message format subsection also lists all the status codes
+    23     (and  subcodes)  possible  for  the  command  and describes their
+    24     meaning.  Note that the omission of a status description  implies
+    25     that its meaning is defined in Section 5.5, "Status Codes."  Note
+    26     also that the MSCP protocol violation "Invalid Command"  statuses
+    27     are  not  listed  for any of the commands but can occur for each.
+    28     See "Invalid Command" status in Section 5.5, "Status  Codes"  for
+    29     more detail.
+
+    30     The description subsection defines the purpose of the command and
+    31     provides  details  about  the command's operation.  In some cases
+    32     the  description  subsection  relies   heavily   on   the   other
+    33     subsections  associated with the command for specific operational
+    34     details.
+
+    35     6.1.1  No-Operation Commands
+
+    36     The opcodes listed in the "Standard Command Messages" portion  of
+    37     Appendix  A, "Opcode, Flag, and Offset Definitions," Table A-1 as
+    38     "allocated for future use" act as place holders  in  anticipation
+    39     of  their  use  in  Volume Shadowing and/or Data Caching.  Volume
+    40     Shadowing and Data Caching have not yet been formalized for  disk
+    41     class  devices.   Data  Caching has, however, been formalized for
+    42     tape class devices (see the Tape Mass  Storage  Control  Protocol
+    43     Specification   (TMSCP)   for   details).   In  order  to  ensure
+    44     compatibility with  controllers  that  support  Volume  Shadowing
+    45     and/or  Data  Caching, controllers which do not shall treat those
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-3
+           Overview                                             11 June 1992
+
+
+     1     opcodes as no-operations.  In other words,  controllers  that  do
+     2     not  support Volume Shadowing and/or Data Caching shall recognize
+     3     those opcodes and not  reject  them  as  "Invalid  Commands"  and
+     4     furthermore shall not perform any action that alters the state of
+     5     the unit, its media, or the controller.
+
+     6     The particular actions such a controller may perform in  treating
+     7     those opcodes as no-operations are as follows:
+
+     8          1.  Treat those opcodes as either Immediate commands  or  as
+     9              Non-Sequential  commands  at  its option.  (Host command
+    10              timeout algorithms shall  treat  them  as  non-Immediate
+    11              commands.)
+
+    12          2.  Construct  the  end  message  by  making  the  following
+    13              changes to the command message:
+
+    14              a.  Substitute the proper "endcode" for the "opcode." 
+
+    15              b.  Clear the  "flags"  (end  flags)  byte,  unless  the
+    16                  controller   has   previously   verified   that  the
+    17                  corresponding "rsvd" byte in the command message was
+    18                  zero.    (Most  controllers  will  verify  that  the
+    19                  corresponding  "rsvd"  byte  is  zero  as  part   of
+    20                  checking for invalid opcodes.)
+
+    21              c.  Substitute the "Success" status code  combined  with
+    22                  the  "Normal"  status  subcode for the "modifiers." 
+    23                  The controller may  optionally  validate  the  "byte
+    24                  count"  and  "logical block number" fields to see if
+    25                  they  are  within  proper  limits  and  return   the
+    26                  "Invalid Command" status with appropriate subcode if
+    27                  they are  not  (i.e.,  validate  the  command  as  a
+    28                  transfer  command  similar  to  the ACCESS and ERASE
+    29                  commands).
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-4
+           ABORT Command                                        11 June 1992
+
+
+     1     6.2  ABORT Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     6.2.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     | outstanding reference number  |
+    14                     +-------------------------------+
+
+
+    15         unit number
+
+    16             Shall be the same as  the  "unit  number"  field  in  the
+    17             outstanding   command   to   be   aborted.   This  allows
+    18             controllers to optimize their search for the  outstanding
+    19             command.   If the incorrect unit number is supplied, some
+    20             controllers may erroneously conclude that the command  is
+    21             no   longer  outstanding  and  therefore  not  abort  the
+    22             command.
+
+    23         outstanding reference number
+
+    24             Command reference number of the command to be aborted.
+
+
+    25     Allowable modifiers:
+
+    26         none
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-5
+           ABORT Command                                        11 June 1992
+
+
+     1     6.2.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     | outstanding reference number  |
+    11                     +-------------------------------+
+
+    12         outstanding reference number
+
+    13             The command reference number  of  the  command  that  was
+    14             aborted.   Identical  to  the value supplied in the ABORT
+    15             command message.
+
+
+    16     Status Codes:
+
+    17         Success (subcode "Normal")
+
+
+
+
+
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-6
+           ABORT Command                                        11 June 1992
+
+
+     1     6.2.3  Description
+
+     2     The ABORT command causes a specified command to be aborted at the
+     3     earliest  time  convenient  for  the  controller.   The specified
+     4     command shall, however, either be aborted or be completed  within
+     5     the  controller  timeout  interval.   See  Section 4.10, "Command
+     6     Timeouts," for a discussion of the interaction between ABORT  and
+     7     command timeouts.
+
+     8     The ABORT command always succeeds.  If the command to be  aborted
+     9     is  not known to the controller, this implies that it has already
+    10     completed and the ABORT command will be ignored.  The  controller
+    11     always returns the Success (subcode "Normal") status code/subcode
+    12     in the ABORT command's end message.
+
+    13     The controller may ignore the ABORT command if the command  being
+    14     aborted  will  always  complete  within  the  controller  timeout
+    15     interval.
+
+    16     The class  driver  shall  wait  for  the  aborted  command's  end
+    17     message,  or  else  resynchronize  with  the  MSCP server, before
+    18     reusing  the  aborted  command's  command  reference  number   or
+    19     releasing  the  aborted  command's  context.   If the command was
+    20     aborted, its end  message  will  contain  the  "Command  Aborted"
+    21     status  code.   Otherwise,  the command was completed.  The class
+    22     driver may ignore the ABORT command's end message.  Note that the
+    23     class  driver  may receive the ABORT command's end message either
+    24     before or after the aborted command's end message.
+
+
+
+
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+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-7
+           ACCESS Command                                       11 June 1992
+
+
+     1     6.3  ACCESS Command
+
+     2     Command Category:
+
+     3         Non-Sequential
+
+     4     6.3.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  caa  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |          byte count           |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---                         ---+
+    17                     |           reserved            |
+    18                     +---                         ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |      logical block number     |
+    22                     +-------------------------------+
+
+
+    23     Allowable modifiers:
+
+    24         Express Request
+    25         Clear Serious Exception (ignored for disk class devices)
+    26         Suppress Error Correction
+    27         Suppress Error Recovery
+
+
+
+
+
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+
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+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-8
+           ACCESS Command                                       11 June 1992
+
+
+     1     6.3.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |          byte count           |
+    11                     +-------------------------------+
+    12                     |                               |
+    13                     +---                         ---+
+    14                     |           undefined           |
+    15                     +---                         ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |        first bad block        |
+    19                     +-------------------------------+
+
+
+    20     Status Codes:
+
+    21         Success (subcode "Normal")
+    22         Success (subcode "Duplicate Unit Number")
+    23         Invalid Command (subcode "Invalid Byte Count")
+    24         Invalid Command (subcode "Invalid Logical Block Number")
+    25         Command Aborted
+    26         Unit-Offline
+    27         Unit-Available
+    28         Data Error
+    29         Host Buffer Access Error (subcode "Odd Byte Count")
+
+    30             If the communications mechanism does not allow  odd  byte
+    31             transfers,   the   controller   may  return  this  status
+    32             code/subcode despite there being no  host  buffer.   This
+    33             allows  the  use of common code for command validation of
+    34             all transfer commands.  Alternatively, the controller may
+    35             round  the byte count up to the nearest whole sector size
+    36             and perform the operation.
+
+    37         Controller Error
+    38         Drive Error
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                 Page 6-9
+           ACCESS Command                                       11 June 1992
+
+
+     1     6.3.3  Description
+
+     2     Data is read from the unit, checked for  errors,  and  discarded.
+     3     The purpose of this command is to verify that the designated data
+     4     can be accessed (read) without error.
+
+     5     This command is exactly equivalent in function, although  not  in
+     6     performance, to a READ whose data is ignored by the host.
+
+     7     The caa field will be ignored and the backing store will be read.
+
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-10
+           AVAILABLE Command                                    11 June 1992
+
+
+     1     6.4  AVAILABLE Command
+
+     2     Command Category:
+
+     3         Sequential
+
+     4     6.4.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable modifiers:
+
+    14         All Class Drivers       (ignored in single host case)
+    15         Clear Serious Exception (ignored for disk class devices)
+
+    16         Spin-down
+
+    17             Requests that the disk stop spinning and that  the  heads
+    18             be  unloaded.  Note that the command completes as soon as
+    19             the spin-down has been initiated, rather than waiting for
+    20             the  disk  to  stop  spinning.  The spin-down will not be
+    21             initiated if this unit belongs to a multiunit  drive  and
+    22             this  unit  shares a spindle with some other unit that is
+    23             "Unit-Online."  See Section 4.16.2, "Multiunit Drives and
+    24             Formatters."   Regardless of whether or not the spin-down
+    25             is actually initiated, AVAILABLE attention messages  will
+    26             be suppressed for this unit, both for this controller and
+    27             for any other  controllers  to  which  the  unit  may  be
+    28             connected.  See Section 4.3, "Unit States." 
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-11
+           AVAILABLE Command                                    11 June 1992
+
+
+     1     6.4.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status Codes:
+
+    11         Success (subcode "Normal")
+    12         Success (subcode "Duplicate Unit Number")
+
+    13         Success (subcode "Spin-down Ignored")
+
+    14             The "Spin-down Ignored" subcode bit flag is  set  if  and
+    15             only if the "Spin-down" modifier was specified and one or
+    16             more other units with which this unit shares a spindle is
+    17             still  "Unit-Online,"  preventing this unit from spinning
+    18             down.   See  Section  4.16.2,   "Multiunit   Drives   and
+    19             Formatters," for an explanation of shared spindles.
+
+    20         Success (subcode "Still Connected")
+
+    21             The "Still Connected" subcode bit flag is set if and only
+    22             if  this  unit  may  potentially  be accessed via another
+    23             controller and one or more other units  with  which  this
+    24             unit  shares  an  access  path  is  still  "Unit-Online,"
+    25             preventing this unit from being accessed  via  the  other
+    26             controller  (if  any).  The "Still Connected" subcode bit
+    27             flag will always be set if the  "Spin-down  Ignored"  bit
+    28             flag  is  set.  See Section 4.16.1, "Multiaccess Drives,"
+    29             for a discussion of access paths.
+
+    30         Command Aborted
+
+    31             The command has been rejected and the  unit's  state  has
+    32             not  changed.   If  the  unit  was "Unit-Online" prior to
+    33             issuing  this   command,   then   the   unit   is   still
+    34             "Unit-Online" and it is still spinning.
+
+    35         Unit-Offline
+    36         Controller Error
+    37         Drive Error
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-12
+           AVAILABLE Command                                    11 June 1992
+
+
+     1     6.4.3  Description
+
+     2     All outstanding commands for the specified  unit  are  completed,
+     3     then  the  unit  becomes  "Unit-Available."   If  the "Spin-down"
+     4     modifier  was  not   specified,   the   unit   is   not   already
+     5     "Unit-Available,"  and  no  other  units  that  share this unit's
+     6     access path are "Unit-Online" (i.e., the "Still Connected" status
+     7     subcode  bit  flag is clear), then an AVAILABLE attention message
+     8     is sent by any other controller to which the unit  is  connected.
+     9     The  controller  to  which  this command was sent need not itself
+    10     send an AVAILABLE attention message.
+
+    11     If the "Spin-down" modifier is specified, the disk spins down and
+    12     its  heads  are  unloaded, unless some other unit with which this
+    13     unit shares a spindle is "Unit-Online."  The disk may be spun  up
+    14     with   an  ONLINE  command  or  by  operator  intervention.   The
+    15     "Spin-down" modifier also suppresses AVAILABLE attention messages
+    16     for this unit, both for this controller and any other controllers
+    17     to which the unit may  be  connected.   See  Section  4.3,  "Unit
+    18     States,"  for  a  discussion  of  suppressing AVAILABLE attention
+    19     messages.
+
+    20     This command will be accepted if the  unit  is  "Unit-Online"  or
+    21     "Unit-Available."  It is nugatory to issue this command to a unit
+    22     that is  "Unit-Available"  unless  the  "Spin-down"  modifier  is
+    23     specified.   Assuming no other errors occur, the "Success" status
+    24     code  will  be  returned  regardless  of  whether  the  unit  was
+    25     previously "Unit-Online" or "Unit-Available." 
+
+    26     If the unit was "Unit-Online" but had  a  duplicate  unit  number
+    27     prior  to  the  AVAILABLE  command  being  issued,  the AVAILABLE
+    28     command may complete, at the controller's option, with  either  a
+    29     "Success"  or  a  "Unit-Offline" status code.  The "Unit-Offline"
+    30     status code shall have the "Duplicate Unit Number"  subcode  flag
+    31     set.    The  "Success"  status  code  may  or  may  not,  at  the
+    32     controller's option, have the  "Duplicate  Unit  Number"  subcode
+    33     flag  set.   Subsequent  attempts  to access the unit will return
+    34     "Unit-Offline" status with the "Duplicate  Unit  Number"  subcode
+    35     flag set unless the duplicate unit number has been eliminated.
+
+    36     When a unit with controller-based write-back cache ceases  to  be
+    37     Online,  it  should  write to the media all data contained in the
+    38     cache but not yet written to the backing store.
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-13
+           COMPARE HOST DATA Command                            11 June 1992
+
+
+     1     6.5  COMPARE HOST DATA Command
+
+     2     Command Category:
+
+     3         Non-Sequential
+
+     4     6.5.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  caa  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |          byte count           |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---         buffer          ---+
+    17                     |                               |
+    18                     +---       descriptor        ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |      logical block number     |
+    22                     +-------------------------------+
+
+
+    23     Allowable modifiers:
+
+    24         Clear Serious Exception (ignored for disk class devices)
+    25         Express Request
+    26         Suppress Error Correction
+    27         Suppress Error Recovery
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-14
+           COMPARE HOST DATA Command                            11 June 1992
+
+
+     1     6.5.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |          byte count           |
+    11                     +-------------------------------+
+    12                     |                               |
+    13                     +---                         ---+
+    14                     |           undefined           |
+    15                     +---                         ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |        first bad block        |
+    19                     +-------------------------------+
+
+
+    20     Status Codes:
+
+    21         Success (subcode "Normal")
+    22         Success (subcode "Duplicate Unit Number")
+    23         Invalid Command (subcode "Invalid Byte Count")
+    24         Invalid Command (subcode "Invalid Logical Block Number")
+    25         Command Aborted
+    26         Unit-Offline
+    27         Unit-Available
+    28         Compare Error
+    29         Data Error
+    30         Host Buffer Access Error
+    31         Controller Error
+    32         Drive Error
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-15
+           COMPARE HOST DATA Command                            11 June 1992
+
+
+     1     6.5.3  Description
+
+     2     Data is read from the unit and compared with the data in the host
+     3     buffer.   A  "Compare  Error"  is  reported  unless  the  data is
+     4     identical.  Note that the occurrence of any other error, except a
+     5     "Forced  Error,"  at  the  same point as the "Compare Error" will
+     6     override the  "Compare  Error."   Note  also  that  any  "Compare
+     7     Errors"  detected  by  this  command shall NOT be reported to the
+     8     error log.  Controllers may retry compare errors detected by this
+     9     command.   Provided the host does not modify the buffer while the
+    10     transfer is in progress, such retries  will  have  a  deleterious
+    11     effect  on  performance but cannot alter the final outcome of the
+    12     command.
+
+    13     If a controller-based cache exists, the cache manager may use the
+    14     Cache  Access  Attribute  advice  in  managing  the  caching  and
+    15     comparing of the requested data area.
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-16
+           DETERMINE ACCESS PATHS Command                       11 June 1992
+
+
+     1     6.6  DETERMINE ACCESS PATHS Command
+
+     2     Command Category:
+
+     3         Controller implementation dependent.  See Section 6.6.3.
+
+     4     6.6.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable modifiers:
+
+    14         none
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-17
+           DETERMINE ACCESS PATHS Command                       11 June 1992
+
+
+     1     6.6.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status codes:
+
+    11         Success (subcode "Normal")
+    12         Success (subcode "Duplicate Unit Number")
+    13         Command Aborted
+    14         Unit-Offline
+    15         Unit-Available
+    16         Controller Error
+    17         Drive Error
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-18
+           DETERMINE ACCESS PATHS Command                       11 June 1992
+
+
+     1     6.6.3  Description
+
+     2     Class drivers use this  command  to  determine  the  topology  of
+     3     multiaccess  drive  configurations  (described in Section 4.16.1,
+     4     "Multiaccess  Drives").   When   sent   to   a   unit   that   is
+     5     "Unit-Online," it causes that unit and any other units that share
+     6     its access path to identify themselves to any other controller to
+     7     which  they  are  connected.   The  MSCP  servers  in  the  other
+     8     controllers will, as a result, become  aware  that  the  unit  is
+     9     online via the controller receiving this command.  They will then
+    10     send   an   ACCESS    PATH    attention    message    to    their
+    11     "Controller-Online" class drivers, informing the class drivers of
+    12     alternate access paths  to  the  unit.   This  command  shall  be
+    13     treated  as a no-op that always succeeds if the unit is incapable
+    14     of being connected to more than one controller.
+
+    15     The actual notification  to  another  controller,  and  thus  the
+    16     sending  of an ACCESS PATH attention message, is dependent on the
+    17     proper  functioning   of   the   unit   and   both   controllers.
+    18     Furthermore, it need not be 100% reliable.  That is, assuming the
+    19     unit and both controllers are functioning  properly,  there  need
+    20     only  be  a  high  probability  (better  than 50%) that the other
+    21     controller will in fact be  notified  and  send  an  ACCESS  PATH
+    22     attention  message.   For  this  reason,  plus  the fact that the
+    23     topology may change while the unit remains  "Unit-Online,"  hosts
+    24     that need to know multiaccess drive topology must issue DETERMINE
+    25     ACCESS PATHS commands to all "Unit-Online" units  on  a  periodic
+    26     basis, such as once every 5 to 15 minutes.
+
+    27     This command in no way  affects  the  unit's  actual  state  with
+    28     respect  to  any  controller.  The unit remains "Unit-Online" via
+    29     the receiving controller and  remains  "Unit-Offline"  via  other
+    30     controllers.   Note,  however,  that  this command may affect the
+    31     unit's  "Unit-Offline"  substate  that  is  perceived  by   other
+    32     controllers.
+
+    33     The processing of this command  may,  and  often  will,  cause  a
+    34     transient   performance   degradation  for  the  specified  unit.
+    35     Controllers shall  consider  this  performance  degradation  when
+    36     specifying their controller timeout interval.
+
+    37     A controller  may,  at  its  option,  treat  this  as  either  an
+    38     Immediate,  Sequential,  or Non-Sequential command.  Host command
+    39     timeout algorithms shall treat this as a non-Immediate command.
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-19
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     6.7  DISK COPY DATA Command
+
+     2     Command Category:
+
+     3         Non-Sequential (except as described in Section 6.7.3)
+
+
+     4     6.7.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |      logical block count      |
+    14                     +---------------+---------------+
+    15                     |   reserved    |src unit number|
+    16                     +---------------+---------------+
+    17                     |            source             |
+    18                     +---          unit           ---+
+    19                     |          identifier           |
+    20                     +-------------------------------+
+    21                     |        destination lbn        |
+    22                     +---------------+---------------+
+    23                     |   entry id    | hrn or entloc |
+    24                     +---------------+---------------+
+    25                     |           reserved            |
+    26                     +-------------------------------+
+    27                     |          source lbn           |
+    28                     +-------------------------------+
+    29                     |source unit's storage subsystem|
+    30                     +---          port           ---+
+    31                     |            address            |
+    32                     +-------------------------------+
+    33                     |source unit's storage subsystem|
+    34                     +---         system          ---+
+    35                     |            address            |
+    36                     +-------------------------------+
+
+
+    37         unit number
+
+    38             Identifies the destination unit -- i.e.,  the  device  to
+    39             which data is to be copied.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-20
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1  |  Allowable modifiers:
+
+     2         Clear Serious Exception (ignored)
+
+     3         Establish Communications Paths
+
+     4             To perform a copy  operation  the  server  establishes  a
+     5             communications path to the source unit in the same manner
+     6             as  a  host  class  driver   --   i.e.,   establishes   a
+     7             "Controller-Online"  connection  with  the  source unit's
+     8             server (possibly itself) and brings the source unit  into
+     9             the  "Unit-Online"  state.   The  server  already  has  a
+    10             communications path established to  the  destination;  if
+    11             not,  it  would  reject  the  command  with  a  status of
+    12             "Unit-Available" or  "Unit-Offline."  The  communications
+    13             path  is  established on behalf of the issuing host class
+    14             driver.  See Section  6.7.3,  "DISK  COPY  DATA  Command,
+    15             Description"  for details of the establish communications
+    16             paths operation.
+
+    17             The server rejects the command as  an  "Invalid  Command"
+    18             ("invalid   modifier"   protocol   error.   See  "Invalid
+    19             Command" status in Section 5.5, "Status Codes") if either
+    20             of the following conditions exist:
+
+    21                 1.  This modifier is clear and communications paths
+    22                     have  not  been  established  on  behalf of the
+    23                     issuing host class driver.
+
+    24                 2.  This modifier is set and  communications  paths
+    25                     previously established on behalf of the issuing
+    26                     host class driver are still intact.
+
+
+    27         History Log
+    28         Reuse Entry
+
+    29             The  History  Log   and   Reuse   Entry   modifiers   are
+    30             interdependent  and  described  together  here  to  avoid
+    31             confusion.
+
+    32             Note that the  History  Log  modifier,  the  Reuse  Entry
+    33             modifier,  the  "hrn or entloc" field, and the "entry id"
+    34             field are valid only  for  servers  that  provide  "write
+    35             history  logging"  support  (see Section 5.6, "Controller
+    36             Flags" for related information).
+
+    37             The  setting  of  the  History  Log  modifier  determines
+    38             whether information about this command is to be stored in
+    39             a "write history entry."
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-21
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             The  setting  of  the  Reuse  Entry  modifier  determines
+     2             whether  the  server  is to allocate a new "write history
+     3             entry" or reuse one that was previously allocated.
+
+     4             If "write history logging" is supported, the History  Log
+     5             modifier  is  set, and the Reuse Entry modifier is clear,
+     6             the server allocates a new "write history entry" and then
+     7             stores  the  value  contained in the "hrn (host reference
+     8             number)" field, "entry id" field, and  other  information
+     9             related  to  this  command into the "write history entry"
+    10             just allocated.  If the  server  has  no  "write  history
+    11             entry"  to  allocate,  it  shall remember this fact.  See
+    12             Section  6.19.3,  "WRITE  HISTORY   MANAGEMENT   Command,
+    13             Description" for more details.  To report this allocation
+    14             failure to the host, the server returns FFFF(hex) in  the
+    15             "entry  locator"  field  of  the end message and sets the
+    16             Allocation Failure  end  flag.   See  Section  5.4,  "End
+    17             Messages," for a description of this flag.
+
+    18             If "write history logging" is supported, the History  Log
+    19             modifier is set, and the Reuse Entry modifier is set, the
+    20             server uses the value contained  in  the  "entloc  (entry
+    21             locator)"  field  to locate the associated "write history
+    22             entry" and then stores the other information  related  to
+    23             this command into that "write history entry." 
+
+    24             See Section 6.19.3, "WRITE  HISTORY  MANAGEMENT  Command,
+    25             Description"  for  details  regarding the maintenance and
+    26             format of "write history entries." 
+
+    27             If "write history logging" is supported and  the  History
+    28             Log  modifier is clear, the server ignores the setting of
+    29             the Reuse Entry modifier and the contents of the "hrn  or
+    30             entloc" and "entry id" fields.
+
+    31             If "write history  logging"  is  not  supported  and  the
+    32             History  Log modifier or the Reuse Entry modifier is set,
+    33             the server rejects the command as  an  "Invalid  Command"
+    34             ("invalid   modifier"   protocol   error.   See  "Invalid
+    35             Command" status in Section 5.5, "Status Codes").
+
+    36             If "write history logging" is supported, the History  Log
+    37             modifier  is  set, and the "logical block count" field is
+    38             zero, the server reject the command as an Invalid Command
+    39             ("invalid  logical  block  count"  parameter  error.  See
+    40             "Invalid Command" status in Section 5.5, "Status Codes").
+
+    41             The server shall set the History Logged end flag in  this
+    42             command's  end message if it has caused the write history
+    43             log to contain a valid "write  history  entry"  for  this
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-22
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             command.    See   Section   5.4,   "End   Messages,"  for
+     2             descriptions of all end message flags.
+
+     3         Local Source Unit
+
+     4             Host class drivers set this modifier to indicate that the
+     5             source  unit  is  served by the server that received this
+     6             command.  If this modifier is set, the server ignores the
+     7             contents  of  the  "source unit's subsystem port address"
+     8             and the "source unit's subsystem system address" fields.
+
+     9             Host class drivers clear this modifier to  indicate  that
+    10             the  source unit is served by a server located in another
+    11             storage subsystem.
+
+    12             This modifier is ignored by  servers  that  only  support
+    13             Local Disk Copy Data.
+
+    14         Retain Communications Paths
+
+    15             See the description of the Establish Communications Paths
+    16             modifier   above   for   information  about  establishing
+    17             communications paths.
+
+    18             The setting  of  this  modifier  determines  whether  the
+    19             server should retain the communications paths established
+    20             on behalf of the issuing host class driver after the copy
+    21             operation has been aborted, completed, or terminated.
+
+    22             If  this  modifier  is  set,  the  server   retains   the
+    23             communications paths established on behalf of the issuing
+    24             host class driver.  A host  class  driver  may  therefore
+    25             issue   subsequent   DISK   COPY  DATA  commands  without
+    26             incurring  the  overhead  associated  with   the   server
+    27             re-establishing the communications paths.
+
+    28             If this  modifier  is  clear,  the  server  performs  the
+    29             operations  necessary  to  disconnect  the communications
+    30             paths established on behalf of  the  issuing  host  class
+    31             driver,  upon  completion  of  the  request.  See Section
+    32             6.7.3, "DISK COPY DATA Command, Description" for  details
+    33             of the disconnect operation.
+
+    34             Note that under certain  circumstances  the  server  will
+    35             disconnect the communications paths established on behalf
+    36             of the  issuing  host  class  driver  regardless  of  the
+    37             setting  of  this modifier.  The particular circumstances
+    38             are described in Section 6.7.3, "DISK COPY DATA  Command,
+    39             Description." 
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-23
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             Note also that the Communications Paths Retained end flag
+     2             reflects  the  state of the communications paths relative
+     3             to the issuing host as described  in  the  "flags"  field
+     4             description  contained  in Section 6.7.2, "DISK COPY DATA
+     5             Command, End Message Format." 
+
+     6  |      logical block count
+
+     7             Zero or the total number of logical blocks to  be  copied
+     8             from the source unit to the destination unit.
+
+     9             If  this  value   is   nonzero,   certain   architectural
+    10             constraints  imposed  by  the  destination  unit  and the
+    11             source unit shall be met.  The constraints depend on  the
+    12             contents of the "destination lbn" or "source lbn" fields.
+    13             Those fields shall identify a logical block in  the  host
+    14             area  of  the  associated disk volume (i.e., the "lbn" is
+    15             less than the "unit size"  returned  in  the  ONLINE  end
+    16             message) and the "logical block count" shall be less than
+    17             or equal to the following maximum:
+
+    18                         unit size - logical block number
+
+    19             where "unit size" is the unit's host area size  (returned
+    20             in  the ONLINE end message) and "logical block number" is
+    21             the contents of the associated "lbn" field in the command
+    22             message.   If  the  "logical block count" is greater than
+    23             the above maximum for either the destination unit or  the
+    24             source  unit,  the  server  rejects  the  command  as  an
+    25             "Invalid  Command"   ("invalid   logical   block   count"
+    26             parameter error.  See "Invalid Command" status in Section
+    27             5.5, "Status Codes").
+
+    28             A host class driver should specify a zero value  in  this
+    29             field   when   it   is   necessary   to   manipulate  the
+    30             communications paths established on  its  behalf  without
+    31             performing a copy operation.  See the descriptions of the
+    32             Establish Communications Paths and Retain  Communications
+    33             Paths   modifiers  later  in  this  section  for  related
+    34             information.  The server  ignores  the  contents  of  the
+    35             "source lbn" and "destination lbn" fields when this field
+    36             is zero.  In addition, if this  field  is  zero  and  the
+    37             Local  Source  Unit  modifier  is  set or the server only
+    38             supports Local Disk Copy Data,  the  server  ignores  the
+    39             contents  of  the  "source  unit's storage subsystem port
+    40             address" and  "source  unit's  storage  subsystem  system
+    41             address" fields as well as the other fields listed above.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-24
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         src unit number
+
+     2             Identifies the source unit -- i.e., the device from which
+     3             data is to be copied.
+
+     4             If the server does not support local copies between units
+     5             with  unlike  geometries  and copies where the source and
+     6             destination are the same unit, has indicated this lack of
+     7             support   by   setting  the  Restricted  DISK  COPY  DATA
+     8             Operations controller flag (see Section 5.6,  "Controller
+     9             Flags")   in   the  SET  CONTROLLER  CHARACTERISTICS  end
+    10             message, and receives a local DISK COPY DATA  command  in
+    11             which  either  of these conditions exists, it rejects the
+    12             command  as  an  "Invalid  Command"  ("invalid  src  unit
+    13             number" parameter error.  See "Invalid Command" status in
+    14             Section 5.5, "Status Codes").
+
+    15             If the server supports Remote Disk Copy Data  the  source
+    16             unit  may be the same as the destination unit, one of the
+    17             other units served  by  the  server  that  received  this
+    18             command, or a unit served by a (disk) MSCP server located
+    19             in  another  storage  subsystem,  depending  on  the  the
+    20             setting  of the Local Source Unit modifier and the values
+    21             specified in  this  field,  the  "source  unit's  storage
+    22             subsystem  port  address"  field,  and the "source unit's
+    23             storage  subsystem  system  address"  field.   Note  that
+    24             servers  that  provide  Remote  Disk  Copy  Data  support
+    25             implicitly provide Local Disk Copy Data support.
+
+    26         source unit identifier
+
+    27             The unique identifier assigned  to  the  source  unit  as
+    28             returned  in the "unit identifier" end message field of a
+    29             GET UNIT STATUS,  ONLINE,  or  SET  UNIT  CHARACTERISTICS
+    30             command addressed to the source unit.
+
+    31             Host  class  drivers  should  obtain   this   information
+    32             directly  from  the  source unit, via one of the commands
+    33             listed above, prior to issuing this command.
+
+    34             The server compares the value of this field  against  the
+    35             "unit  identifier"  it  obtains  directly from the source
+    36             unit.  If the values do not  match  exactly,  the  server
+    37             rejects  the  command  as  an "Invalid Command" ("invalid
+    38             source unit identifier" parameter  error.   See  "Invalid
+    39             Command" status in Section 5.5, "Status Codes").
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-25
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         destination lbn
+
+     2             The logical block number (position)  on  the  destination
+     3             unit's  disk  volume  at  which  the copy operation is to
+     4             start.  Write  access  to  the  destination  unit's  disk
+     5             volume  Replacement  Control  Table  as  part of the copy
+     6             operation is prohibited.   Therefore,  this  value  shall
+     7             identify  a  logical  block  in  the  host  area  of  the
+     8             destination unit's disk volume  (i.e.,  the  "destination
+     9             lbn"  is less than the "unit size" returned in the ONLINE
+    10             end message).  If this value is not in the host area, the
+    11             server  rejects  the  command  as  an  "Invalid  Command"
+    12             ("invalid destination lbn" parameter error.  See "Invalid
+    13             Command" status in Section 5.5, "Status Codes").
+
+    14         hrn or entloc
+    15         entry id
+
+    16             These fields are used in conjunction with the History Log
+    17             and  Reuse Entry modifiers.  See the description of those
+    18             modifiers below.
+
+    19         source lbn
+
+    20             The logical block number (position) on the source  unit's
+    21             disk  volume  at  which  the  copy operation is to start.
+    22             Read access to the source unit's disk volume  Replacement
+    23             Control   Table   as   part  of  the  copy  operation  is
+    24             prohibited.   Therefore,  this  value  shall  identify  a
+    25             logical  block in the host area of the source unit's disk
+    26             volume (i.e., the "source lbn" is  less  than  the  "unit
+    27             size" returned in the ONLINE end message).  If this value
+    28             is not in the host area, the server rejects  the  command
+    29             as  an  "Invalid Command" ("invalid source lbn" parameter
+    30             error.  See "Invalid  Command"  status  in  Section  5.5,
+    31             "Status Codes").
+
+    32             If the server does not support local DISK COPY  DATA  for
+    33             transfers  in  which  the source and destination LBNs are
+    34             different and has  indicated  this  lack  of  support  by
+    35             setting   the   Restricted   DISK  COPY  DATA  Operations
+    36             controller flag (see Section 5.6, "Controller Flags")  in
+    37             the  SET  CONTROLLER  CHARACTERISTICS  end  message,  the
+    38             server  rejects  the  command  as  an  "Invalid  Command"
+    39             ("invalid source lbn" parameter error).
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-26
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         source unit's storage subsystem port address
+
+     2             The communications mechanism dependent  port  address  of
+     3             the  storage  subsystem  to  which  the  source  unit  is
+     4             physically connected.  The information encoded in this  8
+     5             byte  (64  bit)  field  is  the port address formatted as
+     6             required by the communications mechanism being used.  See
+     7             Appendix F, "Port Address And System Address Formats" for
+     8             a description of the formats currently defined.
+
+     9             The server ignores this field if either of the  following
+    10             conditions exists:
+
+    11                 1.  The server does not provide  Remote  Disk  Copy
+    12                     Data support -- i.e., only Local Disk Copy Data
+    13                     is supported.
+
+    14                 2.  The server supports Remote Disk Copy  Data  but
+    15                     the  Local  Source  Unit modifier is set.  Note
+    16                     that servers that provide Remote Disk Copy Data
+    17                     support implicitly provide Local Disk Copy Data
+    18                     support.
+
+
+    19             If Remote Disk Copy  Data  is  supported  and  the  Local
+    20             Source  Unit  modifier  is  clear, the server rejects the
+    21             command as an "Invalid Command" ("source  unit's  storage
+    22             subsystem  port  address"  parameter error.  See "Invalid
+    23             Command" status in Section 5.5, "Status  Codes")  if  the
+    24             content of this field:
+
+    25                 1.  Is   not   formatted    correctly    for    the
+    26                     communications mechanism being used.
+
+    27                 2.  Identifies the subsystem in  which  the  server
+    28                     receiving this command is located.  (Host class
+    29                     drivers  shall  set  the  Local   Source   Unit
+    30                     modifier  to  request  a  Local  Disk Copy Data
+    31                     operation.)
+
+
+    32         source unit's storage subsystem system address
+
+    33             The communications protocol dependent system  address  of
+    34             the  storage  subsystem  to  which  the  source  unit  is
+    35             physically connected.  The information encoded in this  8
+    36             byte (64 bit) field is the system address value formatted
+    37             as required by the communications  protocol  being  used.
+    38             See Appendix F, "Port Address And System Address Formats"
+    39             for a description of the formats currently defined.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-27
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             The server ignores this field if either of the  following
+     2             conditions exists:
+
+     3                 1.  The server does not provide  Remote  Disk  Copy
+     4                     Data support -- i.e., only Local Disk Copy Data
+     5                     is supported.
+
+     6                 2.  The server supports Remote Disk Copy  Data  but
+     7                     the  Local  Source  Unit modifier is set.  Note
+     8                     that servers that provide Remote Disk Copy Data
+     9                     support implicitly provide Local Disk Copy Data
+    10                     support.
+
+    11             If Remote Disk Copy Data is supported, the  Local  Source
+    12             Unit  modifier  is clear, and this field is not formatted
+    13             correctly for the communications protocol being used, the
+    14             server  rejects  the  command  as  an  "Invalid  Command"
+    15             ("source  unit's  storage   subsystem   system   address"
+    16             parameter error.  See "Invalid Command" status in Section
+    17             5.5, "Status Codes").
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-28
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     6.7.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |      logical block count      |
+    11                     +-------------------------------+
+    12                     |                               |
+    13                     +---                         ---+
+    14                     |                               |
+    15                     +---    subcommand status    ---+
+    16                     |                               |
+    17                     +---                         ---+
+    18                     |                               |
+    19                     +---------------+---------------+
+    20                     |   entry id    | entry locator |
+    21                     +---------------+---------------+
+
+
+    22         flags
+
+    23               Communications Paths Retained
+
+    24                 The setting of this end flag  indicates  whether  the
+    25                 communications paths the server established on behalf
+    26                 of the issuing host class driver have been  retained.
+    27                 If  this  end  flag  is set, the communications paths
+    28                 have been retained.  If this end flag is  clear,  the
+    29                 communications paths have been disconnected.
+
+    30             Note that the Communications Paths Retained end  flag  is
+    31             only  valid  in  this end message.  Furthermore, only the
+    32             Communications  Paths  Retained,  Error  Log   Generated,
+    33             Allocation  Failure,  and  Bad Block Unreported end flags
+    34             may be returned set in this end message;  all  other  end
+    35             flags are reserved.
+
+    36         logical block count
+
+    37             The number of logical blocks successfully copied from the
+    38             source unit to the destination unit.
+
+    39             If an unrecoverable error occurs, the  server  sets  this
+    40             value  equal  to  the number of contiguous logical blocks
+    41             beginning  with   the   "destination   lbn"   that   were
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-29
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             successfully  copied to the destination unit prior to the
+     2             occurrence of the unrecoverable error.
+
+     3             If  no  unrecoverable  errors  occur  during   the   copy
+     4             operation,  the server sets this value equal to the exact
+     5             "logical block count" that was specified in  the  command
+     6             message.
+
+     7         subcommand status
+
+     8             This field contains information related to the failure of
+     9             a  subcommand  issued by the server to the source unit or
+    10             the destination unit.
+
+    11             The contents of this  field  are  undefined  unless  this
+    12             command  was  terminated  with  a Subcommand Error status
+    13             code.  If this command is  terminated  with  that  status
+    14             code,  the  "Destination" or "Source" bit flag (contained
+    15             in the subcode portion of the  status  field)  identifies
+    16             which  device  encountered  the  condition  or error.  In
+    17             cases where multiple conditions or errors have  occurred,
+    18             either on one or on both devices, the server only records
+    19             and reports the first condition or  error  detected;  all
+    20             other condition or error context is discarded.
+
+    21             The information supplied when this field is valid  (i.e.,
+    22             defined)  is  a  portion  of  the  command or end message
+    23             (depending on the Subcommand Error subcode reported;  see
+    24             the  descriptions  of  those subcodes for more detail) of
+    25             the subcommand that encountered the condition  or  error.
+    26             The portion of the command or end message supplied begins
+    27             with the message's "unit number" field and ends with  the
+    28             highest  field  capable  of  being  stored  in  the space
+    29             allotted for this field.
+
+    30             The purpose of  this  field  is  to  provide  host  class
+    31             drivers  with  information  to  aid  in  recovering  from
+    32             failures that occur during the copy operation.   However,
+    33             when  a  failure  occurs information is only provided for
+    34             one of the devices involved in the copy  operation.   The
+    35             state  of  the  other  device cannot be deduced from that
+    36             information.  Host class drivers should  therefore  query
+    37             the  other  device  for  its  current  state to determine
+    38             whether copy operation recovery should be attempted.
+
+    39         entry locator
+
+    40             If "write history logging" is supported and  the  History
+    41             Log  modifier  is  set in the command message, this field
+    42             contains the value assigned by the server  that  uniquely
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-30
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             identifies  the  internal location of the "history entry"
+     2             where  this  command's  information  was   stored.    If,
+     3             however,  an  allocation  failure  occurred,  this  field
+     4             contains the value FFFF(hex).  Otherwise, this  field  is
+     5             undefined.
+
+     6             This field contains a valid entry  locator  of  a  "write
+     7             history  entry"  for  this  command  if  and  only if the
+     8             History Logged end flag is set in the  "flags"  field  of
+     9             this end message.
+
+    10             See the description of the History Log  and  Reuse  Entry
+    11             modifiers  in  Section  6.7.1,  "DISK  COPY DATA Command,
+    12             Command  Message  Format"  and  Section  6.19.3,   "WRITE
+    13             HISTORY MANAGEMENT Command, Description" for more detail.
+
+    14         entry id
+
+    15             If "write history logging" is supported and  the  History
+    16             Log  modifier is set in the command message, the contents
+    17             of the corresponding  command  message  field  is  copied
+    18             without modification to this field.  Otherwise this field
+    19             is undefined.
+
+    20             See the description of the History Log  and  Reuse  Entry
+    21             modifies  in  Section  6.7.1,  "DISK  COPY  DATA Command,
+    22             Command  Message  Format"  and  Section  6.19.3,   "WRITE
+    23             HISTORY MANAGEMENT Command, Description" for more detail.
+
+    24     Status Codes:
+
+    25         Success (subcode "Normal")
+    26         Success (subcode "Duplicate Unit Number")
+
+    27             Before returning this  status  the  server  performs  the
+    28             following:
+
+    29                 1.  Sets the  "logical  block  count"  end  message
+    30                     field as described in that field's description.
+
+    31                 2.  If the the Retain Communication Paths  modifier
+    32                     is  clear, performs the operations necessary to
+    33                     disconnect the communications paths established
+    34                     on  behalf  of  the  issuing host class driver.
+    35                     See Section 6.7.3,  "DISK  COPY  DATA  Command,
+    36                     Description"  for  details  of  the  disconnect
+    37                     operations.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-31
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         Invalid Command (subcode "Invalid Modifier")
+
+     2             See the description of the Establish Communications Paths
+     3             modifier  in  Section  6.7.1,  "DISK  COPY  DATA Command,
+     4             Command Message Format" for  specific  information  about
+     5             this  error.   Note  that if the command is rejected with
+     6             this status, communications paths are  retained  only  if
+     7             already established by a previous DISK COPY DATA command.
+
+     8         Invalid Command (subcode "Invalid Logical Block Count")
+     9         Invalid Command (subcode "Invalid Destination LBN")
+    10         Invalid Command (subcode "Invalid Source LBN")
+
+    11             See  the  descriptions  of  the  "logical  block  count,"
+    12             "destination  lbn,"  and  "source  lbn,"  command message
+    13             fields in Section 6.7.1, "DISK COPY DATA Command, Command
+    14             Message  Format"  for  specific  information  about these
+    15             errors.
+
+    16             Note that communications paths are retained even when the
+    17             command is rejected with one of these status codes.
+
+    18         Invalid Command (subcode "Invalid Src Unit Number")
+    19         Invalid Command (subcode "Invalid Source Unit Identifier")
+
+    20             See the description of the "src unit number" and  "source
+    21             unit identifier" command message fields in Section 6.7.1,
+    22             "DISK COPY DATA  Command,  Command  Message  Format"  for
+    23             specific  information  about  these errors.  Note that if
+    24             the command is rejected with one of these  status  codes,
+    25             the  communications paths are not retained, regardless of
+    26             the setting of the  Retain  Communications  Path  command
+    27             modifier.
+
+    28         Invalid Command (subcode "Invalid   Source   Unit's   Storage
+    29                          Subsystem Port Address")
+    30         Invalid Command (subcode "Invalid   Source   Unit's   Storage
+    31                          Subsystem System Address")
+
+    32             See  the  descriptions  of  the  "source  unit's  storage
+    33             subsystem  port  address,"  and  "source  unit's  storage
+    34             subsystem  system  address"  command  message  fields  in
+    35             Section  6.7.1,  "DISK COPY DATA Command, Command Message
+    36             Format" for specific information about these errors.
+
+    37         Invalid Command (subcode "Invalid Entry Locator")
+
+    38             The value specified in the "entloc (entry locator)" field
+    39             is  not  within the limits of the server's "write history
+    40             entry" set.  Note that if the command  is  rejected  with
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-32
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             this  status,  communications  paths are retained only if
+     2             already established by a previous DISK COPY DATA command.
+
+     3             See the description of the History Log  and  Reuse  Entry
+     4             modifiers  in  Section  6.7.1,  "DISK  COPY DATA Command,
+     5             Command  Message  Format"  and  Section  6.19.3,   "WRITE
+     6             HISTORY MANAGEMENT Command, Description" for more detail.
+
+     7             Note that this error can only  occur  if  "write  history
+     8             logging" is supported and the History Log and Reuse Entry
+     9             modifiers are both set in the command message.
+
+    10         Invalid Command (subcode "Invalid Entry Id")
+
+    11             The server located the "write history  entry"  identified
+    12             in  the "entloc (entry locator)" field but found that the
+    13             "entry id" contained in that entry  does  not  equal  the
+    14             value supplied by a host class driver when that entry was
+    15             allocated.  This status  may  also  be  returned  if  the
+    16             History  Log  modifier  was set, the Reuse Entry modifier
+    17             was clear, and the host supplied a value of FFFF (hex) in
+    18             the  "entry  id"  field.   Note  that  if  the command is
+    19             rejected  with  this  status,  communications  paths  are
+    20             retained  only  if already established by a previous DISK
+    21             COPY DATA command.
+
+    22             See the description of the History Log  and  Reuse  Entry
+    23             modifiers  in  Section  6.7.1,  "DISK  COPY DATA Command,
+    24             Command  Message  Format"  and  Section  6.19.3,   "WRITE
+    25             HISTORY MANAGEMENT Command, Description" for more detail.
+
+    26             Note that this error can only  occur  if  "write  history
+    27             logging" is supported and the History Log modifier is set
+    28             in the command message.
+
+    29         Command Aborted
+
+    30             Before returning this status code the server performs the
+    31             following:
+
+    32                 1.  Ceases  issuing  transfer  subcommands  to  the
+    33                     destination unit and the source unit.
+
+    34                 2.  Allows  all  transfer  subcommands   that   are
+    35                     currently  outstanding  on the destination unit
+    36                     to complete.  Outstanding transfer  subcommands
+    37                     issued  to the source may be aborted or allowed
+    38                     to complete.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-33
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1                 3.  Discards any and all  data  obtained  from  the
+     2                     source  unit  and buffered prior to the receipt
+     3                     of the  ABORT  command  as  well  as  any  data
+     4                     obtained  from  the  source unit as a result of
+     5                     allowing outstanding  transfer  subcommands  to
+     6                     complete in Step 2 above.
+
+     7                 4.  Sets the  "logical  block  count"  end  message
+     8                     field as if an unrecoverable error had occurred
+     9                     as described in that field's description.
+
+    10                 5.  If the Retain Communication Paths  modifier  is
+    11                     clear,  performs  the  operations  necessary to
+    12                     disconnect the communications paths established
+    13                     on  behalf  of  the  issuing host class driver.
+    14                     See Section 6.7.3,  "DISK  COPY  DATA  Command,
+    15                     Description"  for  details  of  the  disconnect
+    16                     operations.
+
+
+    17         Unit-Available
+    18         Unit-Offline
+    19         Write Protected
+
+    20             If any of these conditions is detected on the destination
+    21             unit  prior  to initiating the copy operation, the server
+    22             shall reject the  command  with  the  appropriate  status
+    23             code.
+
+    24             If any of these conditions is detected on the destination
+    25             unit  after  the  copy  operation has been initiated, the
+    26             server reports it via the Subcommand Error status code.
+
+    27             In  the  case  of  the  Unit-Available  and  Unit-Offline
+    28             conditions,  the  server  shall  perform  the  operations
+    29             necessary  to   disconnect   the   communications   paths
+    30             previously  established  on  behalf  of  the issuing host
+    31             class driver, if any.  See Section 6.7.3, "DISK COPY DATA
+    32             Command,  Description"  for  details  of  the  disconnect
+    33             operations.
+
+    34             In  the  case   of   the   Write   Protected   condition,
+    35             communications  paths are not retained, regardless of the
+    36             setting  of  the  Retain  Communications  Paths   command
+    37             modifier.
+
+    38         Controller Error (subcode "Remote Connection Lost")
+
+    39             The "Controller-Online" connection between the server and
+    40             a remote source unit's server was spontaneously lost (for
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-34
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             any reason).
+
+     2             Note that unlike a host class driver the server will  not
+     3             attempt   to   recover  from  a  remote  connection  lost
+     4             condition (e.g., re-establish  the  communications  path,
+     5             reissue previously outstanding commands, etc.).  Instead,
+     6             the server performs the following before  returning  this
+     7             status:
+
+     8                 1.  Ceases  issuing  transfer  subcommands  to  the
+     9                     destination unit.
+
+    10                 2.  Allows  all  transfer  subcommands   that   are
+    11                     currently  outstanding  on the destination unit
+    12                     to complete.
+
+    13                 3.  Discards any and all  data  obtained  from  the
+    14                     source   unit   and   buffered   prior  to  the
+    15                     occurrence of the connection lost condition.
+
+    16                 4.  Sets the  "logical  block  count"  end  message
+    17                     field as if an unrecoverable error had occurred
+    18                     as described in that field's description.
+
+    19                 5.  Performs the operations necessary to disconnect
+    20                     the   destination   unit   communications  path
+    21                     established on behalf of the issuing host class
+    22                     driver (regardless of the setting of the Retain
+    23                     Communication  Paths  modifier).   See  Section
+    24                     6.7.3,  "DISK  COPY  DATA Command, Description"
+    25                     for details of the disconnect operations.
+
+    26                 6.  Sends a DISK COPY DATA CORRELATION  information
+    27                     log  message with the Controller Error (subcode
+    28                     "Remote Connection Lost")  event  code  to  the
+    29                     appropriate  host  class  driver.   See Section
+    30                     6.7.3, "DISK COPY  DATA  Command,  Description,
+    31                     Error  Logs  and Correlation Logs," for related
+    32                     information.
+
+    33             Note also that this error can only occur  if  the  source
+    34             unit is located in a remote subsystem.
+
+    35         Controller Error (subcode "Remote Connection Request Failed,
+    36                           Insufficient Resources to Request  Remote
+    37                           Connection)
+    38         Controller Error (subcode "Remote Connection Request Failed,
+    39                           Subsystem Port Address Responded But
+    40                           Subsystem System Address Does Not Match")
+    41         Controller Error (subcode "Remote Connection Request Failed,
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-35
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1                           Subsystem Port Address Responded But No
+     2                           MSCP Server Present")
+     3         Controller Error (subcode "Remote Connection Request Failed,
+     4                           Subsystem Port Address Responded But No
+     5                           Resources Available")
+     6         Controller Error (subcode "Remote Connection Request Failed,
+     7                           Subsystem Port Address Responded But No
+     8                           Credits Available")
+     9         Controller Error (subcode "Remote Connection Request Failed,
+    10                           No Response From Subsystem Port Address")
+    11         Controller Error (subcode "Remote Connection Request Failed,
+    12                           Subsystem  Port  Address  Responding  But
+    13                           Negative  Acknowledge   Retry   Algorithm
+    14                           Exhausted")
+    15         Controller Error (subcode "Remote Connection Request Failed,
+    16                           Subsystem Port Request Timeout Exceeded")
+    17         Controller Error (subcode "Remote Connection Request Failed,
+    18                           Subsystem Port Address Responded But
+    19                           Rejected With Disconnected")
+
+    20             The   server's   initial   attempt   to    establish    a
+    21             "Controller-Online"   connection  with  a  remote  source
+    22             unit's server failed for  the  reason  indicated  by  the
+    23             subcode value.  New subcode values for this status may be
+    24             added  as  the  need  arises   via   an   ECO   to   this
+    25             specification.
+
+    26             The server should  check  for  and  report  a  "subsystem
+    27             system   address"   mismatch   before  checking  for  and
+    28             reporting the unavailability of an MSCP server.  If  that
+    29             is  not possible, a statement to that effect shall appear
+    30             in the server's (controller's) Functional Specification.
+
+    31             Before returning this status code the server shall send a
+    32             DISK  COPY  DATA CORRELATION information log message with
+    33             the  appropriate  Controller   Error   (subcode   "Remote
+    34             Connection Request Failed") event code to the appropriate
+    35             host class driver.  See Section 6.7.3.2, "DISK COPY  DATA
+    36             Command,  Description,  Error Logs and Correlation Logs,"
+    37             for related information.
+
+    38             Note that these errors can only occur if the source  unit
+    39             is located in a remote subsystem.
+
+    40         Controller Error (subcode "Local Connection Request  Failed,
+    41                           Insufficient  Resources  to Request Local
+    42                           Connection)
+
+    43             The   server's   initial   attempt   to    establish    a
+    44             "Controller-Online" connection with a local source unit's
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-36
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             server failed due to insufficient  controller  resources.
+     2             Note that this error can only occur if the source unit is
+     3             located in the same subsystem as the server.
+
+     4         Write History Entry Access Error (subcode "Allocation Failure
+     5                                           Table Full")
+
+     6             The server was unable to allocate a "write history entry"
+     7             and  could  not  record  this fact because the Allocation
+     8             Failure Table was full.  This is a  transient  condition;
+     9             therefore,  the  host  should  reissue  the command.  See
+    10             Section  6.19.3,  "WRITE  HISTORY   MANAGEMENT   Command,
+    11             Description" for more details.
+
+    12             Note that this error can occur  only  if  "write  history
+    13             logging" is supported and the History Log modifier is set
+    14             and Reuse Entry modifier is clear in the command message.
+
+    15             Note  also  that  communications  paths   are   retained,
+    16             regardless  of  the  setting of the Retain Communications
+    17             Paths command modifier.
+
+    18         Subcommand Error (subcode "Destination -- Command Timed Out")
+    19         Subcommand Error (subcode "Source -- Command Timed Out")
+
+    20             The oldest outstanding subcommand the  server  issued  to
+    21             the  destination  unit or the source unit timed out.  The
+    22             subcode bit flag, "Destination" or  "Source,"  identifies
+    23             the device on which the time out occurred.
+
+    24             Note that the server shall implement the command  timeout
+    25             algorithm  described  in Section 4.10, "Command Timeouts"
+    26             for all outstanding subcommands in the same manner  as  a
+    27             host   class   driver,  with  one  exception.   The  only
+    28             exception is that the server makes no attempt to  recover
+    29             from  a  command time out failure (e.g., re-establish the
+    30             communications  path,  reissue   previously   outstanding
+    31             commands, etc.).
+
+    32             Note also that the server's command timeout interval  may
+    33             be  shorter  than  the  command  timeout  interval of the
+    34             remote  server.   Therefore,  the  server  may   indicate
+    35             "progress" to the host while waiting for the subcommand's
+    36             longer interval to lapse.
+
+    37             Before returning this  status  the  server  performs  the
+    38             following:
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-37
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1                 1.  Ceases  issuing  transfer  subcommands  to  the
+     2                     destination unit and the source unit.
+
+     3                 2.  Allows  all  transfer  subcommands   that   are
+     4                     currently  outstanding  on the destination unit
+     5                     and not affected  by  the  command  timeout  to
+     6                     complete.    Outstanding  transfer  subcommands
+     7                     issued to the source and not  affected  by  the
+     8                     command  timeout  may  be aborted or allowed to
+     9                     complete.
+
+    10                 3.  Discards any and all  data  obtained  from  the
+    11                     source   unit   and   buffered   prior  to  the
+    12                     occurrence of the command timeout  as  well  as
+    13                     any  data  obtained  from  the source unit as a
+    14                     result   of   allowing   outstanding   transfer
+    15                     subcommands to complete in Step 2 above.
+
+    16                 4.  Places the appropriate portion of the timed out
+    17                     command's  command  message  in the "subcommand
+    18                     status" end message field.
+
+    19                 5.  Sets the  "logical  block  count"  end  message
+    20                     field as if an unrecoverable error had occurred
+    21                     as described in that field's description.
+
+    22                 6.  Performs the operations necessary to disconnect
+    23                     the  communications paths established on behalf
+    24                     of the issuing host class driver (regardless of
+    25                     the  setting  of the Retain Communication Paths
+    26                     modifier).  See Section 6.7.3, "DISK COPY  DATA
+    27                     Command,   Description"   for  details  of  the
+    28                     disconnect operations.
+
+    29                 7.  Sends a DISK COPY DATA CORRELATION  information
+    30                     log  message  with  the  appropriate Subcommand
+    31                     Error event  code/subcode  to  the  appropriate
+    32                     host   class   driver.   The  "event  dependent
+    33                     information" field should contain a copy of the
+    34                     MSCP  command  message  for the subcommand that
+    35                     timed out.  See  Section  6.7.3.2,  "DISK  COPY
+    36                     DATA   Command,  Description,  Error  Logs  and
+    37                     Correlation Logs," for related information.
+
+
+    38         Subcommand   Error   (subcode  "Destination  --  Inconsistent
+    39                               State")
+    40         Subcommand Error (subcode "Source -- Inconsistent State")
+
+    41             The server detected an  operational  inconsistency  as  a
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-38
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1             result  of the execution of a subcommand it issued to the
+     2             destination unit or the source  unit.   The  subcode  bit
+     3             flag, "Destination" or "Source," identifies the device on
+     4             which the inconsistent state was detected.
+
+     5             This status is returned under the following conditions:
+
+     6                 A.  The  source  unit  is  located  in   a   remote
+     7                     subsystem  and  the  Controller  Initiated  Bad
+     8                     Block Replacement (CIBBR) flag is  not  set  in
+     9                     the   end   message   of   the  SET  CONTROLLER
+    10                     CHARACTERISTICS  command  issued  to  a  remote
+    11                     source  unit's  server  or  the  ONLINE command
+    12                     issued to a  remote  source  unit.   Note  that
+    13                     CIBBR  is  required  on  a  remote  source unit
+    14                     because  the  server  will  not  provide  "Host
+    15                     Initiated Bad Block Replacement." 
+
+    16                 B.  The  server  determines  via  ONLINE   commands
+    17                     issued  to  the source unit and the destination
+    18                     unit that the sector format (512 bytes  or  576
+    19                     bytes)  of  the source unit is not identical to
+    20                     that of the destination unit.
+
+    21                 C.  An ONLINE command  issued  to  the  destination
+    22                     unit  or  the  source  unit is completed with a
+    23                     status of Success (subcode "Already Online").
+
+    24                 D.  An Invalid Command status was  returned  for  a
+    25                     subcommand  issued  to  the destination unit or
+    26                     the source unit.
+
+    27                 E.  
+
+    28  |                  A READ command issued to the source unit, or  a
+    29  |                  WRITE command issued to the destination unit is
+    30  |                  terminated with a status of Host Buffer  Access
+    31  |                  Error   (all   subcodes   except   "Cause   Not
+    32                     Available" and "Host Memory Parity Error").
+
+    33                 F.  An AVAILABLE command issued to the  destination
+    34                     unit   or  the  source  unit  is  completed  or
+    35                     rejected with  a  status  of  Success  (subcode
+    36                     "Spin-down  Ignored"),  Success (subcode "Still
+    37                     Online/Unload  Ignored"),   or   Unit-Available
+    38                     (subcode "Already In Use").
+
+    39             Before returning this  status  the  server  performs  the
+    40             following:
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-39
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1                 1.  Ceases  issuing  transfer  subcommands  to  the
+     2                     destination unit and the source unit.
+
+     3                 2.  Allows  all  transfer  subcommands   that   are
+     4                     currently  outstanding  on the destination unit
+     5                     to complete.  Outstanding transfer  subcommands
+     6                     issued  to the source may be aborted or allowed
+     7                     to complete.
+
+     8                 3.  Discards any and all  data  obtained  from  the
+     9                     source unit and buffered prior to the detection
+    10                     of  the  inconsistency  as  well  as  any  data
+    11                     obtained  from  the  source unit as a result of
+    12                     allowing outstanding  transfer  subcommands  to
+    13                     complete in Step 2 above.
+
+    14                 4.  Places the appropriate portion  of  the  failed
+    15                     command's   end   message  in  the  "subcommand
+    16                     status" end message field.
+
+    17                 5.  Sets the  "logical  block  count"  end  message
+    18                     field as if an unrecoverable error had occurred
+    19                     as described in that field's description.
+
+    20                 6.  Performs the operations necessary to disconnect
+    21                     the  communications paths established on behalf
+    22                     of the issuing host class driver (regardless of
+    23                     the  setting  of the Retain Communication Paths
+    24                     modifier).  See Section 6.7.3, "DISK COPY  DATA
+    25                     Command,   Description"   for  details  of  the
+    26                     disconnect operations.
+
+    27                 7.  For  the   last   four   "Inconsistent   State"
+    28                     conditions  listed  above  (i.e., C through F),
+    29                     sends a DISK COPY DATA  CORRELATION  error  log
+    30                     message  with  the appropriate Subcommand Error
+    31                     event  code/subcode  to  the  appropriate  host
+    32                     class    driver.     The    "event    dependent
+    33                     information" field should contain a copy of the
+    34                     subcommand's end message.  See Section 6.7.3.2,
+    35                     "DISK COPY  DATA  Command,  Description,  Error
+    36                     Logs   and   Correlation   Logs,"  for  related
+    37                     information.
+
+    38                 8.  The last four "Inconsistent  State"  conditions
+    39                     listed  above  (i.e.,  C  through F) indicate a
+    40                     serious malfunction in the server's operation.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-40
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1                     After attempting to deliver the end message for
+     2                     the  DISK  COPY  DATA Command, the server shall
+     3                     enter the "Controller-Available" state relative
+     4                     to all hosts.
+
+
+     5         Subcommand  Error  (subcode  "Destination  --  Unrecoverable
+     6                             Error")
+     7         Subcommand Error (subcode "Source -- Unrecoverable Error")
+
+     8             An unrecoverable error occurred during the execution of a
+     9             subcommand  issued  by the server to the destination unit
+    10             or the source unit.  The subcode bit flag,  "Destination"
+    11             or   "Source,"   identifies   the  device  on  which  the
+    12             unrecoverable error occurred.
+
+    13             The following are classed as unrecoverable errors:
+
+    14                 A.  Any subcommand issued to the  destination  unit
+    15                     or  the source unit is rejected with any of the
+    16                     following  status  codes:   Controller   Error,
+    17                     Drive     Error,     Media     Format    Error,
+    18                     Unit-Available, or Unit-Offline.
+
+    19                 B.  
+
+    20  |                  A READ command issued to the source unit, or  a
+    21  |                  WRITE command issued to the destination unit is
+    22  |                  terminated with a status of Host Buffer  Access
+    23  |                  Error  (subcode  "Cause Not Available" or "Host
+    24                     Memory Parity Error").
+
+    25                 C.  
+
+    26  |                  A READ subcommand issued to the source unit  is
+    27  |                  rejected  with  either  of the following status
+    28  |                  codes:  Compare Error or Data Error.
+
+    29                 D.  A WRITE subcommand issued  to  the  destination
+    30                     unit  is rejected or terminated with any of the
+    31                     following status codes:   Compare  Error,  Data
+    32                     Error, or Write Protected.
+
+
+    33             Before returning this  status  the  server  performs  the
+    34             following:
+
+    35                 1.  Ceases  issuing  transfer  subcommands  to  the
+    36                     destination unit and the source unit.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-41
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1                 2.  Allows  all  transfer  subcommands   that   are
+     2                     currently  outstanding  on the destination unit
+     3                     or the source unit to complete.
+
+     4                 3.  Discards any and all  data  obtained  from  the
+     5                     source unit and buffered prior to the detection
+     6                     of the unrecoverable error.
+
+     7                 4.  Allows  all  transfer  subcommands   that   are
+     8                     currently  outstanding  on the destination unit
+     9                     to complete.  Outstanding transfer  subcommands
+    10                     issued  to the source may be aborted or allowed
+    11                     to complete.
+
+    12                 5.  Discards any and all  data  obtained  from  the
+    13                     source unit and buffered prior to the detection
+    14                     of the unrecoverable error as well as any  data
+    15                     obtained  from  the  source unit as a result of
+    16                     allowing outstanding  transfer  subcommands  to
+    17                     complete in Step 2 above.
+
+    18                 6.  Places the appropriate portion  of  the  failed
+    19                     command's   end   message  in  the  "subcommand
+    20                     status" end message field.
+
+    21                 7.  Sets the  "logical  block  count"  end  message
+    22                     field as described in that field's description.
+
+    23                 8.  With the exception of  the  occurrence  of  the
+    24                     Media   Format   Error,   Unit-Available,   and
+    25                     Unit-Offline  error  conditions  listed  in   A
+    26                     above,  performs  the  operations  necessary to
+    27                     disconnect the communications paths established
+    28                     on  behalf  of the issuing host class driver if
+    29                     the  Retain  Communication  Paths  modifier  is
+    30                     clear.
+
+    31                     If the Media Format Error, Unit-Available,  and
+    32                     Unit-Offline error conditions listed in A above
+    33                     occur, performs  the  operations  necessary  to
+    34                     disconnect the communications paths established
+    35                     on behalf of  the  issuing  host  class  driver
+    36                     (regardless   of  the  setting  of  the  Retain
+    37                     Communication  Paths  modifier).   See  Section
+    38                     6.7.3,  "DISK  COPY  DATA Command, Description"
+    39                     for details of the disconnect operations.
+
+    40                 9.  Sends a DISK COPY DATA CORRELATION  information
+    41                     log  message  with  the  appropriate Subcommand
+    42                     Error event  code/subcode  to  the  appropriate
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-42
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1                     host   class   driver.   The  "event  dependent
+     2                     information" field should contain a copy of the
+     3                     subcommand's end message.  See Section 6.7.3.2,
+     4                     "DISK COPY  DATA  Command,  Description,  Error
+     5                     Logs   and   Correlation   Logs,"  for  related
+     6                     information.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-43
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     6.7.3  Description
+
+
+     2     Support of the DISK COPY DATA command is a server  implementation
+     3     dependent option.  Servers that do not provide such support shall
+     4     reject this command as an  "Invalid  Command"  ("invalid  opcode"
+     5     protocol  error.   See  "Invalid  Command" status in Section 5.5,
+     6     "Status Codes").
+
+     7     For servers that support only local DISK  COPY  DATA  operations,
+     8     the minimum command length is 44 bytes.  For servers that support
+     9     remote DISK COPY DATA operations, the minimum command  length  is
+    10     60  bytes.   All DISK COPY DATA commands issued by the host shall
+    11     be at least as long  as  the  minimum  length  supported  by  the
+    12     server.   If  a  server receives a DISK COPY DATA command that is
+    13     shorter than the minimum command length, it rejects  the  command
+    14     as an "Invalid Command" ("invalid message length" protocol error.
+    15     See "Invalid Command" status in Section 5.5, "Status Codes").
+
+    16     Servers that support remote DISK COPY DATA shall  support  remote
+    17     copies  between  units  with  unlike geometries and remote copies
+    18     where the source LBN and destination  LBN  differ.   All  servers
+    19     should  support  copies  between  units  with  unlike geometries,
+    20     copies where the source and destination LBN  differ,  and  copies
+    21     where the source and destination are the same unit.  Servers that
+    22     do not support local copies between units with unlike geometries,
+    23     local  copies  where  the  source and destination LBN differ, and
+    24     copies where  the  source  and  destination  are  the  same  unit
+    25     indicate this lack of support by setting the Restricted DISK COPY
+    26     DATA  Operations  controller   flag   in   the   SET   CONTROLLER
+    27     CHARACTERISTICS   end   message  (see  Section  5.6,  "Controller
+    28     Flags").
+
+    29     This command allows data to be copied from  one  disk  device  to
+    30     another  without  host  intervention.   A  server may support two
+    31     different types of copy operations:   local  and  remote.   In  a
+    32     local copy operation, both the source and destination disk drives
+    33     are physically connected to the storage subsystem  in  which  the
+    34     server  that  receives this command is located.  In a remote copy
+    35     operation, the destination disk drive is physically connected  to
+    36     the  storage  subsystem  in  which  the server that received this
+    37     command is located  and  the  source  disk  drive  is  physically
+    38     connected  to  another  storage  subsystem  connected to the same
+    39     communications mechanism.  Local copy operations may be supported
+    40     by  servers  that provide Multihost Support as well as those that
+    41     do not.  A server that provides Multihost  Support  may,  at  its
+    42     option,  support  both  local  and remote copy operations or only
+    43     local copy operations.  However, support of both local and remote
+    44     copy  operations  is  preferred.   A server signifies the type of
+    45     copy operation it supports by setting the Local  Disk  Copy  Data
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-44
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     and Remote Disk Copy Data controller flags appropriately.
+
+     2     6.7.3.1  Operation
+
+     3     To perform a copy operation a host class driver merely  specifies
+     4     the  devices  that  are  to  act  as  the  data  source  and data
+     5     destination, the starting logical block numbers on the source and
+     6     destination,  and  the  quantity  of  data  to be copied from the
+     7     source  to  the  destination.   The  server  then  performs   the
+     8     following  operations  to  fulfill  the  host's  DISK  COPY  DATA
+     9     request:
+
+    10           1.  Checks  the  contents  of   the   command   message
+    11               modifiers  and  fields  to ensure that a valid copy
+    12               operation can be performed.  The  checks  performed
+    13               are  described  in  (or  implied  by)  the  various
+    14               modifier and field descriptions in  Section  6.7.1,
+    15               "DISK  COPY DATA Command, Command Message Format." 
+    16               If any of the checks fail, the command is  rejected
+    17               as an "Invalid Command." 
+
+    18               Some of the checks require information provided  by
+    19               operations  performed  in  Steps 4, 5, and 6 below.
+    20               Command message field validation is  therefore  not
+    21               complete until those steps have been completed.
+
+    22           2.  If communications paths to the destination unit and
+    23               the  source  unit  have already been established on
+    24               behalf of the issuing host class driver  and  those
+    25               paths remain intact, proceeds to Step 6.
+
+    26               Otherwise, proceeds to Step 3.
+
+    27           3.  Performs  the  internal  operations  necessary   to
+    28               establish  a  "Controller-Online"  connection  with
+    29               itself   to   enable   communications   with    the
+    30               destination  unit  and  in the case of a Local Disk
+    31               Copy Data (i.e., the Local Source Unit modifier  is
+    32               set  or  the  server  only supports Local Disk Copy
+    33               Data) with the source unit.
+
+    34               If the source unit is local, proceeds to Step 4.
+
+    35               If the source unit is remote (i.e., it is connected
+    36               to  a storage subsystem other than the one in which
+    37               the server is located), performs the  communication
+    38               protocol    dependent   operations   necessary   to
+    39               establish a "Controller-Online" connection with the
+    40               source  unit's  server  (i.e., the remote disk MSCP
+    41               server) and then proceeds to Step 4.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-45
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1           4.  Issues a SET CONTROLLER CHARACTERISTICS commands to
+     2               itself  and,  if  the  source  unit is located in a
+     3               remote subsystem, to the source unit's  server,  to
+     4               set  an  appropriate "host timeout," to request the
+     5               generation  of  error  logs  if  required,  and  to
+     6               determine the maximum byte count supported.
+
+     7               Note that processing related to "host timeout"  and
+     8               error logging is discussed later in this section.
+
+     9           5.  Issues an ONLINE command  to  the  source  unit  to
+    10               bring  it into the "Unit-Online" state (relative to
+    11               the server) and ensures that the  destination  unit
+    12               is "Unit-Online."
+
+    13               Note that the server shall issue  only  one  ONLINE
+    14               command  if the destination unit is the same as the
+    15               source unit.
+
+    16           6.  Issues GET UNIT STATUS commands to the  destination
+    17               unit   and   the   source  unit  to  obtain  device
+    18               characteristics (i.e., disk volume geometry) useful
+    19               for optimizing the copy operation.
+
+    20               Note  that  during  this  step  the  server   shall
+    21               validate   the  "source  unit  identifier"  command
+    22               message field using the value returned in the "unit
+    23               identifier"  end  message  field  of  the  GET UNIT
+    24               STATUS command issued.  See the description of  the
+    25               "source unit identifier" field in Section 6.7.1 for
+    26               more detail.
+
+    27           7.  Issues a READ command to the source unit to  obtain
+    28               the source data.
+
+    29           8.  Issues a WRITE command to the destination  unit  to
+    30               copy the source data.
+
+    31           9.  Steps 7 and 8 are repeated until the copy operation
+    32               is aborted, completed, or terminated.
+
+    33               Note that for subsequent DISK  COPY  DATA  commands
+    34               issued  by  the  same  host class driver the server
+    35               will not perform Steps 3, 4, and  5  (provided  the
+    36               communications   paths   involved  remain  intact).
+    37               However, the server shall perform Steps 1 and 6 (as
+    38               well  as  Steps  7  and  8) for each DISK COPY DATA
+    39               command received.   The  server  shall  retain  all
+    40               information  acquired  in  Steps  4  and  5 that is
+    41               required to perform Step 1 until the communications
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-46
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1               paths involved are no longer intact.
+
+     2       The operations described in the  preceding  steps  provide  a
+     3       simplified  model  of  operation  that  is solely intended to
+     4       identify and describe functions that  are  implied  in  other
+     5       sections  of this command's description.  Any deviations from
+     6       the model shall be implemented in a  manner  that  guarantees
+     7       that  all  host class driver visible results are identical to
+     8       those described in this command description.
+
+     9     The actions the server takes after the copy  operation  has  been
+    10     aborted,  completed,  or  terminated are described in the various
+    11     status code  descriptions  in  Section  6.7.2,  "DISK  COPY  DATA
+    12     Command, End Message Format." 
+
+    13     Throughout this command description numerous references are  made
+    14     to  the  effect that the server issues standard MSCP commands (in
+    15     the same manner as a host class driver).  Note that the server is
+    16     expressly  permitted to perform any equivalent internal operation
+    17     instead of issuing a standard command provided that the following
+    18     rules are followed:
+
+    19         A.  The result of the internal operation shall be identical
+    20             to that of the standard command.
+
+    21         B.  If the internal operation  results  in  an  error,  the
+    22             "subcommand  status"  end message field shall be set as
+    23             if a standard command had been issued.
+
+    24     Naturally,  the  use  of  internal  operations  is   limited   to
+    25     transactions  the server can perform on itself and the unit(s) it
+    26     serves.  Standard commands shall  be  used  in  all  transactions
+    27     dealing with a remote source unit and its server.
+
+    28     The  server  shall  provide  "Controller  Initiated   Bad   Block
+    29     Replacement" (CIBBR) support on all disk drives that it serves --
+    30     i.e., the CIBBR controller flag shall be set.
+
+    31     The  server  will  not  perform   "Host   Initiated   Bad   Block
+    32     Replacement" for a remote source unit.  Host class drivers should
+    33     verify that CIBBR is supported on a remote source unit  prior  to
+    34     issuing  this  command.   Failure  to  do  so  will result in the
+    35     rejection of this command  with  a  status  of  Subcommand  Error
+    36     (subcode "Source -- Inconsistent State").  See the description of
+    37     that status in  Section  6.7.2,  "DISK  COPY  DATA  Command,  End
+    38     Message Format," for more detail.
+
+    39     As mentioned in the description of the Subcommand Error  (subcode
+    40     "Destination/Source  --  Command Timeout) status in Section 6.7.2
+    41     the  server  shall  implement  the  command   timeout   algorithm
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-47
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     described in Section 4.10, "Command Timeouts" for all outstanding
+     2     subcommands in the same manner as a host class driver,  with  one
+     3     exception.   The  only  exception  is  that  the  server makes no
+     4     attempt to recover from a command time out failure (e.g., reissue
+     5     previously outstanding commands).
+
+     6     With regard to GET  COMMAND  STATUS  commands  that  specify  the
+     7     "outstanding  reference  number"  of a DISK COPY DATA command the
+     8     server shall provide status based on the  progress  of  the  copy
+     9     operation in the same manner required for any other command.  See
+    10     Sections 6.11, "GET COMMAND STATUS Command" and  7.5,  "Multihost
+    11     GET  COMMAND  STATUS Command" for more detail.  Note that certain
+    12     communications protocols (e.g., SCA)  do  not  provide  a  direct
+    13     method  for  determining  whether  a remote subsystem exists.  If
+    14     such a communications protocol is in use, the server's request to
+    15     establish  a  "Controller-Online" connection with a remote source
+    16     unit's server will never  complete  if  the  subsystem  does  not
+    17     exist.   To  allow  host  class  drivers  to  react  to  such  an
+    18     occurrence the server shall not show progress  for  a  DISK  COPY
+    19     DATA  command  where  the  source  unit  is  located  in a remote
+    20     subsystem until after the "Controller-Online" connection  to  the
+    21     remote  subsystem has been successfully established.  That action
+    22     will eventually cause  the  host  class  driver  to  timeout  the
+    23     affected  DISK  COPY  DATA  command as described in Section 4.10,
+    24     "Command Timeouts."  In such a case it is highly recommended that
+    25     a  host  class  driver  not resynchronize with the server without
+    26     first checking that it can still access the remote  source  unit.
+    27     If  the  remote  source can no longer be accessed, the host class
+    28     driver should issue an ABORT  command  referencing  the  affected
+    29     DISK COPY DATA command.  In response to such an ABORT command the
+    30     server shall cancel  the  connection  establishment  request  and
+    31     terminate  the  command  in  the  normal  fashion.  If the remote
+    32     source unit can still be accessed, the host class  driver  should
+    33     assume  that  the  server  may  be  insane and perform the normal
+    34     command timeout operations as prescribed.
+
+    35     The server shall not limit the use of  a  particular  destination
+    36     unit  or  source  unit to copy operations requested by any single
+    37     host class driver.  Communications paths established on behalf of
+    38     multiple class drivers may be managed in any manner convenient to
+    39     the server.  Regardless of the  method  used  for  communications
+    40     path management the server shall:
+
+    41         A.  Maintain state context separately for each  host  class
+    42             driver  to  give  the  appearance that a communications
+    43             path is established solely on an individual host  class
+    44             driver's behalf.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-48
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         B.  Associate the communications  paths  established  on  a
+     2             particular   host   class   driver's  behalf  with  the
+     3             "Controller-Online" connection established between  the
+     4             server  and  that host class driver.  (This association
+     5             is provided to ensure that the communications paths are
+     6             dissolved  following  the loss of a "Controller-Online"
+     7             connection.)
+
+     8     The server shall disconnect the communications paths  established
+     9     on   behalf   of   a   host  class  driver  under  the  following
+    10     circumstances if and only  if  the  Retain  Communications  Paths
+    11     modifier is clear:
+
+    12         A.  This command is aborted  --  i.e.,  status  is  Command
+    13             Aborted.
+
+    14         B.  This command is completed successfully -- i.e.,  status
+    15             is Success (subcode "Normal").
+
+    16         C.  An unrecoverable error occurred during the execution of
+    17             a  subcommand  issued  by the server to the destination
+    18             unit or the  source  unit  --  i.e.,  this  command  is
+    19             terminated    with    a   Subcommand   Error   (subcode
+    20             "Destination/Source --  Unrecoverable  Error")  status,
+    21             all  causes  except Media Format Error, Unit-Available,
+    22             and Unit-Offline.
+
+    23     Regardless of the setting  of  the  Retain  Communications  Paths
+    24     modifier  the  server  shall  disconnect the communications paths
+    25     established on behalf of a host class driver under the  following
+    26     circumstances:
+
+    27         A.  The "Controller-Online" connection to  the  host  class
+    28             driver   is   lost   --   i.e.,   the   server  becomes
+    29             "Controller-Available" or "Controller-Offline" relative
+    30             to the issuing host class driver.
+
+    31         B.  The destination unit  enters  the  "Unit-Available"  or
+    32             "Unit-Offline"  state relative to the host class driver
+    33             -- i.e., this command is rejected with a Unit-Available
+    34             or Unit-Offline status.
+
+    35         C.  An unrecoverable error occurred during the execution of
+    36             a  subcommand  issued  by the server to the destination
+    37             unit or the  source  unit  --  i.e.,  this  command  is
+    38             terminated    with    a   Subcommand   Error   (subcode
+    39             "Destination/Source --  Unrecoverable  Error")  status,
+    40             only  if  the  error  status  is  Media  Format  Error,
+    41             Unit-Available, or Unit-Offline.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-49
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         D.  The "Controller-Online" connection to a  remote  source
+     2             unit's   server  is  lost  --  i.e.,  this  command  is
+     3             terminated with a  Controller  Error  (subcode  "Remote
+     4             Connection Lost") status.
+
+     5         E.  A subcommand issued by the server to the destination or
+     6             the  source  unit is timed out -- i.e., this command is
+     7             terminated   with   a   Subcommand    Error    (subcode
+     8             "Destination/Source -- Command Timed Out") status.
+
+     9         F.  An inconsistent state is  detected  by  the  server  --
+    10             i.e.,  this  command  is  terminated  with a Subcommand
+    11             Error  (subcode  "Destination/Source  --   Inconsistent
+    12             State") status.
+
+    13     The server performs the following operations  to  disconnect  the
+    14     communications  paths  established  on  behalf  of  a  host class
+    15     driver:
+
+    16         A.  Discards the communications path  state  context  being
+    17             maintained on behalf of the issuing host class driver.
+
+    18         B.  Issues an AVAILABLE command to the destination unit.
+
+    19             Note that the server shall not issue  this  command  if
+    20             the  disconnect  is  being  performed  as a result of a
+    21             command time out failure on the destination unit.
+
+    22         C.  Issues an AVAILABLE command to the source unit.
+
+    23             Note that the server shall not issue  this  command  if
+    24             the  disconnect  is  being  performed  as a result of a
+    25             command time out failure on the source unit or  if  the
+    26             destination unit is the same as the source unit.
+
+    27         D.  Performs   the   internal   operations   necessary   to
+    28             disconnect  the internal "Controller Online" connection
+    29             to itself.
+
+    30         E.  If the source unit is located on  a  remote  subsystem,
+    31             performs    the   communications   protocol   dependent
+    32             operations  necessary  to  disconnect  the  "Controller
+    33             Online" connection between itself and the source unit's
+    34             server.
+
+    35     Note that if a communications path is shared among multiple  host
+    36     class drivers, the server shall not perform any of the disconnect
+    37     steps described  above  that  would  result  in  the  loss  of  a
+    38     communications  path established on behalf of a host class driver
+    39     other than the issuing host class driver.  That  is,  the  server
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-50
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     shall  not  disconnect  the  communications  path  established on
+     2     behalf of a host class driver unless the server is  processing  a
+     3     DISK COPY DATA command issued by that host class driver.
+
+     4     To prevent needless consumption  of  remote  source  unit  server
+     5     resources  in  cases where the server spontaneously loses context
+     6     (e.g., crashes, power  failures)  after  having  established  and
+     7     retained  a communications path on behalf of a host class driver,
+     8     the server shall perform the following:
+
+     9         A.  Supply an implementation dependent "host timeout" value
+    10             in the SET CONTROLLER CHARACTERISTICS command issued to
+    11             the remote source unit's server in Step 4 above.
+
+    12             The "host  timeout"  value  supplied  should  be  large
+    13             enough  to  not  cause  a timeout on the communications
+    14             path established and retained on behalf of a host class
+    15             driver while the "Controller-Online" connection between
+    16             that host class driver and the  server  remains  intact
+    17             and operations appear to be normal.
+
+    18         B.  Whenever a DISK COPY DATA  command  that  involves  the
+    19             remote  source  unit (and the associated communications
+    20             path) is not currently in progress, periodically  issue
+    21             an  innocuous  command  (e.g.,  GET UNIT STATUS) to the
+    22             remote source unit at intervals  no  greater  than  the
+    23             "host timeout" supplied.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-51
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     The chart below summarizes the condition under which  the  server
+     2     retains or terminates communications paths prior to returning the
+     3     DISK COPY DATA end message.
+
+     4     +-----------------------------------+------------+--------------+
+     5     |                                   | Retain CP  |  Comm Paths  |
+     6     | End Message Status Code/Subcode   | modifier   |   Retained   |
+     7     |                                   |in cmd mess |   end flag   |
+     8     +-----------------------------------+------------+--------------+
+     9     | Success, subcodes                 |            |              |
+    10     |  "Normal"                         |            |              |
+    11     |  "Duplicate Unit Number"          |    Set     |              |
+    12     | Command Aborted                   |            |              |
+    13     | Subcommand Error, subcode         +------------+--------------+
+    14     |  "Src/Dst Unrecoverable Error" (1)|   Clear    |    Clear     |
+    15     +-----------------------------------+------------+--------------+
+    16     | Invalid Command, subcodes         |            |              |
+    17     |  "Invalid Source LBN"             |            |     Set      |
+    18     |  "Invalid Destination LBN"        |            | (Comm Paths  |
+    19     |  "Invalid Logical Block Count"    |            |    always    |
+    20     |  "Invalid Entry ID"               |            |   retained)  |
+    21     |  "Invalid Entry Locator"          |            |              |
+    22     | Write Hist Entry Access Error     |            |              |
+    23     |  "Allocation Failure Table Full"  |            |              |
+    24     +-----------------------------------+            +--------------+
+    25     | Invalid Command, subcodes         |            | Set if Comm  |
+    26     |  "Invalid Modifier"               |            | Paths up from|
+    27     |                                   |            | previous DCD;|
+    28     |                                   |            | no otherwise.|
+    29     +-----------------------------------+            +--------------+
+    30     | Invalid Command, subcodes         |            |              |
+    31     |  "Invalid Src Unit Number"        |            |    Clear     |
+    32     |  "Invalid Src Unit Identifier"    |  Ignored   | (Comm Paths  |
+    33     | Controller Error (all subcodes)   |            |    never     |
+    34     | Subcommand Error, subcodes        |            |   retained)  |
+    35     |  "Src/Dst Unrecoverable Error" (2)|            |              |
+    36     |  "Src/Dst Command Timed Out"      |            |              |
+    37     |  "Src/Dst Inconsistent State"     |            |              |
+    38     +-----------------------------------+            +--------------+
+    39     | Invalid Command, subcodes         |            |              |
+    40     |  "Invalid Source Unit's Storage   |            | Clear. Server|
+    41     |    Subsystem Port Address"        |            | rejects the  |
+    42     |  "Invalid Source Unit's Storage   |            | DCD command. |
+    43     |    Subsystem System Address"      |            | Comm Paths   |
+    44     | Unit Available                    |            | do not get   |
+    45     | Unit Offline                      |            | established. |
+    46     | Write Protected                   |            |              |
+    47     +-----------------------------------+------------+--------------+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-52
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         Notes:
+
+     2         1.  All cases except Media Format Error, Unit-Available, Unit-
+     3             Offline.   So, errors like Data Error on a READ subcommand
+     4             only  cause  the  server  to  terminate the communications
+     5             paths if the Retain CP modifier is clear.
+             
+     6         2.  Reports  cases  where  a subcommand is rejected with Media 
+     7             Format Error, Unit-Available, or Unit-Offline status.
+
+     8     The server shall not request attention messages  (i.e.,  set  the
+     9     Enable  Attention  Messages controller flag) from a remote source
+    10     unit's server via  the  SET  CONTROLLER  CHARACTERISTICS  command
+    11     issued in Step 4 above.  Therefore, host class drivers should set
+    12     the  "Enable  Attention  Messages"  flag  via  a  SET  CONTROLLER
+    13     CHARACTERISTICS     command     issued     over     a    separate
+    14     "Controller-Online" connection to the remote source unit's server
+    15     if attention messages related to that unit are desired.
+
+    16     The server  shall  not  attempt  to  change  the  current  device
+    17     characteristics  of the source unit via the ONLINE command issued
+    18     in Step 5 above.  If a host class driver requires certain  source
+    19     unit  characteristics  to  be  set,  it  should set them prior to
+    20     issuing this command.
+
+    21     The  server  shall  execute  a  DISK  COPY  DATA  command  as   a
+    22     Non-Sequential command -- i.e., in relation to all other commands
+    23     addressed to the destination unit, the transfer  subcommands  the
+    24     server  issues  to  the  destination  unit  as  part  of the copy
+    25     operation may  be  reordered  to  optimize  performance,  may  be
+    26     interleaved  with  other Non-Sequential commands, and all of them
+    27     shall be completed before the execution of a  Sequential  command
+    28     can  be  initiated.  Host software developers are advised to give
+    29     careful consideration to the effects of issuing  a  lengthy  DISK
+    30     COPY DATA command on Sequential command execution.
+
+    31     The only exception to the Non-Sequential execution just described
+    32     is  that  access  to  the  server's  "immediate  target"  on  the
+    33     destination unit is controlled.  The server's "immediate  target"
+    34     is  the  range of logical blocks on the destination unit that the
+    35     server is about to act upon or is currently acting upon.   To  be
+    36     more  specific  the  "immediate  target"  is the range of logical
+    37     blocks specified in the READ subcommand used to obtain a fragment
+    38     of  source  data  and  the  WRITE  subcommand  used  to  copy the
+    39     resultant data to the destination unit.
+
+    40     Access to the server's "immediate target" on the destination unit
+    41     shall be controlled according to the following rules:
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-53
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1         a.  If a command that specifies a range of  logical  blocks
+     2             that  is  within  or  overlaps  the server's "immediate
+     3             target" is currently in progress, the server  postpones
+     4             issuing  any  subcommands  that  affect  the "immediate
+     5             target" until such an in  progress  command  completes,
+     6             terminates, or is aborted.
+
+     7         b.  If a command that specifies a range of  logical  blocks
+     8             that  is  within  or  overlaps  the server's "immediate
+     9             target"  is  received  while  server  action   on   the
+    10             "immediate  target"  is  postponed (as described in "a"
+    11             above), the  server  postpones  the  execution  of  the
+    12             command  received until the subcommands that affect the
+    13             "immediate target" have been initiated  and  have  then
+    14             completed or terminated.
+
+    15         c.  If a command that specifies a range of  logical  blocks
+    16             that  is  within  or  overlaps  the server's "immediate
+    17             target" is received while the subcommands  that  affect
+    18             the  "immediate  target" are currently in progress, the
+    19             server postpones the execution of the command  received
+    20             until   the  subcommands  that  affect  the  "immediate
+    21             target" have been completed or terminated.
+
+    22     The effect of these rules is to  treat  access  to  the  server's
+    23     "immediate  target" as Sequential operations.  When selecting the
+    24     number of logical blocks to be included in a server's  "immediate
+    25     target,"   server   developers   are   advised  to  give  careful
+    26     consideration to the  effects  of  postponing  host  I/O  to  the
+    27     "immediate target."
+
+    28     The reason for providing the "immediate target"  access  behavior
+    29     described  above  is to allow host class drivers to perform write
+    30     type operations (i.e., DISK COPY DATA, ERASE, and WRITE)  on  the
+    31     destination  unit  in  a manner that guarantees that while a DISK
+    32     COPY DATA command is in progress, all  data  written  to  logical
+    33     blocks  on  the  destination  unit  will  be  identical  to those
+    34     contained on the source unit.  To fulfill  that  guarantee,  host
+    35     class  drivers  shall ensure that write operations that specify a
+    36     range of logical blocks that is within or overlaps the  range  of
+    37     logical  blocks  specified in a DISK COPY DATA command are issued
+    38     to the source unit and  completed  there  before  the  equivalent
+    39     operation is issued to the destination unit.
+
+    40     Note that the result of a  read  type  operation  (i.e.,  ACCESS,
+    41     COMPARE  HOST DATA, and READ) issued to the destination unit that
+    42     specifies a range of logical blocks that is  within  or  overlaps
+    43     the range of logical blocks specified in a DISK COPY DATA command
+    44     is unpredictable.   Host  class  drivers  should  therefore  rely
+    45     solely  on  the  contents  of  the  source  unit  for  read  type
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-54
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     operations.
+
+     2     6.7.3.2  Error Logs And Correlation Logs
+
+     3     All error logs generated due to the execution of a DISK COPY DATA
+     4     command are handled by the server and returned to the appropriate
+     5     host class drivers, irrespective of whether the error  originated
+     6     from  the  destination  unit,  a  local  source unit, or a remote
+     7     source unit.
+
+     8     For error logs generated as a result of subcommands issued to the
+     9     destination  and  source  units the server performs the following
+    10     operations:
+
+    11          A.  Sets the "command reference number" field equal  to  the
+    12              value  supplied  in the "command reference number" field
+    13              of the related DISK COPY DATA command message.
+
+    14          B.  If the error log message is from the source  unit,  sets
+    15              the Informational error log message flag.
+
+    16          C.  If the error log message is from a remote  source  unit,
+    17              sets the Disk Copy Data Correlate error log message flag
+    18              and remembers that at least  one  error  log  associated
+    19              with  this  command  has been delivered so that one DISK
+    20              COPY DATA CORRELATION error log message may be delivered
+    21              to  the  appropriate class drivers prior to the delivery
+    22              of the end message for this particular  DISK  COPY  DATA
+    23              command.
+
+    24          D.  Sends the modified message to the appropriate host class
+    25              drivers.
+
+    26  |       E.  If a  source  unit  communications  path  is  shared  by
+    27  |           multiple  host  class  drivers,  the  server shall treat
+    28              error logs received over  that  path  according  to  the
+    29              following rules (assuming that the host class driver has
+    30              properly enabled error logging):
+
+    31              1.  Error logs generated by subcommands issued on behalf
+    32                  of  an  individual  host  class driver shall be sent
+    33                  only to that particular host class driver.
+
+    34              2.  Error logs generated by subcommands issued on behalf
+    35                  of  all  the  host class drivers that share the same
+    36                  path shall be sent to all  the  host  class  drivers
+    37                  sharing that path.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-55
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1     Prior to sending the end message for a particular DISK COPY  DATA
+     2     command,  the server generates a DISK COPY DATA CORRELATION error
+     3     log and delivers it to the  appropriate  class  drivers  for  the
+     4     following cases:
+
+     5          1.  The server determines that  the  source  or  destination
+     6              unit  is in an inconsistent state for any of the reasons
+     7              described in Section 6.7.2, "DISK COPY DATA Command, End
+     8              Message Format," except:  Controller Initiated Bad Block
+     9              Replacement is not supported by the remote source unit's
+    10              server   or   the  source  and  destination  units  have
+    11              different formats.
+
+    12              The event code is the same as the status  code  returned
+    13              in the DISK COPY DATA command's end message (see Section
+    14              6.7.2, "DISK COPY DATA Command,  End  Message  Format").
+    15              The "event dependent information" field should contain a
+    16              copy of the subcommand's end message.
+
+    17          2.  The status  code  returned  in  the  end  message  of  a
+    18              subcommand  issued  to  the  source  or destination unit
+    19              indicates  an  unrecoverable  error  other   than   Unit
+    20              Available, Unit Offline, or Write Protected.
+
+    21              The event code is the same as the status  code  returned
+    22              in the DISK COPY DATA command's end message (see Section
+    23              6.7.2, "DISK COPY DATA Command,  End  Message  Format").
+    24              The "event dependent information" field should contain a
+    25              copy of the subcommand's end message.
+
+    26          3.  The  server  fails  to  establish  a  connection  to  or
+    27              spontaneously  loses  an  established  connection to the
+    28              remote source unit's server.
+
+    29              The event code is  the  same  as  the  Controller  Error
+    30              status code returned in the DISK COPY DATA command's end
+    31              message (see Section 6.7.2, "DISK COPY DATA Command, End
+    32              Message Format").
+
+    33          4.  The server times out the oldest outstanding  command  to
+    34              either the source or destination unit.
+
+    35              The event code is the same as the status  code  returned
+    36              in the DISK COPY DATA command's end message (see Section
+    37              6.7.2, "DISK COPY DATA Command,  End  Message  Format").
+    38              The "event dependent information" field should contain a
+    39              copy of the subcommand's command message.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-56
+           DISK COPY DATA Command                               11 June 1992
+
+
+     1          5.  At least one error log has been delivered  as  described
+     2              in item C of the previous list.
+
+     3              The event code is Informational (subcode "Disk Copy Data
+     4              Correlation").
+
+
+     5     Note that if more than one of the above  conditions  exists,  the
+     6     DISK COPY DATA CORRELATION error log shall be as specified by the
+     7     lowest numbered condition.  That is, the conditions listed  above
+     8     are ordered by priority.
+
+     9     The server shall send the DISK COPY DATA  CORRELATION  error  log
+    10     and the error logs modified as described above only to host class
+    11     drivers that have enabled error logging  by  setting  the  Enable
+    12     This  Host's  Error  Logs  controller  flag or setting the Enable
+    13     Other Host's Error Logs  controller  flag  in  a  SET  CONTROLLER
+    14     CHARACTERISTICS command.
+
+    15     The server performs the following special actions with regard  to
+    16     the  source  unit  communications path established on behalf of a
+    17     host class driver when error logging  is  enabled  as  previously
+    18     described:
+
+    19          A.  If a host class driver enables error  logging  before  a
+    20              source  unit  communications  path has been established,
+    21              the server shall set the Enable This Host's  Error  Logs
+    22              controller  flag  via the SET CONTROLLER CHARACTERISTICS
+    23              command issued to the source unit's  server  in  Step  4
+    24              above.
+
+    25          B.  If a host class driver enables  error  logging  after  a
+    26              source  unit  communications  path has been established,
+    27              the server shall issue a SET CONTROLLER  CHARACTERISTICS
+    28              command  to  the  source unit's server to set the Enable
+    29              This  Host's  Error  Logs  controller  flag   for   that
+    30              connection  before  issuing  any other subcommand to the
+    31              source unit.
+
+    32          C.  If any host class driver or server action  occurs  which
+    33              causes error logging to be disabled after the server has
+    34              issued a SET CONTROLLER CHARACTERISTICS command  to  the
+    35              source unit's server to set the Enable This Host's Error
+    36              Logs controller flag on a connection, the  server  shall
+    37              issue  a  SET  CONTROLLER CHARACTERISTICS command to the
+    38              source unit's server to clear  the  Enable  This  Host's
+    39              Error  Logs  controller  flag for that connection before
+    40              issuing any other subcommand to the source unit.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-57
+           DISPLAY Command                                      11 June 1992
+
+
+     1     6.8  DISPLAY Command
+
+     2     Command Category:
+
+     3         Controller implementation dependent.  See Section 6.8.3
+
+
+     4     6.8.1  Command Message Format
+
+     5                     31                             0
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |     mode      |      item     |
+    14                     +---------------+---------------+
+    15                     |                               |
+    16                     /            display            /
+    17                     /             text              /
+    18                     |                               |
+    19                     +-------------------------------+
+
+
+    20         item
+
+    21  |          Identifies the information meant to  be  conveyed  to  an
+    22  |          operator.   Qualifies  the interpretation of the mode and
+    23             display text fields.  See description below.
+
+    24         mode
+
+    25  |          Identifies the mode or state of the information meant  to
+    26  |          be conveyed to an operator.  Often represented as an icon
+    27             or by the  visual  rendition  of  the  information.   See
+    28             description below.
+
+    29         display text
+
+    30  |          One or more text strings containing the information meant
+    31  |          to  be conveyed to an operator.  The contents of the item
+    32             and mode fields determine the  expected  number  of  text
+    33             strings  and their interpretation; see description below.
+    34             Each text string consists  of  one  byte  containing  the
+    35             length   followed   by  that  number  of  bytes  encoding
+    36             characters from the ISO Latin-1 character set.  A  length
+    37             byte  that contains zero is followed by no characters and
+    38             denotes a null or zero length string.  While ISO  Latin-1
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-58
+           DISPLAY Command                                      11 June 1992
+
+
+     1  |          is  the  character  set  encoding, operator communication
+     2  |          devices need only implement  those  characters  that  are
+     3             meaningful  for  the  specified  item  and  mode  on that
+     4             device.  Multiple text strings are  concatenated  without
+     5             any  intervening  bytes.  Any bytes between the last text
+     6             string and the end of the command message  shall  contain
+     7             zeros, denoting null strings.
+
+     8             If the host supplies fewer than the  expected  number  of
+     9             text  strings,  then  the  MSCP  server shall append null
+    10             strings (zero bytes) to make up the difference.   If  the
+    11             host  supplies  more  than  the  expected  number of text
+    12             strings, and the extra strings are null, then  the  extra
+    13             strings  shall  be  ignored.   If any extra string is not
+    14             null, the server shall ignore the string if the "Must  Be
+    15             Implemented"  modifier  is clear or reject the command if
+    16             that modifier is set.  If the length  byte  of  a  string
+    17             specifies  that  the  string  extends past the end of the
+    18             command message, the server shall truncate the string  at
+    19             the end of the command message; this is not considered an
+    20             error.
+
+    21             The following  example  display  text  field  shows  each
+    22             byte's  value  in  hexadecimal  and  the  equivalent  ISO
+    23             Latin-1 character in parentheses.  This example specifies
+    24             "VMSRL5"  as the first string followed by "Placez" as the
+    25             second string.  These are  followed  by  null  third  and
+    26             fourth strings; these two strings would be ignored if the
+    27             item and mode field values implied that only two  strings
+    28             were expected.
+
+    29                         +-------+-------+-------+-------+
+    30                         | 73(S) | 4D(M) | 56(V) |  06   |
+    31                         +-------+-------+-------+-------+
+    32                         |  06   | 35(5) | 4C(L) | 52(R) |
+    33                         +-------+-------+-------+-------+
+    34                         | 63(c) | 61(a) | 6C(l) | 50(P) |
+    35                         +-------+-------+-------+-------+
+    36                         |  00   |  00   | 7A(z) | 65(e) |
+    37                         +-------+-------+-------+-------+
+
+
+    38     Allowable modifiers:
+
+    39         Must Be Implemented
+
+    40             If clear, the MSCP server shall ignore and return success
+    41             for an unimplemented DISPLAY operation.  If set, the MSCP
+    42             server shall reject and return the  appropriate  "Invalid
+    43             Command" status for an unimplemented DISPLAY operation.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-59
+           DISPLAY Command                                      11 June 1992
+
+
+     1     6.8.2  End Message Format
+
+     2                     31                             0
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status Codes:
+
+    11         Success (subcode "Normal")
+
+    12             Either the requested DISPLAY operation is implemented and
+    13             has   been   successfully  accomplished,  or  it  is  not
+    14             implemented and the "Must Be  Implemented"  modifier  was
+    15             clear.
+
+    16         Invalid Command (subcode "Invalid Opcode")
+
+    17             The unit or controller does  not  implement  the  DISPLAY
+    18             command in its entirety.
+
+    19         Invalid Command (subcode "Invalid Item")
+
+    20             The specified display item code is  not  implemented  and
+    21             the "Must Be Implemented" modifier was set.
+
+    22         Invalid Command (subcode "Invalid Mode")
+
+    23             The specified display mode is  not  implemented  for  the
+    24             specified  display  item  and  the  "Must Be Implemented"
+    25             modifier was set.
+
+    26         Invalid Command (subcodes "Invalid Display Text")
+
+    27             The specified display text is invalid or not  implemented
+    28             for  the specified display item and mode.  Note that many
+    29             different  subcodes  may  be  returned,  as  the  subcode
+    30             indicates  the location of the invalid display text.  The
+    31             following conditions return these subcodes:
+
+
+    32             1.  The host supplied a non-null value for  an  extra  or
+    33                 unimplemented  string  and  the "Must Be Implemented"
+    34                 modifier was set.  The subcode indicates the location
+    35                 of the offending string's length byte.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-60
+           DISPLAY Command                                      11 June 1992
+
+
+     1             2.  The host did not supply a value or it supplied a null
+     2                 value  for  a  string that requires a non-null value.
+     3                 The subcode indicates the location of  the  offending
+     4                 string's  length  byte.   Note  that  the subcode may
+     5                 indicate a location  past  the  end  of  the  command
+     6                 message  if  the  host did not supply a value for the
+     7                 string.
+
+     8             3.  The host supplied a value  that  is  invalid  due  to
+     9                 being  either  too  long  or  too short.  The subcode
+    10                 indicates the  location  of  the  offending  string's
+    11                 length byte.
+
+    12             4.  The host supplied a value that  contains  an  invalid
+    13                 character.  The subcode indicates the location of the
+    14                 offending character.
+
+
+    15         Command Aborted
+
+    16         Unit-Offline
+    17         Unit-Available
+
+    18             The specified display operation  is  implemented  but  is
+    19             inappropriate  in the unit's current state.  These status
+    20             codes shall not be  returned  for  unimplemented  display
+    21             operations.
+
+    22         Controller Error
+    23         Drive Error
+    24  |      Media Loader Error
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-61
+           DISPLAY Command                                      11 June 1992
+
+
+     1     6.8.3  Description
+
+     2  |  A controller may, at its option, treat  the  DISPLAY  command  as
+     3  |  either an Immediate, Sequential, or Non-sequential command.  Host
+     4  |  command timeout algorithms shall  treat  it  as  a  non-Immediate
+     5  |  command.
+
+     6  |  The DISPLAY command conveys information meant to be  communicated
+     7  |  to  an  operator.   Some  devices  implement  device  displays to
+     8  |  communicate with human operators.  A device  display  provides  a
+     9  |  means  of displaying text and other information associated with a
+    10  |  particular  device  or  unit.   Other  devices   may   pass   the
+    11  |  information  to a robotic operator, such as a media loader.  This
+    12  |  specification architects the intent or meaning of the information
+    13  |  being  conveyed.   Whether  or  not particular information can be
+    14  |  communicated, the means  of  communication,  the  nature  of  the
+    15  |  operator  communicated  to,  and the result of such communication
+    16  |  are device and operator dependent.
+
+    17     The conceptual  model  for  use  of  the  DISPLAY  command  is  a
+    18     simplified version of the model for forms management systems such
+    19  |  as DECforms.  Information is presented in a manner analogous to a
+    20     pre-printed form.  A form dictates the arrangement of fields, how
+    21     they appear or are separated, and can provide  fixed  explanatory
+    22     text.   Forms  also  contain  variable  fields whose contents are
+    23     supplied by an application program.
+
+    24     A key feature of forms management systems is that the format  and
+    25     appearance  of a form is kept separate from supplying information
+    26     contained in the form.  The application passes information to the
+    27     run-time  control  portion  of the forms system, called the Forms
+    28     Control  System.   The  Forms   Control   System   presents   the
+    29     information  using  a  form  layout appropriate to the particular
+    30     display device.  Form layouts can  be  defined  for  new  display
+    31     devices or be redefined for existing devices at any time, without
+    32     any application modifications.  Also, display devices need not be
+    33     visual   displays:    voice  output,  tactile  output,  or  other
+    34     nonvisual devices might be used.
+
+    35     In this  forms  management  system  model,  the  DISPLAY  command
+    36     corresponds  to  the  interface  between  the application and the
+    37     Forms Control System.  The host class driver, together with other
+    38     host  software,  acts as the application.  The DISPLAY command is
+    39     the means by which the  host  passes  information  to  the  Forms
+    40     Control System associated with a drive.
+
+    41     The Forms Control System associated with  a  drive  has  complete
+    42     control  of  the  appearance of the device display.  It takes the
+    43     information provided by the host  via  the  DISPLAY  command  and
+    44     presents  it  in  whatever  manner  the  drive  designers  deemed
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-62
+           DISPLAY Command                                      11 June 1992
+
+
+     1     appropriate.  In effect, each new drive design incorporates a new
+     2     display  device and the drive designers must define the layout of
+     3     the MSCP DISPLAY command form on that display device.  Note  that
+     4     not all information need appear in the layout.  That is, the form
+     5     layout  for  a  device  might  ignore  and  not  display  certain
+     6  |  information  that  is unimportant for that device.  A device with
+     7  |  suitable  hardware  capability  might  act  on  the   information
+     8  |  directly,  rather  than  waiting  for a human operator to perform
+     9  |  equivalent actions.  Or the drive designers might prioritize  the
+    10     available  information  and  only  display  the  most  important,
+    11     perhaps using drive state or other internal information  sources.
+    12     Such  techniques are typically used to fit within the constraints
+    13     of a small physical display.
+
+    14     Whereas general purpose forms management systems  typically  deal
+    15  |  with  many  forms, the DISPLAY command only deals with one.  That
+    16     is, there is a single form, a single set of variable fields whose
+    17     contents can be specified by the host.  The logical definition of
+    18     this single form, its  variable  fields  and  their  meaning,  is
+    19     specified  here  as  part  of  the  DISPLAY  command  definition.
+    20     Changes  to  this  logical  form  definition  are  ECOs  to  this
+    21     specification.    However,   the  layout  or  appearance  of  the
+    22     information is part of individual device designs; new or  changed
+    23     layouts  for  this  information  need  not  be  reflected in this
+    24     specification.
+
+    25     The item field of the DISPLAY command designates a variable field
+    26     in the device display form.  Item field values are defined below;
+    27     those  definitions  specify  the  meaning,   interpretation,   or
+    28     semantic  content of the information in that field.  The mode and
+    29     display text fields provide the information for that field.   The
+    30     mode  field  provides  nontextual  or  state  information.  It is
+    31     typically used with items that can be in one of several states or
+    32     modes.    The   drive  designer  might  choose  to  present  this
+    33     information  by  displaying  a  keyword  describing   the   mode,
+    34     displaying  an  icon designating the mode, by changing the visual
+    35     rendition of the field, or by any other means.  The display  text
+    36     field  provides  textual  information  that might be displayed in
+    37     that field.  The drive designer may alter the text or  choose  to
+    38     not  display  it  at  all,  so  long as its meaning is preserved.
+    39     Often, state information is conveyed in both the mode  field  and
+    40     as a string in the display text field.
+
+    41     A unit or controller that does not implement the DISPLAY  command
+    42     at  all  shall  return  "Invalid Command" status with an "Invalid
+    43     Opcode" subcode for this command.  A controller may implement the
+    44     DISPLAY  command  for  some  units but not for others.  A unit or
+    45     controller  that  does  implement   the   DISPLAY   command   may
+    46     selectively  implement the item and mode field values appropriate
+    47     to that device, subject to the restrictions  on  individual  item
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-63
+           DISPLAY Command                                      11 June 1992
+
+
+     1     and  mode combinations described below.  Not implementing an item
+     2     or mode field  value  is  equivalent  to  determining  that  that
+     3     information  is  irrelevant  to  the  device  and  need  never be
+     4  |  displayed  or  utilized.   The  host  can  use   the   "Must   Be
+     5     Implemented"  modifier  to  indicate  that the host considers the
+     6     information to be important, so that  the  item  and  mode  field
+     7     value  must  be implemented and presented to the operator or else
+     8     an error returned.
+
+     9     If the "Must  Be  Implemented"  modifier  is  clear,  a  unit  or
+    10     controller  that  implements  the  DISPLAY  command  shall ignore
+    11     DISPLAY commands that request unimplemented  display  operations.
+    12     Hosts  will  typically  use this for advisory messages, where the
+    13     host will continue with the same actions  regardless  of  whether
+    14  |  the  information  is  actually displayed or otherwise utilized by
+    15  |  the device.
+
+    16     If  the  "Must  Be  Implemented"  modifier  is  set,  a  unit  or
+    17     controller  that  implements  the  DISPLAY  command  shall return
+    18     "Invalid  Command"  status  for  DISPLAY  commands  that  request
+    19     unimplemented  display operations.  Hosts will typically use this
+    20  |  when absence of a device display or other device  specific  means
+    21  |  of  satisfying the request requires that the host communicate the
+    22     information via another means.
+
+    23     All item and mode field values are defined below.  An ECO  adding
+    24     new  item  and  mode  field  values to this list must be approved
+    25     before any device can implement the new values.
+
+
+    26     Item = 0, Mode = any
+
+    27     Item 0 shall never be implemented by any display  device.   Hosts
+    28     may use this item field value to determine if the DISPLAY command
+    29     is implemented with the assurance  that  no  display  information
+    30     will be altered.
+
+
+    31     Item = any, Mode = 0
+
+    32     Mode 0 shall be implemented for every implemented item; it is not
+    33     implemented   for  unimplemented  items.   Mode  0  restores  the
+    34     specified display  item  to  its  default  state  upon  reset  or
+    35     power-up,  typically  blank  or  empty.   Mode  0 expects no text
+    36     strings regardless of the item field value.
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-64
+           DISPLAY Command                                      11 June 1992
+
+
+     1  |  Item = 1    Volume Identification
+
+     2  |  Item 1 identifies the volume associated  with  the  unit  or  the
+     3     volume  mounted  on  the  unit.   For all nonzero mode values the
+     4  |  first text string is the volume  label.   Device  displays  shall
+     5     support  at least the number of characters and character set that
+     6     the applicable volume format standard allows for  volume  labels.
+     7  |  If  a device display is present, any character not displayable by
+     8     the device  display  is  invalid.   For  most  disk  devices  the
+     9     applicable standard is Files-11, which in the past has allowed up
+    10     to 12 characters from the upper  case  alphabet,  digits,  dollar
+    11     sign,   underscore,   and  space.   For  most  tape  devices  the
+    12     applicable standard is the ANSI tape label standard, which in the
+    13     past has allowed up to 6 characters from the upper case alphabet,
+    14     digits, dollar sign, underscore, space, and the following special
+    15     characters:
+
+    16         ! " % ' ( ) * + , - . / : ; < = > ?
+
+    17     Implementors are responsible  for  verifying  that  their  device
+    18     supports  the  current  version of the applicable standard.  They
+    19     shall not assume that the above description is current.
+
+    20     Any unit that implements display item 1 shall implement the three
+    21     modes 0, 1, and 2 for that display item.  This display item shall
+    22     function whenever the unit is "Unit-Online," "Unit-Available," or
+    23     "Unit-Offline,  Known."   This  display  item  shall not function
+    24     whenever the unit is "Unit-Offline" for  any  other  reason;  the
+    25     DISPLAY  command shall be rejected with "Unit-Offline" status and
+    26  |  the appropriate subcode.  A unit may optionally  reject  mode  2,
+    27  |  Mount   Request   Volume   Identification,   when   the  unit  is
+    28  |  "Unit-Online" to any host.  If the unit chooses  to  reject  this
+    29  |  mode  when "Unit-Online", it should treat it as unimplemented and
+    30  |  return either "Success" status  (subcode  "Normal")  or  "Invalid
+    31  |  Command"  status (subcode "Invalid Mode"), depending on the state
+    32  |  of the "Must Be Implemented" modifier.
+
+    33  |  The unit or device display shall clear its volume  identification
+    34     information on reset, power-up, or any other loss of context.  It
+    35  |  shall also clear its volume identification information whenever a
+    36     volume  is  inserted  or  removed  from the drive.  It should not
+    37  |  otherwise alter  its  volume  identification  information  unless
+    38     directed to by a host (via this command).
+
+    39  |  Item = 1, Mode = 0    Clear Volume Identification
+
+    40  |      Clears the volume  identification  information.   Expects  no
+    41         text strings.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-65
+           DISPLAY Command                                      11 June 1992
+
+
+     1  |  Item = 1, Mode = 1    Current Volume Identification
+
+     2         Conveys the volume label currently associated with  the  unit
+     3  |      or the volume mounted on the unit.  Expects one non-null text
+     4  |      string containing the volume label.
+
+     5  |  Item = 1, Mode = 2    Mount Request Volume Identification
+
+     6  |      Identifies the volume that an operator should  mount  on  the
+     7         unit.   Typically the drive should highlight or emphasize the
+     8  |      volume identification to notify the  operator  of  the  mount
+     9  |      request.   Units  that are associated with a media loader may
+    10  |      satisfy  mount  requests  directly,  without  passing   mount
+    11  |      requests to a human operator.  Expects three text strings.
+
+    12  |      The first  text  string  is  either  null  or  the  requested
+    13  |      volume's  label.   The  second  text string is optional; this
+    14         mode shall function regardless of whether the  second  string
+    15         is  null or non-null.  If the second text string is non-null,
+    16         then it  is  the  command  "Mount"  expressed  in  the  local
+    17         language.   The entire ISO Latin-1 character set is valid for
+    18         the second text string; the device display  may  ignore  this
+    19         text  string  (treat  it  as  if  it  were  null), display it
+    20         unmodified, or translate it to a form that can be  displayed.
+    21         For  example, a device display might translate the lower case
+    22         alphabet to upper case.  If  a  device  display  can  neither
+    23         display  the second text string nor translate the string to a
+    24         form that can be displayed, then it shall ignore  the  string
+    25         as if it were null.
+
+    26  |      The third  text  string  is  either  null  or  the  requested
+    27  |      volume's location.  With device displays and human operators,
+    28  |      this would typically use the same character set as the volume
+    29  |      label.   Any  character not displayable by the device display
+    30  |      is invalid.  The format and meaning of the location codes are
+    31  |      outside the scope of T/MSCP; they will typically be chosen by
+    32  |      the individual site  or  installation.   With  robotic  media
+    33  |      loaders,  the  location  will  typically  be  a decimal digit
+    34  |      string encoding the number of the element within  the  loader
+    35  |      that  contains  the requested volume.  If the string does not
+    36  |      identify a valid or  potentially  valid  element  within  the
+    37  |      media loader, then some character in it is invalid.
+
+    38  |      If both the first and third text strings are null, then  this
+    39  |      is  a  request to mount an unidentified volume.  If the first
+    40  |      is non-null and the third null, then this  is  a  request  to
+    41  |      mount a volume with the specified volume label.  If the first
+    42  |      is null and the third non-null, then this  is  a  request  to
+    43  |      mount  the  volume  that  normally  resides  at the specified
+    44  |      location.  If both are non-null, then this is  a  request  to
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-66
+           DISPLAY Command                                      11 June 1992
+
+
+     1  |      mount  a  volume  that  both has the specified label and that
+     2  |      normally resides at the specified location.  A unit need only
+     3  |      use  the  information  that  is  consistent  with its display
+     4  |      device's or media loader's capabilities.
+
+     5  |      The means by which a mount request is resolved, resulting  in
+     6  |      a  specific  physical  volume  being located and mounted, are
+     7  |      outside the scope of T/MSCP.  In general, mount requests  may
+     8  |      specify  volumes  that  do  not  exist,  ambiguously  specify
+     9  |      multiple volumes, or be  self-contradictory.   With  a  human
+    10  |      operator,  treatment  of  these conditions will depend on the
+    11  |      operator's intelligence  and  local  site-specific  policies.
+    12  |      With  robotic  operators,  treatment of these conditions will
+    13  |      depend on the cleverness of the  device's  designers.   While
+    14  |      not  required,  it  is recommended that robotic media loaders
+    15  |      detect any mount request  that  does  not  specify  a  unique
+    16  |      volume  and  either  treat it as unimplemented or return some
+    17  |      other error.
+
+    18  |      Note that the DISPLAY command with this mode and item  merely
+    19  |      delivers  a mount request.  Completion of the DISPLAY command
+    20  |      does not mean that the mount request itself can  or  will  be
+    21  |      satisfied.   If  the  mount  request  is  in  fact eventually
+    22  |      satisfied, the unit will  become  "Unit-Available"  when  the
+    23  |      volume  is  mounted  and  an  attention  message delivered if
+    24  |      enabled.  However there is no guarantee that the  correct  or
+    25  |      requested volume was indeed mounted, since operators (whether
+    26  |      human  or  robotic)  make  mistakes.   The  host   must   use
+    27  |      non-T/MSCP  means, such as label verification, to ensure that
+    28  |      the correct or a suitable volume  has  been  mounted.   If  a
+    29  |      mount request cannot be satisfied, there is in general no way
+    30  |      to report that with T/MSCP.   Hosts  might  use  timeouts  or
+    31  |      similar means to detect mount request failures.
+
+    32  |      Notwithstanding the above, if a device can determine,  during
+    33  |      the  normal  execution  of  the DISPLAY command, that a mount
+    34  |      request cannot or will  not  be  satisfied,  then  it  should
+    35  |      report  that  failure with the DISPLAY command's status code.
+    36  |      This is  most  likely  to  be  possible  with  robotic  media
+    37  |      loaders.    However  T/MSCP  do  not  preclude  such  failure
+    38  |      reporting with human operators as well,  if  any  device  can
+    39  |      implement  it.   The ability to detect mount request failures
+    40  |      and which failures are or are not reported as DISPLAY command
+    41  |      status codes are all extremely device dependent.
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-67
+           ERASE Command                                        11 June 1992
+
+
+     1     6.9  ERASE Command
+
+     2     Command Category:
+
+     3         Non-Sequential
+
+     4     6.9.1  Command Message Format
+
+     5              31                             0              
+     6              +-------------------------------+             
+     7              |   command reference number    |             
+     8              +---------------+---------------+             
+     9              |   reserved    |  unit number  |             
+    10              +---------------+-------+-------+             
+    11              |   modifiers   |  caa  | opcode|             
+    12              +---------------+-------+-------+             
+    13              |          byte count           |             
+    14              +-------------------------------+             
+    15              |                               |             
+    16              +---                         ---+             
+    17              |           reserved            |             
+    18              +---                         ---+             
+    19              |                               |             
+    20              +-------------------------------+             
+    21              |      logical block number     |             
+    22              +---------------+---------------+             
+    23              |   entry id    | hrn or entloc | (optional)  
+    24              +---------------+---------------+             
+
+
+    25         hrn (host reference number) or entloc (entry locator)
+    26         entry id
+
+    27             Optional fields, used in conjunction with the History Log
+    28             and Reuse Entry modifiers.  See the descriptions of those
+    29             modifiers below.
+
+    30     Allowable modifiers:
+
+    31         Clear Serious Exception (ignored for disk class devices)
+    32         Express Request
+    33         Force Error
+    34         Suppress Error Recovery
+    35         History Log
+    36         Reuse Entry
+
+    37             The  History  Log   and   Reuse   Entry   modifiers   are
+    38             interdependent  and  described  together  here  to  avoid
+    39             confusion.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-68
+           ERASE Command                                        11 June 1992
+
+
+     1             Note that the  History  Log  modifier,  the  Reuse  Entry
+     2             modifier,  the  "hrn or entloc" field, and the "entry id"
+     3             field are valid only  for  servers  that  provide  "write
+     4             history logging" support (i.e., the server sets the Write
+     5             History Logging Support controller flag; see Section 5.6,
+     6             "Controller Flags" for related information).
+
+     7             The  setting  of  the  History  Log  modifier  determines
+     8             whether information about this command is to be stored in
+     9             a "write history entry."
+
+    10             The  setting  of  the  Reuse  Entry  modifier  determines
+    11             whether  the  server  is to allocate a new "write history
+    12             entry" or reuse one that was previously allocated.
+
+    13             If "write history logging" is supported and  the  History
+    14             Log  modifier  is  set  and  the  Reuse Entry modifier is
+    15             clear, the server allocates a new "write  history  entry"
+    16             and  then  stores  the  value contained in the "hrn (host
+    17             reference number)" field, "entry  id"  field,  and  other
+    18             information  related  to  this  command  into  the "write
+    19             history entry" just allocated.   If  the  server  has  no
+    20             "write history entry" to allocate, it shall remember this
+    21             fact.  See  Section  6.19.3,  "WRITE  HISTORY  MANAGEMENT
+    22             Command,  Description"  for more details.  To report this
+    23             allocation  failure  to  the  host,  the  server  returns
+    24             FFFF(hex) in the "entry locator" field of the end message
+    25             and sets the Allocation Failure end  flag.   See  Section
+    26             5.4, "End Messages," for a description of this flag.
+
+    27             If "write history logging" is supported and  the  History
+    28             Log  modifier is set and the Reuse Entry modifier is set,
+    29             the server uses the value contained in the "entloc (entry
+    30             locator)"  field  to locate the associated "write history
+    31             entry" and then stores the other information  related  to
+    32             this command into that "write history entry." 
+
+    33             See Section 6.19.3, "WRITE  HISTORY  MANAGEMENT  Command,
+    34             Description"  for  details  regarding the maintenance and
+    35             format of "write history entries." 
+
+    36             Note that when "write history logging" is  supported  and
+    37             the History Log modifier is set, host class drivers shall
+    38             supply both the  "hrn  or  entloc"  and  the  "entry  id"
+    39             fields.   If  the  History  Log modifier is set and those
+    40             fields are not supplied, the server rejects  the  command
+    41             as   an   "Invalid  Command"  ("invalid  message  length"
+    42             protocol error.  See "Invalid Command" status in  Section
+    43             5.5, "Status Codes").
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-69
+           ERASE Command                                        11 June 1992
+
+
+     1             If "write history logging" is supported and  the  History
+     2             Log  modifier is clear, the server ignores the setting of
+     3             the Reuse Entry modifier and the contents of the "hrn  or
+     4             entloc" and "entry id" fields if they are supplied.  Note
+     5             that host class drivers need not supply those fields when
+     6             the History Log modifier is clear.
+
+     7             If "write history  logging"  is  not  supported  and  the
+     8             History  Log modifier or the Reuse Entry modifier is set,
+     9             the server rejects the command as  an  "Invalid  Command"
+    10             ("invalid   modifier"   protocol   error.   See  "Invalid
+    11             Command" status in Section 5.5, "Status Codes").
+
+    12             If "write history logging" is supported, the History  Log
+    13             modifier  is set, and the "byte count" field is zero, the
+    14             server  rejects  the  command  as  an   Invalid   Command
+    15             ("invalid  byte  count"  parameter  error.   See "Invalid
+    16             Command" status in Section 5.5, "Status Codes").
+
+    17             The server shall set the History Logged end flag in  this
+    18             command's  end message if it has caused the write history
+    19             log to contain a valid "write  history  entry"  for  this
+    20             command.    See   Section   5.4,   "End   Messages,"  for
+    21             descriptions of all end message flags.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-70
+           ERASE Command                                        11 June 1992
+
+
+     1     6.9.2  End Message Format
+
+     2         31                             0                         
+     3         +-------------------------------+                        
+     4         |   command reference number    |                        
+     5         +---------------+---------------+                        
+     6         |sequence number|  unit number  |                        
+     7         +---------------+-------+-------+                        
+     8         |    status     | flags |endcode|                        
+     9         +---------------+-------+-------+                        
+    10         |          byte count           |                        
+    11         +-------------------------------+                        
+    12         |                               |                        
+    13         +---                         ---+                        
+    14         |           undefined           |                        
+    15         +---                         ---+                        
+    16         |                               |                        
+    17         +-------------------------------+                        
+    18         |        first bad block        |                        
+    19         +---------------+---------------+                        
+    20         |   entry id    | entry locator | (conditionally present)
+    21         +---------------+---------------+                        
+
+
+    22         entry locator
+
+    23             This field is present if and only  if  the  corresponding
+    24             command  message  field  is  present.  If present and the
+    25             History Log modifier is set in the command message,  this
+    26             field  contains  the  value  assigned  by the server that
+    27             uniquely identifies the internal location of the "history
+    28             entry"  where this command's information was stored.  If,
+    29             however,  an  allocation  failure  occurred,  this  field
+    30             contains the value FFFF(hex).  If present and the History
+    31             Log modifier is clear in the command message, this  field
+    32             is undefined.
+
+    33             This field contains a valid entry  locator  of  a  "write
+    34             history  entry"  for  this  command  if  and  only if the
+    35             History Logged end flag is set in the  "flags"  field  of
+    36             this end message.
+
+    37             See the description of the History Log  and  Reuse  Entry
+    38             modifiers  in  Section  6.9.1,  "ERASE  Command,  Command
+    39             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+    40             MANAGEMENT Command, Description" for more detail.
+
+    41         entry id
+
+    42             This field is present if and only  if  the  corresponding
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-71
+           ERASE Command                                        11 June 1992
+
+
+     1             command  message  field  is  present.  If present and the
+     2             History Log modifier is set in the command  message,  the
+     3             contents  of  the  corresponding command message field is
+     4             copied without modification to this  field.   If  present
+     5             and  the  History  Log  modifier  is clear in the command
+     6             message, this field is undefined.
+
+     7             See the description of the History Log  and  Reuse  Entry
+     8             modifies   in  Section  6.9.1,  "ERASE  Command,  Command
+     9             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+    10             MANAGEMENT Command, Description" for more detail.
+
+
+    11     Status Codes:
+
+    12         Success (subcode "Normal")
+    13         Success (subcode "Duplicate Unit Number")
+    14         Invalid Command (subcode "Invalid Byte Count")
+
+    15             A byte count of zero was specified with the  History  Log
+    16             modifier set.
+
+    17         Invalid Command (subcode "Invalid Entry Locator")
+
+    18             The value specified in the "entloc (entry locator)" field
+    19             is  not  within the limits of the server's "write history
+    20             entry" set.
+
+    21             See the description of the History Log  and  Reuse  Entry
+    22             modifiers  in  Section  6.9.1,  "ERASE  Command,  Command
+    23             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+    24             MANAGEMENT Command, Description" for more detail.
+
+    25             Note that this error can only  occur  if  "write  history
+    26             logging" is supported and the History Log and Reuse Entry
+    27             modifiers are both set in the command message.
+
+    28         Invalid Command (subcode "Invalid Entry Id")
+
+    29             The server located the "write history  entry"  identified
+    30             in  the "entloc (entry locator)" field but found that the
+    31             "entry id" contained in that entry  does  not  equal  the
+    32             value supplied by a host class driver when that entry was
+    33             allocated.  This status  may  also  be  returned  if  the
+    34             History  Log  modifier  was set, the Reuse Entry modifier
+    35             was clear, and the host supplied a value of FFFF (hex) in
+    36             the "entry id" field.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-72
+           ERASE Command                                        11 June 1992
+
+
+     1             See the description of the History Log  and  Reuse  Entry
+     2             modifiers  in  Section  6.9.1,  "ERASE  Command,  Command
+     3             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+     4             MANAGEMENT Command, Description" for more detail.
+
+     5             Note that this error can only  occur  if  "write  history
+     6             logging" is supported and the History Log modifier is set
+     7             in the command message.
+
+     8         Invalid Command (subcode "Invalid Logical Block Number")
+     9         Command Aborted
+    10         Unit-Offline
+    11         Unit-Available
+    12         Write Protected
+    13         Host Buffer Access Error (subcode "Odd Byte Count")
+
+    14             If the communications mechanism does not allow  odd  byte
+    15             transfers,   the   controller   may  return  this  status
+    16             code/subcode despite there being no  host  buffer.   This
+    17             allows  the  use of common code for command validation of
+    18             all transfer commands.  Alternatively, the controller may
+    19             round  the byte count up to the nearest whole sector size
+    20             and perform the operation.
+
+    21         Controller Error
+    22         Drive Error
+    23         Write History Entry Access Error (subcode "Allocation Failure
+    24                                           Table Full")
+
+    25             The server was unable to allocate a "write history entry"
+    26             and  could  not  record  this fact because the Allocation
+    27             Failure Table was full.  This is a  transient  condition;
+    28             therefore,  the  host  should  reissue  the command.  See
+    29             Section  6.19.3,  "WRITE  HISTORY   MANAGEMENT   Command,
+    30             Description" for more details.
+
+    31             Note that this error can occur  only  if  "write  history
+    32             logging" is supported and the History Log modifier is set
+    33             and Reuse Entry modifier is clear in the command message.
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-73
+           ERASE Command                                        11 June 1992
+
+
+     1     6.9.3  Description
+
+     2     All data in the  specified  region  of  the  unit  is  erased  by
+     3     overwriting it with zero.
+
+     4     This command is exactly equivalent in function, although  not  in
+     5     performance,  to  a  WRITE command which references a buffer that
+     6     the host has zeroed.
+
+     7     If a controller-based cache exists, the cache manager may use the
+     8     Cache  Access  Attribute  advice  in  managing  the  caching  and
+     9     comparing of the requested data area.
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-74
+           FORMAT Command                                       11 June 1992
+
+
+     1     6.10  FORMAT Command
+
+     2     Command Category:
+
+     3         Sequential
+
+     4     6.10.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |          byte count           |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---         buffer          ---+
+    17                     |                               |
+    18                     +---       descriptor        ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |        format function        |
+    22                     +-------------------------------+
+
+
+    23         byte count
+    24         buffer descriptor
+
+    25             If  "byte  count"  is  non-zero,  these  two   parameters
+    26             identify    a   buffer   containing   additional   format
+    27             information.  If  "byte  count"  is  zero,  then  "buffer
+    28             descriptor" is reserved.  See description below.
+
+    29         format function
+
+    30             A  code  from  Table  A-13,  "Format   Function   Codes,"
+    31             indicating the desired format for the volume.
+
+
+    32     Allowable modifiers:
+
+    33         Clear Serious Exception (ignored for disk class devices)
+    34         Compare                             -- ignored
+    35         Express Request                     -- ignored
+    36         Suppress Error Correction           -- ignored
+    37         Suppress Error Recovery             -- ignored
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-75
+           FORMAT Command                                       11 June 1992
+
+
+     1     6.10.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+
+    10     Status Codes:
+
+    11         Success (subcode "Normal")
+    12         Invalid Command
+    13         Command Aborted
+    14         Unit-Offline
+    15         Unit-Available
+    16         Media Format Error
+    17         Write Protected
+    18         Host Buffer Access Error
+    19         Controller Error
+    20         Drive Error
+    21         Invalid Parameter
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-76
+           FORMAT Command                                       11 June 1992
+
+
+     1     6.10.3  Description
+
+     2     This command formats or  reformats  the  volume  mounted  on  the
+     3     specified unit.  Support for this command is completely optional.
+     4     Controllers that do not implement this command, or  that  do  not
+     5     implement  it  for  the  specified  unit,  shall  reject  it with
+     6     "Invalid Command -- Invalid Opcode" status.
+
+     7     A  controller  may  choose  to  implement  this  command,  a  DUP
+     8     formatting  program,  both,  or  neither.   All  combinations are
+     9     allowed  and  reasonable  for  various  devices.   However,  this
+    10     command  is  the  preferred means of formatting devices for which
+    11     formatting  is  considered  a  normal,  user-invoked   operation.
+    12     Floppy diskettes are common examples of such devices.
+
+    13     Hosts shall use the following  command  sequence  to  format  the
+    14     volume  on  a  unit using this command.  This command sequence is
+    15     the only context in which the FORMAT command shall be issued.
+
+    16          1.  The unit should be "Unit-Available" or "Unit-Offline" to
+    17              all  hosts  at  the start of the command sequence.  That
+    18              is, the unit should not be "Unit-Online" to any host.
+
+    19          2.  The host brings the unit "Unit-Online" using  an  ONLINE
+    20              command  with  the  "Ignore Media Format Error" modifier
+    21              set.  The host shall exit (not continue  the  formatting
+    22              sequence)  if  this  command  does  not  succeed.  It is
+    23              recommended, although not required, that the  host  also
+    24              set the "Exclusive Access" modifier.
+
+    25          3.  The  host  may  optionally  issue  any  number  of  READ
+    26              commands  to  the unit.  These commands might be used to
+    27              verify that the correct volume is mounted.  The host may
+    28              exit  or  continue  as  it  sees  fit, regardless of the
+    29              success or failure of these  READ  commands.   The  host
+    30              should  issue  an  AVAILABLE  command  to the unit if it
+    31              chooses to exit.  Note that  these  READ  commands  will
+    32              normally  fail  if  the  volume  has not been previously
+    33              formatted.
+
+    34          4.  The host issues a FORMAT command to the unit.
+
+
+    35     The  controller  shall  verify  that   the   unit   was   brought
+    36     "Unit-Online"  by an ONLINE command with the "Ignore Media Format
+    37     Error"  modifier  set.   If  the  unit   is   "Unit-Offline"   or
+    38     "Unit-Available"  the  controller shall reject the FORMAT command
+    39     with the corresponding status  code  and  leave  the  unit  state
+    40     unaltered.   If  the  unit is "Unit-Online" but the "Ignore Media
+    41     Format Error" modifier was not  specified,  then  the  controller
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-77
+           FORMAT Command                                       11 June 1992
+
+
+     1     shall  reject  the  FORMAT command with a "Unit-Available" status
+     2     code and the unit becomes "Unit-Available."
+
+     3     The controller shall return  "Invalid  Command,"  "Unit-Offline,"
+     4     "Unit-Available,"   "Write  Protected,"  or  "Invalid  Parameter"
+     5     status if a FORMAT command is rejected.  These status  codes  all
+     6     imply that the volume on the unit has not been altered.  The unit
+     7     state  is  unaltered,  except  for  the  case  described  in  the
+     8     preceding paragraph.
+
+     9     The controller  shall  return  "Success"  status  if  the  format
+    10     operation is successful.  The unit is left "Unit-Available."
+
+    11     Any other status code implies  that  the  volume  may  have  been
+    12     partially formatted.  The volume must be successfully reformatted
+    13     before  it  can  be  used  reliably.   The  unit  may  be  either
+    14     "Unit-Available" or "Unit-Offline."
+
+    15     The FORMAT command often  causes  the  unit  to  transition  from
+    16     "Unit-Online"   to   "Unit-Available."   This  is  a  synchronous
+    17     transition.   Therefore  the  controller  need  not  generate  an
+    18     AVAILABLE attention message.
+
+    19     The FORMAT command neither obtains nor releases Exclusive  Access
+    20     to the unit.
+
+    21     The actual formatting  operation  is  controlled  by  the  format
+    22     function  code value and the additional format information in the
+    23     buffer described by the  "byte  count"  and  "buffer  descriptor"
+    24     fields.   Format  function  code values are listed in Table A-13.
+    25     The  format  function  determines   the   use,   structure,   and
+    26     interpretation of any additional format information.
+
+    27     Most format functions do not use additional  format  information.
+    28     The   controller  shall  ignore  the  "byte  count"  and  "buffer
+    29     descriptor" fields, as well as any buffer described by them, when
+    30     the  format  function does not use additional format information.
+    31     If the format function does use  additional  format  information,
+    32     the  controller  shall validate the additional format information
+    33     before it begins the formatting process.   The  controller  shall
+    34     reject the FORMAT command with an "Invalid Parameter" status code
+    35     if the additional format information is invalid.
+
+    36     Format  function  code  value  zero  specifies  that  a   default
+    37     formatting  operation  should  be  performed.   A  default format
+    38     function shall be supported by all  controller/unit  combinations
+    39     that  support  the  FORMAT  command.  The exact definition of the
+    40     default format function is device specific.  It does not  require
+    41     additional format information.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-78
+           FORMAT Command                                       11 June 1992
+
+
+     1     Support for non-default format  functions  is  device  dependent.
+     2     Table  A-13 indicates which functions are applicable to different
+     3     classes of devices, but  the  final  determination  of  supported
+     4     format  functions  is device dependent.  Controllers shall reject
+     5     FORMAT commands that specify an unsupported format function  with
+     6     "Invalid Command -- Invalid Format Function" status.
+
+     7                                   NOTE
+
+     8             At the present time there are no format functions
+     9             that  use  additional  format  information.   The
+    10             definition of additional format  information  has
+    11             been  included  in  this  command  for those host
+    12             operating  systems  that  choose   to   implement
+    13             support   for   it   in  advance  of  any  device
+    14             requirements  for  such  support.    Valid   host
+    15             implementations   need   not  support  additional
+    16             format information.  Any device that  anticipates
+    17             using    additional   format   information   must
+    18             negotiate for host support, just as if they  were
+    19             proposing  a  totally  new feature.  The ECO that
+    20             defines  the  first  use  of  additional   format
+    21             information should also remove this note.
+
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-79
+           GET COMMAND STATUS Command                           11 June 1992
+
+
+     1     6.11  GET COMMAND STATUS Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     6.11.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     | outstanding reference number  |
+    14                     +-------------------------------+
+
+
+    15         unit number
+
+    16             Shall be the same as  the  "unit  number"  field  in  the
+    17             outstanding command whose status is to be obtained.  This
+    18             allows controllers  to  optimize  their  search  for  the
+    19             outstanding  command.   If  the  incorrect unit number is
+    20             supplied, some controllers may erroneously conclude  that
+    21             the   command   is  no  longer  outstanding,  leading  to
+    22             erroneous command timeouts.
+
+    23         outstanding reference number
+
+    24             The command reference number of the command whose  status
+    25             is to be obtained.
+
+
+    26     Allowable modifiers:
+
+    27         none
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-80
+           GET COMMAND STATUS Command                           11 June 1992
+
+
+     1     6.11.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     | outstanding reference number  |
+    11                     +---------------+---------------+
+    12                     |        command status         |
+    13                     +---------------+---------------+
+
+
+    14         outstanding reference number
+
+    15             The command reference number of the command whose  status
+    16             has  been  returned.   Identical to the value supplied in
+    17             the GET COMMAND STATUS command message.
+
+    18         command status
+
+    19             The amount of work remaining to be done to  complete  the
+    20             command, expressed as an unsigned integer.  This field is
+    21             zero if the command is not known to the controller,  such
+    22             as  when  the  command has already completed.  This field
+    23             should also be zero if the command has been aborted.  The
+    24             controller  may  also return zero in this field if it can
+    25             guarantee that  the  command  will  complete  within  the
+    26             controller  timeout interval.  The controller shall never
+    27             return a value of all ones (2**32-1) in  this  field,  as
+    28             that  value  is  used  to  initialize the command timeout
+    29             algorithm.
+
+    30             The units in which this value is measured  are  arbitrary
+    31             and  may  be  controller,  device  type,  and/or  command
+    32             dependent.  However, the units shall remain the same  for
+    33             a  particular  command  for  as  long  as that command is
+    34             outstanding.
+
+
+    35     Status Codes:
+
+    36         Success (subcode "Normal")
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-81
+           GET COMMAND STATUS Command                           11 June 1992
+
+
+     1     6.11.3  Description
+
+     2     The GET COMMAND STATUS command is used to monitor the progress of
+     3     a  command  towards  completion.  The command status measures the
+     4     "doneness" of the command.  The value  returned  in  the  command
+     5     status   field   is   guaranteed   to  not  increase  over  time.
+     6     Furthermore, the  command  status  of  an  MSCP  server's  oldest
+     7     outstanding   command   is  guaranteed  to  decrease  within  the
+     8     controller timeout interval.  This last feature may be used by  a
+     9     host   class   driver  to  detect  an  insane  or  malfunctioning
+    10     controller.  See  Section  4.10,  "Command  Timeouts,"  for  more
+    11     details.
+
+    12     The GET COMMAND STATUS command always succeeds.  If  the  command
+    13     referenced  by the "outstanding reference number" is not known to
+    14     the MSCP server or has been aborted, then the MSCP server  should
+    15     return  zero  for  its  command status.  The MSCP server may also
+    16     return zero as the command status of any command that will always
+    17     complete within the controller timeout interval.  The MSCP server
+    18     always returns the "Normal" status code in the GET COMMAND STATUS
+    19     command's end message.
+
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-82
+           GET UNIT STATUS Command                              11 June 1992
+
+
+     1     6.12  GET UNIT STATUS Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     6.12.1  Command Message Format
+
+     5              31                             0               
+     6              +-------------------------------+              
+     7              |   command reference number    |              
+     8              +---------------+---------------+              
+     9              |   reserved    |  unit number  |              
+    10              +---------------+-------+-------+              
+    11              |   modifiers   | rsvd  | opcode|              
+    12              +---------------+-------+-------+              
+
+    13  |  Allowable modifiers:
+
+    14         Clear Serious Exception (ignored for disk class devices)
+
+    15         Next Unit
+
+    16             Requests that the controller return  the  status  of  the
+    17             next  unit  (in order of ascending unit numbers) that the
+    18             controller knows  to  exist  and  whose  unit  number  is
+    19             greater than or equal to the unit number specified in the
+    20             command.  See Section 4.3, "Unit States," for a  detailed
+    21             definition  of  which  units  are  acknowledged  by  this
+    22             modifier.
+
+    23             If this modifier is set, the "unit number" field  in  the
+    24             end message corresponds to the unit whose characteristics
+    25             are being returned, and is typically not the same as  the
+    26             "unit number" field in the command message.  If there are
+    27             no units that are both known to the controller and  whose
+    28             unit numbers are greater than or equal to the unit number
+    29             specified in the command message, then zero  is  returned
+    30             in  the  "unit  number"  field  of  the end message.  The
+    31             remaining fields of the end message are identical to  the
+    32             values  that  would  be  returned  for  a GET UNIT STATUS
+    33             command with a "unit number" of zero and the "Next  Unit"
+    34             modifier left clear.
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-83
+           GET UNIT STATUS Command                              11 June 1992
+
+
+     1     6.12.2  End Message Format
+
+     2              31                             0               
+     3              +-------------------------------+              
+     4              |   command reference number    |              
+     5              +---------------+---------------+              
+     6              |sequence number|  unit number  |              
+     7              +---------------+-------+-------+              
+     8              |    status     | flags |endcode|              
+     9              +---------------+-------+-------+              
+    10              |  unit flags   |multiunit code |              
+    11              +---------------+---------------+              
+    12              |      reserved         |spndles|              
+    13              +-------------------------------+              
+    14              |                               |              
+    15              +---     unit identifier     ---+              
+    16              |                               |              
+    17              +-------------------------------+              
+    18              |     media type identifier     |              
+    19              +-------------------------------+              
+    20              |           reserved            |              
+    21              +-------------------------------+              
+    22              |  group size   |   track size  |              
+    23              +-------+-------+---------------+              
+    24              |uhvrsn | usvrsn| cylinder size |              
+    25              +-------+-------+---------------+              
+    26              |copies | RBNs  |    RCT size   |              
+    27              +-------+-------+---------------+              
+
+
+
+    28  |      status
+
+    29             The validity of the unit characteristics varies with  the
+    30             unit's  state implied by the contents of this field.  See
+    31             the Status Codes listed later in this section and Section
+    32             6.12.3,   "GET  UNIT  STATUS  Command,  Description"  for
+    33             complete details.
+
+    34         multiunit code
+
+    35             The low byte of this field  identifies  the  access  path
+    36             between  the  controller  and the unit.  The high byte of
+    37             this field identifies  the  spindle,  within  the  access
+    38             path,  to  which  the  unit belongs.  See Section 4.16.2,
+    39             "Multiunit Drives and Formatters," for more information.
+
+    40         unit flags
+
+    41             See Section 5.7, "Unit Flags." 
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-84
+           GET UNIT STATUS Command                              11 June 1992
+
+
+     1         spndles
+
+     2             The number minus one of spindles or spindle  "groups"  in
+     3             the  unit.  The value of zero indicates one spindle.  See
+     4             Section 4.12, "Disk Geometry and Format."
+
+     5                                IMPLEMENTATION NOTE
+
+     6             The "spndles" field was defined in February 1991.  Before
+     7             that  it  was  a  reserved field, but not all controllers
+     8             zeroed it.  To ensure validity of  the  "spndles"  field,
+     9             class   drivers   should   send  GET UNIT STATUS  command
+    10             messages of at least 20 bytes and zero all  bytes  beyond
+    11             byte 12.
+
+
+    12         unit identifier
+
+    13             Uniquely identifies the unit among all devices accessible
+    14             via   MSCP.   See  Section  4.17,  "Controller  and  Unit
+    15             Identifiers." 
+
+    16         media type identifier
+
+    17             Identifies the type of media used by this unit,  for  use
+    18             by   host  generic  device  allocation  mechanisms.   See
+    19             Section 4.18, "Media Type Identifiers." 
+
+    20         track size
+
+    21             The number of logical blocks in a track.   The  value  of
+    22             zero  is  reserved  and illegal.  See Section 4.12, "Disk
+    23             Geometry and Format." 
+
+    24         group size
+
+    25             The number of tracks in a group.  The value  of  zero  is
+    26             reserved  and  illegal.  See Section 4.12, "Disk Geometry
+    27             and Format." 
+
+    28         cylinder size
+
+    29             The number of groups in a cylinder.  The value of zero is
+    30             reserved  and  illegal.  See Section 4.12, "Disk Geometry
+    31             and Format." 
+
+    32         usvrsn
+
+    33             The unit's  software,  firmware,  or  microcode  revision
+    34             number.   Always  zero  if  the product's development and
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-85
+           GET UNIT STATUS Command                              11 June 1992
+
+
+     1             maintainability groups determine that there is no benefit
+     2             from supporting machine readable revision numbers.
+
+     3         uhvrsn
+
+     4             The unit's hardware revision number.  Always zero if  the
+     5             product's    development   and   maintainability   groups
+     6             determine  that  there  is  no  benefit  from  supporting
+     7             machine readable revision numbers.
+
+     8         RCT size
+
+     9             The difference between the starting logical block numbers
+    10             of  successive  copies  of the unit's Replacement Control
+    11             Table (RCT).  Excepting only the last copy, this value is
+    12             also  the  size  of  each  copy  of the RCT.  See Digital
+    13             Storage Architecture Disk  Format  Specification  (DSDF).
+    14             Zero  if  this  disk  has  no RCT whatsoever; see Section
+    15             4.12, "Disk Geometry and Format." 
+
+    16         RBNs
+
+    17             The number of replacement blocks per track.
+
+    18             See   Digital   Storage    Architecture    Disk    Format
+    19             Specification  (DSDF).   Zero  if  this  disk  has no RCT
+    20             whatsoever;  see  Section  4.12,   "Disk   Geometry   and
+    21             Format." 
+
+    22         copies
+
+    23             The number of copies of  the  Replacement  Control  Table
+    24             that  are  stored  on  the  unit.   See  Digital  Storage
+    25             Architecture Disk Format Specification (DSDF).   Zero  if
+    26             this  disk has no RCT whatsoever; see Section 4.12, "Disk
+    27             Geometry and Format." 
+
+
+    28     Status Codes:
+
+    29         Success (subcode "Normal")
+    30         Success (subcode "Duplicate Unit Number")
+
+    31             Both  of   these   codes   imply   that   the   unit   is
+    32             "Unit-Online." 
+
+    33         Unit-Offline
+    34         Unit-Available
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-86
+           GET UNIT STATUS Command                              11 June 1992
+
+
+     1         Controller Error
+     2         Drive Error
+
+     3             For both of these status codes the actual drive state  is
+     4             unknown  (although  it will usually be "Unit-Offline" due
+     5             to being inoperative).  Therefore the class driver should
+     6             assume that the unit is "Unit-Offline." 
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-87
+           GET UNIT STATUS Command                              11 June 1992
+
+
+     1     6.12.3  Description
+
+     2     The GET UNIT STATUS command returns the current state of  a  unit
+     3     plus  certain  unit characteristics.  In particular, the GET UNIT
+     4     STATUS command is used to  obtain  host-settable  characteristics
+     5     and those fixed unit characteristics that are not normally needed
+     6     by the class driver.
+
+     7     Class  drivers  can  determine  which  of   the   returned   unit
+     8     characteristics  are  valid  from  the  unit state implied by the
+     9     returned "status" (see Status Codes listed above) as follows:
+
+    10          1.  Unit is "Unit-Offline" and unknown to  controller.   All
+    11              characteristic fields and all unit flags are undefined.
+
+    12          2.  Unit is "Unit-Offline" and known to the controller or it
+    13              is   "Unit-Available."    The  "multiunit  code,"  "unit
+    14              identifier,"   "media   type   identifier,"    "usvrsn,"
+    15              "uhvrsn," and "spndles" characteristic fields are valid.
+    16              The "Removable Media" unit flag  is  valid.   All  other
+    17              characteristic fields and unit flags are undefined.
+
+    18          3.  Unit is "Unit-Online."  All characteristics  fields  and
+    19              all unit flags are valid.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-88
+           ONLINE Command                                       11 June 1992
+
+
+     1     6.13  ONLINE Command
+
+     2     Command categories:
+
+     3         Sequential
+
+     4     6.13.1  Command Message Format
+
+     5              31                             0               
+     6              +-------------------------------+              
+     7              |   command reference number    |              
+     8              +---------------+---------------+              
+     9              |   reserved    |  unit number  |              
+    10              +---------------+-------+-------+              
+    11              |   modifiers   | rsvd  | opcode|              
+    12              +---------------+-------+-------+              
+    13              |  unit flags   |    reserved   |              
+    14              +---------------+---------------+              
+    15              |           reserved            |              
+    16              +-------------------------------+              
+    17              |                               |              
+    18              +---        reserved         ---+              
+    19              |                               |              
+    20              +-------------------------------+              
+    21              |  device dependent parameters  |              
+    22              +-------------------------------+              
+    23              |           reserved            |              
+    24              +-------------------------------+              
+
+
+
+    25  |      unit flags
+
+    26             Host-settable  unit  flags.   See  Section   5.7,   "Unit
+    27             Flags." 
+
+    28             If the unit is already "Unit-Online" to the class driver,
+    29             then  the  class driver shall specify the same values for
+    30             controller supported  host-settable  unit  flags  as  are
+    31             currently  in effect on the unit.  The controller may, at
+    32             its option, check that the flag values are the same.   If
+    33             it  does  check,  then  it shall reject the command as an
+    34             "Invalid Command" ("invalid unit flags" parameter  error.
+    35             See "Invalid Command" status in Section 5.5), if they are
+    36             different.  If the controller does not  check  that  they
+    37             are  the  same,  then  it  shall  ignore  the  unit flags
+    38             specified by the  class  driver  and  preserve  the  flag
+    39             settings currently in effect on the unit.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-89
+           ONLINE Command                                       11 June 1992
+
+
+     1         device dependent parameters
+
+     2             Device  and/or   controller   dependent   device   tuning
+     3             parameters.   The  value  zero  in  this field means that
+     4             default or  normal  tuning  parameters  should  be  used.
+     5             Nonzero   values   for  this  field  should  normally  be
+     6             established through  the  system  startup  command  file.
+     7             Examples  of  the  use  of  this  field include selecting
+     8             alternative  optimization  algorithms  or  enabling   and
+     9             disabling automatic (online) diagnosis of the unit.
+
+
+    10     Allowable modifiers:
+
+    11         Allow Self Destruction
+
+    12             Some controllers and/or drives are able to predict that a
+    13             unit  is  in  danger  of  imminent  self destruction, and
+    14             automatically spin-down and disable the unit  to  prevent
+    15             its  destruction.   Such  mechanisms  typically  sense an
+    16             exponentially  increasing   (correctable)   error   rate,
+    17             indicating  that  the  disk surface has been contaminated
+    18             with dust or other foreign objects.  Units that have been
+    19             disabled  for  this  reason  appear to be "Unit-Offline,"
+    20             with a subcode indicating that they have been disabled by
+    21             field  service  or  a  diagnostic.  Therefore such a unit
+    22             cannot normally be brought "Unit-Online." 
+
+    23             This modifier allows a host to bring a unit that has been
+    24             so  disabled  "Unit-Online," even though the consequences
+    25             for the unit may be fatal.  For this reason THIS MODIFIER
+    26             SHALL  NEVER  BE  USED  UNLESS  FIELD  SERVICE EXPLICITLY
+    27             DIRECTS A SITE TO DO SO.  When imminent self  destruction
+    28             has  been predicted for a unit, it is usually possible to
+    29             make one "last ditch" copy of the  unit  before  it  dies
+    30             completely,  recovering  all  or  most of the data on the
+    31             unit.  This modifier exists primarily to simplify support
+    32             of such a "last ditch" copy.  This modifier also provides
+    33             a means, if necessary, to work around a  diagnostic  that
+    34             is erroneously disabling a unit.
+
+    35             This modifier shall be supported by:
+
+    36             1.  Any controller that may disable a unit for the reason
+    37                 mentioned above.
+
+    38             2.  Any controller that may potentially be connected to a
+    39                 unit that can disable itself.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-90
+           ONLINE Command                                       11 June 1992
+
+
+     1             3.  Any controller that does not  disable  units  itself,
+     2                 but that will respond properly if a drive is disabled
+     3                 by some other (more capable) controller.
+
+     4             This modifier shall be ignored if the unit has  not  been
+     5             disabled  or  if the controller does not fall into any of
+     6             the above categories.
+
+     7         Clear Serious Exception (ignored for disk class devices)
+
+     8         Ignore Media Format Error
+
+     9             Suppresses most checking for "Media  Format  Errors"  and
+    10             causes   the   "576   Byte   Sectors"  unit  flag  to  be
+    11             host-settable.
+
+    12             If either the controller or the  unit  only  support  512
+    13             byte   sectors,   then   setting   this  modifier  merely
+    14             suppresses Media Format  Error  checking.   The  unit  is
+    15             brought  "Unit-Online" as a 512 byte formatted volume and
+    16             the "576 Byte Sectors" unit flag is returned  clear.   If
+    17             both  the  controller  and  the  unit  support  576  byte
+    18             sectors, then the state of the "576  Byte  Sectors"  unit
+    19             flag  (in  the  unit  flags field of this ONLINE command)
+    20             specifies whether the volume is formatted with 512 or 576
+    21             byte  sectors.   The unit is brought "Unit-Online" in the
+    22             specified format, rather than the format being determined
+    23             from the volume itself.
+
+    24             Use of this modifier allows the host to set a unit to the
+    25             wrong  block  or sector size.  Reading a unit that is set
+    26             to the wrong block or sector size  may  yield  a  mix  of
+    27             erroneous  "Data Errors," "Drive Errors," and "Controller
+    28             Errors."  Writing a unit that is set to the  wrong  block
+    29             or  sector  size may permanently corrupt the volume.  The
+    30             volume must be reformatted if this occurs.
+
+    31         Enable Set Write Protect
+
+    32             Causes the "Write Protect" unit flag to be host-settable.
+    33             This  modifier  causes  the  state  of the "Write Protect
+    34             (software)" unit flag to be copied to the Software  Write
+    35             Protect  flag  for  this  unit.  See Section 4.14, "Write
+    36             Protection." 
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-91
+           ONLINE Command                                       11 June 1992
+
+
+     1     6.13.2  End Message Format
+
+     2              31                             0               
+     3              +-------------------------------+              
+     4              |   command reference number    |              
+     5              +---------------+---------------+              
+     6              |sequence number|  unit number  |              
+     7              +---------------+-------+-------+              
+     8              |    status     | flags |endcode|              
+     9              +---------------+-------+-------+              
+    10              |  unit flags   |multiunit code |              
+    11              +---------------+---------------+              
+    12              |      reserved         |spndles|              
+    13              +-------------------------------+              
+    14              |                               |              
+    15              +---     unit identifier     ---+              
+    16              |                               |              
+    17              +-------------------------------+              
+    18              |     media type identifier     |              
+    19              +-------------------------------+              
+    20              |           reserved            |              
+    21              +-------------------------------+              
+    22              |           unit size           |              
+    23              +-------------------------------+              
+    24              |      volume serial number     |              
+    25              +-------------------------------+              
+
+
+
+    26  |      status
+
+    27             The validity of the unit characteristics varies with  the
+    28             unit's  state implied by the contents of this field.  See
+    29             Status Codes listed below and Section 6.17.3,  "SET  UNIT
+    30             CHARACTERISTICS   Command,   Description"   for  complete
+    31             details.
+
+    32         All other fields of the  ONLINE  command's  end  message  are
+    33         identical  to  the  SET  UNIT  CHARACTERISTICS  command's end
+    34         message.  Refer to Section 6.17.2, "SET UNIT  CHARACTERISTICS
+    35         Command, End Message Format" for a description of the fields.
+
+    36     Status Codes:
+
+    37         Success (subcode "Normal")
+
+    38             Implies that the unit is "Unit-Online." 
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-92
+           ONLINE Command                                       11 June 1992
+
+
+     1         Success (subcode "Already Online")
+
+     2             The "Already Online" subcode bit flag is set if and  only
+     3             if  the  unit  is already "Unit-Online" to the requesting
+     4             class driver.  The unit's state and  characteristics  are
+     5             not  altered.   When the unit is already "Unit-Online" to
+     6             the  requesting  class  driver,  the  controller   merely
+     7             returns  the unit characteristics in the end message with
+     8             this status bit flag set, without  performing  any  other
+     9             actions.
+
+    10         Invalid Command (subcode "Invalid Unit Flags")
+
+    11             The unit is already "Unit-Online"  and,  for  those  unit
+    12             flags  that  are  both host-settable and supported by the
+    13             controller, the  class  driver  has  specified  different
+    14             values  from  those currently in effect on the unit.  The
+    15             unit remains "Unit-Online."  The host-settable unit flags
+    16             are  not  changed,  and  the end message returns all unit
+    17             flags.  Controllers may, at their option,  omit  checking
+    18             that  the  unit  flags  are  the same and just ignore the
+    19             class driver specified unit  flags  for  units  that  are
+    20             already  "Unit-Online," thus returning a "Success" status
+    21             code with "Already Online" subcode.
+
+    22         Command Aborted
+
+    23             The command has been rejected and the  unit's  state  has
+    24             not been changed.  If the unit was "Unit-Available" prior
+    25             to   issuing   this   command,   then   it    is    still
+    26             "Unit-Available."   However,  as  the unit's state is not
+    27             reported to the host, the host  should  assume  that  the
+    28             returned unit characteristics are invalid as the unit may
+    29             be "Unit-Offline." 
+
+    30         Unit-Offline
+
+    31             Note that some causes of a unit being "Unit-Offline"  may
+    32             be   overridden   (suppressed)   by   the   "Allow   Self
+    33             Destruction" command modifier.
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-93
+           ONLINE Command                                       11 June 1992
+
+
+     1         Media Format Error
+
+     2             The  unit  is  and  remains  "Unit-Available."   However,
+     3             attention  messages  are suppressed for this unit and the
+     4             controller attempts to spin-down this unit exactly as  if
+     5             an  AVAILABLE  command  with the "Spin-down" modifier set
+     6             were issued.  Note, however,  that  this  error  will  be
+     7             suppressed  and  the unit brought "Unit-Online" anyway if
+     8             the "Ignore Media Format Error" command modifier is  set.
+     9             See the modifier description above.
+
+    10         Controller Error
+
+    11             The actual drive state is unknown.  Therefore  the  class
+    12             driver should assume that the unit is "Unit-Offline." 
+
+    13         Drive Error
+
+    14             The unit is "Unit-Offline" due to being inoperative.  The
+    15             controller  shall  suppress  AVAILABLE attention messages
+    16             for the unit and attempt to spin-down the  unit,  exactly
+    17             as  if an AVAILABLE command with the "Spin-down" modifier
+    18             set  were  issued  for  the  unit.   The  controller  may
+    19             subsequently    report   the   unit   as   being   either
+    20             "Unit-Offline" or "Unit-Available."   The  reported  unit
+    21             state  will depend upon exactly how the unit is "broken,"
+    22             and the interactions of the failure with the controller's
+    23             perception of the unit's state.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-94
+           ONLINE Command                                       11 June 1992
+
+
+     1     6.13.3  Description
+
+     2     The ONLINE command is used to bring  a  unit  "Unit-Online,"  set
+     3     host-settable   unit   characteristics,  and  obtain  those  unit
+     4     characteristics  that  are  essential  for  proper  class  driver
+     5     operation   (see   Section   5.7,  "Unit  Flags"  for  additional
+     6     information).  The unit is spun-up, if necessary, and  its  heads
+     7     are  loaded  prior to returning the ONLINE command's end message.
+     8     Host-settable characteristics are set exactly as if  a  SET  UNIT
+     9     CHARACTERISTICS  command  were  issued  (see Section 6.17.2, "SET
+    10     UNIT CHARACTERISTICS  Command").   Host-settable  characteristics
+    11     are  set  after  the  unit  has been successfully spun-up and any
+    12     other validity checks  have  succeeded.   Note  that  the  unit's
+    13     host-settable  characteristics  are  NOT  altered  if the unit is
+    14     already "Unit-Online." 
+
+    15     For details of class driver responsibilities with respect to Host
+    16     Initiated   Bad   Block  Replacement,  see  the  Digital  Storage
+    17     Architecture Disk Format (DSDF) Specification  section  on  "Host
+    18     Initiated Bad Block Replacement."
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-95
+           READ Command                                         11 June 1992
+
+
+     1     6.14  READ Command
+
+     2     Command Category:
+
+     3         Non-Sequential
+
+     4     6.14.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  caa  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |          byte count           |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---         buffer          ---+
+    17                     |                               |
+    18                     +---       descriptor        ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |      logical block number     |
+    22                     +-------------------------------+
+
+
+    23     Allowable modifiers:
+
+    24         Clear Serious Exception (ignored for disk class devices)
+    25         Compare
+    26         Express Request
+    27         Suppress Error Correction
+    28         Suppress Error Recovery
+
+
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+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-96
+           READ Command                                         11 June 1992
+
+
+     1     6.14.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |          byte count           |
+    11                     +-------------------------------+
+    12                     |                               |
+    13                     +---                         ---+
+    14                     |           undefined           |
+    15                     +---                         ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |        first bad block        |
+    19                     +-------------------------------+
+
+
+    20     Status Codes:
+
+    21         Success (subcode "Normal")
+    22         Success (subcode "Duplicate Unit Number")
+    23         Invalid Command (subcode "Invalid Byte Count")
+    24         Invalid Command (subcode "Invalid Logical Block Number")
+    25         Command Aborted
+    26         Unit-Offline
+    27         Unit-Available
+    28         Compare Error
+    29         Data Error
+    30         Host Buffer Access Error
+    31         Controller Error
+    32         Drive Error
+
+
+
+
+
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+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-97
+           READ Command                                         11 June 1992
+
+
+     1     6.14.3  Description
+
+     2     Data is read from the unit and transferred to  the  host  buffer.
+     3     For  host  buffer  access  details see Section 4.11, "Host Buffer
+     4     Access."
+
+     5     If a controller-based cache exists, the cache manager may use the
+     6     Cache  Access  Attribute  advice  in  managing  the  caching  and
+     7     comparing of the requested data area.
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-98
+           REPLACE Command                                      11 June 1992
+
+
+     1     6.15  REPLACE Command
+
+     2     Command Category:
+
+     3         Non-Sequential
+
+     4     6.15.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |    replacement block number   |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---                         ---+
+    17                     |           reserved            |
+    18                     +---                         ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |      logical block number     |
+    22                     +-------------------------------+
+
+
+    23         replacement block number
+
+    24             Identifies the replacement block that has been  allocated
+    25             to replace the bad logical block.
+
+    26         logical block number
+
+    27             Identifies the bad logical block that is being replaced.
+
+
+    28     Allowable modifiers:
+
+    29         Clear Serious Exception (ignored for disk class devices)
+    30         Express Request
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                                Page 6-99
+           REPLACE Command                                      11 June 1992
+
+
+     1         Primary Replacement Block
+
+     2             Shall be set  if  and  only  if  the  "replacement  block
+     3             number"  specifies  the  primary  replacement  block  for
+     4             "logical block number."  I.e., set if  and  only  if  the
+     5             following expression is true:
+
+     6                  replacement block number =
+     7                            logical block number / track size * RBNs
+
+     8             where "track size" and "RBNs"  are  unit  characteristics
+     9             obtained via the GET UNIT CHARACTERISTICS command and "/"
+    10             denotes  integer  (truncating)  division.   See   Digital
+    11             Storage Architecture Disk Format Specification (DSDF) for
+    12             more information.  Note that this modifier  is  redundant
+    13             information   provided   for   the   convenience  of  the
+    14             controller.
+
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-100
+           REPLACE Command                                      11 June 1992
+
+
+     1     6.15.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status Codes:
+
+    11         Success (subcode "Normal")
+    12         Success (subcode "Duplicate Unit Number")
+    13         Invalid Command (subcode "Invalid Replacement Block Number")
+
+    14             Replacement block number is outside the  range  supported
+    15             by this disk.
+
+    16         Invalid Command (subcode "Invalid Logical Block Number")
+    17         Command Aborted
+    18         Unit-Offline
+    19         Unit-Available
+    20         Write Protected
+    21         Controller Error
+    22         Drive Error
+
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-101
+           REPLACE Command                                      11 June 1992
+
+
+     1     6.15.3  Description
+
+     2     The specified logical block is flagged to indicate  that  it  has
+     3     been replaced with the specified replacement block.  The volume's
+     4     Replacement Control Table (RCT) shall have been updated prior  to
+     5     using   this   command,  and  the  replacement  block  should  be
+     6     initialized with a write command to the same logical block number
+     7     after   using   this  command.   See  Section  4.13,  "Bad  Block
+     8     Replacement,"  and  Digital  Storage  Architecture  Disk   Format
+     9     Specification (DSDF) for more information on the use and function
+    10     of this command.
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-102
+           SET CONTROLLER CHARACTERISTICS Command               11 June 1992
+
+
+     1     6.16  SET CONTROLLER CHARACTERISTICS Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     6.16.1  Command Message Format
+
+     5              31                             0              
+     6              +-------------------------------+             
+     7              |   command reference number    |             
+     8              +---------------+---------------+             
+     9              |   reserved    |    reserved   |             
+    10              +---------------+-------+-------+             
+    11              |   modifiers   | rsvd  | opcode|             
+    12              +---------------+-------+-------+             
+    13              | cntrlr. flags |  MSCP version |             
+    14              +---------------+---------------+             
+    15              |   reserved    |  host timeout |             
+    16              +---------------+---------------+             
+    17              |           quad-word           |             
+    18              +---                         ---+             
+    19              |         time and date         |             
+    20              +-------------------------------+             
+    21              |controller dependent parameters|             
+    22              +---------------+---------------+             
+    23                              |      crn      | (optional)  
+    24                              +---------------+             
+
+
+    25         MSCP version
+
+    26             Host class drivers shall supply the value  zero  in  this
+    27             field.   MSCP  servers shall verify this value and, if it
+    28             is not zero, reject the command as an  "Invalid  Command"
+    29             ("invalid  MSCP  version"  protocol  error.  See "Invalid
+    30             Command" status in Section  5.5).   This  value  will  be
+    31             incremented if MSCP is ever modified in a way that is not
+    32             upwards compatible.
+
+    33         cntrlr. flags
+
+    34             Host-settable  controller  flags.    See   Section   5.6,
+    35             "Controller Flags." 
+
+    36         host timeout
+
+    37             The time interval that the controller should use for  the
+    38             host  access  timeout  with  this  host,  or  zero if the
+    39             controller should disable the  host  access  timeout  for
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-103
+           SET CONTROLLER CHARACTERISTICS Command               11 June 1992
+
+
+     1             this  host.   Expressed  as an unsigned binary integer in
+     2             units of seconds.  Controllers should use a default  host
+     3             access  timeout of 60 seconds if they have not received a
+     4             SET CONTROLLER  CHARACTERISTICS  command  since  becoming
+     5             "Controller-Online." 
+
+     6             Even though this is a sixteen bit field, controllers  may
+     7             treat  all  values  greater  than  255 as if 255 had been
+     8             specified, and all values between 1 and 9 as  if  10  had
+     9             been   specified.    See   Section   4.9,   "Host  Access
+    10             Timeouts." 
+
+    11         quad-word time and date
+
+    12             The current time and date, expressed  as  the  number  of
+    13             clunks  since  00:00  o'clock,  November 17, 1858 (in the
+    14             local time zone), or zero if the current time and date is
+    15             not  available.  A clunk is 100 nanoseconds.  This is the
+    16             standard VAX/VMS time and date format.  The use  that  is
+    17             made of the current time and date and the action taken if
+    18             it is  not  supplied  (i.e.,  if  zero  is  supplied)  is
+    19             controller  dependent,  and  should  be described in each
+    20             controller's Functional Specification.  Controllers shall
+    21             not  require  that a time and date be supplied for proper
+    22             operation.
+
+    23         controller dependent parameters
+
+    24             Controller dependent tuning parameters.  The  value  zero
+    25             in  this  field  means  that  default  or  normal  tuning
+    26             parameters should be used.  Nonzero values for this field
+    27             should normally be established through the system startup
+    28             command file.
+
+    29         crn (connection reference number)
+
+    30             The nonzero value the server is  to  associate  with  the
+    31             "Controller-Online" connection over which this command is
+    32             received  for  the  purpose  of  terminating  the   class
+    33             driver/server  connection a host has established with the
+    34             server.  See Section  7.10,  "Multihost  TERMINATE  CLASS
+    35             DRIVER/SERVER  CONNECTION  Command"  for more information
+    36             regarding the use of this field for that purpose.
+
+    37             This field is valid only for servers that  provide  Write
+    38             History  Logging Support or servers that support just the
+    39             TERMINATE CLASS DRIVER/SERVER  CONNECTION  command.   See
+    40             Section 5.6, "Controller Flags" for related information.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-104
+           SET CONTROLLER CHARACTERISTICS Command               11 June 1992
+
+
+     1             If  a  server  provides  such  support,  this  field   is
+     2             optional.   A  host  class  driver  need only supply this
+     3             field if the  TERMINATE  CLASS  DRIVER/SERVER  CONNECTION
+     4             command is to be used.  If a value of zero is supplied in
+     5             this field, the server rejects the command as an "Invalid
+     6             Command" ("invalid crn" parameter error).
+
+     7             If the Connection Reference Number  Present  modifier  is
+     8             set  and  the  "crn"  field  is  not supplied, the server
+     9             rejects  the  command  as  an  Invalid  Command  (subcode
+    10             "Invalid  Message  Length")  protocol  error (see Section
+    11             5.5, "Status Codes").
+
+
+
+    12     Allowable modifiers:
+
+    13         Connection Reference Number Present
+
+    14             Shall be set  if  and  only  if  the  host  provides  the
+    15             optional  "crn  (connection  reference  number)"  command
+    16             message field.  See the description of that field above.
+
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-105
+           SET CONTROLLER CHARACTERISTICS Command               11 June 1992
+
+
+     1     6.16.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|    reserved   |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     | cntrlr. flags |  MSCP version |
+    11                     +-------+-------+---------------+
+    12                     |chvrsn | csvrsn|cntrlr. timeout|
+    13                     +-------+-------+---------------+
+    14                     |                               |
+    15                     +---  controller identifier  ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     | maximum byte count (optional) |
+    19                     +-------------------------------+
+
+
+    20         cntrlr.  flags
+
+    21             See Section 5.6, "Controller Flags." 
+
+    22         cntrlr.  timeout
+
+    23             The controller timeout interval; the  minimum  amount  of
+    24             time  that  the  controller  needs  to  guarantee it will
+    25             accomplish useful work on its oldest outstanding command.
+    26             Expressed  as  an  unsigned  binary  integer  in units of
+    27             seconds.  This value may not exceed 255 (one byte),  even
+    28             though  a  sixteen  bit  field  has  been  provided.  See
+    29             Section 4.10, "Command Timeouts." 
+
+    30         csvrsn
+
+    31             The  controller's  software,   firmware,   or   microcode
+    32             revision   number.    Always   zero   if   the  product's
+    33             development and  maintainability  groups  determine  that
+    34             there  is  no  benefit  from  supporting machine readable
+    35             revision numbers.
+
+    36         chvrsn
+
+    37             The controller's hardware revision number.   Always  zero
+    38             if  the  product's development and maintainability groups
+    39             determine  that  there  is  no  benefit  from  supporting
+    40             machine readable revision numbers.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-106
+           SET CONTROLLER CHARACTERISTICS Command               11 June 1992
+
+
+     1         controller identifier
+
+     2             Uniquely identifies  the  controller  among  all  devices
+     3             accessible  via  MSCP.  See Section 4.17, "Controller and
+     4             Unit Identifiers." 
+
+     5         maximum byte count (optional)
+
+     6             The maximum byte count value  supported  by  the  server.
+     7             Class  drivers  should  not  issue transfer commands with
+     8             byte counts larger than this value; byte counts equal  to
+     9             this value are supported.
+
+    10             This value shall be at least as large as the following:
+
+    11              o  For disk class devices, 16,777,216 (2**24) bytes.
+
+    12              o  For tape class devices, 65535 (2**16-1) bytes.
+
+
+    13             That is, all servers shall support transfers at least  as
+    14             large  as the above values.  Support for longer transfers
+    15             is optional.
+
+    16             This field is optional.  Class drivers determine  whether
+    17             or  not  this  field  is  included  in the end message by
+    18             checking   the   message   length   supplied    by    the
+    19             communications mechanism.  If this field is not supplied,
+    20             or if its contents are zero,  then  the  server  supports
+    21             byte  counts  up  to  the  values  shown  in the previous
+    22             paragraph.
+
+
+    23     Status Codes:
+
+    24         Success (subcode "Normal")
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-107
+           SET CONTROLLER CHARACTERISTICS Command               11 June 1992
+
+
+     1     6.16.3  Description
+
+     2     The SET CONTROLLER CHARACTERISTICS command is  used  to  set  and
+     3     obtain   controller   characteristics.   The  default  value  for
+     4     "cntrlr. flags" is all flags clear (i.e., all messages disabled).
+     5     The  default  value  for  "host  timeout"  is  60 seconds.  These
+     6     default values are used from the time that the controller becomes
+     7     "Controller-Online"    to   a   host   until   it   stops   being
+     8     "Controller-Online" or until the host  issues  a  SET  CONTROLLER
+     9     CHARACTERISTICS command.
+
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-108
+           SET UNIT CHARACTERISTICS Command                     11 June 1992
+
+
+     1     6.17  SET UNIT CHARACTERISTICS Command
+
+     2     Command Category:
+
+     3         Sequential
+
+     4     6.17.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |  unit flags   |    reserved   |
+    14                     +---------------+---------------+
+    15                     |           reserved            |
+    16                     +-------------------------------+
+    17                     |                               |
+    18                     +---        reserved         ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |  device dependent parameters  |
+    22                     +-------------------------------+
+    23                     |           reserved            |
+    24                     +-------------------------------+
+
+
+    25         unit flags
+
+    26             Host-settable  unit  flags.   See  Section   5.7,   "Unit
+    27             Flags." 
+
+    28         device dependent parameters
+
+    29             Device  and/or   controller   dependent   device   tuning
+    30             parameters.   The  value  zero  in  this field means that
+    31             default or  normal  tuning  parameters  should  be  used.
+    32             Nonzero   values   for  this  field  should  normally  be
+    33             established through  the  system  startup  command  file.
+    34             Examples  of  the  use  of  this  field include selecting
+    35             alternative  optimization  algorithms  or  enabling   and
+    36             disabling automatic (online) diagnosis of the unit.
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-109
+           SET UNIT CHARACTERISTICS Command                     11 June 1992
+
+
+     1     Allowable modifiers:
+
+     2         Clear Serious Exception (ignored for disk class devices)
+
+     3         Enable Set Write Protect
+
+     4             Causes the "Write Protect" unit flag to be host-settable.
+     5             This  modifier  causes  the  state  of the "Write Protect
+     6             (software)" unit flag to be copied to the Software  Write
+     7             Protect  flag  for  this  unit.  See Section 4.14, "Write
+     8             Protection." 
+
+
+
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-110
+           SET UNIT CHARACTERISTICS Command                     11 June 1992
+
+
+     1     6.17.2  End Message Format
+
+     2              31                             0               
+     3              +-------------------------------+              
+     4              |   command reference number    |              
+     5              +---------------+---------------+              
+     6              |sequence number|  unit number  |              
+     7              +---------------+-------+-------+              
+     8              |    status     | flags |endcode|              
+     9              +---------------+-------+-------+              
+    10              |  unit flags   |multiunit code |              
+    11              +---------------+---------------+              
+    12              |      reserved         |spndles|              
+    13              +-------------------------------+              
+    14              |                               |              
+    15              +---     unit identifier     ---+              
+    16              |                               |              
+    17              +-------------------------------+              
+    18              |     media type identifier     |              
+    19              +-------------------------------+              
+    20              |           reserved            |              
+    21              +-------------------------------+              
+    22              |           unit size           |              
+    23              +-------------------------------+              
+    24              |      volume serial number     |              
+    25              +-------------------------------+              
+
+
+
+    26  |      status
+
+    27             The validity of the unit characteristics varies with  the
+    28             unit's  state implied by the contents of this field.  See
+    29             Status Codes listed later in  this  section  and  Section
+    30             6.17.3,  "SET  UNIT CHARACTERISTICS Command, Description"
+    31             for complete details.
+
+    32         multiunit code
+
+    33             The low byte of this field  identifies  the  access  path
+    34             between  the  controller  and the unit.  The high byte of
+    35             this field identifies  the  spindle,  within  the  access
+    36             path,  to  which  the  unit belongs.  See Section 4.16.2,
+    37             "Multiunit Drives and Formatters," for more information.
+
+    38         unit flags
+
+    39             See Section 5.7, "Unit Flags." 
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-111
+           SET UNIT CHARACTERISTICS Command                     11 June 1992
+
+
+     1         spndles
+
+     2             The number minus one of spindles or spindle  "groups"  in
+     3             the  unit.  The value of zero indicates one spindle.  See
+     4             Section 4.12, "Disk Geometry and Format."
+
+     5         unit identifier
+
+     6             Uniquely identifies the unit among all devices accessible
+     7             via   MSCP.   See  Section  4.17,  "Controller  and  Unit
+     8             Identifiers." 
+
+     9         media type identifier
+
+    10             Identifies the type of media used by this unit,  for  use
+    11             by   host  generic  device  allocation  mechanisms.   See
+    12             Section 4.18, "Media Type Identifiers." 
+
+    13         unit size
+
+    14             The number of logical blocks in the  host  area  of  this
+    15             unit.   This  value  does  NOT  include the logical block
+    16             range occupied by the unit's  Replacement  Control  Table
+    17             (RCT).   The  logical  block number of the first block of
+    18             the unit's RCT is equal to this value.
+
+    19         volume serial number
+
+    20             The low order 32 bits of the serial number of the  volume
+    21             that  is  mounted  on this unit.  Zero if the volume does
+    22             not have  a  serial  number.   When  displayed  in  human
+    23             readable  form,  this number should be formatted as a ten
+    24             digit  decimal  number  with   leading   zeros   printed.
+    25             Undefined    if    the    unit   is   "Unit-Offline"   or
+    26             "Unit-Available," or if the "Ignore Media  Format  Error"
+    27             modifier was set in the ONLINE command that first brought
+    28             this unit "Unit-Online." 
+
+    29             Hosts may not assume that  the  value  returned  in  this
+    30             field  uniquely  identifies  the  volume.   Although this
+    31             value  often  will  be  unique,  the  uniqueness  is  not
+    32             guaranteed, especially across media obtained from several
+    33             independent  suppliers.   Also,  media  that  must   meet
+    34             external  (industry  compatible)  format  standards  will
+    35             typically be unable to implement a volume serial  number.
+    36             This field will always be zero for such media.
+
+
+    37     Status Codes:
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-112
+           SET UNIT CHARACTERISTICS Command                     11 June 1992
+
+
+     1         Success (subcode "Normal")
+     2         Success (subcode "Duplicate Unit Number")
+
+     3             Imply that the unit is "Unit-Online." 
+
+     4         Command Aborted
+
+     5             The command has been rejected and the  unit's  state  and
+     6             context  have not been changed.  That is, the unit's unit
+     7             flags and  device  dependent  parameters  are  unchanged.
+     8             Since  the  unit's state is not reported to the host, the
+     9             host should assume that the returned unit characteristics
+    10             are invalid as the unit may be "Unit-Offline." 
+
+    11         Unit-Offline
+    12         Unit-Available
+
+    13         Controller Error
+    14         Drive Error
+
+    15             For both of these status codes the actual drive state  is
+    16             unknown  (although  it will usually be "Unit-Offline" due
+    17             to being inoperative).  Therefore the class driver should
+    18             assume that the unit is "Unit-Offline." 
+
+
+
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+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-113
+           SET UNIT CHARACTERISTICS Command                     11 June 1992
+
+
+     1     6.17.3  Description
+
+     2     The SET UNIT CHARACTERISTICS command is used to set host-settable
+     3     unit  characteristics  and obtain those unit characteristics that
+     4     are essential for proper class driver  operation.   This  command
+     5     never  alters  the unit's state ("Unit-Online," "Unit-Available,"
+     6     "Unit-Offline").   It  is  meaningless   to   set   host-settable
+     7     characteristics   for   a   unit   that  is  "Unit-Available"  or
+     8     "Unit-Offline." 
+
+     9     Class  drivers  can  determine  which  of   the   returned   unit
+    10     characteristics  are  valid  from  the  unit state implied by the
+    11     returned "status" (see Status Codes listed above) as follows:
+
+    12          1.  Unit is "Unit-Offline" and unknown to  controller.   All
+    13              characteristic fields and all unit flags are undefined.
+
+    14          2.  Unit is "Unit-Offline" and known to the controller or it
+    15              is   "Unit-Available."    The  "multiunit  code,"  "unit
+    16              identifier," and "media type identifier"  characteristic
+    17              fields  are  valid.   The "Removable Media" unit flag is
+    18              valid.  All other characteristic fields and  unit  flags
+    19              are undefined.
+
+    20          3.  Unit is "Unit-Online".  All characteristics  fields  and
+    21              all unit flags are valid.
+
+
+    22     Note that the format of the SET  UNIT  CHARACTERISTICS  command's
+    23     end  message  is  identical  to  that of the ONLINE command's end
+    24     message  because  the  ONLINE  command  performs   a   SET   UNIT
+    25     CHARACTERISTICS operation after bringing a unit "Unit-Online." 
+
+
+
+
+
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+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-114
+           WRITE Command                                        11 June 1992
+
+
+     1     6.18  WRITE Command
+
+     2     Command Category:
+
+     3         Non-Sequential
+
+     4     6.18.1  Command Message Format
+
+     5              31                             0              
+     6              +-------------------------------+             
+     7              |   command reference number    |             
+     8              +---------------+---------------+             
+     9              |   reserved    |  unit number  |             
+    10              +---------------+-------+-------+             
+    11              |   modifiers   |  caa  | opcode|             
+    12              +---------------+-------+-------+             
+    13              |          byte count           |             
+    14              +-------------------------------+             
+    15              |                               |             
+    16              +---         buffer          ---+             
+    17              |                               |             
+    18              +---       descriptor        ---+             
+    19              |                               |             
+    20              +-------------------------------+             
+    21              |      logical block number     |             
+    22              +---------------+---------------+             
+    23              |   entry id    | hrn or entloc | (optional)  
+    24              +---------------+---------------+             
+
+
+    25         hrn (host reference number) or entloc (entry locator)
+    26         entry id
+
+    27             Optional fields, used in conjunction with the History Log
+    28             and Reuse Entry modifiers.  See the descriptions of those
+    29             modifiers below.
+
+    30     Allowable modifiers:
+
+    31         Clear Serious Exception (ignored for disk class devices)
+    32         Compare
+    33         Express Request
+    34         Force Error
+    35         History Log
+    36         Reuse Entry
+
+    37             The  History  Log   and   Reuse   Entry   modifiers   are
+    38             interdependent  and  described  together  here  to  avoid
+    39             confusion.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-115
+           WRITE Command                                        11 June 1992
+
+
+     1             Note that the  History  Log  modifier,  the  Reuse  Entry
+     2             modifier,  the  "hrn or entloc" field, and the "entry id"
+     3             field are valid only  for  servers  that  provide  "write
+     4             history logging" support (i.e., the server sets the Write
+     5             History Logging Support controller flag; see Section 5.6,
+     6             "Controller Flags" for related information).
+
+     7             The  setting  of  the  History  Log  modifier  determines
+     8             whether information about this command is to be stored in
+     9             a "write history entry."
+
+    10             The  setting  of  the  Reuse  Entry  modifier  determines
+    11             whether  the  server  is to allocate a new "write history
+    12             entry" or reuse one that was previously allocated.
+
+    13             If "write history logging" is supported and  the  History
+    14             Log  modifier  is  set  and  the  Reuse Entry modifier is
+    15             clear, the server allocates a new "write  history  entry"
+    16             and  then  stores  the  value contained in the "hrn (host
+    17             reference number)" field, "entry  id"  field,  and  other
+    18             information  related  to  this  command  into  the "write
+    19             history entry" just allocated.   If  the  server  has  no
+    20             "write history entry" to allocate, it shall remember this
+    21             fact.  See  Section  6.19.3,  "WRITE  HISTORY  MANAGEMENT
+    22             Command,  Description"  for more details.  To report this
+    23             allocation  failure  to  the  host,  the  server  returns
+    24             FFFF(hex) in the "entry locator" field of the end message
+    25             and sets the Allocation Failure end  flag.   See  Section
+    26             5.4, "End Messages," for a description of this flag.
+
+    27             If "write history logging" is supported and  the  History
+    28             Log  modifier is set and the Reuse Entry modifier is set,
+    29             the server uses the value contained in the "entloc (entry
+    30             locator)"  field  to locate the associated "write history
+    31             entry" and then stores the other information  related  to
+    32             this command into that "write history entry." 
+
+    33             See Section 6.19.3, "WRITE  HISTORY  MANAGEMENT  Command,
+    34             Description"  for  details  regarding the maintenance and
+    35             format of "write history entries." 
+
+    36             Note that when "write history logging" is  supported  and
+    37             the  History Log modifier is set host class drivers shall
+    38             supply both the  "hrn  or  entloc"  and  the  "entry  id"
+    39             fields.   If  the  History  Log modifier is set and those
+    40             fields are not supplied, the server rejects  the  command
+    41             as   an   "Invalid  Command"  ("invalid  message  length"
+    42             protocol error.  See "Invalid Command" status in  Section
+    43             5.5, "Status Codes").
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-116
+           WRITE Command                                        11 June 1992
+
+
+     1             If "write history logging" is supported and  the  History
+     2             Log  modifier is clear, the server ignores the setting of
+     3             the Reuse Entry modifier and the contents of the "hrn  or
+     4             entloc" and "entry id" fields if they are supplied.  Note
+     5             that host class drivers need not supply those fields when
+     6             the History Log modifier is clear.
+
+     7             If "write history  logging"  is  not  supported  and  the
+     8             History  Log modifier or the Reuse Entry modifier is set,
+     9             the server rejects the command as  an  "Invalid  Command"
+    10             ("invalid   modifier"   protocol   error.   See  "Invalid
+    11             Command" status in Section 5.5, "Status Codes").
+
+    12             If "write history logging" is supported, the History  Log
+    13             modifier  is set, and the "byte count" field is zero, the
+    14             server  rejects  the  command  as  an   Invalid   Command
+    15             ("invalid  byte  count"  parameter  error.   See "Invalid
+    16             Command" status in Section 5.5, "Status Codes").
+
+    17             The server shall set the History Logged end flag in  this
+    18             command's  end message if it has caused the write history
+    19             log to contain a valid "write  history  entry"  for  this
+    20             command.    See   Section   5.4,   "End   Messages,"  for
+    21             descriptions of all end message flags.
+
+    22  |      Supplement Write Log
+
+    23  |          The Supplement Write log  modifier  allows  the  transfer
+    24  |          length  field of an existing write log in a controller to
+    25  |          be modified.  When this modifier  is  set,  the  transfer
+    26  |          length  of  the  current  WRITE  command  is added to the
+    27  |          transfer length field of the specified write  log  entry.
+    28  |          No  other fields in the write log entry are modified.  In
+    29  |          particular, the unit number field, the flags  field,  the
+    30  |          logical  block  number field, the entry id field, and the
+    31  |          hrn field shall remain unchanged.
+
+    32  |          This modifier is valid only if the History Log and  Reuse
+    33  |          Entry modifiers are both set, and the controller supports
+    34  |          Write History Logging; Write History Logging Support flag
+    35  |          set, otherwise it shall be ignored.
+
+    36  |          Proper use of this modifier requires  that  hosts  should
+    37  |          not  issue more than one MSCP command at a time using the
+    38  |          same entry locator.  Requests for a given  entry  locator
+    39  |          using  this  modifier  shall  be  in  exact logical block
+    40  |          number sequence order with each successive logical  block
+    41  |          number  being  higher  by  precisely  the transfer length
+    42  |          (converted to sectors) of the previous WRITE command.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-117
+           WRITE Command                                        11 June 1992
+
+
+     1         Suppress Error Correction
+
+     2             Note that this modifier only affects the compare pass  of
+     3             a write-compare operation.  It has no affect on the write
+     4             operation itself.
+
+     5         Suppress Error Recovery
+
+
+
+
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-118
+           WRITE Command                                        11 June 1992
+
+
+     1     6.18.2  End Message Format
+
+     2         31                             0                         
+     3         +-------------------------------+                        
+     4         |   command reference number    |                        
+     5         +---------------+---------------+                        
+     6         |sequence number|  unit number  |                        
+     7         +---------------+-------+-------+                        
+     8         |    status     | flags |endcode|                        
+     9         +---------------+-------+-------+                        
+    10         |          byte count           |                        
+    11         +-------------------------------+                        
+    12         |                               |                        
+    13         +---                         ---+                        
+    14         |           undefined           |                        
+    15         +---                         ---+                        
+    16         |                               |                        
+    17         +-------------------------------+                        
+    18         |        first bad block        |                        
+    19         +---------------+---------------+                        
+    20         |   entry id    | entry locator | (conditionally present)
+    21         +---------------+---------------+                        
+
+
+    22         entry locator
+
+    23             This field is present if and only  if  the  corresponding
+    24             command  message  field  is  present.  If present and the
+    25             History Log modifier is set in the command message,  this
+    26             field  contains  the  value  assigned  by the server that
+    27             uniquely identifies the internal location of the "history
+    28             entry"  where this command's information was stored.  If,
+    29             however,  an  allocation  failure  occurred,  this  field
+    30             contains the value FFFF(hex).  If present and the History
+    31             Log modifier is clear in the command message, this  field
+    32             is undefined.
+
+    33             This field contains a valid entry  locator  of  a  "write
+    34             history  entry"  for  this  command  if  and  only if the
+    35             History Logged end flag is set in the  "flags"  field  of
+    36             this end message.
+
+    37             See the description of the History Log  and  Reuse  Entry
+    38             modifiers  in  Section  6.18.1,  "WRITE  Command, Command
+    39             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+    40             MANAGEMENT Command, Description" for more detail.
+
+    41         entry id
+
+    42             This field is present if and only  if  the  corresponding
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-119
+           WRITE Command                                        11 June 1992
+
+
+     1             command  message  field  is  present.  If present and the
+     2             History Log modifier is set in the command  message,  the
+     3             contents  of  the  corresponding command message field is
+     4             copied without modification to this  field.   If  present
+     5             and  the  History  Log  modifier  is clear in the command
+     6             message, this field is undefined.
+
+     7             See the description of the History Log  and  Reuse  Entry
+     8             modifiers  in  Section  6.18.1,  "WRITE  Command, Command
+     9             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+    10             MANAGEMENT Command, Description" for more detail.
+
+
+    11     Status Codes:
+
+    12         Success (subcode "Normal")
+    13         Success (subcode "Duplicate Unit Number")
+    14         Invalid Command (subcode "Invalid Byte Count")
+
+    15             A byte count of zero was specified with the  History  Log
+    16             modifier set.
+
+    17         Invalid Command (subcode "Invalid Entry Locator")
+
+    18             The value specified in the "entloc (entry locator)" field
+    19             is  not  within the limits of the server's "write history
+    20             entry" set.
+
+    21             See the description of the History Log  and  Reuse  Entry
+    22             modifiers  in  Section  6.18.1,  "WRITE  Command, Command
+    23             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+    24             MANAGEMENT Command, Description" for more detail.
+
+    25             Note that this error can only  occur  if  "write  history
+    26             logging" is supported and the History Log and Reuse Entry
+    27             modifiers are both set in the command message.
+
+    28         Invalid Command (subcode "Invalid Entry Id")
+
+    29             The server located the "write history  entry"  identified
+    30             in  the "entloc (entry locator)" field but found that the
+    31             "entry id" contained in that entry  does  not  equal  the
+    32             value supplied by a host class driver when that entry was
+    33             allocated.  This status  may  also  be  returned  if  the
+    34             History  Log  modifier  was set, the Reuse Entry modifier
+    35             was clear, and the host supplied a value of FFFF (hex) in
+    36             the "entry id" field.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-120
+           WRITE Command                                        11 June 1992
+
+
+     1             See the description of the History Log  and  Reuse  Entry
+     2             modifiers  in  Section  6.18.1,  "WRITE  Command, Command
+     3             Message  Format"  and  Section  6.19.3,  "WRITE   HISTORY
+     4             MANAGEMENT Command, Description" for more detail.
+
+     5             Note that this error can only  occur  if  "write  history
+     6             logging" is supported and the History Log modifier is set
+     7             in the command message.
+
+     8         Invalid Command (subcode "Invalid Logical Block Number")
+     9         Command Aborted
+    10         Unit-Offline
+    11         Unit-Available
+    12         Write Protected
+    13         Compare Error
+    14         Data Error
+    15         Host Buffer Access Error
+    16         Controller Error
+    17         Drive Error
+    18         Write History Entry Access Error (subcode "Allocation Failure
+    19                                           Table Full")
+
+    20             The server was unable to allocate a "write history entry"
+    21             and  could  not  record  this fact because the Allocation
+    22             Failure Table was full.  This is a  transient  condition;
+    23             therefore,  the  host  should  reissue  the command.  See
+    24             Section  6.19.3,  "WRITE  HISTORY   MANAGEMENT   Command,
+    25             Description" for more details.
+
+    26             Note that this error can occur  only  if  "write  history
+    27             logging" is supported and the History Log modifier is set
+    28             and Reuse Entry modifier is clear in the command message.
+
+
+
+
+
+
+
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+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-121
+           WRITE Command                                        11 June 1992
+
+
+     1     6.18.3  Description
+
+     2     Data is fetched from the host data  buffer  and  written  to  the
+     3     unit.   For  host  buffer  access details see Section 4.11, "Host
+     4     Buffer Access."
+
+     5     If a controller-based cache exists, the cache manager may use the
+     6     Cache  Access  Attribute  advice  in  managing  the  caching  and
+     7     comparing of the requested data area.
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-122
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1     6.19  WRITE HISTORY MANAGEMENT Command
+
+     2     Command Category:
+
+     3         Sequential
+
+     4     6.19.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |           byte count          |
+    14                     +---------------+---------------+
+    15                     |        "write history"        |
+    16                     +---                         ---+
+    17                     |            buffer             |
+    18                     +---                         ---+
+    19                     |          descriptor           |
+    20                     +---------------+---------------+
+    21                     |    offset     |   operation   |
+    22                     +---------------+---------------+
+    23                     |   entry id    | hrn or entloc |
+    24                     +---------------+---------------+
+
+
+    25         byte count
+
+    26             The total requested length of the data  transfer  ("write
+    27             history entries") in bytes; that is, the number of "write
+    28             history entries" requested times the  size  of  a  "write
+    29             history entry" (16 bytes).  If value in this field is not
+    30             a multiple of the "write history entry" size, the  server
+    31             shall  reject  the command as an Invalid Command (subcode
+    32             "Invalid  Byte  Count")   parameter   error.    See   the
+    33             description  of  the "byte count" field in Section 5.3.1,
+    34             "Transfer Command Message Format," for  more  details  on
+    35             architectural and server constraints.
+
+    36             The interpretation of this field varies depending on  the
+    37             "write  history management" operation specified.  See the
+    38             description of the READ ALL and READ  BY  HOST  REFERENCE
+    39             NUMBER  operations  in  the "operation" field description
+    40             below for complete details.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-123
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1         "write history" buffer descriptor
+
+     2             Identification of the host buffer  where  "write  history
+     3             entries"  are  to  be stored when a READ ALL or a READ BY
+     4             HOST REFERENCE NUMBER operation is  specified.   See  the
+     5             description  of those operations in the "operation" field
+     6             above for complete details.
+
+     7             The format of the "'write history' buffer descriptor"  is
+     8             identical  to  the "buffer descriptor" field described in
+     9             Section 5.3.1, "Transfer Command Message Format." 
+
+    10         operation
+
+    11             The "write history management" operation to be performed:
+
+    12               DEALLOCATE ALL
+
+    13                 The server deallocates  all  of  the  "write  history
+    14                 entries"  associated  with the unit identified in the
+    15                 "unit number" field.
+
+    16                 This  operation  clears  any   "previous   allocation
+    17                 failure"  condition associated with this unit for all
+    18                 host reference  numbers.   This  is  accomplished  by
+    19                 deleting  all  entries with this unit number from the
+    20                 controller's Allocation Failure Table.
+
+    21                 The  server  ignores  the  "'write  history'   buffer
+    22                 descriptor," "byte count," "offset," "hrn or entloc,"
+    23                 and  "entry  id"  fields  when  this   operation   is
+    24                 specified.
+
+    25               DEALLOCATE BY HOST REFERENCE NUMBER
+
+    26                 The server deallocates  all  of  the  "write  history
+    27                 entries"  that  are  associated  with  both the value
+    28                 supplied in the "hrn (host reference  number)"  field
+    29                 and the unit identified in the "unit number" field.
+
+    30                 This  operation  clears  any   "previous   allocation
+    31                 failure" condition associated with this unit for this
+    32                 host reference number.  That is, the  server  deletes
+    33                 the  entry associated with this host reference number
+    34                 and unit in its Allocation Failure Table if  such  an
+    35                 entry   exists.    This  does  not  affect  "previous
+    36                 allocation failures" associated with other  units  or
+    37                 other host reference numbers.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-124
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1                 The  server  ignores  the  "'write  history'   buffer
+     2                 descriptor,"  "byte  count," "offset," and "entry id"
+     3                 fields when this operation is specified.
+
+     4               DEALLOCATE BY ENTRY LOCATOR
+
+     5                 The server deallocates the "write history entry" that
+     6                 is  located at the internal server location specified
+     7                 in the "entloc (entry locator)" field.
+
+     8                 If the "entry locator" is not within  the  limits  of
+     9                 the  server's  "write  history entry" set, the server
+    10                 rejects the command as an "Invalid Command" ("invalid
+    11                 entry   locator"   parameter   error.   See  "Invalid
+    12                 Command" status in Section 5.5, "Status Codes").
+
+    13                 If the "write history entry" is already  deallocated,
+    14                 the   server  rejects  the  command  as  an  "Invalid
+    15                 Command" ("invalid entry id" parameter error).
+
+    16                 If the value contained in the "entry id" field of the
+    17                 command  message  is  FFFF (hex) and the entry is not
+    18                 already  deallocated,  the  server  deallocates   the
+    19                 "write history entry" and does not compare this value
+    20                 with the "entry id" value associated with the  "write
+    21                 history entry."
+
+    22                 If the value contained in the "entry id" field of the
+    23                 command  message  is  not  FFFF (hex) and it does not
+    24                 equal the "entry id" value associated with the "write
+    25                 history  entry"  located via the "entry locator," the
+    26                 server rejects the command as  an  "Invalid  Command"
+    27                 ("invalid  entry  id"  parameter error.  See "Invalid
+    28                 Command" status in Section 5.5, "Status Codes").
+
+    29                 The  server  ignores  the  "'write  history'   buffer
+    30                 descriptor,"  "byte  count," and "offset" fields when
+    31                 this operation is specified.
+
+    32               DECREMENT ALLOCATION FAILURE COUNT
+
+    33                 The server decrements the count field  of  the  entry
+    34                 associated  with  this host reference number and unit
+    35                 in its Allocation Failure  Table  if  such  an  entry
+    36                 exists.   If  this  causes  the  count field to go to
+    37                 zero, the entry is deleted.  If no such entry exists,
+    38                 the server rejects the command with a status of Write
+    39                 History Entry Access Error (subcode "No Such Entry").
+    40                 Otherwise, the server returns Success.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-125
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1                 The server may, at its option, treat a WRITE  HISTORY
+     2                 MANAGEMENT  command  with  the  DECREMENT  ALLOCATION
+     3                 FAILURE COUNT operation as an Immediate command.
+
+     4                 The  server  ignores  the  "'write  history'   buffer
+     5                 descriptor,"  "byte  count," "offset," and "entry id"
+     6                 fields when this operation is specified.
+
+     7               READ ALL
+
+     8                 If the "byte count" field is zero,  the  server  sets
+     9                 the  command's end message "count" field equal to the
+    10                 number of "write history entries" that are associated
+    11                 with  the unit identified in the "unit number" field,
+    12                 and then completes  the  command  with  a  status  of
+    13                 Success  (subcode "Normal").  In this case the server
+    14                 ignores  the  "'write  history'  buffer  descriptor,"
+    15                 "offset," "hrn or entloc," and "entry id" fields.
+
+    16                 If the "byte count"  field  is  nonzero,  the  server
+    17                 transfers  the  number  of  "write  history  entries"
+    18                 implied by the "byte count" field to the  host  class
+    19                 driver's  buffer  (specified  in the "'write history'
+    20                 buffer descriptor" field)  beginning  at  the  offset
+    21                 specified  in  the  "offset"  field into the group of
+    22                 entries associated with the unit  identified  in  the
+    23                 "unit  number"  field.   There  may  be  fewer "write
+    24                 history entries" than implied  by  the  "byte  count"
+    25                 field.   The  server  indicates  the number of "write
+    26                 history entries" actually transferred in the  "count"
+    27                 field  of  the  end  message.   Note  that only those
+    28                 "write history entries" that are associated with  the
+    29                 unit  identified  in  the  "unit  number"  field  are
+    30                 included in the transfer.  In this  case  the  server
+    31                 ignores  the  "hrn or entloc," and "entry id" fields.
+    32                 Note also that the "count" returned by the server may
+    33                 be zero if there are no more entries beginning at the
+    34                 specified offset but there  is  at  least  one  entry
+    35                 associated  with  the  unit  identified  in the "unit
+    36                 number" field.  If there is no  entry  in  the  write
+    37                 history  log  associated  with the unit identified in
+    38                 the "unit number" field,  the  server  completes  the
+    39                 command  with  a status of Write History Entry Access
+    40                 Error (subcode "No Such Entry").
+
+    41               READ BY HOST REFERENCE NUMBER
+
+    42                 If the "byte count" field is zero,  the  server  sets
+    43                 the  command's end message "count" field equal to the
+    44                 number of "write history entries" that are associated
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-126
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1                 with  both  the  value  contained  in  the "hrn (host
+     2                 reference number)" field and the unit  identified  in
+     3                 the  "unit  number"  field,  and  then  completes the
+     4                 command with a status of Success (subcode  "Normal").
+     5                 In  this case the server ignores the "'write history'
+     6                 buffer descriptor," "offset," and "entry id" fields.
+
+     7                 If the "byte count"  field  is  nonzero,  the  server
+     8                 transfers  the  number  of  "write  history  entries"
+     9                 implied by the "byte count" field to the  host  class
+    10                 driver's  buffer  (specified  in the "'write history'
+    11                 buffer descriptor" field)  beginning  at  the  offset
+    12                 specified  in  the  "offset"  filed into the group of
+    13                 entries associated with both the value  contained  in
+    14                 the  "hrn (host reference number)" field and the unit
+    15                 identified in the "unit number" field.  There may  be
+    16                 fewer  "write  history entries" than specified in the
+    17                 "count" field.  The server indicates  the  number  of
+    18                 "write  history  entries" actually transferred in the
+    19                 "count" field of the end  message.   Note  that  only
+    20                 those  "write  history  entries"  that are associated
+    21                 with both the  value  contained  in  the  "hrn  (host
+    22                 reference  number)"  field and the unit identified in
+    23                 the "unit number" field are included in the transfer.
+    24                 In this case the server ignores the "entry id" field.
+    25                 Note also that the "count" returned by the server may
+    26                 be zero if there are no more entries beginning at the
+    27                 specified offset but there  is  at  least  one  entry
+    28                 associated  with  both  the  value  in the "hrn (host
+    29                 reference number)" field and the unit  identified  in
+    30                 the "unit number" field.  If there is no entry in the
+    31                 write history log associated with the specified  host
+    32                 reference  number  and unit, the server completes the
+    33                 command with a status of Write History  Entry  Access
+    34                 Error (subcode "No Such Entry").
+
+    35             See Appendix A, "Opcode, Flag, and  Offset  Definitions,"
+    36             Table  A-14,  "Write History Management Command Operation
+    37             Codes" for the values and  mnemonics  assigned  to  those
+    38             operations.
+
+    39             If an operation not listed in Table  A-14  is  specified,
+    40             the  server  rejects  the command as an "Invalid Command"
+    41             ("invalid  operation"  protocol  error.    See   "Invalid
+    42             Command" status in Section 5.5, "Status Codes").
+
+    43             Note  that  the  server  shall  perform   the   specified
+    44             operation  when  the  unit  is in the "Unit-Offline" (all
+    45             substates except "Exclusive Use"), "Unit-Online," or  the
+    46             "Unit-Available" state relative to the issuing host class
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-127
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1             driver.
+
+     2         offset
+
+     3             The offset (in "write history entries") into the group of
+     4             "write  history  entries" specified by a READ ALL or READ
+     5             BY HOST REFERENCE NUMBER operation.  See the  description
+     6             of  these  WRITE  HISTORY  MANAGEMENT  operations  in the
+     7             "operation" field above for complete details.
+
+     8         hrn (host reference number) or entloc (entry locator)
+
+     9             The interpretation of this field varies depending on  the
+    10             "write  history management" operation specified.  See the
+    11             descriptions of the DEALLOCATE BY HOST REFERENCE  NUMBER,
+    12             DEALLOCATE BY ENTRY LOCATOR, DECREMENT ALLOCATION FAILURE
+    13             COUNT, and READ BY HOST REFERENCE  NUMBER  operations  in
+    14             the "operation" field above for complete details.
+
+    15             See Sections 6.19.3, "WRITE HISTORY  MANAGEMENT  Command,
+    16             Description,"  6.7, "DISK COPY DATA Command," 6.9, "ERASE
+    17             Command," and 6.18, "WRITE Command" for more  information
+    18             regarding this field.
+
+    19         entry id
+
+    20             The interpretation of this field varies depending on  the
+    21             "write  history management" operation specified.  See the
+    22             description of the DEALLOCATE BY ENTRY LOCATOR  operation
+    23             in the "operation" field above for complete details.
+
+    24             See Sections 6.19.3, "WRITE HISTORY  MANAGEMENT  Command,
+    25             Description,"  6.7, "DISK COPY DATA Command," 6.9, "ERASE
+    26             Command," and 6.18, "WRITE Command" for more  information
+    27             regarding this field.
+
+    28     Allowable modifiers:
+
+    29         none
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-128
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1     6.19.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |           byte count          |
+    11                     +---------------+---------------+
+    12                     | server alloc  |  unit alloc   |
+    13                     +---------------+---------------+
+    14                     |               |server unalloc |
+    15                     +               +---------------+
+    16                     |           undefined           |
+    17                     +---------------+---------------+
+    18                     |    offset     |     count     |
+    19                     +---------------+---------------+
+    20                     |   entry id    | hrn or entloc |
+    21                     +---------------+---------------+
+
+
+    22         byte count
+
+    23             The  number  of  bytes  successfully  transferred.    See
+    24             Section 5.3.1, "Transfer Command End Message Format," for
+    25             more details.
+
+    26         unit alloc
+
+    27             The total number of  "write  history  entries"  currently
+    28             allocated  and associated with the unit identified in the
+    29             command message  "unit  number"  field.   This  field  is
+    30             undefined  if  the operation specified in the command was
+    31             DEALLOCATE  BY  ENTRY  LOCATOR  or  DECREMENT  ALLOCATION
+    32             FAILURE COUNT.
+
+    33         server alloc
+
+    34             The total number of  "write  history  entries"  currently
+    35             allocated  across  all  units served by the server.  This
+    36             field is undefined if  the  operation  specified  in  the
+    37             command  was  DEALLOCATE  BY  ENTRY  LOCATOR or DECREMENT
+    38             ALLOCATION FAILURE COUNT.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-129
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1         server unalloc
+
+     2             The total number of unallocated "write  history  entries"
+     3             currently  available.   This  field  is  undefined if the
+     4             operation specified in  the  command  was  DEALLOCATE  BY
+     5             ENTRY LOCATOR or DECREMENT ALLOCATION FAILURE COUNT.
+
+     6         count
+
+     7             The interpretation of this field varies depending on  the
+     8             "operation"  specified and the outcome of the operation's
+     9             execution.  See  the  descriptions  of  those  operations
+    10             above  and  the  descriptions of the various status codes
+    11             below for specific details.
+
+    12         offset
+
+    13             Copied  without  modification  from   the   corresponding
+    14             command message field.
+
+    15         hrn (host reference number) or entloc (entry locator)
+
+    16             The interpretation of this field varies depending on  the
+    17             "operation"  specified  and  possibly  the outcome of the
+    18             operation's execution.
+
+    19             If a DEALLOCATE BY HOST REFERENCE NUMBER,  DEALLOCATE  BY
+    20             ENTRY  LOCATOR,  DECREMENT  ALLOCATION  FAILURE COUNT, or
+    21             READ BY HOST REFERENCE NUMBER  operation  was  specified,
+    22             this  field  is  copied  without  modification  from  the
+    23             corresponding command message field.
+
+    24             If a DEALLOCATE ALL or READ ALL operation was  specified,
+    25             this field is undefined.
+
+    26         entry id
+
+    27             If a DEALLOCATE BY ENTRY LOCATOR operation was specified,
+    28             this  field  is  copied  without  modification  from  the
+    29             corresponding command  message  field.   Otherwise,  this
+    30             field is undefined.
+
+    31     Status Codes:
+
+    32         Success (subcode "Normal")
+    33         Success (subcode "Duplicate Unit Number")
+
+    34             If a DEALLOCATE ALL, DEALLOCATE BY HOST REFERENCE NUMBER,
+    35             or  DEALLOCATE  BY ENTRY LOCATOR operation was specified,
+    36             the server sets the end message "count"  field  equal  to
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-130
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1             the number of "write history entries" deallocated.
+
+     2             If a DECREMENT ALLOCATION  FAILURE  COUNT  operation  was
+     3             specified,  the server sets the end message "count" field
+     4             to the decremented value in the count field of the  entry
+     5             in the Allocation Failure Table.
+
+     6             If a READ ALL operation with a zero command message "byte
+     7             count"  field  was  specified,  the  server  sets the end
+     8             message "count" field  equal  to  the  number  of  "write
+     9             history  entries"  associated with the unit identified in
+    10             the "unit number" command message field.
+
+    11             If a READ BY HOST REFERENCE NUMBER operation with a  zero
+    12             command  message  "byte  count"  field was specified, the
+    13             server sets the end message "count" field  equal  to  the
+    14             number  of  "write  history entries" associated with both
+    15             the value contained in the "hrn (host reference  number)"
+    16             command  message  field  and  the  unit identified in the
+    17             "unit number" command message field.
+
+    18             If a READ ALL or READ BY HOST REFERENCE NUMBER  operation
+    19             with  a  nonzero  command  message "byte count" field was
+    20             specified, the server sets the end message "count"  field
+    21             equal  to  the number of "write history entries" actually
+    22             transferred to the host's buffer.
+
+    23         Invalid Command (subcode "Invalid Byte Count")
+    24         Invalid Command (subcode "Invalid Operation")
+    25         Invalid Command (subcode "Invalid Entry Locator")
+    26         Invalid Command (subcode "Invalid Entry Id")
+    27         Command Aborted
+
+    28         Unit-Offline (subcode "Exclusive Use")
+
+    29             The unit is under the "Exclusive Access" of a host  class
+    30             driver  other than the host class driver that issued this
+    31             command.
+
+    32             As described in Section 6.19.3, "WRITE HISTORY MANAGEMENT
+    33             Command, Description," all of the "write history entries"
+    34             associated with the unit are only accessible by the  host
+    35             class  driver  that was granted "Exclusive Access" to the
+    36             unit.  The server will therefore take no  action  on  the
+    37             "write  history  entries"  associated  with the unit when
+    38             this condition exists.
+
+    39         Host Buffer Access Error
+
+    40             Before returning this status  the  server  sets  the  end
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-131
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1             message  "count"  field  equal  to  the  number of "write
+     2             history entries" transferred before the error occurred.
+
+     3             Note that this error can  only  occur  if  the  operation
+     4             specified  is either a READ ALL or READ BY HOST REFERENCE
+     5             NUMBER with a nonzero "byte count" field.
+
+     6         Write History Entry Access Error (subcode "No Such Entry")
+
+     7             The  server  returns  this  status  under  the  following
+     8             conditions:
+
+     9             1.  A READ ALL operation was  specified  but  the  server
+    10                 could not find any "write history entries" associated
+    11                 with the unit identified in the "unit number" command
+    12                 message field.
+
+    13             2.  A  READ  BY  HOST  REFERENCE  NUMBER  operation   was
+    14                 specified  but  the  server could not find any "write
+    15                 history  entries"  associated  with  both  the  value
+    16                 contained   in  the  "hrn  (host  reference  number)"
+    17                 command message field and the unit identified in  the
+    18                 "unit number" command message field.
+
+    19             3.  A DEALLOCATE BY ENTRY LOCATOR operation was specified
+    20                 but  the  specified  "write  history  entry"  is  not
+    21                 currently associated with the unit identified in  the
+    22                 "unit number" command message field.
+
+    23             4.  A DECREMENT ALLOCATION FAILURE  COUNT  operation  was
+    24                 specified  but  no  entry  associated  with this host
+    25                 reference number and unit was found in the Allocation
+    26                 Failure Table.
+
+    27             In all the above cases, the server zeros the end  message
+    28             "count" field.
+
+    29         Write  History  Entry  Access   Error   (subcode    "Previous
+    30                                                  Allocation Failure")
+
+    31             A READ BY HOST REFERENCE NUMBER operation  was  specified
+    32             but  a  previous "write history entry" allocation failure
+    33             condition exists for this host reference number and  unit
+    34             or  a  READ  ALL  operation  was specified and a previous
+    35             allocation failure exists for some host reference  number
+    36             and  this  unit.   In  either  case,  no  "write  history
+    37             entries" are transferred to the host.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-132
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1     6.19.3  Description
+
+     2     Support of "write history logging"  is  a  server  implementation
+     3     dependent option.  Servers that do not provide such support shall
+     4     reject this command as an  "Invalid  Command"  ("invalid  opcode"
+     5     protocol  error.   See  "Invalid  Command" status in Section 5.5,
+     6     "Status Codes").
+
+     7     Servers that provide "write history logging" support  shall  also
+     8     support  the  TERMINATE CLASS DRIVER/SERVER CONNECTION command as
+     9     described   in   Section   7.10,   "Multihost   TERMINATE   CLASS
+    10     DRIVER/SERVER   CONNECTION  Command."   Note  that  servers  that
+    11     support "write history logging"  but  do  not  provide  Multihost
+    12     Support   treat  the  TERMINATE  CLASS  DRIVER/SERVER  CONNECTION
+    13     command  as  a  no-operation  as  described  in  Section   6.1.1,
+    14     "No-Operation Commands." 
+
+    15     "Write history logging" and  the  TERMINATE  CLASS  DRIVER/SERVER
+    16     CONNECTION  command  were  defined  primarily  for the purpose of
+    17     assisting  in  the  operation  of  host  based  volume  shadowing
+    18     implementations.    All   servers  that  support  "write  history
+    19     logging"  (and  therefore  the  TERMINATE   CLASS   DRIVER/SERVER
+    20     CONNECTION  command)  shall set the Write History Logging Support
+    21     controller flag.
+
+    22     "Write history logging" provides host class drivers with a  means
+    23     for  detecting  and  recovering  from the incomplete execution of
+    24     DISK COPY DATA, ERASE, and WRITE commands  addressed  to  a  disk
+    25     unit  caused  by  unexpected host failure (e.g., a system crash).
+    26     The server provides a set of "write history entries" for  logging
+    27     information related to write type commands issued by a host.  The
+    28     number of "write history entries" contained in the  set  provided
+    29     by  a  server  is  implementation dependent.  The number provided
+    30     depends on an implementation's memory availability and  speed  of
+    31     I/O request execution.  In a multihost environment the host class
+    32     drivers are responsible for  managing  the  distribution  of  the
+    33     "write history entry" set among themselves.
+
+    34     The server also provides a table, the Allocation  Failure  Table,
+    35     for  recording  "write  history  entry"  allocation  failures.  A
+    36     "write history entry" allocation failure occurs when  the  server
+    37     cannot  allocate  a  new  "write  history entry" in response to a
+    38     write type command requesting  such  an  allocation  because  all
+    39     "write  history  entries"  are already allocated.  The Allocation
+    40     Failure Table is initially empty (i.e., contains no entries).  An
+    41     entry  has three fields:  "host reference number," "unit number,"
+    42     and "count."  When  an  allocation  failure  occurs,  the  server
+    43     searches  the  Allocation  Failure  Table for entry with the host
+    44     reference number and unit number associated with the  write  type
+    45     command.   If  such  an entry is found, the server increments the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-133
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1     "count" field of that  entry  and  completes  processing  of  the
+     2     allocation  failure as detailed below.  If no such entry is found
+     3     and the table is not full, the server creates a  new  entry  with
+     4     the  host  reference  number  and unit number associated with the
+     5     write type command and sets the "count" field to one.  The server
+     6     then  completes  processing of the allocation failure as detailed
+     7     below.  If no such entry is found and  the  table  is  full,  the
+     8     server  rejects  the  write  type  command with a status of Write
+     9     History Entry Access Error  (subcode  "Allocation  Failure  Table
+    10     Full").
+
+    11     The host can use the WRITE HISTORY MANAGEMENT command to clear an
+    12     allocation   failure   condition   using   either  the  DECREMENT
+    13     ALLOCATION FAILURE COUNT operation  or  a  DEALLOCATE  operation.
+    14     Normally,  the  host  will  use  the  former  method  after being
+    15     notified of the condition by the end message of  the  write  type
+    16     command that caused it.  Therefore, allocation failure conditions
+    17     are usually  transitory  in  nature.   Note  that  an  allocation
+    18     failure  condition  exists  for a given host reference number and
+    19     unit exactly when there exists an entry in the Allocation Failure
+    20     Table  for  that  host  reference  number  and  unit.  Therefore,
+    21     clearing an  allocation  failure  condition  using  a  DEALLOCATE
+    22     operation amounts to deleting the entry in the Allocation Failure
+    23     Table associated  with  that  host  reference  number  and  unit,
+    24     regardless of the value in the "count" field in that entry.
+
+    25     If a write type command  is  rejected  with  a  status  of  Write
+    26     History  Entry  Access  Error  (subcode "Allocation Failure Table
+    27     Full"), the host should reissue the command since this  condition
+    28     is  transitory  in  nature  and the reissued command is likely to
+    29  |  succeed.  The server may maintain the "write history  entry"  set
+    30  |  in  volatile  memory  and  shall  initialize  all  "write history
+    31  |  entries" after each power-up.
+
+    32     The  initialization  process  the  server  performs  consists  of
+    33     setting  the  "write  history entries" into the deallocated state
+    34     and deleting all entries in the Allocation Failure Table.  "Write
+    35     history  entries"  are  allocated only via a History Log modified
+    36     write type  command.   A  "write  history  entry"  is  considered
+    37     deallocated  exactly  when it is not accessible to a host via the
+    38     WRITE HISTORY MANAGEMENT command or via a History Log  and  Reuse
+    39     Entry  modified  write  type  command.   The  exact mechanism for
+    40     tracking  the  status  of  a  "write  history  entry"  is  server
+    41     implementation  dependent.  However, in order to provide one such
+    42     mechanism, the host shall never supply a value of FFFF  (hex)  in
+    43     the  "entry  id"  field.  The server may, at its option, reject a
+    44     command with an "entry id" field of FFFF  (hex)  as  an  "Invalid
+    45     Command" ("invalid entry id" parameter error).
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-134
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1     For the  purposes  of  retrieving  and  deallocating  the  "write
+     2     history  entries,"  when an entry is associated with a write type
+     3     command it is also associated with the unit to which the  command
+     4     was addressed.  (The "unit number" is stored within the entry.)
+
+     5     The loss of the "Controller-Online" connection between the server
+     6     and  a  host  class  driver shall have no effect on any allocated
+     7     "write history entry" or Allocation Failure  Table  entry.   Such
+     8     entries   will  remain  allocated  until  they  are  specifically
+     9     deallocated
+
+    10           o  by a host  class  driver  request  (i.e.,  via  a  WRITE
+    11              HISTORY MANAGEMENT command that specifies an appropriate
+    12              operation)
+
+    13           o  by the occurrence  of  a  condition  that  requires  the
+    14              server  to initialize its set of "write history entries"
+    15              as described earlier in this section
+
+
+    16     Host class drivers associate a write type command with  a  "write
+    17     history  entry"  by  setting  the  History  Log  modifier  in the
+    18     command's message.  The class driver also supplies information in
+    19     the command message which allows the host to:
+
+    20         1.  Associate an entry with a  group  of  entries  --  "hrn
+    21             (host reference number)" field.
+
+    22         2.  Reuse an entry it  has  already  allocated  --  "entloc
+    23             (entry locator)" field.
+
+    24         3.  Uniquely identify an entry among all of the entries  in
+    25             a  group  that  are available for its use -- "entry id"
+    26             field.
+
+    27     See Sections 6.7, "DISK COPY DATA Command," 6.9, "ERASE Command,"
+    28     and  6.18,  "WRITE Command" for descriptions of that modifier and
+    29     those fields.  The actions a server  takes  when  a  History  Log
+    30     modified  write  type  command  is received are described in more
+    31     detail later in this section.
+
+    32     The WRITE HISTORY MANAGEMENT command allows host class drivers to
+    33     manage the "write history entries" associated with a unit via the
+    34     following operations:
+
+    35           o  Deallocate all entries.
+
+    36           o  Deallocate  the  group  of  entries  associated  with  a
+    37              particular "host reference number." 
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-135
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1           o  Deallocate a specific entry.
+
+     2           o  Read all entries.
+
+     3           o  Read the group of entries associated with  a  particular
+     4              "host reference number." 
+
+     5           o  Decrement the  count  of  an  Allocation  Failure  Table
+     6              entry.
+
+     7     See the description of the "operation" field in  Section  6.19.1,
+     8     "WRITE  HISTORY  MANAGEMENT  Command, Command Message Format" for
+     9     complete descriptions of those operations.
+
+    10     A server may use any appropriate internal  format  for  a  "write
+    11     history  entry."  A  "write history entry" presented to a host in
+    12     response to a WRITE HISTORY MANAGEMENT READ ALL or  READ  BY  HRN
+    13     operation shall have the following format:
+
+    14                     31                             0 
+    15                     +---------------+---------------+
+    16                     |  unit number  | entry locator |
+    17                     +---------------+---------------+
+    18                     |        transfer length        |
+    19                     +-------------------------------+
+    20                     | starting logical block number |
+    21                     +---------------+---------------+
+    22                     |  entry flags  |      hrn      |
+    23                     +---------------+---------------+
+
+
+    24         entry locator
+
+    25             The value assigned by the server that uniquely identifies
+    26             the internal location of the "write history entry." 
+
+    27         unit number
+
+    28             Identifies the unit to which the command associated  with
+    29             the "write history entry" was addressed.
+
+    30         transfer length
+
+    31             The format of this field varies depending on the  setting
+    32             of  the  "Transfer  Length  in  Blocks" entry flag in the
+    33             "write history entry." The server  may,  at  its  option,
+    34             store  the  transfer  length of the associated command in
+    35             bytes or blocks.  If the byte  count  in  the  associated
+    36             command  was not a multiple of the unit's block size, the
+    37             server may store the exact byte  count,  the  byte  count
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-136
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1             rounded up to the next multiple of the unit's block size,
+     2             or the block count computed by rounding the byte count up
+     3             to a multiple of the unit's block size.
+
+     4         starting logical block number
+
+     5             The logical block number (position) on the disk volume at
+     6             which the write operation is to start as specified in the
+     7             associated command.
+
+     8         hrn (host reference number)
+
+     9             The value assigned by a host class driver for referencing
+    10             a group of "write history entries." 
+
+    11         entry flags
+
+    12             Bit flags, collectively known as  entry  flags,  used  to
+    13             indicate  the  state  of  a  "write  history entry."  The
+    14             following flags are defined:
+
+    15                 Error
+
+    16                     Set if and only if the  write  type  command  for
+    17                     which  the  "write  history entry" was created or
+    18                     updated  did  not  complete  with  a  status   of
+    19                     Success.
+
+    20                 Transfer Length in Blocks
+
+    21                     Set if and only if the "transfer length" field of
+    22                     the  "write history entry" is specified in blocks
+    23                     rather than in bytes.  Note  that  a  server  may
+    24                     convert  the  transfer  length  of a command from
+    25                     bytes to blocks or vice versa before  storing  it
+    26                     in  the "transfer length" field.  Such conversion
+    27                     shall not truncate partial blocks.
+
+    28             Refer  to  Appendix  A,   "Opcode,   Flag,   and   Offset
+    29             Definitions," Table A-15, "Write History Entry Flags" for
+    30             the values assigned to  the  defined  flags.   All  other
+    31             entry  flag  bits  are  reserved, and shall be treated in
+    32             accordance with  the  requirements  for  reserved  fields
+    33             described   in   Section  5.2,  "Reserved  and  Undefined
+    34             Fields."
+
+    35         entry id
+
+    36             The value assigned by a host  class  driver  to  uniquely
+    37             identify  a "write history entry" among all of the "write
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-137
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1             history entries" in a group that are  available  for  its
+     2             use.   This  field  is  NOT  present  in the host-visible
+     3             "write  history  entry"  pictured  above.   However,  the
+     4             server  shall internally maintain the association between
+     5             the "entry id" specified  in  the  History  Log  modified
+     6             command and its corresponding "write history entry."
+
+     7             Note that the host may not supply a value of  FFFF  (hex)
+     8             for  this  field.   That  value  is  reserved  for use by
+     9             servers to mark a "write history entry" as unallocated in
+    10             its  internal  format.   Supplying an "entry id" value of
+    11             FFFF  (hex)  will  result  in   unpredictable   behavior,
+    12             possibly leading to user data corruption.
+
+    13     Upon receipt of a host class driver issued History  Log  modified
+    14     write type command the server performs the following operations:
+
+    15         1.  Validates the command message  fields  and  checks  the
+    16             unit  state  in  the  normal  manner.   Note  that this
+    17             implies having an established  communications  path  in
+    18             the case of History Log modified DISK COPY DATA command
+    19             (see Section 6.7, "DISK COPY DATA Command").
+
+    20             If the field validation or unit state checks fail,  the
+    21             server rejects the command with the appropriate status.
+    22             Otherwise, the server proceeds to Step 2.
+
+    23         2.  Checks the setting of the Reuse Entry modifier  to  see
+    24             whether  the  command  is to be associated with a newly
+    25             allocated "write history entry" or  with  a  previously
+    26             allocated "write history entry." 
+
+    27             If the the Reuse Entry modifier is  clear,  the  server
+    28             proceeds  to  Step  3  (allocate  new).  Otherwise, the
+    29             server proceeds to Step 4 (reuse previously allocated).
+
+    30         3.  Searches the set of  "write  history  entries"  for  an
+    31             entry that is not currently allocated.
+
+    32             If an unallocated "write history entry" is  not  found,
+    33             the  server  shall  record  this fact in the Allocation
+    34             Failure Table as described above  and  continue  normal
+    35             processing  of  the  command.   The server shall return
+    36             FFFF(hex) in the  "entry  locator"  field  of  the  end
+    37             message  and set the Allocation Failure end flag in the
+    38             end message.
+
+    39             If  the  server  cannot  record  the  failure  in   the
+    40             Allocation  Failure  Table, it shall reject the command
+    41             with a status  of  Write  History  Entry  Access  Error
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-138
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1             (subcode  "Allocation Failure Table Full").  Due to the
+     2             transitory nature of this condition,  the  host  should
+     3             reissue the failed write type command as it will likely
+     4             succeed on retry.
+
+     5             If an unallocated "write history  entry"  is  found,  the
+     6             server performs the following operations:
+
+     7                 a.  Copies the contents of the associated command's
+     8                     command  message  "unit  number"  field  to the
+     9                     "unit number" field in that entry.
+
+    10                 b.  Preserves  the  associated  command's  transfer
+    11                     length  ("byte count" or "logical block count")
+    12                     in that entry as detailed in the description of
+    13                     "transfer length" above.
+
+    14                 c.  Copies the contents of the associated command's
+    15                     command  message  "logical  block  number"  (or
+    16                     "destination  lbn")  field  to  the   "starting
+    17                     logical block number" field within that entry.
+
+    18                 d.  Copies the contents of the associated command's
+    19                     command  message  "hrn (host reference number)"
+    20                     field to  the  "hrn  (host  reference  number)"
+    21                     field within that entry.
+
+    22                 e.  Associates  the  associated  command's  command
+    23                     message "entry id" field with that entry.
+
+    24                 f.  Continues normal processing of the command.
+
+
+    25         4.  Checks the value contained in the "entry locator" command
+    26             message  field  to ensure that it is within the limits of
+    27             the set of "write history entries." 
+
+    28             If the "entry locator" is not within the  limits  of  the
+    29             set,  the  server  rejects  the  command  as  an "Invalid
+    30             Command" ("invalid entry locator" parameter  error.   See
+    31             "Invalid Command" status in Section 5.5, "Status Codes").
+
+    32             If the "entry locator" is within the limits of  the  set,
+    33             the  server  uses  the value of the "entry locator" as an
+    34             index into the set of "write history  entries"  and  then
+    35             proceeds to Step 5.
+
+    36         5.  Compares the "entry id" value associated with the located
+    37             "write  history  entry"  with  the value contained in the
+    38             "entry id" command message field.  Note  that  the  value
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MINIMAL MSCP COMMAND SET                               Page 6-139
+           WRITE HISTORY MANAGEMENT Command                     11 June 1992
+
+
+     1             FFFF (hex) is reserved for servers to mark "write history
+     2             entries" as unallocated.
+
+     3             If those values do not  match,  the  server  rejects  the
+     4             command  as  an  "Invalid  Command"  ("invalid  entry id"
+     5             parameter error.  See "Invalid Command" status in Section
+     6             5.5,  "Status  Codes").   The server may optionally check
+     7             that the value supplied in the "entry id" command message
+     8             field is not FFFF (hex) and, if it is, reject the command
+     9             with this same status.  If the command  is  not  rejected
+    10             and the values do match, the server proceeds to Step 6.
+
+    11         6.  The server performs the following operations:
+
+    12                 a.  Copies the contents of the associated command's
+    13                     command  message  "unit  number"  field  to the
+    14                     "unit number" field in the located entry.
+
+    15                 b.  Preserves  the  associated  command's  transfer
+    16                     length  ("byte count" or "logical block count")
+    17                     in that entry as detailed in the description of
+    18                     "transfer length" above.
+
+    19                 c.  Copies the contents of the associated command's
+    20                     command  message  "logical  block  number"  (or
+    21                     "destination  lbn")  field  to  the   "starting
+    22                     logical  block  number"  field  in  the located
+    23                     entry.
+
+    24                 d.  Continues normal processing of the command.
+
+
+    25     After a command associated  with  a  "write  history  entry"  has
+    26     aborted,  terminated, or completed, the server shall set or clear
+    27     the "Error" flag in the "entry flags" field of the "write history
+    28     entry"  and  continue  processing.  The server clears the "Error"
+    29     flag if and only if  the  associated  command  completed  with  a
+    30     status of "Success" and sets the "Error" flag in all other cases.
+
+    31     In addition, prior to returning the  end  message  of  a  command
+    32     associated with a "write history entry" the server shall copy the
+    33     "entry id" field of the command message to the "entry  id"  field
+    34     of  the  end  message  and  copy the "entry locator" field of the
+    35     "write history entry" to the "entry locator"  field  of  the  end
+    36     message.
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 7
+
+     2                             MULTIHOST SUPPORT
+
+
+     3     7.1  Overview
+
+     4     Multihost Support is an  MSCP  option  that  allows  simultaneous
+     5     communication  between  multiple  host  class drivers and an MSCP
+     6     server across separate logical connections.
+
+     7     Various aspects of Multihost Support have already been  discussed
+     8     in  previous  chapters.   A  brief  review  of the more important
+     9     aspects, along with specific section references, follows:
+
+    10        o  Controllers shall maintain each  host  class  driver/MSCP
+    11           server  connection  as  a separate communications entity.
+    12           See Section 3.3, "Multihost  Communication."   Controller
+    13           state   actually   reflects   the  state  of  each  class
+    14           driver/MSCP  server   connection.    See   Section   4.1,
+    15           "Controller States." 
+
+    16        o  Controllers  shall  maintain  the   state   of   a   unit
+    17           ("Unit-Offline,"   "Unit-Available,"  and  "Unit-Online")
+    18           separately for each class driver/MSCP server  connection.
+    19           See  Section  4.3,  "Unit  States."   That  section  also
+    20           contains important details regarding  "Exclusive  Access"
+    21           of a unit (a mandatory feature of Multihost Support).
+
+    22        o  Command execution ordering is performed on a  unit  basis
+    23           rather than a single connection basis.  Controllers shall
+    24           therefore  enforce  the  Sequential   command   execution
+    25           guarantees  across  all  "Controller-Online" connections.
+    26           Controllers  are  expressly  permitted  to  include   all
+    27           Non-Sequential     commands     received     from     all
+    28           "Controller-Online" class drivers, directed to  the  same
+    29           unit,  in  any  optimization algorithm being performed on
+    30           the unit.  See 4.5,  "Command  Categories  and  Execution
+    31           Order." 
+
+    32        o  Controllers shall maintain and operate according  to  the
+    33           setting  of  a  separate  set of host-settable controller
+    34           flags for each host class driver/MSCP server  connection.
+    35           Each  class  driver  is  expressly  permitted  to specify
+    36           different settings for the host-settable controller flags
+    37           and  as  a result realize different controller operation.
+    38           See Section 5.6, "Controller Flags." 
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-2
+           Overview                                             11 June 1992
+
+
+     1        o  Controllers shall maintain only one set of  host-settable
+     2           unit flags for a particular unit regardless of the number
+     3           of host class drivers that have brought the unit into the
+     4           "Unit-Online"   state.   The  unit  will  operate  in  an
+     5           identical  manner  (according  to  the  setting  of   the
+     6           host-settable  unit flags) for each host that has brought
+     7           the unit "Unit-Online."  Any host may alter  the  setting
+     8           of  the  host-settable  unit  flags  (for  itself and all
+     9           others) only via the SET UNIT CHARACTERISTICS command and
+    10           only  after bringing the unit "Unit-Online."  See Section
+    11           5.7, "Unit Flags." 
+
+
+    12     Controllers that provide Multihost Support  shall  implement  all
+    13     the commands described in Chapter 6, "Minimal MSCP Command Set." 
+    14     Command operation is identical to what is described  there,  with
+    15     the  exception  of  the the ABORT, AVAILABLE, GET COMMAND STATUS,
+    16     GET UNIT STATUS, ONLINE, SET CONTROLLER CHARACTERISTICS, and  SET
+    17     UNIT  CHARACTERISTICS  commands.   The operation of each of those
+    18     commands within a Multihost environment  is  described  later  in
+    19     this chapter.  The same major section/subsection format described
+    20     in Section 6.1, "[Minimal MSCP Command Set] Overview" is used for
+    21     the   command  descriptions  in  this  chapter.   The  references
+    22     contained in Section  6.1  for  message  offset  definitions  and
+    23     treatment  of  invalid  and  no-operation  commands  apply to the
+    24     Multihost  environment  as  well.   In  cases  where  information
+    25     contained  in the Chapter 6 command descriptions is identical for
+    26     both the single host  and  multihost  operation,  the  Chapter  6
+    27     information  is  referenced  rather  than  repeating  it  in this
+    28     chapter.  In addition, the  ACCESS  NONVOLATILE  MEMORY  command,
+    29     which   shall  be  supported  by  all  controllers  that  provide
+    30     Multihost Support, is also described.  Note that controllers that
+    31     do  not  provide  Multihost  Support may optionally implement the
+    32     ACCESS NONVOLATILE MEMORY command.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-3
+           Multihost ABORT Command                              11 June 1992
+
+
+     1     7.2  Multihost ABORT Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     7.2.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     | outstanding reference number  |
+    14                     +-------------------------------+
+
+    15         unit number
+
+    16             See  Section  6.2.1,  "ABORT  Command,  Command   Message
+    17             Format." 
+
+    18         outstanding reference number
+
+    19             The "command reference number" of the command that is  to
+    20             be aborted.
+
+    21             As described in Section 5.3, "Command Messages," "command
+    22             reference numbers" need not be unique for commands issued
+    23             by different class drivers -- i.e.,  commands  issued  by
+    24             different  hosts  or  commands for different MSCP servers
+    25             from the same host.  Controllers shall therefore use  the
+    26             combination of the "outstanding reference number" and the
+    27             connection on which the ABORT  command  was  received  in
+    28             order  to  uniquely identify commands issued by different
+    29             hosts.
+
+
+    30     Allowable modifiers:
+
+    31         none
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-4
+           Multihost ABORT Command                              11 June 1992
+
+
+     1     7.2.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     | outstanding reference number  |
+    11                     +-------------------------------+
+
+    12         outstanding reference number
+
+    13             See Section 6.2.2, "ABORT Command, End Message Format." 
+
+
+    14     Status Codes:
+
+    15         Success (subcode "Normal")
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-5
+           Multihost ABORT Command                              11 June 1992
+
+
+     1     7.2.3  Description
+
+     2     See Section 6.2.3, "ABORT Command, Description." 
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-6
+           Multihost ACCESS NONVOLATILE MEMORY Command          11 June 1992
+
+
+     1     7.3  Multihost ACCESS NONVOLATILE MEMORY Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     7.3.1  Command Message Format
+
+     5                     31                             0
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |    reserved   |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |           reserved            |
+    14                     +-------------------------------+
+    15                     |            offset             |
+    16                     +---------------+---------------+
+    17                     |    length     |   operation   |
+    18                     +---------------+---------------+
+    19                     |                               |
+    20                     /         nonvolatile           /
+    21                     /         memory data           /
+    22                     |                               |
+    23                     +-------------------------------+
+
+    24         offset
+
+    25             The byte offset within the server's nonvolatile memory at
+    26             which to perform the operation.
+
+    27         operation
+
+    28             The operation to be performed on the server's nonvolatile
+    29             memory:
+
+    30                   Read
+
+    31                   Exchange
+
+    32                   Test and Set
+
+
+    33             See Appendix A, "Opcode, Flag, and  Offset  Definitions,"
+    34             Table  A-12, "Access Nonvolatile Memory Command Operation
+    35             Codes" for the values and  mnemonics  assigned  to  those
+    36             operations.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-7
+           Multihost ACCESS NONVOLATILE MEMORY Command          11 June 1992
+
+
+     1             If any other value is specified the  server  rejects  the
+     2             command  as  an  "Invalid  Command"  ("invalid operation"
+     3             protocol error.  See "Invalid Command" status in  Section
+     4             5.5, "Status Codes").
+
+     5         length
+
+     6             The number of bytes of nonvolatile memory  data  included
+     7             in  this  command  message  or  to  be  returned  in this
+     8             command's end message.  This should be an even  value  if
+     9             the  operation  is "Test and Set."  If an odd "length" is
+    10             specified with the "Test and Set"  operation  the  server
+    11             rejects  the  command  as  an "Invalid Command" ("invalid
+    12             length" parameter error.  See "Invalid Command" status in
+    13             Section 5.5, "Status Codes").
+
+    14             The maximum valid "length" is server dependent.  However,
+    15             all  servers  that  implement  this command shall support
+    16             values of "length" in the  range  0  through  32.   If  a
+    17             "length"  larger  than  the server's maximum is specified
+    18             the server rejects the command as  an  "Invalid  Command"
+    19             ("invalid length" parameter error.  See "Invalid Command"
+    20             status in Section 5.5, "Status Codes").
+
+    21         nonvolatile memory data
+
+    22             "Length" bytes of data to be used in the operation.
+
+
+    23     Allowable modifiers:
+
+    24         none
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-8
+           Multihost ACCESS NONVOLATILE MEMORY Command          11 June 1992
+
+
+     1     7.3.2  End Message Format
+
+     2                     31                             0
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|    reserved   |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |    nonvolatile memory size    |
+    11                     +-------------------------------+
+    12                     |            offset             |
+    13                     +---------------+---------------+
+    14                     |     length    |   operation   |
+    15                     +---------------+---------------+
+    16                     |                               |
+    17                     /         nonvolatile           /
+    18                     /         memory data           /
+    19                     |                               |
+    20                     +-------------------------------+
+
+    21         nonvolatile memory size
+
+    22             The number of bytes of nonvolatile memory implemented  by
+    23             the server.
+
+    24         offset
+    25         operation
+    26         length
+
+    27             Copied without alteration from the command message.
+
+    28         nonvolatile memory data
+
+    29             "Length" bytes of data returned by the operation.
+
+    30     Status Codes:
+
+    31         Success (subcode "Normal")
+    32         Invalid Command (subcode "Invalid Length")
+    33         Invalid Command (subcode "Invalid Operation")
+    34         Compare Error
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                        Page 7-9
+           Multihost ACCESS NONVOLATILE MEMORY Command          11 June 1992
+
+
+     1     7.3.3  Description
+
+     2     The ACCESS  NONVOLATILE  MEMORY  command  provides  class  driver
+     3     access  to  a  nonvolatile memory region within the controller or
+     4     server.  Hosts may use this memory to store arbitrary  parameters
+     5     or  information.  MSCP servers merely store the data; they do not
+     6     use it in any way.
+
+     7     The  presence  and  size  of  a  nonvolatile  memory  is   server
+     8     dependent.   However,  whenever  a  server provides a nonvolatile
+     9     memory, the memory must be at least 16 bytes long.  Servers  that
+    10     provide  Multihost  Support  shall  always implement a 16 byte or
+    11     larger nonvolatile memory.  Single host  servers  may  optionally
+    12     implement a nonvolatile memory.  If a single host server does not
+    13     implement a nonvolatile memory, then it need not  implement  this
+    14     command.  In that case the controller shall reject this command's
+    15     opcode as an "Invalid Command" ("invalid opcode" protocol  error.
+    16     See "Invalid Command" status in Section 5.5, "Status Codes").
+
+    17     The nonvolatile memory is addressed at  offsets  0  through  N-1,
+    18     where  N is the size of the server's nonvolatile memory in bytes.
+    19     Class drivers may access offsets N and higher, even  though  they
+    20     are  not  included  in the implemented memory.  Attempts to write
+    21     offsets N and higher are ignored and always succeed.  Attempts to
+    22     read offsets N and higher return zero.
+
+    23     Three operations  are  provided  for  accessing  the  nonvolatile
+    24     memory.   The  detailed  semantics  of  these  operations  are as
+    25     follows:
+
+    26         Read
+
+    27             The server reads "length" bytes beginning  at  offset
+    28             "offset"  and returns them as the "nonvolatile memory
+    29             data" in the end message.   The  "nonvolatile  memory
+    30             data" in the command message is ignored.
+
+    31         Exchange
+
+    32             The server reads "length" bytes beginning  at  offset
+    33             "offset"  and returns them as the "nonvolatile memory
+    34             data" in the end message.  Next,  the  server  writes
+    35             "length"  bytes beginning at offset "offset" with the
+    36             "nonvolatile memory data" from the command message.
+
+    37             This operation exchanges the nonvolatile memory  data
+    38             with  the  memory contents stored in the server.  The
+    39             command message provides  the  new  memory  contents,
+    40             while the old memory contents are returned in the end
+    41             message.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-10
+           Multihost ACCESS NONVOLATILE MEMORY Command          11 June 1992
+
+
+     1         Test and Set
+
+     2             The server reads "length"/2 bytes beginning at offset
+     3             "offset"  and  returns  them as the first half of the
+     4             "nonvolatile memory data" in the end message.   Next,
+     5             the  server  compares  these "length"/2 bytes against
+     6             the first half of the "nonvolatile memory data"  from
+     7             the  command  message.   If  they  are different, the
+     8             server returns a "Compare  Error"  status  code.   If
+     9             they are the same, the server writes "length"/2 bytes
+    10             beginning at offset "offset" with the second half  of
+    11             the   "nonvolatile  memory  data"  from  the  command
+    12             message and returns a "Success" status code.  In both
+    13             cases,  the  second  half  of the "nonvolatile memory
+    14             data" in the end message is copied without alteration
+    15             from the command message.
+
+    16             This  operation  provides  an   atomic   test-and-set
+    17             operation.   The "nonvolatile memory data" is divided
+    18             into equal length halves.  The first half is compared
+    19             against  the values stored in the server to determine
+    20             the  "Success"  or  "Compare  Error"  status  of  the
+    21             command.   If comparison shows both values are equal,
+    22             then the second half of "nonvolatile memory data"  is
+    23             stored  into the nonvolatile memory.  In either case,
+    24             the values that were stored in the  server  prior  to
+    25             this  operation are returned in the first half of the
+    26             end message's "nonvolatile memory data." 
+
+    27     For all three operations, a "length" of zero  is  legal.   It  is
+    28     treated as a no-operation and always succeeds.  This is primarily
+    29     useful as a means for class drivers to determine the size of  the
+    30     server's nonvolatile memory.
+
+    31     The server shall  implement  all  operations  requested  by  this
+    32     command  as  atomic  operations.   If  several ACCESS NONVOLATILE
+    33     MEMORY commands are outstanding simultaneously,  the  server  may
+    34     perform  them in any sequence, but shall process only one of them
+    35     at a time.
+
+    36     The server shall remember the contents of its nonvolatile  memory
+    37     across  power failures, reinitializations, losses of context, and
+    38     other normal failures.  However, the  following  events  are  not
+    39     normal and may result in loss of the nonvolatile memory contents:
+
+    40       1.  Field Service attention or other maintenance actions.  If
+    41           the   component  containing  the  nonvolatile  memory  is
+    42           replaced, its contents may become undefined.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-11
+           Multihost ACCESS NONVOLATILE MEMORY Command          11 June 1992
+
+
+     1       2.  A power  failure,  reinitialization,  or  other  loss  of
+     2           context  while  the nonvolatile memory is being modified.
+     3           The offset being modified, plus some controller dependent
+     4           number of adjacent locations, may become undefined.
+
+
+     5     The nonvolatile memory should contain all zeros in newly  shipped
+     6     servers.
+
+     7     One characteristic of commonly available nonvolatile memories  is
+     8     that they can only be reliably written a limited number of times.
+     9     Therefore class drivers may not treat the nonvolatile  memory  as
+    10     they  would  standard  RAM  or disk memory.  Class drivers should
+    11     design their algorithms  for  using  the  nonvolatile  memory  to
+    12     ensure  that  they  do not alter it more often than an average of
+    13     once per day.  Short periods of frequent updates are  acceptable,
+    14     provided they are balanced by long periods of few updates.
+
+
+
+
+
+
+
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+
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+
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+
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+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-12
+           Multihost AVAILABLE Command                          11 June 1992
+
+
+     1     7.4  Multihost AVAILABLE Command
+
+     2     Command Category:
+
+     3         Sequential
+
+     4     7.4.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable modifiers:
+
+    14         All Class Drivers
+
+    15             If the "All Class Drivers" modifier is set and  the  unit
+    16             is  "Unit-Online"  to  the issuing host as well as one or
+    17             more other hosts, the unit  becomes  "Unit-Available"  to
+    18             all hosts.
+
+    19             If the "All Class Drivers" modifier is set and  the  unit
+    20             is  "Unit-Online"  only  to  the  issuing  host, the unit
+    21             becomes "Unit-Available" to the issuing host.
+
+    22             If the "All Class Drivers" modifier is clear and the unit
+    23             is  "Unit-Online"  to  the issuing host as well as one or
+    24             more other hosts, the unit  becomes  "Unit-Available"  to
+    25             the issuing host only.  The state of the unit relative to
+    26             all other hosts is unaffected.
+
+    27             If the unit is "Unit-Offline" or already "Unit-Available"
+    28             to  the  issuing  host,  the  setting  of  the "All Class
+    29             Drivers" modifier is ignored.
+
+    30         Clear Serious Exception (ignored for disk class devices)
+
+    31         Exclusive Access
+
+    32             If the "Exclusive Access" modifier is set and the unit is
+    33             "Unit-Online"   to   the   issuing   host   and   is  not
+    34             "Unit-Online"  to  any  other  host,  the  unit   becomes
+    35             "Unit-Available"  to  and under the "Exclusive Access" of
+    36             the  issuing  host.   This  specifically   includes   the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-13
+           Multihost AVAILABLE Command                          11 June 1992
+
+
+     1             possibility  that the unit became "Unit-Available" to the
+     2             issuing host as well as to other hosts as a result of the
+     3             "All  Class  Drivers"  modifier  also  being  set  in the
+     4             command.  Note that controllers shall  service  the  "All
+     5             Class  Drivers"  modifier before servicing the "Exclusive
+     6             Access" modifier.
+
+     7             If the "Exclusive Access" modifier is set and the unit is
+     8             "Unit-Available"   to   the   issuing  host  and  is  not
+     9             "Unit-Online" to any other host  or  the  unit  is  "Unit
+    10             Offline  (substate  'No Media Loaded')", the issuing host
+    11             is granted "Exclusive Access" of the  unit.   The  unit's
+    12             state is otherwise unchanged.
+
+    13             If the "Exclusive Access" modifier is set and the unit is
+    14             "Unit-Online"  to and under the "Exclusive Access" of the
+    15             issuing host, the unit becomes  "Unit-Available"  to  the
+    16             issuing   host.   "Exclusive  Access"  of  the  unit  is,
+    17             however, retained.
+
+    18             If the "Exclusive Access" modifier is clear and the  unit
+    19             is  "Unit-Online"  to and under the "Exclusive Access" of
+    20             the issuing host, the unit  becomes  "Unit-Available"  to
+    21             the  issuing  host  and "Exclusive Access" of the unit is
+    22             released.
+
+    23             If the "Exclusive Access" modifier is clear and the  unit
+    24             is  "Unit-Available" or "Unit Offline (substate 'No Media
+    25             Loaded')" to and also under the "Exclusive Access" of the
+    26             issuing host, "Exclusive Access" to the unit is released.
+    27             The unit's state is otherwise unchanged.
+
+    28             If the unit is "Unit-Offline" to the issuing host for any
+    29             reason  other  than  "No  Media  Loaded,"  the "Exclusive
+    30             Access" modifier is ignored.
+
+    31             Note that granting "Exclusive Access" of a unit to a host
+    32             implicitly   causes  the  unit  to  become  "Unit-Offline
+    33             (substate 'Exclusive Use')" to all other hosts  connected
+    34             via  this  controller and "Unit-Offline (substate 'Online
+    35             To Another Controller')" to hosts which  may  potentially
+    36             have   access   to   the  unit  via  another  controller.
+    37             Conversely, the release of a  unit  from  the  "Exclusive
+    38             Access"  of  a  host implicitly causes the unit to become
+    39             "Unit-Available" to all other hosts.
+
+    40             Note also that controllers ignore duplicate  unit  number
+    41             conditions  when  a  host retains "Exclusive Access" of a
+    42             unit.  That is, if the unit is "Unit-Online" to and under
+    43             the  "Exclusive  Access"  of  the  issuing  host  and  in
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-14
+           Multihost AVAILABLE Command                          11 June 1992
+
+
+     1             addition had a duplicate unit number prior to the receipt
+     2             of  an  AVAILABLE  command  with  the  "Exclusive Access"
+     3             modifier set, the unit will not be become  "Unit-Offline"
+     4             nor  will  it  be  spun-down.   Instead  the unit becomes
+     5             "Unit-Available" to and "Exclusive Access" is retained by
+     6             the issuing host.
+
+     7  |          Note also that "Exclusive Access" is released if  any  of
+     8  |          the following events occur:
+
+     9  |            1.  The controller loses or breaks the connection  to
+    10  |                the  host  that  has  "Exclusive  Access"  of the
+    11  |                drive.
+
+    12  |            2.  The  drive  becomes  unknown  to  the  controller
+    13  |                (e.g., port button disabled.) 
+
+    14  |            3.  The  drive  becomes  inoperative,  "Unit-Offline,
+    15  |                Inoperative." 
+
+    16  |            4.  The controller is 'rebooted', by  itself,  by  an
+    17  |                operator, or by any host.
+
+
+    18         Spin-down
+
+    19             Requests that the disk stop spinning and that  the  heads
+    20             be  unloaded.  Note that the command completes as soon as
+    21             the spin-down has been initiated, rather than waiting for
+    22             the  disk  to  stop  spinning.  The spin-down will not be
+    23             initiated if either or both of the  following  conditions
+    24             exist:
+
+    25               1.  The unit belongs to a multiunit drive and  shares
+    26                   a   spindle   with   some   other  unit  that  is
+    27                   "Unit-Online"  (See  Section  4.16.2,  "Multiunit
+    28                   Drives and Formatters").
+
+    29               2.  The unit is still "Unit-Online" to  one  or  more
+    30                   other  hosts and the "All Class Drivers" modifier
+    31                   is clear.
+
+    32             Note that a spin-down request is  honored  and  performed
+    33             (unless  prohibited  as  just described) when the unit is
+    34             "Unit-Available" to the issuing host as well as  when  it
+    35             is "Unit-Online" to the issuing host.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-15
+           Multihost AVAILABLE Command                          11 June 1992
+
+
+     1     7.4.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status Codes:
+
+    11         Success (subcode "Normal")
+
+    12             This status code (i.e., subcode zero) is  returned  under
+    13             the following conditions:
+
+    14               1.  The   unit   changes   from   "Unit-Online"    to
+    15                   "Unit-Available"  and  the  action or actions (if
+    16                   any) requested by the setting of the  "All  Class
+    17                   Drivers,"  "Exclusive  Access,"  and  "Spin-down"
+    18                   modifiers were successfully completed.
+
+    19               2.  The unit  was  already  "Unit-Available"  to  the
+    20                   issuing  host  and the action or actions (if any)
+    21                   requested  by  the  setting  of  the   "Exclusive
+    22                   Access"    and    "Spin-down"    modifiers   were
+    23                   successfully completed.
+
+
+    24             Separate bits within the "Success" status  subcode  field
+    25             are  used  to  indicate that the actions requested by the
+    26             setting  of  the  "All  Class  Drivers"  and  "Spin-down"
+    27             modifiers  could  not  be  performed  but the command was
+    28             otherwise successful.  See the  "Spin-down  Ignored"  and
+    29             "Still Online" subcode bit flag descriptions below.
+
+    30             Another "Success" status subcode bit is used to  indicate
+    31             that  the  command  was successful and that the unit will
+    32             remain  connected  to  the  controller  (see  the  "Still
+    33             Connected" bit flag description).
+
+    34             Note that if the unit was "Unit-Online"  (and  not  under
+    35             the  "Exclusive  Access"  of  the issuing host) but had a
+    36             duplicate unit number  prior  to  the  AVAILABLE  command
+    37             being  issued,  the  controller  may  optionally return a
+    38             status code of:  "Success (subcode 'Normal')" or "Success
+    39             (subcode   'Duplicate  Unit  Number')"  or  "Unit-Offline
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-16
+           Multihost AVAILABLE Command                          11 June 1992
+
+
+     1             (subcode 'Duplicate Unit Number')." 
+
+     2         Success (subcode "Spin-down Ignored")
+
+     3             The "Spin-down Ignored" subcode bit flag is  set  if  the
+     4             unit  could  not be spun-down due to one of the following
+     5             conditions:
+
+     6               1.  One or more other  units  with  which  this  unit
+     7                   shares  a  spindle  is  still  "Unit-Online" (see
+     8                   Section    4.16.2,    "Multiunit    Drives    and
+     9                   Formatters,"   for   an   explanation  of  shared
+    10                   spindles).
+
+    11               2.  The unit is still "Unit-Online" to  one  or  more
+    12                   other hosts.
+
+
+    13         Success (subcode "Still Connected")
+
+    14             The "Still Connected" subcode bit flag is set if the unit
+    15             may  potentially  be  accessed via another controller and
+    16             one or more other units with which this  unit  shares  an
+    17             access  path is still "Unit-Online," preventing this unit
+    18             from being accessed via that other controller  (if  any).
+    19             See   Section   4.16.1,   "Multiaccess   Drives,"  for  a
+    20             discussion of access paths.
+
+    21             The "Still Connected" subcode bit flag will always be set
+    22             if the "Spin-down Ignored" subcode bit flag is set.
+
+    23         Success (subcode "Still Online")
+
+    24             The "Still Online" subcode bit flag is set if the unit is
+    25             still  "Unit-Online"  to one or more other hosts via this
+    26             controller.  If the "Exclusive Access" modifier  was  set
+    27             this  status  code implies that "Exclusive Access" of the
+    28             unit was denied.
+
+    29             The "Still Online" subcode bit flag will always be set if
+    30             the "Spin-down Ignored" subcode bit flag is set.
+
+    31         Command Aborted
+
+    32             The command has been rejected and the  unit's  state  has
+    33             not  changed.   If  the  unit  was "Unit-Online" prior to
+    34             issuing  this   command,   then   the   unit   is   still
+    35             "Unit-Online" and it is still spinning.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-17
+           Multihost AVAILABLE Command                          11 June 1992
+
+
+     1         Unit-Available (subcode "Already In Use")
+
+     2             The unit was "Unit-Available" to  the  issuing  host  and
+     3             "Exclusive  Access"  of the unit was denied because it is
+     4             currently "Unit-Online" to another host.
+
+     5         Unit-Offline
+
+     6             All subcodes may be returned.   Note  that  "Unit-Offline
+     7             (subcode  'No  Media  Loaded')" implies "Success (subcode
+     8             'Normal')"  when  "Exclusive  Access"  of  the  unit  was
+     9             granted  while  the  unit was "Unit-Offline (substate 'No
+    10             Media Loaded')."  Note also that in cases where the  unit
+    11             was  "Unit-Online"  (and not under the "Exclusive Access"
+    12             of the issuing host) but  had  a  duplicate  unit  number
+    13             prior   to   the  AVAILABLE  command  being  issued,  the
+    14             controller may, at its option, return  the  "Unit-Offline
+    15             (subcode  'Duplicate  Unit  Number')  status  code or two
+    16             forms of the "Success" status code (see Success  (subcode
+    17             "Normal") above).
+
+    18         Unit Offline (subcode "Exclusive Use")
+
+    19             The unit is currently under  the  "Exclusive  Access"  of
+    20             some other host connected via this controller.
+
+    21         Controller Error
+    22         Drive Error
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-18
+           Multihost AVAILABLE Command                          11 June 1992
+
+
+     1     7.4.3  Description
+
+     2     The operation of the AVAILABLE command depends on the settings of
+     3     the  "All  Class  Drivers,"  "Exclusive  Access," and "Spin-down"
+     4     modifiers as well as the state of the specified unit relative  to
+     5     the  issuing  host and all other "Controller-Online" hosts (class
+     6     drivers)  as  described   in   the   preceding   sections.    All
+     7     combinations of those modifiers are permissible.
+
+     8     Note that issuing  an  AVAILABLE  command  with  the  "All  Class
+     9     Drivers,"  "Exclusive Access," and "Spin-down" modifiers clear to
+    10     a unit that is already "Unit-Available"  to  and  not  under  the
+    11     "Exclusive   Access"   of  the  issuing  host  is  essentially  a
+    12     sequential no-operation.
+
+    13     Upon completion of the requested AVAILABLE  command  operation(s)
+    14     an  AVAILABLE  attention  message  is  sent by all controllers to
+    15     which  the  specified  unit  is  connected  under  the  following
+    16     conditions (provided the "Spin-down" modifier was clear):
+
+    17          1.  The  unit's  state   changes   from   "Unit-Online"   to
+    18              "Unit-Available"  relative  to  the  the issuing host as
+    19              well as all  other  "Controller-Online"  hosts  and  the
+    20              issuing  host  was not granted "Exclusive Access" of the
+    21              unit in the process.
+
+    22          2.  The  unit's  state   changes   from   "Unit-Online"   to
+    23              "Unit-Available"  relative  to  the issuing class driver
+    24              and "Exclusive Access" of the unit was also released.
+
+    25          3.  The unit was already  "Unit-Available"  to  the  issuing
+    26              host and "Exclusive Access" of the unit was released.
+
+    27     In all the cases just  described  the  controller  to  which  the
+    28     AVAILABLE  command  was sent need not send an AVAILABLE attention
+    29     message.
+
+    30     If the "Spin-down" modifier was set, AVAILABLE attention messages
+    31     will   be  suppressed  for  the  specified  unit  both  for  this
+    32     controller and for any other controllers to which the unit may be
+    33     connected,  regardless  of  whether  the  spin-down  was actually
+    34     initiated.  See Section 4.3, "Unit States" for  a  discussion  of
+    35     suppressing AVAILABLE attention messages.
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-19
+           Multihost GET COMMAND STATUS Command                 11 June 1992
+
+
+     1     7.5  Multihost GET COMMAND STATUS Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     7.5.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     | outstanding reference number  |
+    14                     +-------------------------------+
+
+    15         unit number
+
+    16             See Section 6.11.1, "GET COMMAND STATUS Command,  Command
+    17             Message Format." 
+
+    18         outstanding reference number
+
+    19             The "command  reference  number"  of  the  command  whose
+    20             status is to be obtained.
+
+    21             As described in Section 5.3, "Command Messages," "command
+    22             reference numbers" need not be unique for commands issued
+    23             by different class drivers -- i.e.,  commands  issued  by
+    24             different  hosts  or  commands for different MSCP servers
+    25             from the same host.  Controllers shall therefore use  the
+    26             combination of the "outstanding reference number" and the
+    27             connection on which the GET COMMAND  STATUS  command  was
+    28             received in order to uniquely identify commands issued by
+    29             different hosts.
+
+
+    30     Allowable modifiers:
+
+    31         none
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-20
+           Multihost GET COMMAND STATUS Command                 11 June 1992
+
+
+     1     7.5.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     | outstanding reference number  |
+    11                     +---------------+---------------+
+    12                     |        command status         |
+    13                     +---------------+---------------+
+
+    14         outstanding reference number
+
+    15             See Section 6.11.2,  "GET  COMMAND  STATUS  Command,  End
+    16             Message Format." 
+
+    17         command status
+
+    18             The amount of work remaining to be done to  complete  the
+    19             command, expressed as an unsigned integer.  This field is
+    20             zero if the command is not known to the controller,  such
+    21             as when the command has already completed or was aborted.
+    22             The controller may also return zero in this field  if  it
+    23             can  guarantee  that the command will complete within the
+    24             controller timeout interval.  The controller shall  never
+    25             return  a  value  of all ones (2**32-1) in this field, as
+    26             that value is  used  to  initialize  the  host's  command
+    27             timeout algorithm (see Section 4.10, "Command Timeouts").
+
+    28             Each "Controller-Online" class  driver  has  a  different
+    29             concept  of  which  command  is  the  oldest  outstanding
+    30             command.  As mentioned  earlier  the  command  for  which
+    31             status  is  being  obtained  is uniquely qualified by the
+    32             "outstanding reference  number"  and  the  connection  on
+    33             which  the  command  GET  COMMAND  STATUS  was  received.
+    34             Controllers shall therefore return a  status  value  that
+    35             reflects the "doneness" of the specified command relative
+    36             to the issuing host.
+
+    37             An example of  how  a  controller  might  produce  status
+    38             values  that  meet  all  the  requirements defined in the
+    39             preceding paragraphs follows.  Note  that  the  algorithm
+    40             described  in  the example is driven solely by the host's
+    41             issuance of  the  GET  COMMAND  STATUS  command,  not  an
+    42             internal controller timer.  For the algorithm to function
+    43             properly the host must not issue  a  GET  COMMAND  STATUS
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-21
+           Multihost GET COMMAND STATUS Command                 11 June 1992
+
+
+     1             command for a particular command more frequently than the
+     2             value returned  by  the  controller  in  the  "controller
+     3             timeout"  field  of  the  SET  CONTROLLER CHARACTERISTICS
+     4             command's end message.
+
+     5               The controller  maintains  two  distinctly  different
+     6               command  status values in the context of each command
+     7               received by the controller:
+
+     8                  o  Controller-Host Command Status (CHCS)
+
+     9                       A 32-bit unsigned integer value which  is
+    10                       the   actual   "Command   Status"   value
+    11                       returned to  the  host.   The  controller
+    12                       initializes  CHCS to a reasonable maximum
+    13                       value (e.g., 2**32-2).
+
+    14                  o  Command-Specific Command Status (CSCS)
+
+    15                       An unsigned integer value  consisting  of
+    16                       two  subfields:   the Requested Command's
+    17                       Position   (RCP)   and   the    Requested
+    18                       Command's  Status (RCS).  RCP is the high
+    19                       order field.  (The size of  the  CSCS  as
+    20                       well  as  the  RCP  and  RCS subfields is
+    21                       controller   dependent,   as    discussed
+    22                       later.)  The  controller  initializes the
+    23                       CSCS to its maximum value (all bits set).
+
+    24               Upon receipt of a GET COMMAND STATUS  command  for  a
+    25               particular   command   the  controller  constructs  a
+    26               current copy  of  CSCS  according  to  the  requested
+    27               command's current execution state as follows:
+
+    28                  o  Unknown (e.g., no longer outstanding)
+
+    29                       RCP and RCS are both set to zero.
+
+    30                  o  Already In-progress
+
+    31                       RCP is set to  zero,  implying  that  the
+    32                       requested   command   is   at   execution
+    33                       position 0.
+
+    34                       RCS is set to an encoding of  the  amount
+    35                       of   work  (including  possible  retries,
+    36                       etc.)  remaining  before  the   requested
+    37                       command   completes,   or   zero  if  the
+    38                       controller expects the command to  finish
+    39                       within the "controller timeout" interval.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-22
+           Multihost GET COMMAND STATUS Command                 11 June 1992
+
+
+     1                  o  Sequential Command  Waiting  For  In-progress
+     2                     Commands to Complete
+
+     3                       RCP is set  to  one,  implying  that  the
+     4                       requested   command   will  be  the  next
+     5                       command initiated.
+
+     6                       RCS is set to an encoding of  the  amount
+     7                       of  work  remaining  on  all  in-progress
+     8                       commands.
+
+     9                  o  Non-Sequential   Command   Waiting   For   An
+    10                     In-progress Sequential Command To Complete
+
+    11                       RCP  is  set  equal  to  the  number   of
+    12                       commands    which   are   scheduled   for
+    13                       initiation   ahead   of   the   requested
+    14                       command,  plus  one  to  account  for the
+    15                       in-progress Sequential command.
+
+    16                       RCS  is  set  equal  to  the  in-progress
+    17                       Sequential command's RCS value.
+
+    18                  o  Sequential or Non-Sequential Command  Waiting
+    19                     For   Other   Reasons   (e.g.,  a  controller
+    20                     resource to become available)
+
+    21                       RCP  is  set  equal  to  the  number   of
+    22                       commands    that    are   scheduled   for
+    23                       initiation   ahead   of   the   requested
+    24                       command,  plus  one  to  account  for the
+    25                       requested command.
+
+    26                       RCS is set to an encoding of  the  amount
+    27                       of  work  remaining  on  all  in-progress
+    28                       commands.
+
+    29               The  controller  then  tests   the   value   of   the
+    30               constructed  CSCS.   If  the constructed CSCS is zero
+    31               due to the  command  being  unknown,  the  controller
+    32               zeros  the  "command status" field of the GET COMMAND
+    33               STATUS command's end message, and completes  the  GET
+    34               COMMAND  STATUS  command.  If the constructed CSCS is
+    35               zero and the command is known, the command's CHCS  is
+    36               set  equal  to  zero.   If  the  constructed  CSCS is
+    37               nonzero, the controller compares the constructed CSCS
+    38               to  the  command's  CSCS  stored  previously.  If the
+    39               constructed CSCS is less than the command's CSCS, the
+    40               controller  decrements the command's CHCS by one.  If
+    41               the constructed CSCS is greater than or equal to  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-23
+           Multihost GET COMMAND STATUS Command                 11 June 1992
+
+
+     1               command's  CSCS,  the controller leaves the command's
+     2               CHCS at the same value.  Then the  controller  stores
+     3               the  constructed  CSCS  in  the  requested  command's
+     4               context, obtains the current value of the  CHCS  from
+     5               the  requested command's context and stores it in the
+     6               "command status" field  of  the  GET  COMMAND  STATUS
+     7               command's  end message, and completes the GET COMMAND
+     8               STATUS command.
+
+     9               As mentioned above the size  of  CSCS  is  controller
+    10               dependent.  CSCS size actually depends on the maximum
+    11               values of the RCP and RCS  subfields  the  controller
+    12               can  reasonably  expect  to  encounter, determined as
+    13               follows:
+
+    14                  o  RCP
+
+    15                       The maximum number  of  command  messages
+    16                       the  controller  is capable of queuing at
+    17                       any one time.
+
+    18                  o  RCS
+
+    19                       The maximum amount of  time  the  longest
+    20                       command    execution   (including   error
+    21                       recovery retries) can  possibly  consume,
+    22                       expressed as the number of ticks starting
+    23                       from  the  initiation  of   the   longest
+    24                       command execution through its completion.
+    25                       A tick is the amount of time it takes  to
+    26                       complete  the longest single operation of
+    27                       all the individual operations  which  are
+    28                       used   to  execute  commands  within  the
+    29                       controller.   The  "controller   timeout"
+    30                       interval is equal to one tick.
+
+
+
+    31     Status Codes:
+
+    32         Success (subcode "Normal")
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-24
+           Multihost GET COMMAND STATUS Command                 11 June 1992
+
+
+     1     7.5.3  Description
+
+     2     The GET COMMAND STATUS command is used to monitor the progress of
+     3     a  command  towards  completion.  The command status measures the
+     4     "doneness" of the command.  The value  returned  in  the  command
+     5     status   field   is   guaranteed   to  not  increase  over  time.
+     6     Furthermore, the  command  status  of  an  MSCP  server's  oldest
+     7     outstanding command relative to the issuing host is guaranteed to
+     8     decrease within  the  controller  timeout  interval.   This  last
+     9     feature may be used by a host class driver to detect an insane or
+    10     malfunctioning controller.  See Section 4.10, "Command Timeouts,"
+    11     for more details.
+
+    12     The GET COMMAND STATUS command always succeeds.  If  the  command
+    13     referenced  by the "outstanding reference number" is not known to
+    14     the MSCP server or has been aborted, then the MSCP server  should
+    15     return  zero  for  its  command status.  The MSCP server may also
+    16     return zero as the command status of any command that will always
+    17     complete within the controller timeout interval.  The MSCP server
+    18     always returns the "Normal" status code in the GET COMMAND STATUS
+    19     command's end message.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-25
+           Multihost GET UNIT STATUS Command                    11 June 1992
+
+
+     1     7.6  Multihost GET UNIT STATUS Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     7.6.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable modifiers:
+
+    14         Clear Serious Exception (ignored for disk class devices)
+
+    15         Next Unit
+
+    16             See Section 6.12.1, "GET  UNIT  STATUS  Command,  Command
+    17             Message Format." 
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-26
+           Multihost GET UNIT STATUS Command                    11 June 1992
+
+
+     1     7.6.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |  unit flags   |multiunit code |
+    11                     +---------------+---------------+
+    12                     |      reserved         |spndles|
+    13                     +-------------------------------+
+    14                     |                               |
+    15                     +---     unit identifier     ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |     media type identifier     |
+    19                     +-------------------------------+
+    20                     |           reserved            |
+    21                     +-------------------------------+
+    22                     |  group size   |   track size  |
+    23                     +-------+-------+---------------+
+    24                     |uhvrsn | usvrsn| cylinder size |
+    25                     +-------+-------+---------------+
+    26                     |copies | RBNs  |    RCT size   |
+    27                     +-------+-------+---------------+
+
+    28         status
+
+    29             The validity of the unit characteristics varies with  the
+    30             unit's  state implied by the contents of this field.  See
+    31             the Status Codes listed later in this section and Section
+    32             6.12.3,   "GET  UNIT  STATUS  Command,  Description"  for
+    33             complete details.
+
+    34         multiunit code
+    35         unit identifier
+    36         media type identifier
+    37         track size
+    38         group size
+    39         cylinder size
+    40         usvrsn
+    41         uhvrsn
+    42         RCT size
+    43         RBNs
+    44         copies
+
+    45             See Section 6.12.2, "GET UNIT STATUS Command, End Message
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-27
+           Multihost GET UNIT STATUS Command                    11 June 1992
+
+
+     1             Format." 
+
+     2         unit flags
+
+     3             See Section 5.7, "Unit Flags." 
+
+     4             Exclusive Access
+
+     5                 The Exclusive Access unit flag is  non-host-settable;
+     6                 it  is  set  when  the  unit  is under the "Exclusive
+     7                 Access"  of  this  or  some  other  host   via   this
+     8                 controller and clear when it is not.
+     9         spndles
+
+    10             The number minus one of spindles or spindle  "groups"  in
+    11             the  unit.  The value of zero indicates one spindle.  See
+    12             Section 4.12, "Disk Geometry and Format."
+
+
+    13     Status Codes:
+
+    14         Unit-Offline
+
+    15             All subcodes may be returned.   Note  that  "Unit-Offline
+    16             (subcode  'Exclusive  Use')"  indicates  that the unit is
+    17             under the "Exclusive Access" of some other host connected
+    18             via  this controller.  All characteristic fields and unit
+    19             flags are valid when that status is returned.
+
+    20         Success (subcode "Normal")
+    21         Success (subcode "Duplicate Unit Number")
+    22         Unit-Available
+    23         Controller Error
+    24         Drive Error
+
+    25             See Section 6.12.2, "GET UNIT STATUS Command, End Message
+    26             Format." 
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-28
+           Multihost GET UNIT STATUS Command                    11 June 1992
+
+
+     1     7.6.3  Description
+
+     2     See Section 6.12.3, "GET UNIT STATUS Command, Description." 
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-29
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1     7.7  Multihost ONLINE Command
+
+     2     Command categories:
+
+     3         Sequential
+
+     4     7.7.1  Command Message Format
+
+     5              31                             0               
+     6              +-------------------------------+              
+     7              |   command reference number    |              
+     8              +---------------+---------------+              
+     9              |   reserved    |  unit number  |              
+    10              +---------------+-------+-------+              
+    11              |   modifiers   | rsvd  | opcode|              
+    12              +---------------+-------+-------+              
+    13              |  unit flags   |    reserved   |              
+    14              +---------------+---------------+              
+    15              |           reserved            |              
+    16              +-------------------------------+              
+    17              |                               |              
+    18              +---        reserved         ---+              
+    19              |                               |              
+    20              +-------------------------------+              
+    21              |  device dependent parameters  |              
+    22              +-------------------------------+              
+    23              |           reserved            |              
+    24              +-------------------------------+              
+
+    25  |      unit flags
+
+    26             Host-settable  unit  flags.   See  Section   5.7,   "Unit
+    27             Flags." 
+
+    28             If the unit is already "Unit-Online" to any  host  (class
+    29             driver),  then  the  class  driver shall specify the same
+    30             values for controller supported host-settable unit  flags
+    31             as  are  currently in effect on the unit.  The controller
+    32             shall check that the flag values are the same.   If  they
+    33             are  different  then  it  shall  reject the command as an
+    34             "Invalid Command" ("invalid unit flags" parameter  error.
+    35             See  "Invalid  Command"  status  in  Section 5.5, "Status
+    36             Codes").
+
+    37         device dependent parameters
+
+    38             See Section  6.13.1,  "ONLINE  Command,  Command  Message
+    39             Format." 
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-30
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1             Note that controllers may  either  maintain  the  "device
+     2             dependent     parameters"     separately     for     each
+     3             "Controller-Online" class driver or  singularly  for  all
+     4             "Controller-Online"  class  drivers, at their option.  In
+     5             the latter case the value expressed in this field by  the
+     6             host  that  issued  the  most  recent  ONLINE or SET UNIT
+     7             CHARACTERISTICS command dictates which  device  dependent
+     8             parameters are in effect.
+
+
+
+     9     Allowable modifiers:
+
+    10         Allow Self Destruction
+    11         Clear Serious Exception (ignored for disk class devices)
+    12         Ignore Media Format Error
+
+    13             See Section  6.13.1,  "ONLINE  Command,  Command  Message
+    14             Format." 
+
+    15             The server may, at its option, reject the command  as  an
+    16             Invalid Command (subcode "Invalid Modifiers") if the unit
+    17             is already "Unit-Online" to another host.
+
+    18         Enable Set Write Protect
+
+    19             See "unit flags"  field  description  above  and  Section
+    20             6.13.1, "ONLINE Command, Command Message Format." 
+
+    21         Exclusive Access
+
+    22             If the "Exclusive Access" modifier is set and the unit is
+    23             "Unit-Available"   to   the   issuing  host  and  is  not
+    24             "Unit-Online" to nor under the "Exclusive Access" of  any
+    25             other  host,  the unit becomes "Unit-Online" to and under
+    26             the "Exclusive Access" of the issuing host.
+
+    27             If the "Exclusive Access" modifier is set and the unit is
+    28             "Unit-Available"  to  and under the "Exclusive Access" of
+    29             the issuing host, the unit becomes "Unit-Online"  to  the
+    30             issuing   host.    "Exclusive  Access"  of  the  unit  is
+    31             retained.
+
+    32             If the "Exclusive Access" modifier is set and the unit is
+    33             already  "Unit-Online"  to  and  not under the "Exclusive
+    34             Access" of the issuing host and is not  "Unit-Online"  to
+    35             any  other  host,  the issuing host is granted "Exclusive
+    36             Access" of the unit.  The unit's state  relative  to  the
+    37             issuing host is otherwise unchanged.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-31
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1             If the "Exclusive Access" modifier is set and the unit is
+     2             "Unit-Online"  to and under the "Exclusive Access" of the
+     3             issuing host, the issuing host retains "Exclusive Access"
+     4             of  the  unit.   The unit's state relative to the issuing
+     5             host is otherwise unchanged.
+
+     6             If the "Exclusive Access" modifier is clear and the  unit
+     7             is  "Unit-Available"  to and under the "Exclusive Access"
+     8             of the issuing host, the unit  becomes  "Unit-Online"  to
+     9             the  issuing  host  and "Exclusive Access" of the unit is
+    10             released.
+
+    11             If the "Exclusive Access" modifier is clear and the  unit
+    12             is  "Unit-Online"  to and under the "Exclusive Access" of
+    13             the issuing host,  "Exclusive  Access"  of  the  unit  is
+    14             released.   The unit's state relative to the issuing host
+    15             is otherwise unchanged.
+
+    16             If the unit is "Unit-Offline" to  the  issuing  host  the
+    17             setting of the "Exclusive Access" modifier is ignored.
+
+    18             Note that granting "Exclusive Access" of a unit to a host
+    19             implicitly   causes  the  unit  to  become  "Unit-Offline
+    20             (substate 'Exclusive Use')" to all other hosts  connected
+    21             via  this  controller and "Unit-Offline (substate 'Online
+    22             To Another Controller')" to hosts  that  may  potentially
+    23             have   access   to   the  unit  via  another  controller.
+    24             Conversely, the release of a  unit  from  the  "Exclusive
+    25             Access"  of  a  host implicitly causes the unit to become
+    26             "Unit-Available" to all other hosts.
+
+    27  |          Note also that "Exclusive Access" is released if  any  of
+    28  |          the following events occur:
+
+    29  |            1.  The controller loses or breaks the connection  to
+    30  |                the  host  that  has  "Exclusive  Access"  of the
+    31  |                drive.
+
+    32  |            2.  The  drive  becomes  unknown  to  the  controller
+    33  |                (e.g., port button disabled.) 
+
+    34  |            3.  The  drive  becomes  inoperative,  "Unit-Offline,
+    35  |                Inoperative." 
+
+    36  |            4.  The controller is 'rebooted', by  itself,  by  an
+    37  |                operator, or by any host.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-32
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1     7.7.2  End Message Format
+
+     2              31                             0               
+     3              +-------------------------------+              
+     4              |   command reference number    |              
+     5              +---------------+---------------+              
+     6              |sequence number|  unit number  |              
+     7              +---------------+-------+-------+              
+     8              |    status     | flags |endcode|              
+     9              +---------------+-------+-------+              
+    10              |  unit flags   |multiunit code |              
+    11              +---------------+---------------+              
+    12              |      reserved         |spndles|              
+    13              +-------------------------------+              
+    14              |                               |              
+    15              +---     unit identifier     ---+              
+    16              |                               |              
+    17              +-------------------------------+              
+    18              |     media type identifier     |              
+    19              +-------------------------------+              
+    20              |           reserved            |              
+    21              +-------------------------------+              
+    22              |           unit size           |              
+    23              +-------------------------------+              
+    24              |      volume serial number     |              
+    25              +-------------------------------+              
+
+    26  |      status
+
+    27             The validity of the unit characteristics varies with  the
+    28             unit's  state implied by the contents of this field.  See
+    29             Status Codes listed below and Section 6.17.3,  "SET  UNIT
+    30             CHARACTERISTICS   Command,   Description"   for  complete
+    31             details.
+
+    32         unit flags
+
+    33             See Section 5.7, "Unit Flags." 
+
+    34             Exclusive Access
+
+    35                 The Exclusive Access unit flag is  non-host-settable;
+    36                 it  is  set  when  the  unit  is under the "Exclusive
+    37                 Access"  of  this  or  some  other  host   via   this
+    38                 controller and clear when it is not.
+
+    39         All other fields of the  ONLINE  command's  end  message  are
+    40         identical  to  the  SET  UNIT  CHARACTERISTICS  command's end
+    41         message.   Refer  to  Section  7.9.2,  "Multihost  SET   UNIT
+    42         CHARACTERISTICS   Command,   End   Message   Format"   for  a
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-33
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1         description of the fields.
+
+
+     2     Status Codes:
+
+     3         Success (subcode "Normal")
+
+     4             The unit has become "Unit-Online" to  the  issuing  host.
+     5             The  unit  may  have already been "Unit-Online" to one or
+     6             more other hosts and, for those unit flags that are  both
+     7             host-settable  and supported by the controller, the class
+     8             driver specified values identical to those  currently  in
+     9             effect on the unit.
+
+    10             As described earlier (see "Exclusive Access" modifier  in
+    11             the preceding section), "Exclusive Access" of the unit by
+    12             the issuing host may  have  been  granted,  retained,  or
+    13             released  depending  on  the  setting  of  the "Exclusive
+    14             Access" modifier and whether or not the unit was  already
+    15             under the "Exclusive Access" of the issuing host.
+
+    16             The unit's state relative  to  all  other  hosts  remains
+    17             unchanged  unless  "Exclusive  Access"  of  the  unit was
+    18             granted or released.  See "Exclusive Access" modifier  in
+    19             the preceding section.
+
+    20         Success (subcode "Already Online")
+
+    21             The "Already Online" subcode bit flag is set only if  the
+    22             unit  was  already "Unit-Online" to the issuing host and,
+    23             for those unit flags  that  are  both  host-settable  and
+    24             supported  by  the controller, the class driver specified
+    25             values identical to those  currently  in  effect  on  the
+    26             unit.   The  unit  remains  "Unit-Online"  to the issuing
+    27             host.
+
+    28             As described earlier (see "Exclusive Access" modifier  in
+    29             the  preceding section) "Exclusive Access" of the unit by
+    30             the issuing host may  have  been  granted,  retained,  or
+    31             released  depending  on  the  setting  of  the "Exclusive
+    32             Access" modifier and whether or not the unit was  already
+    33             under the "Exclusive Access" of the issuing host.
+
+    34             The unit's state relative  to  all  other  hosts  remains
+    35             unchanged,  unless  "Exclusive  Access"  of  the unit was
+    36             granted or released.  See "Exclusive Access" modifier  in
+    37             the preceding section.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-34
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1         Invalid Command (subcode "Invalid Unit Flags")
+
+     2             The unit was already "Unit-Online" to one or  more  hosts
+     3             and, for those unit flags that are both host-settable and
+     4             supported by the controller, the class  driver  specified
+     5             different  values  from  those currently in effect on the
+     6             unit.  The  unit  state  relative  to  the  issuing  host
+     7             remains  unchanged.  The host-settable unit flags are not
+     8             changed,  and  the  end  message  returns  their  current
+     9             settings.
+
+    10             As described earlier (see "Exclusive Access" modifier  in
+    11             the  preceding section) "Exclusive Access" of the unit by
+    12             the issuing host may have been denied, granted, retained,
+    13             or  released  depending  on the setting of the "Exclusive
+    14             Access" modifier, whether or not  the  unit  was  already
+    15             under the "Exclusive Access" of the issuing host, and the
+    16             state of the unit relative to all other hosts.  Note that
+    17             controllers shall service the "Exclusive Access" modifier
+    18             before checking for equality of  the  host-settable  unit
+    19             flags.
+
+    20             The unit's state relative  to  all  other  hosts  remains
+    21             unchanged,  unless  "Exclusive  Access"  of  the unit was
+    22             granted or released.  See "Exclusive Access" modifier  in
+    23             the preceding section.
+
+    24         Unit-Offline
+
+    25             All subcodes may be returned.   Note  that  "Unit-Offline
+    26             (subcode 'Exclusive Use')" status indicates that the unit
+    27             is under the "Exclusive Access" of another host via  this
+    28             controller.  All characteristic fields and unit flags are
+    29             valid when that status is returned.  Also note that  some
+    30             causes  of  a unit being "Unit-Offline" may be overridden
+    31             (suppressed) by  the  "Allow  Self  Destruction"  command
+    32             modifier.
+
+    33         Unit-Available (subcode "Already In Use")
+
+    34             The unit was "Unit-Available" to the the issuing host and
+    35             "Exclusive  Access"  of the unit was denied because it is
+    36             currently  "Unit-Online"  to  another   host   via   this
+    37             controller.  All characteristic fields and unit flags are
+    38             valid.
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-35
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1         Command Aborted
+     2         Media Format Error
+     3         Controller Error
+     4         Drive Error
+
+     5             See  Section  6.13.2,  "ONLINE   Command,   End   Message
+     6             Format." 
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-36
+           Multihost ONLINE Command                             11 June 1992
+
+
+     1     7.7.3  Description
+
+     2     The ONLINE command is used to bring  a  unit  "Unit-Online,"  set
+     3     host-settable   unit   characteristics,  and  obtain  those  unit
+     4     characteristics  that  are  essential  for  proper  class  driver
+     5     operation   (see   Section   5.7,  "Unit  Flags"  for  additional
+     6     information).  The unit is spun-up, if necessary, and  its  heads
+     7     are  loaded  prior to returning the ONLINE command's end message.
+     8     Host-settable characteristics are set exactly as if  a  SET  UNIT
+     9     CHARACTERISTICS   command   were   issued   (see  Section  7.9.2,
+    10     "Multihost SET  UNIT  CHARACTERISTICS  Command").   Host-settable
+    11     characteristics  are  set  after  the  unit has been successfully
+    12     spun-up and any other validity checks have succeeded.
+
+    13     In addition, "Exclusive Access" of the unit by the  issuing  host
+    14     may have been denied, granted, retained, or released depending on
+    15     the setting of the "Exclusive Access" modifier,  whether  or  not
+    16     the  unit was already under the "Exclusive Access" of the issuing
+    17     host, and the state of the unit relative to all other hosts.  See
+    18     "Exclusive Access" modifier description above.
+
+    19     Note that if the unit is already  "Unit-Online"  to  the  issuing
+    20     host   or   any   other   host(s),   the   unit's   host-settable
+    21     characteristics and state relative to the issuing host as well as
+    22     all other hosts are not altered.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-37
+           Multihost SET CONTROLLER CHARACTERISTICS Command     11 June 1992
+
+
+     1     7.8  Multihost SET CONTROLLER CHARACTERISTICS Command
+
+     2     Command Category:
+
+     3         Immediate
+
+     4     7.8.1  Command Message Format
+
+     5              31                             0              
+     6              +-------------------------------+             
+     7              |   command reference number    |             
+     8              +---------------+---------------+             
+     9              |   reserved    |    reserved   |             
+    10              +---------------+-------+-------+             
+    11              |   modifiers   | rsvd  | opcode|             
+    12              +---------------+-------+-------+             
+    13              | cntrlr. flags |  MSCP version |             
+    14              +---------------+---------------+             
+    15              |   reserved    |  host timeout |             
+    16              +---------------+---------------+             
+    17              |           quad-word           |             
+    18              +---                         ---+             
+    19              |         time and date         |             
+    20              +-------------------------------+             
+    21              |controller dependent parameters|             
+    22              +---------------+---------------+             
+    23                              |      crn      | (optional)  
+    24                              +---------------+             
+
+    25         MSCP version
+
+    26             See  Section  6.16.1,  "SET  CONTROLLER   CHARACTERISTICS
+    27             Command, Command Message Format." 
+
+    28         cntrlr. flags
+
+    29             Host-settable  controller  flags.    See   Section   5.6,
+    30             "Controller Flags." 
+
+    31             Note  that  controllers  maintain  a  separate   set   of
+    32             host-settable     "controller     flags"     for     each
+    33             "Controller-Online" class driver.
+
+    34         host timeout
+
+    35             See  Section  6.16.1,  "SET  CONTROLLER   CHARACTERISTICS
+    36             Command, Command Message Format." 
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-38
+           Multihost SET CONTROLLER CHARACTERISTICS Command     11 June 1992
+
+
+     1             Note that controllers maintain a separate "host  timeout"
+     2             for each "Controller-Online" class driver.
+
+     3         quad-word time and date
+
+     4             See  Section  6.16.1,  "SET  CONTROLLER   CHARACTERISTICS
+     5             Command, Command Message Format." 
+
+     6             Note that controllers maintain a single  "quad-word  time
+     7             and date" for all "Controller-Online" class drivers.
+
+     8         controller dependent parameters
+
+     9             See  Section  6.16.1,  "SET  CONTROLLER   CHARACTERISTICS
+    10             Command, Command Message Format." 
+
+    11             Note that controllers may either maintain the "controller
+    12             dependent     parameters"     separately     for     each
+    13             "Controller-Online" class driver or  singularly  for  all
+    14             "Controller-Online" class drivers, at their option.
+
+    15         crn (connection reference number)
+
+    16             See  Section  6.16.1,  "SET  CONTROLLER   CHARACTERISTICS
+    17             Command, Command Message Format." 
+
+
+    18     Allowable modifiers:
+
+    19         none
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-39
+           Multihost SET CONTROLLER CHARACTERISTICS Command     11 June 1992
+
+
+     1     7.8.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number| rsvd  | alloc |
+     7                     +---------------+-------+-------+
+     8                     |    status     | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     | cntrlr. flags |  MSCP version |
+    11                     +-------+-------+---------------+
+    12                     |chvrsn | csvrsn|cntrlr. timeout|
+    13                     +-------+-------+---------------+
+    14                     |                               |
+    15                     +---  controller identifier  ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     | maximum byte count (optional) |
+    19                     +-------------------------------+
+
+    20         alloc
+
+    21             Allocation Class.  Zero, unless  manually  altered  in  a
+    22             controller  implementation  dependent  manner.  Multihost
+    23             controllers shall provide an external method for  setting
+    24             this  field (via DUP commands is sufficient).  When a new
+    25             allocation class  value  takes  effect,  all  connections
+    26             shall be broken to allow hosts to recognize the new value
+    27             upon reconnection.  Allocation class shall  be  preserved
+    28             in   nonvolatile   controller  memory  to  allow  correct
+    29             operation across power failure.
+
+    30         cntrlr.  flags
+
+    31             See Section 5.6, "Controller Flags." 
+
+    32             For those controller options that a  controller  supports
+    33             the   corresponding   non-host-settable  flag  and  those
+    34             host-settable flags that are currently in effect for  the
+    35             issuing host will be returned set.  Note that controllers
+    36             shall set the "Multihost Support" flag.
+
+    37         cntrlr.  timeout
+
+    38             See  Section  6.16.2,  "SET  CONTROLLER   CHARACTERISTICS
+    39             Command,  End  Message  Format,"  Section  4.10, "Command
+    40             Timeouts," and Section 7.5, "Multihost GET COMMAND STATUS
+    41             Command." 
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-40
+           Multihost SET CONTROLLER CHARACTERISTICS Command     11 June 1992
+
+
+     1         csvrsn
+     2         chvrsn
+     3         controller identifier
+     4         maximum byte count (optional)
+
+     5             See  Section  6.16.2,  "SET  CONTROLLER   CHARACTERISTICS
+     6             Command, End Message Format." 
+
+
+     7     Status Codes:
+
+     8         Success (subcode "Normal")
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-41
+           Multihost SET CONTROLLER CHARACTERISTICS Command     11 June 1992
+
+
+     1     7.8.3  Description
+
+     2     See Section  6.16.3,  "SET  CONTROLLER  CHARACTERISTICS  Command,
+     3     Description." 
+
+     4     For  those  host-settable  controller  characteristics  that  are
+     5     maintained separately, controllers shall operate according to the
+     6     value(s) specified by each individual  "Controller-Online"  class
+     7     driver.    The   operational  characteristics  requested  by  one
+     8     "Controller-Online" class driver  shall  not  interfere  with  or
+     9     modify   in   any   manner  those  requested  by  another.   Each
+    10     "Controller-Online"  class  driver  is  expressly  permitted   to
+    11     specify  different  operational  characteristics  and as a result
+    12     realize different controller operation.
+
+    13     For  those  host-settable  characteristics  that  are  maintained
+    14     singularly,  controllers  shall operate according to the value(s)
+    15     specified by the host that issued the most recent SET  CONTROLLER
+    16     CHARACTERISTICS command.
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-42
+           Multihost SET UNIT CHARACTERISTICS Command           11 June 1992
+
+
+     1     7.9  Multihost SET UNIT CHARACTERISTICS Command
+
+     2     Command Category:
+
+     3         Sequential
+
+     4     7.9.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |   reserved    |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |  unit flags   |    reserved   |
+    14                     +---------------+---------------+
+    15                     |           reserved            |
+    16                     +-------------------------------+
+    17                     |                               |
+    18                     +---        reserved         ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |  device dependent parameters  |
+    22                     +-------------------------------+
+    23                     |           reserved            |
+    24                     +-------------------------------+
+
+    25         unit flags
+
+    26             Host-settable  unit  flags.   See  Section   5.7,   "Unit
+    27             Flags." 
+
+    28             Note  that  controllers  maintain   a   single   set   of
+    29             host-settable  unit  flags  for  all  "Controller-Online"
+    30             class drivers.
+
+    31         device dependent parameters
+
+    32             See Section 6.17.1, "SET  UNIT  CHARACTERISTICS  Command,
+    33             Command Message Format." 
+
+    34             Note that controllers may  either  maintain  the  "device
+    35             dependent     parameters"     separately     for     each
+    36             "Controller-Online" class driver or  singularly  for  all
+    37             "Controller-Online"  class  drivers, at their option.  In
+    38             the latter case the value expressed in this field by  the
+    39             host  that  issued  the  most  recent  ONLINE or SET UNIT
+    40             CHARACTERISTICS command dictates which  device  dependent
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-43
+           Multihost SET UNIT CHARACTERISTICS Command           11 June 1992
+
+
+     1             parameters are in effect.
+
+
+     2     Allowable modifiers:
+
+     3         Clear Serious Exception (ignored for disk class devices)
+
+     4         Enable Set Write Protect
+
+     5             See Section 6.17.1, "SET  UNIT  CHARACTERISTICS  Command,
+     6             Command Message Format." 
+
+     7         Exclusive Access
+
+     8             If the "Exclusive Access" modifier is set and the unit is
+     9             "Unit-Available"   to   the   issuing  host  and  is  not
+    10             "Unit-Online" to any other host,  "Exclusive  Access"  of
+    11             the unit is granted to the issuing host.
+
+    12             If the "Exclusive Access" modifier is set and the unit is
+    13             "Unit-Available"  to  and under the "Exclusive Access" of
+    14             the issuing host,  "Exclusive  Access"  of  the  unit  is
+    15             retained.
+
+    16             If the "Exclusive Access" modifier is set and the unit is
+    17             "Unit-Online"  to and not under the "Exclusive Access" of
+    18             the issuing host and is not "Unit-Online"  to  any  other
+    19             host,  the  issuing host is granted "Exclusive Access" of
+    20             the unit.
+
+    21             If the "Exclusive Access" modifier is set and the unit is
+    22             "Unit-Online"  to and under the "Exclusive Access" of the
+    23             issuing host, the issuing host retains "Exclusive Access"
+    24             of the unit.
+
+    25             If the "Exclusive Access" modifier is clear and the  unit
+    26             is  "Unit-Available"  to and under the "Exclusive Access"
+    27             of the issuing host, "Exclusive Access" of  the  unit  is
+    28             released.
+
+    29             If the "Exclusive Access" modifier is clear and the  unit
+    30             is  "Unit-Online"  to and under the "Exclusive Access" of
+    31             the issuing host,  "Exclusive  Access"  of  the  unit  is
+    32             released.
+
+    33             If the unit is "Unit-Offline" to  the  issuing  host  the
+    34             setting of the "Exclusive Access" modifier is ignored.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-44
+           Multihost SET UNIT CHARACTERISTICS Command           11 June 1992
+
+
+     1             In all the cases just listed the unit's state relative to
+     2             the issuing host remains unchanged.
+
+     3             Note that granting "Exclusive Access" of a unit to a host
+     4             implicitly   causes  the  unit  to  become  "Unit-Offline
+     5             (substate 'Exclusive Use')" to all other hosts  connected
+     6             via  this  controller and "Unit-Offline (substate 'Online
+     7             To Another Controller')" to hosts which  may  potentially
+     8             have   access   to   the  unit  via  another  controller.
+     9             Conversely, the release of a  unit  from  the  "Exclusive
+    10             Access"  of  a  host implicitly causes the unit to become
+    11             "Unit-Available" to all other hosts.
+
+    12  |          Note also that "Exclusive Access" is released if  any  of
+    13  |          the following events occur:
+
+    14  |            1.  The controller loses or breaks the connection  to
+    15  |                the  host  that  has  "Exclusive  Access"  of the
+    16  |                drive.
+
+    17  |            2.  The  drive  becomes  unknown  to  the  controller
+    18  |                (e.g., port button disabled.) 
+
+    19  |            3.  The  drive  becomes  inoperative,  "Unit-Offline,
+    20  |                Inoperative." 
+
+    21  |            4.  The controller is 'rebooted', by  itself,  by  an
+    22  |                operator, or by any host.
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-45
+           Multihost SET UNIT CHARACTERISTICS Command           11 June 1992
+
+
+     1     7.9.2  End Message Format
+
+     2              31                             0               
+     3              +-------------------------------+              
+     4              |   command reference number    |              
+     5              +---------------+---------------+              
+     6              |sequence number|  unit number  |              
+     7              +---------------+-------+-------+              
+     8              |    status     | flags |endcode|              
+     9              +---------------+-------+-------+              
+    10              |  unit flags   |multiunit code |              
+    11              +---------------+---------------+              
+    12              |       reserved        |spndles|              
+    13              +-------------------------------+              
+    14              |                               |              
+    15              +---     unit identifier     ---+              
+    16              |                               |              
+    17              +-------------------------------+              
+    18              |     media type identifier     |              
+    19              +-------------------------------+              
+    20              |           reserved            |              
+    21              +-------------------------------+              
+    22              |           unit size           |              
+    23              +-------------------------------+              
+    24              |      volume serial number     |              
+    25              +-------------------------------+              
+
+    26  |      status
+
+    27             The validity of the unit characteristics varies with  the
+    28             unit's  state implied by the contents of this field.  See
+    29             Status Codes listed later in  this  section  and  Section
+    30             6.17.3,  "SET  UNIT CHARACTERISTICS Command, Description"
+    31             for complete details.
+
+    32         unit flags
+
+    33             See Section 5.7, "Unit Flags." 
+
+    34             Exclusive Access
+
+    35                 The Exclusive Access unit flag is  non-host-settable;
+    36                 it  is  set  when  the  unit  is under the "Exclusive
+    37                 Access"  of  this  or  some  other  host   via   this
+    38                 controller and clear when it is not.
+
+    39         spndles
+
+    40             The number minus one of spindles or spindle  "groups"  in
+    41             the  unit.  The value of zero indicates one spindle.  See
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-46
+           Multihost SET UNIT CHARACTERISTICS Command           11 June 1992
+
+
+     1             Section 4.12, "Disk Geometry and Format."
+
+     2         multiunit code
+     3         unit identifier
+     4         media type identifier
+     5         unit size
+     6         volume serial number
+
+     7             See Section 6.17.2, "SET  UNIT  CHARACTERISTICS  Command,
+     8             End Message Format." 
+
+
+     9     Status Codes:
+
+    10         Success (subcode "Normal")
+    11         Success (subcode "Duplicate Unit Number")
+
+    12             The unit is "Unit-Online" to the issuing host.  The  unit
+    13             may  also  be  "Unit-Online"  to one or more other hosts.
+    14             The unit will operate according to the setting  of  those
+    15             unit  flags  that are both host-settable and supported by
+    16             the controller, as specified by the issuing class driver,
+    17             for all hosts to which the unit is "Unit-Online." 
+
+    18             As described earlier (see "Exclusive Access" modifier  in
+    19             the  preceding section) "Exclusive Access" of the unit by
+    20             the issuing host may  have  been  granted,  retained,  or
+    21             released  depending  on  the  setting  of  the "Exclusive
+    22             Access" modifier and whether or not the unit was  already
+    23             under the "Exclusive Access" of the issuing host.
+
+    24             The unit's state relative  to  all  other  hosts  remains
+    25             unchanged,  unless  "Exclusive  Access"  of  the unit was
+    26             granted or released.  See "Exclusive Access" modifier  in
+    27             the preceding section.
+
+    28         Unit-Offline
+
+    29             All subcodes may be returned.   Note  that  "Unit-Offline
+    30             (subcode 'Exclusive Use')" status indicates that the unit
+    31             is under the "Exclusive Access" of some  other  host  via
+    32             this  controller.   All  characteristic  fields  and unit
+    33             flags are valid when that status is returned.
+
+    34         Unit-Available (subcode "Available")
+
+    35             This status code  implies  "Success  (subcode  'Normal')"
+    36             when "Exclusive Access" of the unit was granted while the
+    37             unit was "Unit-Available." 
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-47
+           Multihost SET UNIT CHARACTERISTICS Command           11 June 1992
+
+
+     1         Unit-Available (subcode "Already In Use")
+
+     2             The unit was "Unit-Online" or "Unit-Available" to the the
+     3             issuing  host  and  "Exclusive  Access"  of  the unit was
+     4             denied because it is currently "Unit-Online"  to  another
+     5             host  via this controller.  All characteristic fields and
+     6             unit flags are valid.
+
+     7         Command Aborted
+     8         Controller Error
+     9         Drive Error
+
+    10             See Section 6.17.2, "SET  UNIT  CHARACTERISTICS  Command,
+    11             End Message Format." 
+
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-48
+           Multihost SET UNIT CHARACTERISTICS Command           11 June 1992
+
+
+     1     7.9.3  Description
+
+     2     The SET UNIT CHARACTERISTICS command is used to set host-settable
+     3     unit  characteristics  and obtain those unit characteristics that
+     4     are essential for proper class driver operation.  The setting  of
+     5     the  host-settable  unit flags is effective only when the unit is
+     6     "Unit-Online" to the issuing host and affects all hosts to  which
+     7     the unit is "Unit-Online." 
+
+     8     In addition, "Exclusive Access" of the unit by the  issuing  host
+     9     may have been denied, granted, retained, or released depending on
+    10     the setting of the "Exclusive Access" modifier, whether the  unit
+    11     was already under the "Exclusive Access" of the issuing host, and
+    12     the  state  of  the  unit  relative  to  all  other  hosts.   See
+    13     "Exclusive Access" modifier description above.
+
+    14     Note that the format of the SET  UNIT  CHARACTERISTICS  command's
+    15     end  message  is  identical  to  that of the ONLINE command's end
+    16     message  because  the  ONLINE  command  performs   a   SET   UNIT
+    17     CHARACTERISTICS operation after bringing a unit "Unit-Online." 
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-49
+           Multihost TERMINATE CLASS DRIVER/SERVER CONNECTI ... 11 June 1992
+
+
+     1     7.10  Multihost TERMINATE CLASS DRIVER/SERVER CONNECTION Command
+
+
+     2     Command Category:
+
+     3         Immediate
+
+
+     4     7.10.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |   command reference number    |
+     8                     +---------------+---------------+
+     9                     |           reserved            |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+    13                     |                               |
+    14                     +---                         ---+
+    15                     |                               |
+    16                     +---                         ---+
+    17                     |           reserved            |
+    18                     +---                         ---+
+    19                     |                               |
+    20                     +---                         ---+
+    21                     |                               |
+    22                     +---------------+---------------+
+    23                                     |      crn      |
+    24                                     +---------------+
+
+
+    25         crn (connection reference number)
+
+    26             The identification of the host whose class  driver/server
+    27             connection is to be terminated.
+
+    28             Note that host  class  drivers  associate  a  "connection
+    29             reference number" with the "Controller-Online" connection
+    30             they establish with the server  via  the  SET  CONTROLLER
+    31             CHARACTERISTICS  command  as described in Section 6.16.1,
+    32             "SET CONTROLLER CHARACTERISTICS Command, Command  Message
+    33             Format." 
+
+    34             If the value supplied in this field is zero,  the  server
+    35             rejects  the  command  as  an "Invalid Command" ("invalid
+    36             crn" parameter error.  See Section  5.5,  "Status  Codes"
+    37             for complete details.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-50
+           Multihost TERMINATE CLASS DRIVER/SERVER CONNECTI ... 11 June 1992
+
+
+     1     Allowable modifiers:
+
+     2         None
+
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+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-51
+           Multihost TERMINATE CLASS DRIVER/SERVER CONNECTI ... 11 June 1992
+
+
+     1     7.10.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |   command reference number    |
+     5                     +---------------+---------------+
+     6                     |sequence number|   reserved    |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |                               |
+    11                     +---                         ---+
+    12                     |                               |
+    13                     +---                         ---+
+    14                     |           undefined           |
+    15                     +---                         ---+
+    16                     |                               |
+    17                     +---                         ---+
+    18                     |                               |
+    19                     +---------------+---------------+
+    20                                     |      crn      |
+    21                                     +---------------+
+
+
+    22         crn (connection reference number)
+
+    23             Zero or the value supplied in the  command  message  "crn
+    24             (connection reference number)" field.
+
+    25             If this field is zero, the server was unable  to  find  a
+    26             class  driver/server  connection associated with the host
+    27             specified  in  the  command  message   "crn   (connection
+    28             reference number)" field.
+
+    29             If this field  is  nonzero,  the  server  found  a  class
+    30             driver/server   connection   associated   with  the  host
+    31             specified  in  the  command  message   "crn   (connection
+    32             reference  number)"  field  and  performed the operations
+    33             necessary to terminate it.
+
+    34     Status Codes:
+
+    35         Success (subcode "Normal")
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           MULTIHOST SUPPORT                                       Page 7-52
+           Multihost TERMINATE CLASS DRIVER/SERVER CONNECTI ... 11 June 1992
+
+
+     1     7.10.3  Description
+
+
+     2     Support of the TERMINATE CLASS DRIVER/SERVER  CONNECTION  command
+     3     is a Disk MSCP server implementation dependent option.  Disk MSCP
+     4     servers that do  not  provide  such  support  shall  reject  this
+     5     command as an "Invalid Command" ("invalid opcode" protocol error.
+     6     See "Invalid Command" status in  Section  5.5,  "Status  Codes").
+     7     Tape MSCP servers shall not implement this command.
+
+     8     Servers that support "write  history  logging"  as  described  in
+     9     Section  6.19.3,  "WRITE HISTORY MANAGEMENT Command, Description"
+    10     shall also support the TERMINATE CLASS  DRIVER/SERVER  CONNECTION
+    11     command.   Note that servers that support "write history logging"
+    12     but do not provide Multihost Support shall  treat  the  TERMINATE
+    13     CLASS  DRIVER/SERVER  CONNECTION  command  as  a  no-operation as
+    14     described in Section 6.1.1, "No-Operation Commands."
+
+    15     "Write history logging" and  the  TERMINATE  CLASS  DRIVER/SERVER
+    16     CONNECTION  command  were  defined  primarily  for the purpose of
+    17     assisting  in  the  operation  of  host  based  volume  shadowing
+    18     implementations.    All   servers  that  support  "write  history
+    19     logging"  (and  therefore  the  TERMINATE   CLASS   DRIVER/SERVER
+    20     CONNECTION  command)  shall set the Write History Logging Support
+    21     controller flag.  A server that does not support  "write  history
+    22     logging"   may,  at  its  option,  support  the  TERMINATE  CLASS
+    23     DRIVER/SERVER CONNECTION command.
+
+    24     If supported and the server also provides Multihost Support, this
+    25     command  permits  a  host  class  driver  to  terminate the class
+    26     driver/server connection a host has established with  the  server
+    27     receiving this command.
+
+    28     After  performing  the  operations  necessary  to  terminate  the
+    29     connection  between  itself  and  the class driver located in the
+    30     specified    host    system     the     server     enters     the
+    31     "Controller-Available"  state relative to that class driver.  See
+    32     Section 4.1, "Controller States," for information  regarding  the
+    33     processing  of new and outstanding commands received when a class
+    34     driver/server connection is terminated.   Note  that  the  server
+    35     shall   "notice"   connection  terminations  resulting  from  the
+    36     execution of this command prior to returning this  command's  end
+    37     message.
+
+    38     Note that in cases where multiple class drivers  located  in  the
+    39     specified  host system have established separate connections with
+    40     the server receiving this command and each of  those  connections
+    41     is  associated  with  the same "connection reference number," the
+    42     server shall terminate all connections so established.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-1
+                                                                11 June 1992
+
+
+     1                                 CHAPTER 8
+
+     2                CONTROLLER INITIATED BAD BLOCK REPLACEMENT
+
+
+     3     8.1  Controller Initiated Bad Block Replacement Overview
+
+     4     Controller Initiated Bad Block Replacement is an MSCP option that
+     5     relieves host class drivers from the task of performing bad block
+     6     replacement on disk drives.
+
+     7     In a disk MSCP server that does not provide Controller  Initiated
+     8     Bad  Block Replacement the server reports bad blocks to the class
+     9     driver,  and  the  class  driver  and/or  an  auxiliary   process
+    10     associated  with  it  actually  replaces  the  bad  block.   With
+    11     Controller Initiated Bad Block Replacement, the disk MSCP  server
+    12     replaces  bad  blocks  itself,  transparent  to the class driver.
+    13     This requires that the server also take over maintenance  of  the
+    14     Replacement  Control  Table  (RCT),  which  contains  replacement
+    15     context information.
+
+    16     Note that a controller may  implement  Controller  Initiated  Bad
+    17     Block  Replacement  on  some or all of the disk types that may be
+    18     connected  to  it,  at  its  option.   In  addition,   Controller
+    19     Initiated  Bad  Block  Replacement  does  not  apply to Tape Mass
+    20     Storage Control Protocol, as bad block replacement is not used on
+    21     tape class devices.
+
+    22     Conceptually,  Controller  Initiated  Bad  Block  Replacement  is
+    23     implemented  by inserting a Bad Block Replacement layer between a
+    24     Gatekeeper layer and the Disk MSCP layer, as shown in Figure  8-1
+    25     below.    The   Gatekeeper   layer   ensures   that   Sequential,
+    26     Non-Sequential, and Immediate commands behave as  described.   It
+    27     recognizes  when  a  Sequential  command has begun execution, and
+    28     defers  all  Non-Sequential  commands  on  that  unit  until  the
+    29     Sequential  command  completes.   The Bad Block Replacement layer
+    30     performs bad block replacement and  RCT  maintenance  by  issuing
+    31     MSCP commands to the Disk MSCP layer.
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-2
+           Controller Initiated Bad Block Replacement Overview  11 June 1992
+
+
+     1                                     Class Driver              
+     2                                          A                    
+     3                                          |                    
+     4                                          V                    
+     5                       / +----------------------------------+  
+     6                      /  |            Gatekeeper            |  
+     7                     /   +----------------------------------+  
+     8             MSCP server |       Bad Block Replacement      |  
+     9                     \   +----------------------------------+  
+    10                      \  |             Disk MSCP            |  
+    11                       \ +----------------------------------+  
+
+
+    12        Figure 8-1:  Controller  Initiated  Bad  Block  Replacement
+    13                     Layers
+
+
+    14     The remainder of this chapter describes the  algorithms  used  by
+    15     the  Bad  Block  Replacement  layer, the concepts that Controller
+    16     Initiated Bad Block Replacement adds to standard Disk  MSCP,  the
+    17     changes  to MSCP control message formats (described in Chapter 6,
+    18     "Minimal MSCP  Command  Set"  and,  if  the  controller  provides
+    19     Multihost  Support,  Chapter  7,  "Multihost  Support").   Actual
+    20     implementations of Controller  Initiated  Bad  Block  Replacement
+    21     need  not be a distinct layer or use exactly the algorithms given
+    22     here, so long as all class driver visible results are identical.
+
+    23     8.2  Data Safety Write Protection
+
+    24     As described in Section 4.14, "Write Protection," MSCP has  three
+    25     forms  of  write protection:  Hardware Write Protection, Software
+    26     Write Protection, and Data Safety Write Protection.  Data  Safety
+    27     Write  Protection shall be put into effect whenever the Bad Block
+    28     Replacement layer detects that the  volume  has  an  inconsistent
+    29     Replacement  Control  Table  (RCT) or a replacement has failed on
+    30     the volume.
+
+    31     When a unit is brought "Unit-Online," the Bad  Block  Replacement
+    32     layer  examines  block  0 of the RCT to see if the volume must be
+    33     Data Safety Write Protected.  If so, the layer write protects the
+    34     unit  and disables all bad block replacement attempts.  The layer
+    35     provides read-only access to the unit on a "best efforts"  basis,
+    36     so that a host can copy the volume's data to a backup device.
+
+    37     If a volume has an incomplete replacement attempt, and  its  unit
+    38     is Hardware Write Protected when it is brought "Unit-Online," the
+    39     Bad Block  Replacement  layer  cannot  complete  the  replacement
+    40     attempt.   As an incomplete replacement attempt is one form of an
+    41     inconsistent RCT, the unit is Data Safety Write Protected when it
+    42     is  brought "Unit-Online."  If the unit subsequently ceases to be
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-3
+           Data Safety Write Protection                         11 June 1992
+
+
+     1     Hardware Write Protected, the layer shall attempt to complete the
+     2     incomplete  replacement.   If  this  succeeds,  Data Safety Write
+     3     Protection may be cleared for the unit.
+
+     4     The only way to clear Data Safety  Write  Protection  due  to  an
+     5     invalid RCT is to reformat the volume.
+
+     6     8.3  Bad Block Replacement
+
+     7     With Controller Initiated Bad Block Replacement, the MSCP  server
+     8     performs  all  bad  block  replacement.   At  a minimum, the MSCP
+     9     server replaces the first bad block encountered by each  transfer
+    10     command.   That  is, the MSCP server shall replace all bad blocks
+    11     that would have been reported in any  end  message's  "first  bad
+    12     block"  field  by an MSCP server that does not provide Controller
+    13     Initiated Bad Block Replacement.  Replacement of  any  additional
+    14     bad blocks encountered by a transfer is strongly recommended, but
+    15     not required.
+
+    16     Bad blocks can be detected by both  read  and  write  operations.
+    17     For read operations, the success or failure of the read cannot be
+    18     changed by retrying the read after  performing  the  replacement.
+    19     However, with writes it is definitely possible (indeed, probable)
+    20     that a write that fails before the replacement will succeed after
+    21     the replacement.  Consider that most bad blocks detected on write
+    22     operations will be due to  invalid  headers  --  defects  in  the
+    23     header  area  of  a  sector.   A  write to such a block will fail
+    24     before the replacement, but succeed afterwards.
+
+    25     For this reason, if a write operation fails on a bad  block,  the
+    26     Bad Block Replacement layer shall retry the write operation after
+    27     replacing the block.  This implies that the replacement operation
+    28     must  be  performed  before such a write command completes (i.e.,
+    29     before the  end  message  is  returned).   In  all  other  cases,
+    30     however,  the  replacement  operation  may  be  asynchronous with
+    31     respect to completion of the command that detected the bad block.
+    32     That  is,  since  the  replacement  cannot  affect the success or
+    33     failure of the command, the replacement may be  performed  either
+    34     before or after the command completes.
+
+    35     In addition to detecting bad blocks in transfers issued by  hosts
+    36     or  higher  layers  in the MSCP server, the Bad Block Replacement
+    37     layer may  also  check  for  bad  blocks  spontaneously,  without
+    38     receiving  any  actual  commands.   In effect, whenever a unit is
+    39     "Unit-Online," the Bad Block Replacement layer  may  periodically
+    40     generate  reads  to  the  unit.   Any bad blocks detected by such
+    41     internally generated transfers should be  replaced,  exactly  the
+    42     same as if the read were externally generated.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-4
+           Bad Block Replacement                                11 June 1992
+
+
+     1     Bad block replacement  cannot  be  performed  when  the  unit  is
+     2     Hardware  Write  Protected.   If  a  bad  block  is detected on a
+     3     Hardware Write Protected unit, replacement is not attempted.   If
+     4     the  unit becomes Hardware Write Protected after the bad block is
+     5     detected, but before the replacement is completed, some phase  of
+     6     the  bad  block  replacement  algorithm  will  fail with a "Write
+     7     Protect" error.  This error is handled  the  same  as  any  other
+     8     failure   of   the  bad  block  replacement  algorithm;  see  the
+     9     discussion below.
+
+    10     When  both  Disk  MSCP  and  Controller   Initiated   Bad   Block
+    11     Replacement  are  implemented  in  the  same  controller,  it  is
+    12     strongly recommended that the  controller  defer  recognition  of
+    13     Hardware   Write   Protection   until   all  pending  replacement
+    14     operations have completed.  That is, when  an  operator  actuates
+    15     the  unit's  write  protect  mechanism,  the  controller does not
+    16     physically write protect the  unit  until  after  all  previously
+    17     detected  bad  blocks have been replaced.  This is similar to the
+    18     requirement  in  standard  Disk  MSCP  of  providing   a   smooth
+    19     transition to the write protected state (see Section 4.14, "Write
+    20     Protection").
+
+    21     Bad block  replacement  shall  be  performed  regardless  of  the
+    22     Software  Write  Protect  status  of the unit.  If a bad block is
+    23     detected when the unit is Software Write Protected, the Bad Block
+    24     Replacement  layer  shall  write  enable  the  unit,  perform the
+    25     replacement, then re-write protect the  unit.   The  layer  shall
+    26     guarantee  that all host or higher layer write commands are still
+    27     rejected in spite of the unit being temporarily write enabled.
+
+    28     Bad block replacement shall not be attempted  when  the  unit  is
+    29     Data  Safety  Write  Protected  or  when  the  unit  was  brought
+    30     "Unit-Online" with the "Ignore Media Format Error" modifier.
+
+    31     If the unit is  Hardware  Write  Protected  when  it  is  brought
+    32     "Unit-Online"  and  it  subsequently  ceases to be Hardware Write
+    33     Protected, the Bad Block Replacement layer shall perform  certain
+    34     actions  before  it attempts a replacement or modifies the volume
+    35     in any  other  way.   These  actions  are  to  read  and  rewrite
+    36     Replacement  Control  Table  (RCT)  block  0  and  to  attempt to
+    37     complete any incomplete replacement operation.  See Section 8.10,
+    38     "Actions Before First Modification." 
+
+    39     If a bad block replacement fails for any reason,  the  Bad  Block
+    40     Replacement layer handles the failure as follows:
+
+    41          1.  The status code returned for the command  that  detected
+    42              the  bad block is not affected by the failed replacement
+    43              attempt.  That is, the status code appropriate  for  the
+    44              original access is returned.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-5
+           Bad Block Replacement                                11 June 1992
+
+
+     1          2.  The replacement failure is reported with  a  "Bad  Block
+     2              Replacement  Attempt"  error  log  message  (see Section
+     3              5.9.8).
+
+     4              Many of the errors that cause  replacement  failure  are
+     5              themselves   reported  with  error  log  messages.   For
+     6              example, failure of a multiread or multiwrite access  to
+     7              the  RCT  produces  at  least two error log messages:  a
+     8              (usually) "Disk Transfer Error"  message  reporting  the
+     9              access  failure  and  a  separate "Bad Block Replacement
+    10              Attempt" message reporting the replacement failure.  All
+    11              such  error  log  messages  that  report  the cause of a
+    12              replacement  failure  should  have  the  "Error   During
+    13              Replacement"  error  log  flag  set  and  should be sent
+    14              before the "Bad Block  Replacement  Attempt"  error  log
+    15              message.
+
+    16          3.  The Bad Block  Replacement  layer  forces  the  unit  to
+    17              become "Unit-Available" (or "Unit-Offline") with respect
+    18              to all class drivers, exactly as if an AVAILABLE command
+    19              with  the  "All Class Drivers" modifier had been issued.
+    20              This, of course, causes an AVAILABLE  Attention  message
+    21              to be sent to all class drivers that have enabled them.
+
+    22              Note that Data Safety Write Protection will be in effect
+    23              when  the  unit's  volume  is next brought "Unit-Online"
+    24              (unless the problem is no longer present at that time).
+
+    25          4.  Between  the  time  that  the   server   discovers   the
+    26              replacement   failure   and   the   unit  completes  its
+    27              transition to "Unit-Available," the setting of the  Data
+    28              Safety Write Protected unit flag is indeterminate.  This
+    29              flag may be either set or clear if it is returned  to  a
+    30              host during this interval.
+
+    31          5.  Any pending bad block replacement attempts for the  unit
+    32              are  discarded,  exactly  as  if  the unit had been Data
+    33              Safety  Write  Protected  when  those  bad  blocks  were
+    34              detected.  Error log messages or other notifications are
+    35              not generated to report the dropped replacements.
+
+    36          6.  No attempt is made to replace any additional bad  blocks
+    37              that   may   be   detected   during  the  transition  to
+    38              "Unit-Available," exactly  as  if  the  unit  were  Data
+    39              Safety Write Protected.
+
+    40          7.  Subject to the constraint of not attempting further  bad
+    41              block   replacement,   the  server  shall  complete  any
+    42              outstanding commands that it has already initiated.  All
+    43              new  commands  received  after  the  replacement failure
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-6
+           Bad Block Replacement                                11 June 1992
+
+
+     1              shall be rejected.  Outstanding commands that  have  not
+     2              yet   been  initiated  may  be  completed  or  rejected.
+     3              Commands  are  rejected  with  a   "Unit-Available"   or
+     4              "Unit-Offline" status code.
+
+
+     5     The success or failure of read operations is not affected by  bad
+     6     block  replacement  or the replacement's success or failure.  The
+     7     status  code  reported  for  read  operations   is   the   status
+     8     appropriate  to the original read attempt, before any replacement
+     9     was performed.  The success or failure  of  write  operations  is
+    10     affected  by  replacement,  since the write must be retried after
+    11     the replacement is performed.  If the replacement  succeeds,  the
+    12     status code reported is that appropriate to the retry (unless the
+    13     retry  also  encounters  a  bad  block,  in  which  case  another
+    14     replacement  and  another  retry  are  performed, and the process
+    15     repeated until the retry does not encounter a bad block).  If the
+    16     replacement  fails,  the status code reported is that appropriate
+    17     to  the  original  write  attempt  that  caused  the  bad   block
+    18     replacement.   The  cause  of  a bad block replacement failure is
+    19     only reported as an event code in an error  log  message;  it  is
+    20     never reported as a status code in an end message.
+
+    21     8.4  Replacement Control Table Access
+
+    22     The standard Disk MSCP server shall allow read and  write  access
+    23     to  the  Replacement  Control  Table  (RCT),  as  such  access is
+    24     required by higher layers in order to  be  able  to  perform  bad
+    25     block  replacement.   The  Bad  Block  Replacement layer need not
+    26     provide RCT  access,  however,  since  it  implements  bad  block
+    27     replacement itself.
+
+    28     Even though it is not necessary for  the  Bad  Block  Replacement
+    29     layer  to provide RCT access, in many cases it is desirable to do
+    30     so.  Providing such access  allows  a  host  to  examine  certain
+    31     additional  information  contained  in  RCT block 0.  Also, since
+    32     accessing a replacement block is usually slower than accessing  a
+    33     good logical block, a host could use the RCT contents to allocate
+    34     files with critical, real-time access requirements in portions of
+    35     the volume that do not contain bad blocks.
+
+    36     Therefore a server's Bad Block Replacement  layer  shall  provide
+    37     one  of the following RCT access options.  The server may provide
+    38     different options for different disk types.  The options are:
+
+    39          1.  No RCT access.  Any attempt to access any  block  inside
+    40              the RCT will be rejected.
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-7
+           Replacement Control Table Access                     11 June 1992
+
+
+     1          2.  Read-only access to the entire RCT.   Attempts  to  read
+     2              any  block in the first copy of the RCT will be honored.
+     3              That is, a READ command with "logical block  number"  in
+     4              the  range  "unit  size"  through  "unit size" plus "RCT
+     5              size" minus one and "byte count"  equal  to  the  unit's
+     6              block size will be honored.  Any other attempt to access
+     7              the RCT will be rejected.
+
+     8     When an RCT access attempt is rejected the command is rejected as
+     9     an  "Invalid  Command"  parameter  error  (see  "Invalid Command"
+    10     status in Section 5.5, "Status Codes").  If the command is a READ
+    11     command with a valid "logical block number," but the "byte count"
+    12     is not equal to the  unit's  block  size,  then  the  subcode  is
+    13     "Invalid  Byte Count."  Otherwise the subcode is "Invalid Logical
+    14     Block Number."  Attempts to write the RCT are  rejected  with  an
+    15     "Invalid Logical Block Number" subcode.
+
+    16     Note that  option  2,  entire  RCT  access,  only  allows  access
+    17     attempts to the first copy of the RCT.  The Bad Block Replacement
+    18     layer uses the multicopy read algorithm to locate the proper  RCT
+    19     copy to use in satisfying the request.
+
+    20     The  Bad  Block  Replacement  layer  reports  which   option   it
+    21     implements  by altering the "RCT size" and "copies" parameters in
+    22     the GET UNIT STATUS end message when it passes that message on to
+    23     higher layers.  These parameters are altered to describe the host
+    24     accessible portion of the RCT.  The  actual  alterations  are  as
+    25     follows:
+
+    26             RCT Access Option       "RCT size"       "copies"  
+    27             -----------------       ----------       --------  
+
+    28               No RCT access              0               0     
+
+    29               Entire RCT          actual RCT size        1     
+
+    30     It is recommended that high-end controllers provide access to the
+    31     entire  RCT.   Minimal, low-end controllers will probably provide
+    32     no RCT access.  Note that any disk that uses a nonstandard  means
+    33     of  bad  block  replacement,  and therefore does not have an RCT,
+    34     appears identical to a disk that provides no RCT access.
+
+    35     The Bad Block Replacement layer may optionally  allow  additional
+    36     RCT  access when a unit is brought "Unit-Online" with the "Ignore
+    37     Media Format Error" modifier.  This may include read-write access
+    38     and/or  access to individual copies of the RCT.  The layer should
+    39     ensure that the values  returned  for  "RCT  size"  and  "copies"
+    40     reflect  the  RCT  geometry  presented  to  hosts.  Note that any
+    41     additional access provided is intended for diagnostic use, and is
+    42     inherently  controller  dependent.  Attempts to write to the unit
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-8
+           Replacement Control Table Access                     11 June 1992
+
+
+     1     when the "Ignore Media Format Error" modifier has  been  set  may
+     2     have  undefined  results,  including  permanent corruption of the
+     3     volume.   The  errors  produced  by  such   corruption   may   be
+     4     indistinguishable from device hardware failures.
+
+     5     8.5  Atomic Bad Block Replacement
+
+     6     The  Bad  Block  Replacement  layer  shall  perform   bad   block
+     7     replacement  operations  as  atomic operations.  That is, while a
+     8     bad block replacement operation  is  in  progress,  any  transfer
+     9     operations  for either the bad block being replaced or block 0 of
+    10     the Replacement Control Table (RCT) shall not be performed.  Such
+    11     transfer  operations  shall be deferred until after the bad block
+    12     replacement  has  completed.   For  a  multiblock  transfer  that
+    13     includes the block being replaced, only the transfer of the block
+    14     being replaced need be deferred; the blocks before and after  the
+    15     one being replaced may be transferred immediately.
+
+    16     A bad block replacement operation begins just  before  the  "best
+    17     guess"  data  is obtained for the block.  This is not necessarily
+    18     the same transfer that detects the bad block and causes it to  be
+    19     replaced.   The  bad  block replacement operation ends just after
+    20     RCT block 0 is updated to indicate that replacement is no  longer
+    21     in  progress.   The  Controller Bad Block Replacement layer shall
+    22     lock out all host and higher layer  access  to  the  block  being
+    23     replaced and RCT block 0 between these two times.  Note that read
+    24     access must be locked out as well as write access.
+
+    25     8.6  Error Log Messages
+
+    26     Any error severe enough to warrant replacement  of  the  affected
+    27     block  is,  by  definition,  a  "significant" error that shall be
+    28     reported in an error log message.  The error  log  message  shall
+    29     use  the  "Disk  Transfer Error" format or some other format that
+    30     specifies the disk location at which  the  error  occurred.   The
+    31     "Bad  Block  Replacement  Request"  error  log flag is set in the
+    32     error log message to indicate that replacement of the block  will
+    33     be attempted.
+
+    34     When the replacement attempt completes, the Bad Block Replacement
+    35     layer  generates  an  additional  "Bad Block Replacement Attempt"
+    36     format error log message  to  report  the  replacement  attempt's
+    37     success or failure.
+
+    38     Due to the different error  characteristics  of  the  Replacement
+    39     Control  Table  (RCT), controllers should use an altered strategy
+    40     for logging errors on RCT accesses.  In the host area of a  disk,
+    41     the  most  common errors are eliminated by bad block replacement.
+    42     Since bad block replacement is not used in the RCT, these  common
+    43     errors  can  be  expected  to occur, and are therefore relatively
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT               Page 8-9
+           Error Log Messages                                   11 June 1992
+
+
+     1     uninteresting for an error log.
+
+     2     The goal for logging RCT errors is to  not  log  any  error  that
+     3     would  cause bad block replacement in the host area.  Controllers
+     4     that contain both standard Disk MSCP and Controller Initiated Bad
+     5     Block  Replacement can do this precisely, since they know exactly
+     6     which  errors  they  use   to   trigger   replacement.    Layered
+     7     controllers,  where  standard  Disk MSCP and Controller Initiated
+     8     Bad Block Replacement are distinct entities, can only approximate
+     9     this by filtering out "Data Errors."  That is, an independent Bad
+    10     Block Replacement layer should only  log  the  non-"Data  Errors"
+    11     that occur on RCT accesses.
+
+    12     The one exception to not logging RCT "Data Errors" is when an RCT
+    13     access  fails  --  that  is,  when  the  multiread  or multiwrite
+    14     algorithms fail, indicating that the read or write failed to  all
+    15     copies   of  the  RCT.   Whenever  the  multiread  or  multiwrite
+    16     algorithms  fail,  an  error  log  message  shall  be   generated
+    17     reporting  the  error  that  occurred  when the last RCT copy was
+    18     accessed.  If this error is a "Data Error,"  its  event  code  is
+    19     converted  to  a "Media Format Error" with the same subcode value
+    20     before being reported in the error log message.  All other errors
+    21     are reported without event code alteration.
+
+    22     Other than the altered strategy for  logging  RCT  "Data  Errors"
+    23     described  above,  any errors resulting from accesses to the unit
+    24     during bad block  replacement  are  logged  according  to  normal
+    25     controller  policies.   The only change is that the "Error During
+    26     Replacement" error log flag  should  be  set  in  such  messages,
+    27     indicating  that  the  operation  was  requested by the bad block
+    28     replacement process.  All such messages should be  issued  before
+    29     the "Bad Block Replacement Attempt" format error log message that
+    30     reports the replacement attempt's status.
+
+    31     8.7  Unit Context Information
+
+    32     The Bad Block Replacement layer  shall  separately  maintain  the
+    33     following unit flags for each unit it has "Unit-Online":
+
+    34              Write Protected (software)
+
+    35              Write Protected (data safety)
+
+
+    36     The reason for this is that the Bad Block Replacement layer  sets
+    37     the "Write Protected (software)" unit flag in the Disk MSCP layer
+    38     if either of these flags are set.  The  replacement  layer  shall
+    39     keep  track  of  these  flags  itself,  in  order to remember the
+    40     reason(s) for write protecting the unit.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-10
+           Unit Context Information                             11 June 1992
+
+
+     1     The Bad Block Replacement  layer  also  maintains  internal,  per
+     2     unit, flags to record:
+
+     3           o  The individual causes of Data Safety  Write  Protection.
+     4              It   uses  these  flags  to  return  the  proper  status
+     5              subcodes.
+
+     6           o  Whether Replacement Control Table (RCT) block 0 has been
+     7              rewritten  and  any  incomplete  replacements  finished.
+     8              Normally these actions are performed when  the  unit  is
+     9              brought "Unit-Online."  However, if the unit is Hardware
+    10              Write Protected when it is brought  "Unit-Online,"  then
+    11              these  actions  cannot  be performed.  Thus this flag is
+    12              set to remember to perform these actions when or if  the
+    13              unit subsequently ceases to be Hardware Write Protected.
+    14              This flag is further described in Sections 8.8, "Actions
+    15              During   ONLINE"   and   8.10,   "Actions  Before  First
+    16              Modification." 
+
+    17           o  Whether the "Ignore Media Format Error" modifier was set
+    18              when  the  unit was brought "Unit-Online."  It uses this
+    19              flag to inhibit bad block replacement.
+
+
+    20     8.8  Actions During ONLINE
+
+    21     The Bad Block Replacement layer performs two different courses of
+    22     action  for  an  ONLINE command, depending on whether the "Ignore
+    23     Media Format Error" modifier is set.  Normally, the  layer  reads
+    24     block 0 of the Replacement Control Table (RCT) to perform various
+    25     volume consistency checks.  However, if the "Ignore Media  Format
+    26     Error"  modifier is set, the layer does not access the RCT in any
+    27     way.
+
+    28     If an ONLINE command has the "Ignore Media Format Error" modifier
+    29     set, the Bad Block Replacement layer merely passes the unmodified
+    30     command on to the Disk MSCP  layer.   Similarly,  it  passes  the
+    31     unmodified  response from the Disk MSCP layer back to the host or
+    32     higher layer that issued the command.   The  replacement  layer's
+    33     only  additional action is to remember, in an internal flag, that
+    34     the "Ignore Media Format Error" modifier was set.  It  uses  this
+    35     flag  to  inhibit  all  bad  block  replacement  attempts  and to
+    36     optionally allow additional access to the RCT for diagnostics.
+
+    37     When the "Ignore Media Format Error" modifier is not set, the Bad
+    38     Block  Replacement  layer performs the following steps to process
+    39     an ONLINE command:
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-11
+           Actions During ONLINE                                11 June 1992
+
+
+     1          1.  Issue an ONLINE command to the  Disk  MSCP  layer.   All
+     2              fields  except the "modifiers" field are copied from the
+     3              ONLINE command received by the replacement  layer.   The
+     4              "modifiers" field is constructed as follows:
+
+     5              a.  The "Allow Self Destruction" modifier is copied from
+     6                  the received command.
+
+     7              b.  If the Disk MSCP layer provides  Multihost  Support,
+     8                  the "Exclusive Access" modifier is set.
+
+     9              c.  All other modifiers are clear.
+
+
+    10              If the Disk MSCP ONLINE fails, return its end message as
+    11              the result of this layer's ONLINE and exit.  If the Disk
+    12              MSCP  ONLINE  reports   that   the   unit   is   already
+    13              "Unit-Online,"  the ONLINE has succeeded.  Again, return
+    14              its end message as the result of this layer's ONLINE and
+    15              exit.   Otherwise  save its end message as the prototype
+    16              for this layer's ONLINE end message.
+
+    17          2.  Read  block  0  of  the  RCT  with  the  multicopy  read
+    18              algorithm.   If  the  multicopy  read  algorithm  fails,
+    19              report its result as the status of this ONLINE and exit.
+
+    20          3.  If the unit is Hardware Write Protected, set an internal
+    21              flag  and  go to step 6.  This flag is further described
+    22              in Section 8.10, "Actions Before First Modification." 
+
+    23          4.  Write the data read by step 2 back out to block 0 of the
+    24              RCT   with   the  multicopy  write  algorithm.   If  the
+    25              multicopy write algorithm fails, report  its  result  as
+    26              the status of this ONLINE and exit.
+
+    27          5.  If the data read by step 2 indicates that  there  is  an
+    28              incomplete  bad block replacement on the volume, attempt
+    29              to complete it.  If the completion attempt fails, report
+    30              it  with  an  error log message.  Continue with the next
+    31              step  regardless  of  whether  the  completion   attempt
+    32              succeeds or fails.
+
+    33          6.  Examine the contents of RCT block 0, as read in  step  2
+    34              and  possibly  modified  in  step  5.   If  there  is an
+    35              incomplete bad block replacement attempt or the  RCT  is
+    36              invalid, set the "Write Protect (data safety)" unit flag
+    37              in the unit's context.  The checks,  if  any,  that  the
+    38              controller  performs  to  verify  the RCT's validity are
+    39              specified  by  the  Digital  Storage  Architecture  Disk
+    40              Format and/or the controller's functional specification.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-12
+           Actions During ONLINE                                11 June 1992
+
+
+     1          7.  If the "Enable Set Write Protect" modifier  was  set  in
+     2              the  original  ONLINE  command  received from the higher
+     3              layer, then copy the "Write Protected  (software)"  unit
+     4              flag from the original command to the unit's context.
+
+     5          8.  If either of the "Write Protect (data safety)" or "Write
+     6              Protect  (software)"  unit  flags  are set in the unit's
+     7              context, issue a SET UNIT CHARACTERISTICS command to the
+     8              Disk  MSCP layer to software write protect the unit.  If
+     9              the command fails, report its status as  the  status  of
+    10              this ONLINE and exit.
+
+    11          9.  The ONLINE operation has completed  successfully.   Copy
+    12              the  "Write  Protect  (data  safety)" and "Write Protect
+    13              (software)" unit flags from this unit's context  to  the
+    14              prototype  end  message saved in step 1.  If the unit is
+    15              Data Safety Write Protected, set the appropriate  status
+    16              subcodes.   Return the resulting end message to the host
+    17              or higher layer.
+
+
+    18     If the ONLINE  operation  fails,  clean-up  may  or  may  not  be
+    19     necessary, depending on where the failure occurs.  If the failure
+    20     occurs in step 1 (the ONLINE command to the Disk MSCP layer), the
+    21     unit will not be "Unit-Online" through the Disk MSCP layer, so no
+    22     clean-up is necessary.  If the failure occurs anywhere else,  the
+    23     Bad Block Replacement layer shall issue an AVAILABLE command with
+    24     the "Spin-down" modifier set to the Disk MSCP  layer,  since  the
+    25     unit may still be "Unit-Online" through the Disk MSCP layer.
+
+    26     As  part  of  bringing  a  unit  "Unit-Online,"  the  Bad   Block
+    27     Replacement layer attempts to complete any incomplete replacement
+    28     on the volume.  If the unit is not Hardware Write Protected,  the
+    29     attempt  is  made  as part of the ONLINE command.  If the unit is
+    30     Hardware Write Protected, the attempt  is  made  after  the  unit
+    31     ceases to be Hardware Write Protected and no later than the first
+    32     attempt  to  write  to  the  unit   or   the   first   SET   UNIT
+    33     CHARACTERISTICS  command  after  the  unit  ceases to be Hardware
+    34     Write Protected.  This is discussed  in  Section  8.10,  "Actions
+    35     Before  First  Modification."   In either case, there is only one
+    36     attempt to complete the replacement.  If that one attempt  fails,
+    37     no  other  attempt will be made until the next time the volume is
+    38     brought "Unit-Online." 
+
+    39     8.9  Actions During SET UNIT CHARACTERISTICS
+
+    40     The Bad Block Replacement layer performs the following  steps  to
+    41     process a SET UNIT CHARACTERISTICS command:
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-13
+           Actions During SET UNIT CHARACTERISTICS              11 June 1992
+
+
+     1          1.  If the internal flag is set indicating that the unit was
+     2              Hardware   Write   Protected   when   it   was   brought
+     3              "Unit-Online"  and  it  is  no  longer  Hardware   Write
+     4              Protected,  perform  the  actions  described  in Section
+     5              8.10, "Actions Before First Modification" and clear  the
+     6              flag.  If those actions fail, report their result as the
+     7              status of this SET UNIT CHARACTERISTICS and exit.   Note
+     8              that  this  step  may  clear  the  "Write  Protect (data
+     9              safety)" unit flag in the unit's context.
+
+    10          2.  If the "Enable Set Write Protect" modifier  was  set  in
+    11              the  original  SET UNIT CHARACTERISTICS command received
+    12              from the higher layer, then copy  the  "Write  Protected
+    13              (software)"  unit  flag from the original command to the
+    14              unit's context.
+
+    15          3.  Issue a SET UNIT CHARACTERISTICS  command  to  the  Disk
+    16              MSCP  layer.   The  command is identical to the original
+    17              SET  UNIT  CHARACTERISTICS  command  received   by   the
+    18              replacement layer with the following changes:
+
+    19              a.  The "Enable Set Write Protect"  modifier  is  always
+    20                  set.
+
+    21              b.  The "Write Protect (software)" unit flag is  set  to
+    22                  the  "inclusive-or"  of  the  "Write  Protect  (data
+    23                  safety)" and "Write Protect (software)"  unit  flags
+    24                  in the unit's context.
+
+
+    25              If the command fails, report its status as the status of
+    26              this SET UNIT CHARACTERISTICS and exit.
+
+    27          4.  The SET UNIT  CHARACTERISTICS  operation  has  completed
+    28              successfully.   Copy  the  "Write Protect (data safety)"
+    29              and "Write Protect  (software)"  unit  flags  from  this
+    30              unit's  context  to  the  end  message  returned  in the
+    31              previous  step.   If  the  unit  is  Data  Safety  Write
+    32              Protected,  set the appropriate status subcodes.  Return
+    33              the resulting end message to the host or higher layer.
+
+
+    34     If the SET UNIT CHARACTERISTICS operation fails for  any  reason,
+    35     the  Bad Block Replacement layer shall issue an AVAILABLE command
+    36     with all modifiers clear to the  Disk  MSCP  layer  and  send  an
+    37     AVAILABLE  attention message to the host and higher layers.  This
+    38     is necessary to preserve the rule that the unit may not  be  left
+    39     "Unit-Online"  if  a command that might change the unit's context
+    40     fails.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-14
+           Actions Before First Modification                    11 June 1992
+
+
+     1     8.10  Actions Before First Modification
+
+     2     As  part  of  bringing  a  unit  "Unit-Online,"  the  Bad   Block
+     3     Replacement  layer  reads  and rewrites Replacement Control Table
+     4     (RCT) block 0.  The reason for this is that, when the volume  was
+     5     last  in  use, there may have been a failure after some copies of
+     6     RCT block 0 were written but before all of them were written.  If
+     7     this  situation  were to occur, a transient or inconsistent error
+     8     could cause successive reads of RCT block 0 to  return  different
+     9     data, as data was returned from different copies.  By reading and
+    10     rewriting block 0 the layer ensures that all copies  contain  the
+    11     same  data,  so that successive reads will always return the same
+    12     results.
+
+    13     However, if the unit is  Hardware  Write  Protected  when  it  is
+    14     brought "Unit-Online," this problem can still occur since block 0
+    15     cannot be rewritten.  So long as the unit remains Hardware  Write
+    16     Protected,  no  problems  can  occur,  since the volume cannot be
+    17     modified.  However, if the  unit  ceases  to  be  Hardware  Write
+    18     Protected, volume modification becomes possible.  If this occurs,
+    19     RCT block 0 shall be  read  and  rewritten  prior  to  the  first
+    20     modification.
+
+    21     When a unit is brought "Unit-Online," the Bad  Block  Replacement
+    22     layer  sets  an  internal  flag  if  the  unit  is Hardware Write
+    23     Protected and RCT block 0 cannot be rewritten.  This  flag  shall
+    24     be  checked  when  a  host or higher layer attempts to modify the
+    25     volume and, if it is set, the actions described below  performed.
+    26     There are three ways that the volume may be modified:
+
+    27          1.  Bad block replacement.  The actions below are  performed
+    28              after  checking  that  the  unit  is  not Hardware Write
+    29              Protected, but before  checking  if  the  unit  is  Data
+    30              Safety  Write Protected or modifying the RCT in any way.
+    31              That is, these actions are performed after the bad block
+    32              is   detected  but  before  the  bad  block  replacement
+    33              algorithm is initiated.
+
+    34          2.  A SET UNIT CHARACTERISTICS command.  The point at  which
+    35              the  actions below are performed is described in Section
+    36              8.9, "Actions During SET UNIT CHARACTERISTICS." 
+
+    37          3.  A host or higher layer  write  operation.   The  actions
+    38              below  are performed after checking that the unit is not
+    39              Hardware or Software  Write  Protected  but  before  any
+    40              modification is performed on the unit.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-15
+           Actions Before First Modification                    11 June 1992
+
+
+     1     The  Bad  Block  Replacement  layer  need  not   wait   until   a
+     2     modification  is  attempted.   It  may  perform the actions below
+     3     spontaneously  upon  the  unit  ceasing  to  be  Hardware   Write
+     4     Protected.  From the viewpoint of a host or higher layer, this is
+     5     no different from the actions below being performed  in  response
+     6     to an attempted bad block replacement.
+
+     7     The actions that shall be performed before the first modification
+     8     are:
+
+     9          1.  Check the internal flag discussed above.  If the flag is
+    10              clear,  proceed  with  the modification.  If the flag is
+    11              set, clear it and continue with the next step.
+
+    12          2.  Read  block  0  of  the  RCT  with  the  multicopy  read
+    13              algorithm.  If the multicopy read algorithm fails, exit.
+
+    14          3.  Check the internal flags recording the reasons, if  any,
+    15              for  the unit being Data Safety Write Protected.  If the
+    16              incomplete replacement flag is  clear,  then  clear  the
+    17              incomplete  replacement  flags  in  RCT block 0.  If the
+    18              invalid RCT flag is clear, implying  that  the  RCT  was
+    19              valid  when  the unit was brought "Unit-Online," yet the
+    20              current RCT image is  not  valid,  either  exit  with  a
+    21              "Media  Format  Error"  or  "fix"  the  invalid RCT.  An
+    22              example of "fixing" the RCT is zeroing a reserved  field
+    23              that  was  zero when the unit was brought "Unit-Online,"
+    24              but that is now nonzero.
+
+    25          4.  Write RCT block 0 with the value  read  in  step  2  and
+    26              possibly  modified  in step 3, using the multicopy write
+    27              algorithm.  If  the  multicopy  write  algorithm  fails,
+    28              exit.
+
+    29          5.  If the value just written to RCT block 0 indicates there
+    30              is  an  incomplete  bad block replacement on the volume,
+    31              attempt to  complete  it.   If  the  completion  attempt
+    32              fails, report it with an error log message and continue.
+
+    33          6.  Set the "Write Protected (data safety)" unit flag to the
+    34              "inclusive-or"   of   all  internal  flags  representing
+    35              possible causes of Data Safety Write Protection.
+
+    36          7.  Issue a SET UNIT CHARACTERISTICS  command  to  the  Disk
+    37              MSCP  layer  to  set  the  unit's Software Write Protect
+    38              status to the "inclusive-or" of the "Write Protect (data
+    39              safety)"  and  "Write  Protect (software)" unit flags in
+    40              the unit's context.  If the command fails, exit.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-16
+           Actions Before First Modification                    11 June 1992
+
+
+     1          8.  This  algorithm  has  completed.    Proceed   with   the
+     2              modification attempt that caused it to be invoked.
+
+
+     3     There are four places where this algorithm can  fail.   They  are
+     4     the  multicopy  read operation in step 2, the attempt to "fix" an
+     5     invalid RCT in step 3, the multicopy write in step 4, and the SET
+     6     UNIT CHARACTERISTICS in step 7.  If the algorithm fails at one of
+     7     these steps, the modification attempt fails with the status  code
+     8     resulting  from  the  failed  step.   Also,  the  unit  is forced
+     9     "Unit-Available" to all class drivers.   The  handling  of  other
+    10     commands  that  may  be outstanding is identical to that which is
+    11     done for a failed replacement operation.
+
+    12     Note that execution of  the  above  procedure  may  complete  the
+    13     incomplete  replacement that was causing a unit to be Data Safety
+    14     Write Protected.  This yields the possibility  that  Data  Safety
+    15     Write  Protection may disappear spontaneously, as the result of a
+    16     bad block replacement.
+
+    17     8.11  MSCP Control Message Format Changes
+
+    18     This section describes the changes  in  control  message  formats
+    19     between standard Disk MSCP and MSCP with Controller Initiated Bad
+    20     Block  Replacement.   All  messages  and  fields  that  are   not
+    21     mentioned in this section are identical to the descriptions found
+    22     in Chapter 6, "Minimal MSCP Command Set" and  if  the  controller
+    23     provides Multihost Support, Chapter 7, "Multihost Support." 
+
+    24     8.11.1  Controller Flags
+
+    25     The Controller Initiated Bad Block Replacement controller flag is
+    26     returned  set in the "controller flags" field of a SET CONTROLLER
+    27     CHARACTERISTICS command's end message only if the server supports
+    28     Controller Initiated Bad Block Replacement on all disk types that
+    29     can be connected to it.  Note  that  a  controller  is  expressly
+    30     permitted  to implement replacement on some disk types but not on
+    31     others.  In that case this flag shall  be  returned  clear.   See
+    32     also  the  description  of  this flag in Section 5.6, "Controller
+    33     Flags" for additional detail.
+
+    34     8.11.2  Unit Flags
+
+    35     The Controller Initiated  Bad  Block  Replacement  unit  flag  is
+    36     returned  set  in  the "unit flags" field of the GET UNIT STATUS,
+    37     ONLINE, and SET UNIT CHARACTERISTICS command's end messages  only
+    38     if  bad  block replacement for this unit will be performed by the
+    39     controller.   This  flag  is  returned  clear  if  the  host   is
+    40     responsible  for  bad block replacement for this unit.  (As noted
+    41     in the previous section, a controller may  implement  replacement
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-17
+           MSCP Control Message Format Changes                  11 June 1992
+
+
+     1     on  some  disk types but not on others.) See also the description
+     2     of this flag in Section 5.7, "Unit Flags" for additional detail.
+
+     3     8.11.3  Transfer Commands
+
+     4     The changes described  in  this  section  apply  to  the  ACCESS,
+     5     COMPARE HOST DATA, ERASE, READ, and WRITE commands.
+
+     6     The meaning of transfers addressed  to  the  Replacement  Control
+     7     Table  (RCT)  portion  of  the disk is substantially changed; see
+     8     Section 8.4, "Replacement Control Table Access."   For  transfers
+     9     addressed  to  the host area of the disk, the only change is that
+    10     the controller itself replaces bad blocks rather than the host.
+
+    11     Since the controller is replacing  bad  blocks,  the  "Bad  Block
+    12     Reported"  end  flag will always be clear and the contents of the
+    13     "first bad block" end message field  will  always  be  undefined.
+    14     The  "Bad  Block  Unreported"  end  flag  is set, however, if the
+    15     transfer encountered one or more bad blocks.  The specific blocks
+    16     that  were  bad and the results of their replacement are reported
+    17     only via error log messages.
+
+    18     8.11.4  ONLINE and SET UNIT CHARACTERISTICS Commands
+
+    19     Modifiers:
+
+    20         The "Ignore Media Format  Error"  modifier  is  changed  from
+    21         standard  Disk MSCP.  Note that this modifier is only allowed
+    22         on the ONLINE command.  Its revised meaning is as follows:
+
+    23         Ignore Media Format Error
+
+    24             This modifier is only allowed on the ONLINE command.   In
+    25             standard   Disk  MSCP,  this  modifier  suppresses  media
+    26             (volume)  format  validity  checking.   With   Controller
+    27             Initiated  Bad  Block  Replacement,  this  modifier  also
+    28             suppresses the Replacement  Control  Table  (RCT)  checks
+    29             normally performed when a unit is brought "Unit-Online." 
+
+    30             Normally, when a unit is brought "Unit-Online," the  MSCP
+    31             server  makes  several accesses to the unit.  In standard
+    32             Disk MSCP, these accesses  merely  verify  volume  format
+    33             validity.     With   Controller   Initiated   Bad   Block
+    34             Replacement, the MSCP server also  accesses  the  RCT  to
+    35             check  the  context  information stored there.  If any of
+    36             these  accesses  fail,  the  unit   cannot   be   brought
+    37             "Unit-Online" and a Media Format Error is returned.  This
+    38             modifier suppresses all accesses performed when  bringing
+    39             the  unit  "Unit-Online,"  thus allowing it to be brought
+    40             "Unit-Online"  when  these  accesses  would  fail.    The
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-18
+           MSCP Control Message Format Changes                  11 June 1992
+
+
+     1             penalty  for  using  this  modifier  is  that data on the
+     2             volume may be corrupted.
+
+     3             The exact checks that are  suppressed  by  this  modifier
+     4             are:
+
+     5             1.  The format validity  checks  done  by  standard  Disk
+     6                 MSCP.
+
+     7             2.  Checking for  an  incomplete  bad  block  replacement
+     8                 operation, and completing it if possible.
+
+
+     9             The unit is always brought "Unit-Online" with Data Safety
+    10             Write Protection disabled (i.e., writes allowed).
+
+    11             Setting this modifier inhibits all bad block  replacement
+    12             attempts  and may optionally provide additional access to
+    13             the RCT.
+
+    14             Note that these checks exist  to  prevent  data  loss  or
+    15             corruption.   Therefore this modifier should only be used
+    16             in exceptional circumstances, such as backing up a  known
+    17             bad  disk.   In particular, the results of any attempt to
+    18             write to the unit are undefined when  this  modifier  has
+    19             been set.
+
+    20     Status Codes:
+
+    21         The following new subcodes may be returned with the "Success"
+    22         status  code  for  the  ONLINE  and  SET UNIT CHARACTERISTICS
+    23         commands.  Remember that the subcodes are bit flags, and that
+    24         the subcode field contains the "logical-or" of all applicable
+    25         subcodes, including the subcodes  defined  in  standard  Disk
+    26         MSCP.
+
+    27         Success (subcode "Incomplete Replacement")
+
+    28             The  volume  has  a   partially   completed   bad   block
+    29             replacement operation, and the attempt to complete it did
+    30             not succeed.  The  cause  of  the  attempt's  failure  is
+    31             reported  with an error log message.  This subcode merely
+    32             reports that there is a partially  completed  replacement
+    33             on  the  volume.   Note that the unit will be Data Safety
+    34             Write Protected whenever this subcode is returned.
+
+    35         Success (subcode "Invalid RCT")
+
+    36             The volume has an invalid RCT or,  if  the  disk  uses  a
+    37             nonstandard  means  of  bad  block replacement, some data
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-19
+           MSCP Control Message Format Changes                  11 June 1992
+
+
+     1             structure  used  to  control  bad  block  replacement  is
+     2             invalid.    The  checks,  if  any,  that  the  controller
+     3             performs to verify the RCT's validity  are  specified  by
+     4             the  Digital  Storage Architecture Disk Format and/or the
+     5             controller's functional  specification.   Note  that  the
+     6             unit  will  be  Data Safety Write Protected whenever this
+     7             subcode is returned.
+
+     8         "Media  Format  Errors"  may  be  reported   for   SET   UNIT
+     9         CHARACTERISTICS,  which  could  not happen with standard Disk
+    10         MSCP.
+
+    11     8.11.5  GET UNIT STATUS Command
+
+    12     End Message Fields:
+
+    13         RCT size
+    14         copies
+
+    15             The  interpretation  and  meaning  of  these  fields   is
+    16             identical  to  standard  Disk  MSCP.  However, the values
+    17             returned  are  altered  as  described  in  Section   8.4,
+    18             "Replacement Control Table Access." 
+
+    19     Status Codes:
+
+    20         The following new subcodes may be returned with the "Success"
+    21         status code for the GET UNIT STATUS commands.  These subcodes
+    22         are identical to the new subcodes returned for the ONLINE and
+    23         SET  UNIT  CHARACTERISTICS commands; see the previous section
+    24         for their description.  Remember that the  subcodes  are  bit
+    25         flags,  and  that the subcode field contains the "logical-or"
+    26         of all applicable subcodes, including the subcodes defined in
+    27         standard Disk MSCP.
+
+    28             Success (subcode "Incomplete Replacement")
+
+    29             Success (subcode "Invalid RCT")
+
+    30     8.11.6  REPLACE Command
+
+    31     This command is  illegal  with  Controller  Initiated  Bad  Block
+    32     Replacement.   As  its  sole use is to allow hosts to perform bad
+    33     block replacement, it is not needed.  This command, if issued  to
+    34     a   unit   on  which  the  controller  is  performing  bad  block
+    35     replacement is treated as an "Invalid Command" ("invalid  opcode"
+    36     protocol  error.   See  "Invalid  Command" status in Section 5.5,
+    37     "Status Codes").
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           CONTROLLER INITIATED BAD BLOCK REPLACEMENT              Page 8-20
+           Host Support For Controller Initiated Bad Block  ... 11 June 1992
+
+
+     1     8.12  Host Support For Controller Initiated Bad Block Replacement
+
+     2     The only specialized support that host class drivers must provide
+     3     for Controller Initiated Bad Block Replacement is at the time the
+     4     host brings a unit "Unit-Online."  That is, these  actions  shall
+     5     be performed immediately after the ONLINE command completes.  The
+     6     specific actions are:
+
+     7          1.  Check the "Controller Initiated Bad  Block  Replacement"
+     8              unit  flag.   If  this  flag  is clear, the class driver
+     9              shall examine the Replacement Control Table (RCT) for an
+    10              incomplete   replacement  and  complete  it  if  one  is
+    11              present.  If this flag is set, the  class  driver  shall
+    12              skip this step, as the server has already performed it.
+
+    13          2.  If either of the "Incomplete  Replacement"  or  "Invalid
+    14              RCT"  subcode  flags  is  set, report the problem to the
+    15              user or  take  whatever  other  action  is  dictated  by
+    16              operating    system   policy.    For   the   "Incomplete
+    17              Replacement" flag, this might be a  request  to  remount
+    18              the  unit  write  enabled so that the replacement can be
+    19              completed.  For the "Invalid RCT" flag, this might be  a
+    20              warning that the volume has been corrupted.
+
+
+    21     In all other cases,  the  Bad  Block  Replacement  layer  reports
+    22     things in a manner that is transparent to class drivers.  It will
+    23     never set the "Bad Block Reported" end flag, so the class  driver
+    24     will  never  attempt a replacement.  If the class driver attempts
+    25     to read the RCT, the reported geometry is such that the multicopy
+    26     read algorithm will work correctly.
+
+    27     Aside from the above, a class driver that only  supports  servers
+    28     with   Controller   Initiated  Bad  Block  Replacement  need  not
+    29     implement the Host Initiated  Bad  Block  Replacement,  multicopy
+    30     read,  and  multicopy write algorithms.  It also need never check
+    31     the RCT for incomplete replacements.  Such a class driver should,
+    32     however,  check  the "Controller Initiated Bad Block Replacement"
+    33     unit flag whenever it brings a unit "Unit-Online."  If  the  flag
+    34     is  clear,  then that unit does not have Controller Initiated Bad
+    35     Block Replacement, so it cannot be supported by the class driver.
+    36     The class driver should immediately issue an AVAILABLE command to
+    37     the unit and refuse to access it.
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-1
+                                                                                          11 June 1992
+
+
+     1                                             APPENDIX A
+
+     2                                OPCODE, FLAG, AND OFFSET DEFINITIONS
+
+
+     3                                Table A-1:  Control Message Opcodes
+
+     4     +--------------------+--------------------------+-----------------------------------------+
+     5     |    Opcode Value    |                          |                                         |
+     6     |    (see Note 1)    |   Preferred Mnemonics    |                                         |
+     7     |------+------+------+--------+-----------------+     Control Message Type                |
+     8     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+     9     +------+------+------+--------+-----------------+-----------------------------------------+
+    10     |   0  |  00  |  00  |    -   |        -        | reserved -- must not be assigned        |
+    11     +------+------+------+--------+-----------------+-----------------------------------------+
+    12     | Standard Command Messages                     |                                         |
+    13     |   1  |  01  |  01  | OP.ABO | MSCP$K_OP_ABORT | ABORT                                   |
+    14     |  16  |  20  |  10  | OP.ACC | MSCP$K_OP_ACCES | ACCESS                                  |
+    15     |   8  |  10  |  08  | OP.AVL | MSCP$K_OP_AVAIL | AVAILABLE                               |
+    16     |  32  |  40  |  20  | OP.CMP | MSCP$K_OP_COMP  | COMPARE HOST DATA                       |
+    17     |  11  |  13  |  0B  | OP.DAP | MSCP$K_OP_DTACP | DETERMINE ACCESS PATHS                  |
+    18     |  13  |  15  |  0D  | OP.DCD | MSCP$K_OP_DCD   | DISK COPY DATA                          |
+    19     |   6  |   6  |   6  | OP.DSP | MSCP$K_OP_DISPL | DISPLAY                                 |
+    20     |  18  |  22  |  12  | OP.ERS | MSCP$K_OP_ERASE | ERASE                                   |
+    21     |  24  |  30  |  18  | OP.FMT | MSCP$K_OP_FMT   | FORMAT                                  |
+    22     |  17  |  21  |  11  |    -   |        -        | reserved (see Note 4)                   |
+    23     |  19  |  23  |  13  |    -   |        -        | reserved (see Note 4)                   |
+    24     |   2  |  02  |  02  | OP.GCS | MSCP$K_OP_GTCMD | GET COMMAND STATUS                      |
+    25     |   3  |  03  |  03  | OP.GUS | MSCP$K_OP_GTUNT | GET UNIT STATUS                         |
+    26     |   9  |  11  |  09  | OP.ONL | MSCP$K_OP_ONLIN | ONLINE                                  |
+    27     |  33  |  41  |  21  | OP.RD  | MSCP$K_OP_READ  | READ                                    |
+    28     |  20  |  24  |  14  | OP.RPL | MSCP$K_OP_REPLC | REPLACE                                 |
+    29     |   4  |  04  |  04  | OP.SCC | MSCP$K_OP_STCON | SET CONTROLLER CHARACTERISTICS          |
+    30     |  10  |  12  |  0A  | OP.SUC | MSCP$K_OP_STUNT | SET UNIT CHARACTERISTICS                |
+    31     |  48  |  60  |  30  | OP.TEC | MSCP$K_OP_TERCO | TERMINATE CLASS DRIVER/SERVER CONNECTION|
+    32     |  34  |  42  |  22  | OP.WR  | MSCP$K_OP_WRITE | WRITE                                   |
+    33     |  25  |  31  |  19  | OP.WHM | MSCP$K_OP_WRHIM | WRITE HISTORY MANAGEMENT                |
+    34     +------+------+------+--------+-----------------+-----------------------------------------+
+    35     | Optional Command Messages:                                                              |
+    36     |   5  |  05  |  05  | OP.ANM | MSCP$K_OP_ACCNM | ACCESS NONVOLATILE MEMORY               |
+    37     +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-2
+                                                                                          11 June 1992
+
+
+
+     1                             Table A-1: Control Message Opcodes (cont.)
+
+     2     +--------------------+--------------------------+-----------------------------------------+
+     3     |    Opcode Value    |                          |                                         |
+     4     |    (see Note 1)    |   Preferred Mnemonics    |                                         |
+     5     |------+------+------+--------+-----------------+     Control Message Type                |
+     6     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+     7     +------+------+------+--------+-----------------+-----------------------------------------+
+     8     | Implementation Specific Command Messages (see Note 2):                                  |
+     9     |  40  |  50  |  28  | OP.SP1 | MSCP$K_OP_SPEC1 | # 1                                     |
+    10     |  41  |  51  |  29  | OP.SP2 | MSCP$K_OP_SPEC2 | # 2                                     |
+    11     |  42  |  52  |  2A  | OP.SP3 | MSCP$K_OP_SPEC3 | # 3                                     |
+    12     |  43  |  53  |  2B  | OP.SP4 | MSCP$K_OP_SPEC4 | # 4                                     |
+    13     |  44  |  54  |  2C  | OP.SP5 | MSCP$K_OP_SPEC5 | # 5                                     |
+    14     |  45  |  55  |  2D  | OP.SP6 | MSCP$K_OP_SPEC6 | # 6                                     |
+    15     |  46  |  56  |  2E  | OP.SP7 | MSCP$K_OP_SPEC7 | # 7                                     |
+    16     |  47  |  57  |  2F  | OP.SP8 | MSCP$K_OP_SPEC8 | # 8                                     |
+    17     +------+------+------+--------+-----------------+-----------------------------------------+
+    18     | End Messages (see Note 3):  |                 |                                         |
+    19     | 128  | 200  |  80  | OP.END | MSCP$K_OP_END   | End message flag                        |
+    20     +------+------+------+--------+-----------------+-----------------------------------------+
+    21     | Attention Messages:         |                 |                                         |
+    22     |  64  | 100  |  40  | OP.AVA | MSCP$K_OP_AVATN | AVAILABLE                               |
+    23     |  65  | 101  |  41  | OP.DUP | MSCP$K_OP_DUPUN | DUPLICATE UNIT NUMBER                   |
+    24     |  66  | 102  |  42  | OP.ACP | MSCP$K_OP_ACPTH | ACCESS PATH                             |
+    25     +------+------+------+--------+-----------------+-----------------------------------------+
+    26     | Notes: 1. Message  opcode  bits  6 and 7 indicate the type of message (command, end, or |
+    27     |           attention).                                                                   |
+    28     |                                                                                         |
+    29     |        2. The implementation specific command opcodes may be used by manufacturing test |
+    30     |           equipment to verify controller operation.  They shall not be issued by normal |
+    31     |           host  software,  or  when  a  host is running anything other than stand-alone |
+    32     |           diagnostics. The  operation  of these commands when issued under anything but |
+    33     |           the expected manufacturing and/or diagnostic environment is UNDEFINED.        |
+    34     |                                                                                         |
+    35     |        3. End message endcodes are formed by adding the end message flag to the command |
+    36     |           opcode.  For  example,  a  READ  command's end message contains (using 16 bit |
+    37     |           mnemonics)  the  value  OP.RD+OP.END in its endcode field. As described under |
+    38     |           "Invalid  Command" status  in Section 5.5, "Status Codes" an MSCP protocol    |
+    39     |           violation  produces  an  Invalid  Command end message which contains just the |
+    40     |           end message flag (OP.END) in its endcode field.                               |
+    41     |                                                                                         |
+    42     |        4. Opcodes 17 and 19 (decimal) were originally reserved as a place holder for    |
+    43     |           use in Volume Shadowing and Data Caching. Both Host Based Volume Shadowing    |
+    44     |           and Caching Support have been defined in MSCP, however neither opcode was     |
+    45     |           used.  In order to ensure compatibility, controllers shall treat these        |
+    46     |           opcodes as a no-operation (see Section 6.1.1).                                |
+    47     +-----------------------------------------------------------------------------------------+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-3
+                                                                                          11 June 1992
+
+
+     1                                   Table A-2:  Command Modifiers
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+     Command Modifier                   |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (see Note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     | Generic Command Modifier:    |                 |                                        |
+     9     |  13  | 20000 | 2000 | MD.CSE | MSCP$x_MD_CLSEX | Clear Serious Exception (see  Note 3)  |
+    10     +------+-------+------+--------+-----------------+----------------------------------------+
+    11     | ACCESS, COMPARE HOST DATA, ERASE, READ, REPLACE, and WRITE Command Modifier:            |
+    12     |  15  |100000 | 8000 | MD.EXP | MSCP$x_MD_EXPRS | Express Request                        |
+    13     +------+-------+------+--------+-----------------+----------------------------------------+
+    14     | ACCESS, COMPARE HOST DATA, ERASE, READ, and WRITE Command Modifiers:                    |
+    15     |   8  |   400 |  100 | MD.SER | MSCP$x_MD_SEREC | Suppress Error Recovery                |
+    16     +------+-------+------+--------+-----------------+----------------------------------------+
+    17     | ACCESS, COMPARE HOST DATA, READ, and WRITE Command Modifier:                            |
+    18     |   9  |  1000 |  200 | MD.SEC | MSCP$x_MD_SECOR | Suppress Error Correction              |
+    19     +------+-------+------+--------+-----------------+----------------------------------------+
+    20     | AVAILABLE Command Modifiers:                   |                                        |
+    21     |   4  |    20 |   10 |   -    |        -        | "Bundled Shadowing" (see Note 5)       |
+    22     |   1  |     2 |    2 | MD.ALL | MSCP$x_MD_ALLCD | All Class Drivers (see Note 4)         |
+    23     |   0  |     1 |    1 | MD.SPD | MSCP$x_MD_SPNDW | Spin-down                              |
+    24     +------+-------+------+--------+-----------------+----------------------------------------+
+    25     | AVAILABLE, ONLINE, and SET UNIT CHARACTERISTICS Command Modifier:                       |
+    26     |   5  |    40 |   20 | MD.EXC | MSCP$x_MD_EXCAC | Exclusive Access (see Note 4)          |
+    27     +------+-------+------+--------+-----------------+----------------------------------------+
+    28     | DISPLAY Command modifier:    |                 |                                        |
+    29     |   1  |     2 |    2 | MD.MBI | MSCP$x_MD_MBI   | Must Be Implemented                    |
+    30     +------+-------+------+--------+-----------------+----------------------------------------+
+    31     | ERASE and WRITE Command Modifiers:                                                      |
+    32     |  12  | 10000 | 1000 | MD.ERR | MSCP$x_MD_ERROR | Force Error                            |
+    33     +------+-------+------+--------+-----------------+----------------------------------------+
+    34     | GET UNIT STATUS Command Modifier:              |                                        |
+    35     |   0  |     1 |    1 | MD.NXU | MSCP$x_MD_NXUNT | Next Unit                              |
+    36     +------+-------+------+--------+-----------------+----------------------------------------+
+    37     | ONLINE Command Modifiers:    |                 |                                        |
+    38     |   1  |     2 |    2 | MD.IMF | MSCP$x_MD_IGNMF | Ignore Media Format Error              |
+    39     |   0  |     1 |    1 | MD.RIP | MSCP$x_MD_RIP   | Allow Self Destruction                 |
+    40     +------+-------+------+--------+-----------------+----------------------------------------+
+    41     | ONLINE and SET UNIT CHARACTERISTICS Command Modifiers:                                  |
+    42     |   4  |    20 |   10 |   -    |        -        | "Bundled Shadowing" (see Note 5)       |
+    43     |   2  |     4 |    4 | MD.SWP | MSCP$x_MD_STWRP | Enable Set Write Protect               |
+    44     +------+-------+------+--------+-----------------+----------------------------------------+
+    45  |  | READ, and WRITE Command Modifier:                                                       |
+    46     |  14  | 40000 | 4000 | MD.CMP | MSCP$x_MD_COMP  | Compare                                |
+    47     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-4
+                                                                                          11 June 1992
+
+
+
+     1                                Table A-2: Command Modifiers (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+     Command Modifier                   |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (see Note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     | SET CONTROLLER CHARACTERISTICS Command Modifier:                                        |
+     9     |   0  |     1 |    1 | MD.CRN | MSCP$x_MD_CRNPR | Connection Reference Number Present    |
+    10     +------+-------+------+--------+-----------------+----------------------------------------+
+    11     | ERASE, WRITE, and DISK COPY DATA Command Modifiers:                                     |
+    12     |   3  |    10 |    8 | MD.HIL | MSCP$x_MD_HISLO | History Log                            |
+    13     |   7  |   200 |   80 | MD.REU | MSCP$x_MD_REUSE | Reuse Entry                            |
+    14     +------+-------+------+--------+-----------------+----------------------------------------+
+    15     | DISK COPY DATA Command Modifiers:                                                       |
+    16     |   0  |     1 |    1 | MD.LSU | MSCP$x_MD_LOCSU | Local Source Unit                      |
+    17     |   1  |     2 |    2 | MD.ECP | MSCP$x_MD_ESTCP | Establish Communications Paths         |
+    18     |   2  |     4 |    4 | MD.RCP | MSCP$x_MD_RETCP | Retain Communications Paths            |
+    19     +------+-------+------+--------+-----------------+----------------------------------------+
+    20     | REPLACE Command Modifier:    |                 |                                        |
+    21     |   0  |     1 |    1 | MD.PRI | MSCP$x_MD_PRIMR | Primary Replacement Block              |
+    22     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-5
+                                                                                          11 June 1992
+
+
+
+     1                                Table A-2: Command Modifiers (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+     Command Modifier                   |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (see Note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8  |  | WRITE Command Modifier:      |                 |                                        |
+     9  |  |   4  |   20  |  10  | MD.SWL | MSCP$x_MD_SUPWL | Supplement Write Log                   |
+    10  |  +------+-------+------+--------+-----------------+----------------------------------------+
+    11     | Notes: 1. The  "x"  in  a  32  bit mnemonic for a bit flag will be "V" if the symbol is |
+    12     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    13     |                                                                                         |
+    14     |        2. The  command modifiers are listed above according to the command(s) for which |
+    15     |           they  are  allowable.  All  modifiers  that  are not explicitly allowed for a |
+    16     |           command are reserved and shall be treated in accordance with the requirements |
+    17     |           for  reserved  fields  described  in  Section  5.2,  "Reserved  and Undefined |
+    18     |           Fields."                                                                      |
+    19     |                                                                                         |
+    20     |        3. For  disk controllers the Clear Serious Exception modifier is allowed for all |
+    21     |           commands except ABORT, ACCESS NONVOLATILE MEMORY, DETERMINE ACCESS PATHS, GET |
+    22     |           COMMAND STATUS, and SET CONTROLLER  CHARACTERISTICS.   That  modifier  has no |
+    23     |           function  for  disk  class devices.  Although allowed, disk controllers shall |
+    24     |           ignore  its  setting.  Refer  to  the  Tape  Mass  Storage  Control  Protocol |
+    25     |           Specification (TMSCP) for tape class device use details.                      |
+    26     |                                                                                         |
+    27     |        4. The modifiers that reference this note have meaning only for controllers that |
+    28     |           provide  Multihost  Support.  Controllers  which  do  not  shall ignore those |
+    29     |           modifiers.                                                                    |
+    30     |                                                                                         |
+    31     |        5. The modifiers that reference this note have meaning only for VMS and for HSC  |
+    32     |           controllers that provided "Bundled Shadowing" support, See Appendix E,        |
+    33     |           "Waivers and Exceptions", Section E.3.1.1.7, "Bundled Shadowing Definitions". |
+    34     +-----------------------------------------------------------------------------------------+
+
+
+    35                                   Table A-3:  End Message Flags
+
+    36     +------+--------------+--------------------------+----------------------------------------+
+    37     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    38     |Number+-------+------+--------+-----------------+     End Message Flag                   |
+    39     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+    40     |      |       |      |        |   (see Note 1)  |                                        |
+    41     +------+-------+------+--------+-----------------+----------------------------------------+
+    42     |   7  |   200 |   80 | EF.BBR | MSCP$x_EF_BBLKR | Bad Block Reported                     |
+    43     +------+-------+------+--------+-----------------+----------------------------------------+
+    44     |   6  |   100 |   40 | EF.BBU | MSCP$x_EF_BBLKU | Bad Block Unreported                   |
+    45     +------+-------+------+--------+-----------------+----------------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-6
+                                                                                          11 June 1992
+
+
+
+     1                                Table A-3: End Message Flags (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+     End Message Flag                   |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (see Note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     |   5  |    40 |   20 | EF.LOG | MSCP$x_EF_ERLOG | Error Log Generated                    |
+     9     +------+-------+------+--------+-----------------+----------------------------------------+
+    10     |   4  |    20 |   10 | EF.SEX | MSCP$x_EF_SEREX | Serious Exception (see Note 2)         |
+    11     +------+-------+------+--------+-----------------+----------------------------------------+
+    12     |   3  |    10 |    8 | EF.ALF | MSCP$x_EF_ALLOF | Allocation Failure                     |
+    13     +------+-------+------+--------+-----------------+----------------------------------------+
+    14     |   2  |     4 |    4 | EF.HIL | MSCP$x_EF_HISLO | History Logged                         |
+    15     +------+-------+------+--------+-----------------+----------------------------------------+
+    16     |   0  |     1 |    1 | EF.CPR | MSCP$x_EF_CPRET | Communication Paths Retained           |
+    17     +------+-------+------+--------+-----------------+----------------------------------------+
+    18     | Notes: 1. The  "x"  in  a  32  bit mnemonic for a bit flag will be "V" if the symbol is |
+    19     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    20     |                                                                                         |
+    21     |        2. The  Serious  Exception  end  message  flag  is only applicable to tape class |
+    22     |           devices.  It  will  never be returned by disk controllers.  Refer to the Tape |
+    23     |           Mass  Storage  Control  Protocol  Specification (TMSCP) for tape class device |
+    24     |           usage details.                                                                |
+    25     +-----------------------------------------------------------------------------------------+
+
+
+    26                                    Table A-4:  Controller Flags
+
+    27     +------+--------------+--------------------------+----------------------------------------+
+    28     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    29     |Number+-------+------+--------------------------+     Controller Flag                    |
+    30     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+    31     |      |       |      |        |   (see Note 1)  |                                        |
+    32     +------+-------+------+--------+-----------------+----------------------------------------+
+    33     |  15  |100000 | 8000 | CF.RPL | MSCP$x_CF_REPLC | Controller    Initiated    Bad   Block |
+    34     |      |       |      |        |                 | Replacement                            |
+    35     +------+-------+------+--------+-----------------+----------------------------------------+
+    36     |  10  |  2000 |  400 | CF.RDO | MSCP$x_CF_RDO   | Restricted DISK COPY DATA Operations   |
+    37     +------+-------+------+--------+-----------------+----------------------------------------+
+    38     |   9  |  1000 |  200 | CF.WHL | MSCP$x_CF_WHL   | Write History Logging Support          |
+    39     +------+-------+------+--------+-----------------+----------------------------------------+
+    40     |   8  |   400 |  100 | CF.RDC | MSCP$x_CF_RDCD  | Remote Disk Copy Data                  |
+    41     +------+-------+------+--------+-----------------+----------------------------------------+
+    42     |   7  |   200 |   80 | CF.ATN | MSCP$x_CF_ATTN  | Enable Attention Messages              |
+    43     +------+-------+------+--------+-----------------+----------------------------------------+
+    44     |   6  |   100 |   40 | CF.MSC | MSCP$x_CF_MISC  | Enable Miscellaneous Error Log Messages|
+    45     +------+-------+------+--------+-----------------+----------------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-7
+                                                                                          11 June 1992
+
+
+
+     1                                 Table A-4: Controller Flags (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------------------------+     Controller Flag                    |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (see Note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     |   5  |    40 |   20 | CF.OTH | MSCP$x_CF_OTHER | Enable Other Host's Error Log Messages |
+     9     +------+-------+------+--------+-----------------+----------------------------------------+
+    10     |   4  |    20 |   10 | CF.THS | MSCP$x_CF_THIS  | Enable This Host's Error Log Messages  |
+    11     +------+-------+------+--------+-----------------+----------------------------------------+
+    12     |   3  |    10 |    8 | CF.LDC | MSCP$x_CF_LDCD  | Local Disk Copy Data                   |
+    13     +------+-------+------+--------+-----------------+----------------------------------------+
+    14     |   2  |     4 |    4 | CF.MLH | MSCP$x_CF_MLTHS | Multihost Support                      |
+    15     +------+-------+------+--------+-----------------+----------------------------------------+
+    16     |   1  |     2 |    2 |   -    |        -        | "Bundled Shadowing" (see Note 2)       |
+    17     +------+-------+------+--------+-----------------+----------------------------------------+
+    18     |   0  |     1 |    1 | CF.576 | MSCP$x_CF_576   | 576 Byte Sectors                       |
+    19     +------+-------+------+--------+-----------------+----------------------------------------+
+    20     | Notes: 1. The  "x"  in  a  32  bit mnemonic for a bit flag will be "V" if the symbol is |
+    21     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    22     |                                                                                         |
+    23     |        2. The modifiers that reference this note have meaning only for VMS and for HSC  |
+    24     |           controllers that provided "Bundled Shadowing" support, See Appendix E,        |
+    25     |           "Waivers and Exceptions", Section E.3.1.1.7, "Bundled Shadowing Definitions". |
+    26     +-----------------------------------------------------------------------------------------+
+
+
+    27                                       Table A-5:  Unit Flags
+
+    28     +------+--------------+--------------------------+----------------------------------------+
+    29     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    30     |Number+-------+------+--------+-----------------+     Unit Flag                          |
+    31     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+    32     |      |       |      |        |   (see Note 1)  |                                        |
+    33     +------+-------+------+--------+-----------------+----------------------------------------+
+    34     |  15  |100000 | 8000 | UF.RPL | MSCP$x_UF_REPLC | Controller    Initiated    Bad   Block |
+    35     |      |       |      |        |                 | Replacement                            |
+    36     +------+-------+------+--------+-----------------+----------------------------------------+
+    37     |  14  | 40000 | 4000 |   -    |        -        | "Bundled Shadowing" (see Note 2)       |
+    38     +------+-------+------+--------+-----------------+----------------------------------------+
+    39     |  13  | 20000 | 2000 | UF.WPH | MSCP$x_UF_WRTPH | Write Protect (hardware)               |
+    40     +------+-------+------+--------+-----------------+----------------------------------------+
+    41     |  12  | 10000 | 1000 | UF.WPS | MSCP$x_UF_WRTPS | Write Protect (software)               |
+    42     +------+-------+------+--------+-----------------+----------------------------------------+
+    43     |  10  |  2000 |  400 | UF.EXA | MSCP$x_UF_EXACC | Exclusive Access                       |
+    44     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-8
+                                                                                          11 June 1992
+
+
+
+     1                                    Table A-5: Unit Flags (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+     Unit Flag                          |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (see Note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     |   9  |  1000 |  200 |   -    |        -        | "Bundled Shadowing" (see Note 2)       |
+     9     +------+-------+------+--------+-----------------+----------------------------------------+
+    10     |   8  |   400 |  100 | UF.WPD | MSCP$x_UF_WRTPD | Write Protect (data safety)            |
+    11     +------+-------+------+--------+-----------------+----------------------------------------+
+    12     |   7  |   200 |   80 | UF.RMV | MSCP$x_UF_RMVBL | Removable Media                        |
+    13     +------+-------+------+--------+-----------------+----------------------------------------+
+    14     |   3  |    10 |    8 | UF.WHL | MSCP$x_UF_WHL   | Write History Logging Support          |
+    15     +------+-------+------+--------+-----------------+----------------------------------------+
+    16     |   2  |     4 |    4 | UF.576 | MSCP$x_UF_576   | 576 Byte Sectors                       |
+    17     +------+-------+------+--------+-----------------+----------------------------------------+
+    18     |   1  |     2 |    2 | UF.CMW | MSCP$x_UF_CMPWR | Compare Writes                         |
+    19     +------+-------+------+--------+-----------------+----------------------------------------+
+    20     |   0  |     1 |    1 | UF.CMR | MSCP$x_UF_CMPRD | Compare Reads                          |
+    21     +------+-------+------+--------+-----------------+----------------------------------------+
+    22     | Notes: 1. The  "x"  in  a  32  bit mnemonic for a bit flag will be "V" if the symbol is |
+    23     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    24     |                                                                                         |
+    25     |        2. The modifiers that reference this note have meaning only for VMS and for HSC  |
+    26     |           controllers that provided "Bundled Shadowing" support, See Appendix E,        |
+    27     |           "Waivers and Exceptions", Section E.3.1.1.7, "Bundled Shadowing Definitions". |
+    28     +-----------------------------------------------------------------------------------------+
+
+
+    29                                Table A-6:  Command Message Offsets
+
+    30     +--------------------+--------------------------+-------+---------------------------------+
+    31     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+    32     +------+------+------+--------+-----------------+ Size  | Field Description               |
+    33     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+    34     +------+------+------+--------+-----------------+-------+---------------------------------+
+    35     | Generic Command Message Offsets:              |       |                                 |
+    36     |   0  |   0  |   0  | P.CRF  | MSCP$L_CMD_REF  |   4   | Command reference number        |
+    37     |   4  |   4  |   4  | P.UNIT | MSCP$W_UNIT     |   2   | Unit number                     |
+    38     |   6  |   6  |   6  |        |                 |   2   | Reserved                        |
+    39     |   8  |  10  |   8  | P.OPCD | MSCP$B_OPCODE   |   1   | Opcode                          |
+    40     |   9  |  11  |   9  | P.CAA  | MSCP$B_CAA      |   1   | Cache Access Attributes         |
+    41     |  10  |  12  |   A  | P.MOD  | MSCP$W_MODIFIER |   2   | Modifiers                       |
+    42     |  12  |  14  |   C  | P.BCNT | MSCP$L_BYTE_CNT |   4   | Byte count                      |
+    43     |  16  |  20  |  10  | P.BUFF | MSCP$B_BUFFER   |  12   | Buffer descriptor               |
+    44     |  28  |  34  |  1C  | P.LBN  | MSCP$L_LBN      |   4   | Logical Block Number            |
+    45     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                               Page A-9
+                                                                                          11 June 1992
+
+
+
+     1                             Table A-6: Command Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | ABORT and GET COMMAND STATUS Command Message Offsets: |                                 |
+     8     |  12  |  14  |   C  | P.OTRF | MSCP$L_OUT_REF  |   4   | Outstanding reference number    |
+     9     +------+------+------+--------+-----------------+-------+---------------------------------+
+    10     | ACCESS NONVOLATILE MEMORY Command Message Offsets:                                      |
+    11     |  12  |  14  |   C  |        |                 |   4   | Reserved                        |
+    12     |  16  |  20  |  10  | P.ANOF | MSCP$L_ANM_OFFS |   4   | Offset                          |
+    13     |  20  |  24  |  14  | P.ANOP | MSCP$W_ANM_OPER |   2   | Operation (see Table A-12)      |
+    14     |  22  |  26  |  16  | P.ANDL | MSCP$W_ANM_DLGH |   2   | Data length                     |
+    15     |  24  |  30  |  18  | P.ANMD | MSCP$T_ANM_MEMD |   *   | Memory data                     |
+    16     +------+------+------+--------+-----------------+-------+---------------------------------+
+    17     | DISPLAY Command Message Offsets:              |       |                                 |
+    18     |  12  |  14  |  0C  | P.DITM | MSCP$W_DSP_ITEM |   2   | Item                            |
+    19     |  14  |  16  |  0E  | P.DMOD | MSCP$W_DSP_MODE |   2   | Mode                            |
+    20     |  16  |  20  |  10  | P.DTXT | MSCP$Z_DSP_TEXT |   *   | Display Text                    |
+    21     +------+------+------+--------+-----------------+-------+---------------------------------+
+    22     | ONLINE and SET UNIT CHARACTERISTICS Command Message Offsets:                            |
+    23     |  12  |  14  |   C  |        |                 |   2   | Reserved                        |
+    24     |  14  |  16  |   E  | P.UNFL | MSCP$W_UNT_FLGS |   2   | Unit flags                      |
+    25     |  16  |  20  |  10  |        |                 |  12   | Reserved                        |
+    26     |  20  |  24  |  14  |   -    |        -        |   4   | "Bundled Shadowing" (see note 2)|
+    27     |  24  |  30  |  18  |   -    |        -        |   2   | "Bundled Shadowing" (see note 2)|
+    28     |  28  |  34  |  1C  | P.DVPM | MSCP$L_DEV_PARM |   4   | Device dependent parameters     |
+    29     |  32  |  40  |  20  |   -    |        -        |   2   | "Bundled Shadowing" (see note 2)|
+    30     |  34  |  42  |  22  |   -    |        -        |   2   | "Bundled Shadowing" (see note 2)|
+    31     +------+------+------+--------+-----------------+-------+---------------------------------+
+    32     | REPLACE Command Message Offsets:              |       |                                 |
+    33     |  12  |  14  |   C  | P.RBN  | MSCP$L_RBN      |   4   | Replacement block number        |
+    34     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-10
+                                                                                          11 June 1992
+
+
+
+     1                             Table A-6: Command Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | DISK COPY DATA Command Message Offsets:                                                 |
+     8     |  12  |  14  |  0C  | P.LBC  | MSCP$L_LBCOUNT  |   4   | Logical block count             |
+     9     |  16  |  20  |  10  | P.SUN  | MSCP$W_SRC_UNUM |   2   | Source unit number              |
+    10     |  18  |  22  |  12  |        |                 |   2   | Reserved                        |
+    11     |  20  |  24  |  14  | P.SUI  | MSCP$Q_SRC_UID  |   8   | Source unit identifier          |
+    12     |  28  |  34  |  1C  | P.DBN  | MSCP$L_DEST_LBN |   4   | Destination LBN                 |
+    13     |  32  |  40  |  20  | P.HRN  | MSCP$W_HRN      |   2   | HRN or Entloc                   |
+    14     |  34  |  42  |  22  | P.EID  | MSCP$W_ENTRY_ID |   2   | Entry ID                        |
+    15     |  36  |  44  |  24  |        |                 |   4   | Reserved                        |
+    16     |  40  |  50  |  28  | P.SLBN | MSCP$L_SRC_LBN  |   4   | Source LBN                      |
+    17     |  44  |  54  |  2C  | P.PAD  | MSCP$Q_PORT_ADR |   8   | Source unit's subsystem port    |
+    18     |      |      |      |        |                 |       | address                         |
+    19     |  52  |  64  |  34  | P.SAD  | MSCP$Q_SYS_ADR  |   8   | Source unit's subsystem system  |
+    20     |      |      |      |        |                 |       | address                         |
+    21     +------+------+------+--------+-----------------+-------+---------------------------------+
+    22     | TERMINATE CLASS DRIVER/SERVER CONNECTION Command Message Offsets:                       |
+    23     |  12  |  14  |  0C  |                          |  24   | Reserved                        |
+    24     +------+------+------+--------+-----------------+-------+---------------------------------+
+    25     | WRITE HISTORY MANAGEMENT Command Message Offsets:                                       |
+    26     |  16  |  20  |  10  | P.HBD  | MSCP$B_WRHIS_BD |  12   | Write history buffer descriptor |
+    27     |  28  |  34  |  1C  | P.OPER | MSCP$W_OPERAT   |   2   | Operation                       |
+    28     |  30  |  36  |  1E  | P.OFST | MSCP$W_OFFSET   |   2   | Offset                          |
+    29     +------+------+------+--------+-----------------+-------+---------------------------------+
+    30     | TERMINATE CLASS DRIVER/SERVER CONNECTION and SET CONTROLLER CHARACTERISTICS Command     |
+    31     | Message Offset:                                                                         |
+    32     |  32  |  40  |  20  | P.CRN  | MSCP$W_CONN_REF |   2   | Connection reference number     |
+    33     +------+------+------+--------+-----------------+-------+---------------------------------+
+    34     | DISK COPY DATA, WRITE HISTORY MANAGEMENT, ERASE, and WRITE Command Message Offsets:     |
+    35     |  32  |  40  |  20  | P.HRN  | MSCP$W_HRN      |   2   | HRN or Entry Locator            |
+    36     |  34  |  42  |  22  | P.EID  | MSCP$W_ENTRY_ID |   2   | Entry id                        |
+    37     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-11
+                                                                                          11 June 1992
+
+
+
+     1                             Table A-6: Command Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | SET CONTROLLER CHARACTERISTICS Command Message Offsets:                                 |
+     8     |   4  |   4  |   4  | P.ALLC | MSCP$B_CNT_ALCS |   1   | Allocation Class                |
+     9     |                                                            (Multihost only)             |
+    10     |  12  |  14  |   C  | P.VRSN | MSCP$W_VERSION  |   2   | MSCP version                    |
+    11     |  14  |  16  |   E  | P.CNTF | MSCP$W_CNT_FLGS |   2   | Controller flags                |
+    12     |  16  |  20  |  10  | P.HTMO | MSCP$W_HST_TMO  |   2   | Host timeout                    |
+    13     |  18  |  22  |  12  |        |                 |   2   | Reserved                        |
+    14     |  20  |  24  |  14  | P.TIME | MSCP$Q_TIME     |   8   | Quad-word time and date         |
+    15     +------+------+------+--------+-----------------+-------+---------------------------------+
+    16     | Notes: 1. An asterisk (*) in the field size indicates that the size varies.             |
+    17     |                                                                                         |
+    18     |        2. The modifiers that reference this note have meaning only for VMS and for HSC  |
+    19     |           controllers that provided "Bundled Shadowing" support, See Appendix E,        |
+    20     |           "Waivers and Exceptions", Section E.3.1.1.7, "Bundled Shadowing Definitions". |
+    21     +-----------------------------------------------------------------------------------------+
+             
+
+    22                           Table A-7:  End and Attention Message Offsets
+
+    23     +--------------------+--------------------------+-------+---------------------------------+
+    24     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+    25     +------+------+------+--------+-----------------+ Size  | Field Description               |
+    26     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+    27     +------+------+------+--------+-----------------+-------+---------------------------------+
+    28     | Generic End Message Offsets:                  |       |                                 |
+    29     |   0  |   0  |   0  | P.CRF  | MSCP$L_CMD_REF  |   4   | Command reference number        |
+    30     |   4  |   4  |   4  | P.UNIT | MSCP$W_UNIT     |   2   | Unit number                     |
+    31     |   6  |   6  |   6  | P.SEQ  | MSCP$W_SEQ_NUM  |   2   | Sequence number (LAST error log)|
+    32     |   8  |  10  |   8  | P.OPCD | MSCP$B_OPCODE   |   1   | Opcode (also called endcode)    |
+    33     |   9  |  11  |   9  | P.FLGS | MSCP$B_FLAGS    |   1   | End message flags               |
+    34     |  10  |  12  |   A  | P.STS  | MSCP$W_STATUS   |   2   | Status                          |
+    35     |  12  |  14  |   C  | P.BCNT | MSCP$L_BYTE_CNT |   4   | Byte count                      |
+    36     |  16  |  20  |  10  |        |                 |  12   | Reserved                        |
+    37     |  28  |  34  |  1C  | P.FBBK | MSCP$L_FRST_BAD |   4   | First bad block                 |
+    38     +------+------+------+--------+-----------------+-------+---------------------------------+
+    39     | ABORT and GET COMMAND STATUS End Message Offsets:     |                                 |
+    40     |  12  |  14  |   C  | P.OTRF | MSCP$L_OUT_REF  |   4   | Outstanding reference number    |
+    41     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-12
+                                                                                          11 June 1992
+
+
+
+     1                        Table A-7: End and Attention Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | ACCESS NONVOLATILE MEMORY End Message Offsets:                                          |
+     8     |  12  |  14  |   C  | P.ANSZ | MSCP$L_ANM_SIZE |   4   | Memory Size                     |
+     9     |  16  |  20  |  10  | P.ANOF | MSCP$L_ANM_OFFS |   4   | Offset                          |
+    10     |  20  |  24  |  14  | P.ANOP | MSCP$W_ANM_OPER |   2   | Operation (see Table A-12)      |
+    11     |  22  |  26  |  16  | P.ANDL | MSCP$W_ANM_DLGH |   2   | Data length                     |
+    12     |  24  |  30  |  18  | P.ANMD | MSCP$T_ANM_MEMD |   *   | Memory data                     |
+    13     +------+------+------+--------+-----------------+-------+---------------------------------+
+    14     | AVAILABLE Attention  Message Offsets:                                                   |
+    15     |  12  |  14  |   C  | P.MLUN | MSCP$W_MULT_UNT |   2   | Multiunit code                  |
+    16     |  14  |  16  |   E  | P.UNFL | MSCP$W_UNT_FLGS |   2   | Unit flags                      |
+    17     |  16  |  20  |  10  |        |                 |   4   | Undefined                       |
+    18     |  20  |  24  |  14  | P.UNTI | MSCP$Q_UNIT_ID  |   8   | Unit identifier                 |
+    19     |  28  |  34  |  1C  | P.MEDI | MSCP$L_MEDIA_ID |   4   | Media type identifier           |
+    20     |  32  |  40  |  20  |   -    |        -        |   *   | Undefined                       |
+    21     +------+------+------+--------+-----------------+-------+---------------------------------+
+    22     | GET COMMAND STATUS End Message Offsets:       |       |                                 |
+    23     |  16  |  20  |  10  | P.CMST | MSCP$L_CMD_STS  |   4   | Command status                  |
+    24     +------+------+------+--------+-----------------+-------+---------------------------------+
+    25     | GET UNIT STATUS End Message Offsets:          |       |                                 |
+    26     |  12  |  14  |   C  | P.MLUN | MSCP$W_MULT_UNT |   2   | Multiunit code                  |
+    27     |  14  |  16  |   E  | P.UNFL | MSCP$W_UNT_FLGS |   2   | Unit flags                      |
+    28     |  16  |  20  |  10  | P.SPDL | MSCP$B_SPINDLES |   1   | Spindles                        |
+    29     |  17  |  21  |  11  |        |                 |   3   | Reserved                        |
+    30     |  20  |  24  |  14  | P.UNTI | MSCP$Q_UNIT_ID  |   8   | Unit identifier                 |
+    31     |  28  |  34  |  1C  | P.MEDI | MSCP$L_MEDIA_ID |   4   | Media type identifier           |
+    32     |  32  |  40  |  20  |   -    |        -        |   2   | "Bundled Shadowing" (see note 2)|
+    33     |  34  |  42  |  22  |   -    |        -        |   2   | "Bundled Shadowing" (see note 2)|
+    34     |  36  |  44  |  24  | P.TRCK | MSCP$W_TRACK    |   2   | Track size                      |
+    35     |  38  |  46  |  26  | P.GRP  | MSCP$W_GROUP    |   2   | Group size                      |
+    36     |  40  |  50  |  28  | P.CYL  | MSCP$W_CYLINDER |   2   | Cylinder size                   |
+    37     |  42  |  52  |  2A  | P.USVR | MSCP$B_UNIT_SVR |   1   | Unit software version           |
+    38     |  43  |  53  |  2B  | P.UHVR | MSCP$B_UNIT_HVR |   1   | Unit hardware version           |
+    39     |  44  |  54  |  2C  | P.RCTS | MSCP$W_RCT_SIZE |   2   | RCT table size                  |
+    40     |  46  |  56  |  2E  | P.RBNS | MSCP$W_RBNS     |   1   | RBNs / track                    |
+    41     |  47  |  57  |  2F  | P.RCTC | MSCP$B_RCT_CPYS |   1   | RCT copies                      |
+    42     +------+------+------+--------+-----------------+-------+---------------------------------+
+    43     | GET UNIT STATUS, ONLINE, and SET UNIT CHARACTERISTICS End Message and ACCESS PATH and   |
+    44     | AVAILABLE Attention Message offsets:                                                    |
+    45     |  52  |  64  |  34  | P.NAME | MSCP$Z_NAME     |  16   | Unit name (counted string)      |
+    46     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-13
+                                                                                          11 June 1992
+
+
+
+     1                        Table A-7: End and Attention Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | ONLINE and SET UNIT CHARACTERISTICS End Messages:                                       |
+     8     |  12  |  14  |   C  | P.MLUN | MSCP$W_MULT_UNT |   2   | Multiunit code                  |
+     9     |  14  |  16  |   E  | P.UNFL | MSCP$W_UNT_FLGS |   2   | Unit flags                      |
+    10     |  16  |  20  |  10  | P.SPDL | MSCP$B_SPINDLES |   1   | Spindles                        |
+    11     |  17  |  21  |  11  |        |                 |   3   | Reserved                        |
+    12     |  20  |  24  |  14  | P.UNTI | MSCP$Q_UNIT_ID  |   8   | Unit identifier                 |
+    13     |  28  |  34  |  1C  | P.MEDI | MSCP$L_MEDIA_ID |   4   | Media type identifier           |
+    14     |  32  |  40  |  20  |   -    |        -        |   2   | "Bundled Shadowing" (see note 2)|
+    15     |  34  |  42  |  22  |   -    |        -        |   2   | "Bundled Shadowing" (see note 2)|
+    16     |      |      |      |        |                 |       | Note 2)                         |
+    17     |  36  |  44  |  24  | P.UNSZ | MSCP$L_UNT_SIZE |   4   | Unit size                       |
+    18     |  40  |  50  |  28  | P.VSER | MSCP$L_VOL_SER  |   4   | Volume serial number            |
+    19     +------+------+------+--------+-----------------+-------+---------------------------------+
+    20     |  DISK COPY DATA End Message Offsets:                                                    |
+    21     |  12  |  14  |  0C  | P.LBC  | MSCP$L_LBCOUNT  |   4   | Logical block count             |
+    22     |  16  |  20  |  10  | P.SCS  | MSCP$Z_SBC_STS  |  16   | Subcommand status               |
+    23     +------+------+------+--------+-----------------+-------+---------------------------------+
+    24     | TERMINATE CLASS DRIVER/SERVER CONNECTION and SET CONTROLLER CHARACTERISTICS End Message |
+    25     | Offset:                                                                                 |
+    26     |  32  |  40  |  20  | P.CRN  | MSCP$W_CONN_REF |   2   | Connection reference number     |
+    27     +------+------+------+--------+-----------------+-------+---------------------------------+
+    28     | DISK COPY DATA, WRITE HISTORY MANAGEMENT, ERASE, and WRITE End Message Offsets:         |
+    29     |  32  |  40  |  20  | P.ELO  | MSCP$W_ENT_LOC  |   2   | Entry locator                   |
+    30     |  34  |  42  |  22  | P.EID  | MSCP$W_ENT_ID   |   2   | Entry id                        |
+    31     +------+------+------+--------+-----------------+-------+---------------------------------+
+    32     | WRITE HISTORY MANAGEMENT End Message Offsets:                                           |
+    33     |  16  |  20  |  10  | P.UAL  | MSCP$W_UNIT_AL  |   2   | Unit allocated                  |
+    34     |  18  |  22  |  12  | P.SAL  | MSCP$W_SERV_AL  |   2   | Server allocated                |
+    35     |  20  |  24  |  14  | P.SUA  | MSCP$W_SERV_UNAL|   2   | Server unallocated              |
+    36     |  28  |  34  |  1C  | P.CNT  | MSCP$W_COUNT    |   2   | Count                           |
+    37     |  30  |  36  |  1E  | P.OFST | MSCP$W_OFFSET   |   2   | Offset                          |
+    38     |  32  |  40  |  20  | P.HRN  | MSCP$W_HRN      |   2   | HRN or Entry locator            |
+    39     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-14
+                                                                                          11 June 1992
+
+
+
+     1                        Table A-7: End and Attention Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | SET CONTROLLER CHARACTERISTICS End Message Offsets:   |                                 |
+     8     |  12  |  14  |   C  | P.VRSN | MSCP$W_VERSION  |   2   | MSCP version                    |
+     9     |  14  |  16  |   E  | P.CNTF | MSCP$W_CNT_FLGS |   2   | Controller flags                |
+    10     |  16  |  20  |  10  | P.CTMO | MSCP$W_CNT_TMO  |   2   | Controller timeout              |
+    11     |  18  |  22  |  12  | P.CSVR | MSCP$B_CNT_SVR  |   1   | Controller software version     |
+    12     |  19  |  23  |  13  | P.CHVR | MSCP$B_CNT_HVR  |   1   | Controller hardware version     |
+    13     |  20  |  24  |  14  | P.CNTI | MSCP$Q_CNT_ID   |   8   | Controller ID                   |
+    14     |  28  |  34  |  1C  | P.CMBC | MSCP$L_MAXBCNT  |   4   | Controller  maximum  byte count |
+    15     |      |      |      |        |                 |       | (optional)                      |
+    16     +------+------+------+--------+-----------------+-------+---------------------------------+
+    17     | Notes: 1. An asterisk (*) in the field size indicates that the size varies.             |
+    18     |                                                                                         |
+    19     |        2. The modifiers that reference this note have meaning only for VMS and for HSC  |
+    20     |           controllers that provided "Bundled Shadowing" support, See Appendix E,        |
+    21     |           "Waivers and Exceptions", Section E.3.1.1.7, "Bundled Shadowing Definitions". |
+    22     +-----------------------------------------------------------------------------------------+
+
+
+    23                               Table A-8:  Error Log Message Offsets
+
+    24     +--------------------+--------------------------+-------+---------------------------------+
+    25     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+    26     +------+------+------+--------+-----------------+ Size  | Field Description               |
+    27     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+    28     +------+------+------+--------+-----------------+-------+---------------------------------+
+    29     | Generic Error Log Message Offsets:            |       |                                 |
+    30     |   0  |   0  |   0  | L.CRF  | MSLG$L_CMD_REF  |   4   | Command reference number        |
+    31     |   4  |   4  |   4  | L.UNIT | MSLG$W_UNIT     |   2   | Unit number                     |
+    32     |   6  |   6  |   6  | L.SEQ  | MSLG$W_SEQ_NUM  |   2   | Sequence number                 |
+    33     |   8  |  10  |   8  | L.FMT  | MSLG$B_FORMAT   |   1   | Format                          |
+    34     |   9  |  11  |   9  | L.FLGS | MSLG$B_FLAGS    |   1   | Error log message flags         |
+    35     |  10  |  12  |   A  | L.EVNT | MSLG$W_EVENT    |   2   | Event code                      |
+    36     |  12  |  14  |   C  | L.CNTI | MSLG$Q_CNT_ID   |   8   | Controller ID                   |
+    37     |  20  |  24  |  14  | L.CSVR | MSLG$B_CNT_SVR  |   1   | Controller software version     |
+    38     |  21  |  25  |  15  | L.CHVR | MSLG$B_CNT_HVR  |   1   | Controller hardware version     |
+    39     |  22  |  26  |  16  | L.MLUN | MSLG$W_MULT_UNT |   2   | Multiunit code                  |
+    40     |  24  |  30  |  18  | L.UNTI | MSLG$Q_UNIT_ID  |   8   | Unit ID                         |
+    41     |  32  |  40  |  20  | L.USVR | MSLG$B_UNIT_SVR |   1   | Unit software version           |
+    42     |  33  |  41  |  21  | L.UHVR | MSLG$B_UNIT_HVR |   1   | Unit hardware version           |
+    43     |  34  |  42  |  22  |        |                 |   2   | Format dependent                |
+    44     |  36  |  44  |  24  | L.VSER | MSLG$L_VOL_SER  |   4   | Volume serial number            |
+    45     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-15
+                                                                                          11 June 1992
+
+
+
+     1                            Table A-8: Error Log Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | Controller Error with Controller Dependent Information Offsets:                         |
+     8     |  22  |  26  |  16  |        |                 |   *   | Controller dependent information|
+     9     |      |      |      |        |                 |       | (optional)                      |
+    10     +------+------+------+--------+-----------------+-------+---------------------------------+
+    11     | Memory Errors with Host Bus Address Error Log Message Offsets:                          |
+    12     |  24  |  30  |  18  | L.BADR | MSLG$L_BUS_ADDR |   4   | Host bus memory address         |
+    13     |  28  |  34  |  1C  |        |                 |   *   | Controller dependent information|
+    14     |      |      |      |        |                 |       | (optional)                      |
+    15     +------+------+------+--------+-----------------+-------+---------------------------------+
+    16     | Memory Errors with Controller Memory Address Error Log Message Offsets:                 |
+    17     |  24  |  30  |  18  | L.BADR | MSLG$L_BUS_ADDR |   4   | Controller memory address       |
+    18     |  28  |  34  |  1C  |        |                 |   *   | Controller dependent information|
+    19     |      |      |      |        |                 |       | (optional)                      |
+    20     +------+------+------+--------+-----------------+-------+---------------------------------+
+    21     | Disk Transfer Errors Error Log Message Offsets:       |                                 |
+    22     |  34  |  42  |  22  | L.LVL  | MSLG$B_LEVEL    |   1   | Level                           |
+    23     |  35  |  43  |  23  | L.RTRY | MSLG$B_RETRY    |   1   | Retry                           |
+    24     |  36  |  44  |  24  | L.VSER | MSLG$L_VOL_SER  |   4   | Volume serial number            |
+    25     |  40  |  50  |  28  | L.HDCD | MSLG$L_HDR_CODE |   4   | Header code                     |
+    26     |  44  |  54  |  2C  |        |                 |   *   | Controller  or  disk  dependent |
+    27     |      |      |      |        |                 |       | information (optional)          |
+    28     +------+------+------+--------+-----------------+-------+---------------------------------+
+    29     | SDI Errors Error Log Message Offsets:         |       |                                 |
+    30     |  40  |  50  |  28  | L.HDCD | MSLG$L_HDR_CODE |   4   | Header code                     |
+    31     |  44  |  54  |  2C  | L.SDI  | MSLG$Z_SDI      |  12   | SDI information                 |
+    32     +------+------+------+--------+-----------------+-------+---------------------------------+
+    33     | Small Disk Errors Error Log Message Offsets:  |       |                                 |
+    34     |  34  |  42  |  22  | L.SCYL | MSLG$W_SDE_CYL  |   2   | Cylinder                        |
+    35     |  36  |  44  |  24  | L.VSER | MSLG$L_VOL_SER  |   4   | Volume serial number            |
+    36     +------+------+------+--------+-----------------+-------+---------------------------------+
+    37     | Bad Block Replacement Attempt Error Log Message Offsets:                                |
+    38     |  34  |  42  |  22  | L.RPFL | MSLG$W_RPL_FLGS |   2   | Replace Flags (see Table A-11)  |
+    39     |  36  |  44  |  24  | L.VSER | MSLG$L_VOL_SER  |   4   | Volume serial number            |
+    40     |  40  |  50  |  28  | L.LBN  | MSLG$L_BAD_LBN  |   4   | Bad LBN                         |
+    41     |  44  |  54  |  2C  | L.ORBN | MSLG$L_OLD_RBN  |   4   | Old RBN                         |
+    42     |  48  |  60  |  30  | L.NRBN | MSLG$L_NEW_RBN  |   4   | New RBN                         |
+    43     |  52  |  64  |  34  | L.RPEV | MSLG$W_CAUSE    |   2   | Cause (an event code)           |
+    44     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-16
+                                                                                          11 June 1992
+
+
+
+     1                            Table A-8: Error Log Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | Media Loader Errors Error Log Message Offsets: (allocated for future use; see Note 2)   |
+     8     |  36  |  44  |  24  | L.MLID | MSLG$Q_ML_ID    |   8   | Media loader identifier         |
+     9     |  44  |  54  |  2C  | L.LSVR | MSLG$B_ML_SVR   |   1   | Media loader software version   |
+    10     |  45  |  55  |  2D  | L.LHVR | MSLG$B_ML_HVR   |   1   | Media loader hardware version   |
+    11     |  46  |  56  |  2E  | L.MLUN | MSLG$W_ML_UNIT  |   2   | Media loader unit number        |
+    12     |  48  |  60  |  30  |        |                 |   *   | Controller or media loader      |
+    13     |      |      |      |        |                 |       | dependent information (optional)|
+    14     +------+------+------+--------+-----------------+-----------------------------------------+
+    15     | Disk Copy Data Correlation Error Log Message Offsets:                                   |
+    16     |   4  |   4  |   4  | L.DUN  | MSLG$W_DST_UNUM |   2   | Destination unit number         |
+    17     |  12  |  14  |  0C  | L.CNTI | MSLG$Q_CNT_ID   |   8   | Controller identifier           |
+    18     |  20  |  24  |  14  | L.CSVR | MSLG$B_CNT_SVR  |   1   | Controller software version     |
+    19     |  21  |  25  |  15  | L.CHVR | MSLG$B_CNT_HVR  |   1   | Controller hardware version     |
+    20     |  24  |  30  |  18  | L.DUI  | MSLG$Q_DST_UID  |   8   | Destination unit identifier     |
+    21     |  32  |  40  |  20  | L.SUN  | MSLG$W_SRC_UNUM |   2   | Source unit number              |
+    22     |  36  |  44  |  24  | L.SCID | MSLG$Q_SRC_CID  |   8   | Source controller identifier    |
+    23     |  44  |  54  |  2C  | L.SCSV | MSLG$B_SRC_CSVR |   1   | Source controller SW version    |
+    24     |  45  |  55  |  2D  | L.SCHV | MSLG$B_SRC_CHVR |   1   | Source controller HW version    |
+    25     |  48  |  60  |  30  | L.SUI  | MSLG$Q_SRC_UID  |   8   | Source unit identifier          |
+    26     |  56  |  70  |  38  | L.PAD  | MSLG$Q_PORT_ADR |   8   | Source unit's subsystem port    |
+    27     |      |      |      |        |                 |       | address                         |
+    28     |  64  | 100  |  40  | L.SAD  | MSLG$Q_SYS_ADR  |   8   | Source unit's subsystem system  |
+    29     |      |      |      |        |                 |       | address                         |
+    30     |  72  | 110  |  48  |        |                 |   *   | Event dependent information     |
+    31     |      |      |      |        |                 |       | (optional)                      |
+    32     +------+------+------+--------+-----------------+-----------------------------------------+
+    33     | Notes: 1. An asterisk (*) in the field size indicates that the size varies.             |
+    34     |                                                                                         |
+    35     |        2. These  offsets  are defined in anticipation of the adoption of a formal media |
+    36     |           loader architecture.  These offsets are in current use  as  dictated  by  the |
+    37     |           temporary waiver in Section E.6.                                              |
+    38     +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-17
+                                                                                          11 June 1992
+
+
+     1                             Table A-9:  Error Log Message Format Codes
+
+     2     +--------------------+--------------------------+-----------------------------------------+
+     3     |    Format Code     |   Preferred Mnemonics    |                                         |
+     4     +------+------+------+--------+-----------------+   Format Description                    |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |                                         |
+     6     +------+------+------+--------+-----------------+-----------------------------------------+
+     7     |   0  |   0  |   0  | FM.CNT | MSLG$K_CNT_ERR  | Controller Errors                       |
+     8     +------+------+------+--------+-----------------+-----------------------------------------+
+     9     |   1  |   1  |   1  | FM.BAD | MSLG$K_BUS_ADDR | Memory Errors  with  (Host/Controller)  |
+    10     |      |      |      |        |                 | Memory Address                          |
+    11     +------+------+------+--------+-----------------+-----------------------------------------+
+    12     |   2  |   2  |   2  | FM.DSK | MSLG$K_DISK_TRN | Disk Transfer Errors                    |
+    13     +------+------+------+--------+-----------------+-----------------------------------------+
+    14     |   3  |   3  |   3  | FM.SDI | MSLG$K_SDI      | SDI Errors                              |
+    15     +------+------+------+--------+-----------------+-----------------------------------------+
+    16     |   4  |   4  |   4  | FM.SDE | MSLG$K_SML_DSK  | Small Disk Errors                       |
+    17     +------+------+------+--------+-----------------+-----------------------------------------+
+    18     |   9  |  11  |   9  | FM.RPL | MSLG$K_REPLACE  | Bad Block Replacement Attempt           |
+    19     +------+------+------+--------+-----------------+-----------------------------------------+
+    20     |  10  |  12  |   A  | FM.LDR | MSLG$K_LDR_ERR  | Media Loader Errors (see Note below)    |
+    21     +------+------+------+--------+-----------------+-----------------------------------------+
+    22     |  12  |  14  |   C  | FM.CDC | MSLG$K_DCD_CORR | Disk Copy Data Correlation              |
+    23     +------+------+------+--------+-----------------+-----------------------------------------+
+    24     | Note:  This  format  code  is defined in anticipation of the adoption of a formal media |
+    25     |        loader architecture.  It is in current use as dictated by the  temporary  waiver |
+    26     |        in Section E.6.                                                                  |
+    27     +-----------------------------------------------------------------------------------------+
+
+
+    28                                Table A-10:  Error Log Message Flags
+
+    29     +------+--------------+--------------------------+----------------------------------------+
+    30     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    31     |Number+-------+------+--------+-----------------+   Format Description                   |
+    32     |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+    33     +------+-------+------+--------+-----------------+----------------------------------------+
+    34     |   7  |   200 |   80 | LF.SUC | MSLG$x_LF_SUCC  | Operation Successful                   |
+    35     +------+-------+------+--------+-----------------+----------------------------------------+
+    36     |   6  |   100 |   40 | LF.CON | MSLG$x_LF_CONT  | Operation Continuing                   |
+    37     +------+-------+------+--------+-----------------+----------------------------------------+
+    38     |   5  |    40 |   20 | LF.BBR | MSLG$x_LF_BBR   | Bad Block Replacement Request          |
+    39     +------+-------+------+--------+-----------------+----------------------------------------+
+    40     |   4  |    20 |   10 | LF.RCT | MSLG$x_LF_RPLER | Error During Replacement               |
+    41     +------+-------+------+--------+-----------------+----------------------------------------+
+    42     |   3  |    10 |    8 | LF.CDC | MSLG$x_LF_DCDC  | Disk Copy Data Correlated              |
+    43     +------+-------+------+--------+-----------------+----------------------------------------+
+    44     |   1  |     2 |    2 | LF.INF | MSLG$x_LF_INFO  | Informational                          |
+    45     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-18
+                                                                                          11 June 1992
+
+
+
+     1                             Table A-10: Error Log Message Flags (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+   Format Description                   |
+     5     |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+     6     +------+-------+------+--------+-----------------+----------------------------------------+
+     7     |   0  |     1 |    1 | LF.SNR | MSLG$x_LF_SQNRS | Sequence Number Reset                  |
+     8     +------+-------+------+--------+-----------------+----------------------------------------+
+     9     | Note: The  "x"  in  a  32  bit  mnemonic  for  a  bit flag will be "V" if the symbol is |
+    10     |       defined as a bit number (offset) or an "M" if defined as a mask.                  |
+    11     +-----------------------------------------------------------------------------------------+
+
+
+    12                     Table A-11:  Bad Block Replacement Attempt "Replace Flags"
+
+    13     +------+--------------+--------------------------+----------------------------------------+
+    14     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    15     |Number+-------+------+--------+-----------------+   Replace Flag                         |
+    16     |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+    17     +------+-------+------+--------+-----------------+----------------------------------------+
+    18     |  15  |100000 | 8000 | LFR.RP | MSLG$x_LFR_RP   | Replacement attempted.                 |
+    19     |      |       |      |        |                 |   Set  if  block  was verified as bad. |
+    20     |      |       |      |        |                 |   Clear if block checked out OK.       |
+    21     +------+-------+------+--------+-----------------+----------------------------------------+
+    22     |  14  | 40000 | 4000 | LFR.FE | MSLG$x_LFR_FE   | Force error.                           |
+    23     |      |       |      |        |                 |   Set  if data could not be recovered, |
+    24     |      |       |      |        |                 |   so  block  was  rewritten  with the  |
+    25     |      |       |      |        |                 |   Force Error modifier.                |
+    26     +------+-------+------+--------+-----------------+----------------------------------------+
+    27     |  13  | 20000 | 2000 | LFR.TE | MSLG$x_LFR_TE   | Tertiary revector.                     |
+    28     |      |       |      |        |                 |   Set  if  block  was  revectored to a |
+    29     |      |       |      |        |                 |   nonprimary RBN.                      |
+    30     +------+-------+------+--------+-----------------+----------------------------------------+
+    31     |  12  | 10000 | 1000 | LFR.RF | MSLG$x_LFR_RF   | Reformat error.                        |
+    32     |      |       |      |        |                 |   Set  if  the  REPLACE command or its |
+    33     |      |       |      |        |                 |   analogue failed.                     |
+    34     +------+-------+------+--------+-----------------+----------------------------------------+
+    35     |  11  |  4000 |  800 | LFR.RI | MSLG$x_LFR_RI   | RCT inconsistent.                      |
+    36     |      |       |      |        |                 |   Set  if replacement failed due to an |
+    37     |      |       |      |        |                 |   inconsistent RCT.                    |
+    38     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-19
+                                                                                          11 June 1992
+
+
+
+     1                  Table A-11: Bad Block Replacement Attempt "Replace Flags" (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+   Replace Flag                         |
+     5     |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+     6     +------+-------+------+--------+-----------------+----------------------------------------+
+     7     |  10  |  2000 |  400 | LFR.BR | MSLG$x_LFR_BR   | Bad RBN.                               |
+     8     |      |       |      |        |                 |   Set  if LBN was previously replaced, |
+     9     |      |       |      |        |                 |   so a bad RBN was replaced.           |
+    10     +------+-------+------+--------+-----------------+----------------------------------------+
+    11     | Note: The  "x"  in  a  32  bit  mnemonic  for  a  bit flag will be "V" if the symbol is |
+    12     |       defined as a bit number (offset) or an "M" if defined as a mask.                  |
+    13     +-----------------------------------------------------------------------------------------+
+
+
+    14                   Table A-12:  Access Nonvolatile Memory Command Operation Codes
+
+    15     +--------------------+--------------------------+-----------------------------------------+
+    16     |    Opcode Value    |   Preferred Mnemonics    |                                         |
+    17     +------+------+------+--------+-----------------+   Operation                             |
+    18     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+    19     +------+------+------+--------+-----------------+-----------------------------------------+
+    20     |   0  |   0  |   0  | ANM.RD | MSCP$K_ANM_READ | Read                                    |
+    21     +------+------+------+--------+-----------------+-----------------------------------------+
+    22     |   1  |   1  |   1  | ANM.EX | MSCP$K_ANM_EXCG | Exchange                                |
+    23     +------+------+------+--------+-----------------+-----------------------------------------+
+    24     |   2  |   2  |   2  | ANM.TS | MSCP$K_ANM_TSST | Test and Set                            |
+    25     +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+    26                                 Table A-13:  Format Function Codes
+
+    27     +--------------------+--------+--------+--------+--------+-------+------------------------+
+    28     |   Function Code    |        | bits   | tracks | tracks |       |                        |
+    29     +------+------+------+ device | per    | per    | per    | sides |   reference            |
+    30     | Dec. | Oct. | Hex. |        | radian | mm     | inch   |       |                        |
+    31     +------+------+------+--------+--------+--------+--------+-------+------------------------+
+    32     | Applicable to all devices:                                                              |
+    33     |   0  |   0  |   0  | Format using device dependent default.                             |
+    34     +------+------+------+--------+--------+--------+--------+-------+------------------------+
+    35     | Applicable to all devices that support multiple densities or capacities:                |
+    36     |   1  |   1  |   1  | Lowest supported density or capacity.                              |
+    37     |   2  |   2  |   2  | Highest supported density or capacity.                             |
+    38     +------+------+------+--------+--------+--------+--------+-------+------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-20
+                                                                                          11 June 1992
+
+
+
+     1                              Table A-13: Format Function Codes (cont.)
+
+     2     +--------------------+--------+--------+--------+--------+-------+------------------------+
+     3     |   Function Code    |        | bits   | tracks | tracks |       |                        |
+     4     +------+------+------+ device | per    | per    | per    | sides |   reference            |
+     5     | Dec. | Oct. | Hex. |        | radian | mm     | inch   |       |                        |
+     6     +------+------+------+--------+--------+--------+--------+-------+------------------------+
+     7     | Applicable to 200 mm (8 inch) flexible disks:                                           |
+     8     |  261 |  405 |  105 |  RX01  |   6631 |   1.9  |   48   |   1   | ANSI X3.73-1980        |
+     9     |  262 |  406 |  106 |        |   6631 |   1.9  |   48   |   2   | ANSI X3B8-140          |
+    10     |  265 |  411 |  109 |  RX02  |  13262 |   1.9  |   48   |   1   | ANSI X3B8/78-139       |
+    11     |  266 |  412 |  10A |        |  13262 |   1.9  |   48   |   2   | ANSI X3.121-1984       |
+    12     +------+------+------+--------+--------+--------+--------+-------+------------------------+
+    13     | Applicable to 130 mm (5.25 inch) flexible disks:                                        |
+    14     |  269 |  415 |  10D |        |   3979 |   1.9  |   48   |   1   | ANSI X3.82-1980        |
+    15     |  274 |  422 |  112 |        |   7958 |   1.9  |   48   |   2   | ANSI X3.125-1985       |
+    16     |  277 |  425 |  115 |  RX50  |   7958 |   3.8  |   96   |   1   |                        |
+    17     |  278 |  426 |  116 |        |   7958 |   3.8  |   96   |   2   | ANSI X3.126-1986       |
+    18     |  282 |  432 |  11A |  RX33  |  13262 |   3.8  |   96   |   2   | ISO DIS8630-1985       |
+    19     +------+------+------+--------+--------+--------+--------+-------+------------------------+
+    20     | Applicable to 90 mm (3.5 inch) flexible disks:                                          |
+    21     |  286 |  436 |  11E |        |   7958 |   5.3  |  135   |   2   | ANSI X3.137            |
+    22     +------+------+------+--------+--------+--------+--------+-------+------------------------+
+
+    23                   Table A-14:  WRITE HISTORY MANAGEMENT Command Operation Codes
+
+    24     +--------------------+--------------------------+-----------------------------------------+
+    25     |    Opcode Value    |   Preferred Mnemonics    |                                         |
+    26     +------+------+------+--------+-----------------+   Operation                             |
+    27     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+    28     +------+------+------+--------+-----------------+-----------------------------------------+
+    29     |  0   |  0   |  0   |        |                 | Reserved                                |
+    30     +------+------+------+--------+-----------------+-----------------------------------------+
+    31     |  1   |  1   |  1   | WH.DAL | MSCP$K_WHM_DALL | Deallocate All                          |
+    32     +------+------+------+--------+-----------------+-----------------------------------------+
+    33     |  2   |  2   |  2   | WH.DHR | MSCP$K_WHM_DHRN | Deallocate by Host Reference Number     |
+    34     +------+------+------+--------+-----------------+-----------------------------------------+
+    35     |  3   |  3   |  3   | WH.DEL | MSCP$K_WHM_DELO | Deallocate by Entry Locator             |
+    36     +------+------+------+--------+-----------------+-----------------------------------------+
+    37     |  4   |  4   |  4   | WH.RAL | MSCP$K_WHM_RALL | Read All                                |
+    38     +------+------+------+--------+-----------------+-----------------------------------------+
+    39     |  5   |  5   |  5   | WH.RHR | MSCP$K_WHM_RHRN | Read by Host Reference Number           |
+    40     +------+------+------+--------+-----------------+-----------------------------------------+
+    41     |  6   |  6   |  6   | WH.DAF | MSCP$K_WHM_DAFC | Decrement Allocation Failure Count      |
+    42     +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-21
+                                                                                          11 June 1992
+
+
+     1                            Table A-15:  Write History Entry Entry Flags
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+   Format Description                   |
+     5     |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+     6     +------+-------+------+--------+-----------------+----------------------------------------+
+     7     |  15  |100000 | 8000 | ET.ERR | MSCP$x_ET_ERR   | Error                                  |
+     8     +------+-------+------+--------+-----------------+----------------------------------------+
+     9     |  14  | 40000 | 4000 | ET.TLB | MSCP$x_ET_TLIB  | Transfer Length in Blocks              |
+    10     +------+-------+------+--------+-----------------+----------------------------------------+
+    11     | Note: The  "x"  in  a  32  bit  mnemonic  for  a  bit flag will be "V" if the symbol is |
+    12     |       defined as a bit number (offset) or an "M" if defined as a mask.                  |
+    13     +-----------------------------------------------------------------------------------------+
+
+    14                              Table A-16:  Write History Entry Offsets
+
+    15     +--------------------+--------------------------+-------+---------------------------------+
+    16     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+    17     +------+------+------+--------+-----------------+ Size  | Field Description               |
+    18     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+    19     +------+------+------+--------+-----------------+-------+---------------------------------+
+    20     |   0  |   0  |   0  | W.ELO  | WHIS$W_ELO      |   2   | Entry Locator                   |
+    21     |   2  |   2  |   2  | W.UNIT | WHIS$W_UNIT     |   2   | Unit Number                     |
+    22     |   4  |   4  |   4  | W.LEN  | WHIS$L_LENGTH   |   4   | Transfer Length                 |
+    23     |   8  |  10  |   8  | W.LBN  | WHIS$L_LBN      |   4   | Starting Logical Block Number   |
+    24     |  12  |  14  |  0C  | W.HRN  | WHIS$W_HRN      |   2   | Host Reference Number           |
+    25     |  14  |  16  |  0E  | W.EFL  | WHIS$W_ENTFLGS  |   2   | Entry Flags                     |
+    26     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+    27                             Table A-17:  Cache Access Attribute Values
+
+    28     +--------------------+--------------------------+-----------------------------------------+
+    29     |    Cache Access    |                          |                                         |
+    30     |   Attribute Value  |   Preferred Mnemonics    |                                         |
+    31     |------+------+------+--------+-----------------+           Description                   |
+    32     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+    33     +------+------+------+--------+-----------------+-----------------------------------------+
+    34     |   00 |   00 |  00  | CA.DEF | MSCP$K_CA_DEF   | Default                                 |
+    35     |  128 |  200 |  80  | CA.NN  | MSCP$K_CA_NN    | Not Single / Not Random                 |
+    36     |  129 |  201 |  81  | CA.SN  | MSCP$K_CA_SN    | Single / Not Random                     |
+    37     |  130 |  202 |  82  | CA.NR  | MSCP$K_CA_NR    | Not Single / Random                     |
+    38     |  131 |  203 |  83  | CA.SR  | MSCP$K_CA_SR    | Single / Random                         |
+    39     +------+------+------+--------+-----------------+-----------------------------------------+
+    40     | Note:  All unassigned values are interpreted as though they were 00. The interpretation |
+    41     |        of all values, both assigned and unassigned, may be altered by the System        |
+    42     |        Manager.                                                                         |
+    43     +-----------------------------------------------------------------------------------------+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           OPCODE, FLAG, AND OFFSET DEFINITIONS                                              Page A-22
+                                                                                          11 June 1992
+
+
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+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-1
+                                                                                          11 June 1992
+
+
+     1                                             APPENDIX B
+
+     2                                  STATUS AND EVENT CODE DEFINITIONS
+
+
+
+     3                                                NOTES
+
+
+     4             1.  The combination of a status or event  code  with  a  subcode  expressed
+     5                 (assuming 16 bit symbols) as:
+
+     6                                         (subcode *ST.SUB)+code
+
+     7             2.  The subcode values shown in Table B-2 are only returned as status codes
+     8                 in  end  messages.  They are never returned as event codes in error log
+     9                 messages.  In addition, the references to other tables within Table B-2
+    10  |              indicate  that those tables contain subcodes which are only returned as
+    11  |              status codes in end messages.  Some of the tables referenced  by  Table
+    12  |              B-2  show  subcodes  listed as both status and event subcodes, however,
+    13  |              under certain conditions some  subcode  values  are  only  returned  as
+    14  |              status codes.
+
+    15             3.  The subcode values shown in Tables B-3 through B-10, and Table B-12 may
+    16                 be  returned  (as  indicated  in  the  "EV" and "ST" columns) as status
+    17                 codes, event codes or both.  In those tables, an asterisk in  the  "EV"
+    18                 column  indicates that the code and subcode may be returned as an event
+    19                 code.  An asterisk in the "ST"  column  indicates  that  the  code  and
+    20                 subcode may be returned as a status code.
+
+    21             4.  The  subcode  values  shown  in  Table  B-9,  "Bad  Block   Replacement
+    22                 Completion"  Event  Only Subcode Values, are only returned as the event
+    23                 code in "Bad Block Replacement Attempt" error log messages, where  they
+    24                 report  the completion status of a bad block replacement attempt.  They
+    25                 are never returned as status codes.  The subcode values shown in  Table
+    26                 B-11,  "Informational"  Event Only Subcode Values, are only returned as
+    27                 event codes for reporting informational logs.
+
+    28  |          5.  Status and Event code/subcode descriptions  that  reference  this  note
+    29  |              have  meaning  only  for  VMS  and  for  HSC  controllers that provided
+    30                 "Bundled Shadowing" support, See Appendix E, "Waivers and  Exceptions",
+    31                 Section   E.3.1.1.8,   ""Bundled   Shadowing"   Specific   Status  Code
+    32                 Definitions".
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-2
+                                                                                          11 June 1992
+
+
+     1                                 Table B-1:  Status and Event Codes
+
+     2     +--------------------+--------------------------+-----------------------------------------+
+     3     |       Value        |   Preferred Mnemonics    |                                         |
+     4     +------+------+------+--------+-----------------+     Status or Event Code                |
+     5     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+     6     +------+------+------+--------+-----------------+-----------------------------------------+
+     7     |   31 |   37 |   1F | ST.MSK | MSCP$M_ST_MASK  | Status / event code mask                |
+     8     |   32 |   40 |   20 | ST.SUB | MSCP$K_ST_SBCOD | Subcode multiplier                      |
+     9     |      |      |      |        |                 |                                         |
+    10     |    0 |    0 |    0 | ST.SUC | MSCP$K_ST_SUCC  | Success                                 |
+    11     |    1 |    1 |    1 | ST.CMD | MSCP$K_ST_ICMD  | Invalid Command                         |
+    12     |    2 |    2 |    2 | ST.ABO | MSCP$K_ST_ABRTD | Command Aborted                         |
+    13     |    3 |    3 |    3 | ST.OFL | MSCP$K_ST_OFFLN | Unit-Offline                            |
+    14     |    4 |    4 |    4 | ST.AVL | MSCP$K_ST_AVLBL | Unit-Available                          |
+    15     |    5 |    5 |    5 | ST.MFE | MSCP$K_ST_MFMTE | Media Format Error                      |
+    16     |    6 |    6 |    6 | ST.WPR | MSCP$K_ST_WRTPR | Write Protected                         |
+    17     |    7 |    7 |    7 | ST.CMP | MSCP$K_ST_COMP  | Compare Error                           |
+    18     |    8 |   10 |    8 | ST.DAT | MSCP$K_ST_DATA  | Data Error                              |
+    19     |    9 |   11 |    9 | ST.HST | MSCP$K_ST_HSTBF | Host Buffer Access Error                |
+    20     |   10 |   12 |    A | ST.CNT | MSCP$K_ST_CNTLR | Controller Error                        |
+    21     |   11 |   13 |    B | ST.DRV | MSCP$K_ST_DRIVE | Drive Error                             |
+    22     |   12 |   14 |    C |   -    |        -        | "Bundled Shadowing"  (see Note 5)       |
+    23     |   13 |   15 |    D | ST.WHE | MSCP$K_ST_WHEAE | Write History Entry Access Error        |
+    24     |   20 |   24 |   14 | ST.BBR | MSCP$K_ST_BBR   | Bad Block Replacement Completion        |
+    25     |   21 |   25 |   15 | ST.PRM | MSCP$K_ST_IPARM | Invalid Parameter                       |
+    26     |   22 |   26 |   16 | ST.INF | MSCP$K_ST_INFO  | Informational (not an error)            |
+    27     |   23 |   27 |   17 | ST.LDR | MSCP$K_ST_LOADR | Media Loader Error (see note below)     |
+    28     |   24 |   30 |   18 | ST.HER | MSCP$K_ST_HOST  | Host Error                              |
+    29  |  |   25 |   31 |   19 | ST.CAC | MSCP$K_ST_CAC   | Cache Error                             |
+    30     |   30 |   36 |   1E | ST.SCO | MSCP$K_ST_SBCERR| Subcommand Error                        |
+    31     +------+------+------+--------+-----------------+-----------------------------------------+
+
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+
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+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-3
+                                                                                          11 June 1992
+
+
+     1                               Table B-2:  Status Only Subcode Values
+
+     2      +------+---------------------+---------------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |                                                         |
+     4      | code +------+-------+------+                 Status Subcode                          |
+     5      |      | Dec. |  Oct. | Hex. |                                                         |
+     6      +------+------+-------+------+---------------------------------------------------------+
+     7      | "Success" Subcode values:  |                                                         |
+     8      |    0 |    0 |     0 |    0 | Normal                                                  |
+     9      |    1 |   32 |    40 |   20 | Spin-down Ignored                                       |
+    10      |    2 |   64 |   100 |   40 | Still Connected                                         |
+    11      |    4 |  128 |   200 |   80 | Duplicate Unit Number                                   |
+    12      |    8 |  256 |   400 |  100 | Already Online                                          |
+    13      |   16 |  512 |  1000 |  200 | Still Online                                            |
+    14      |   32 | 1024 |  2000 |  400 | Incomplete Replacement                                  |
+    15      |   64 | 2048 |  4000 |  800 | Invalid RCT                                             |
+    16      |  128 | 4096 | 10000 | 1000 | Read Only Volume Format                                 |
+    17      +------+------+-------+------+---------------------------------------------------------+
+    18      | "Invalid Command" Subcode values:                                                    |
+    19      |    0 |    1 |     1 |    1 | Invalid Message Length                                  |
+    20      | many |      |       |      | Other  "Invalid Command" subcodes values are referenced |
+    21      |      |      |       |      | as follows (note that this is combined with the  status |
+    22      |      |      |       |      | code):                                                  |
+    23      |      |      |       |      |                                                         |
+    24      |      |      |       |      |      offset*256.+code                                   |
+    25      |      |      |       |      |                                                         |
+    26      |      |      |       |      | where "offset" is the command message offset symbol for |
+    27      |      |      |       |      | the  field  in  error  and "code" is the symbol for the |
+    28      |      |      |       |      | "Invalid Command" status code.                          |
+    29      +------+------+-------+------+---------------------------------------------------------+
+    30      | "Command Aborted" Subcode values:                                                    |
+    31      |    0 |    2 |     2 |    2 | Subcodes are not used.                                  |
+    32      +------+------+-------+------+---------------------------------------------------------+
+    33      | "Unit-Offline" Subcode values:                                                       |
+    34      |    0 |    3 |     3 |    3 | Unit unknown or online to another controller.           |
+    35      |    1 |   35 |    43 |   23 | No  volume  mounted  or  drive  disabled  via  RUN/STOP |
+    36      |      |      |       |      | switch (unit is in known substate).                     |
+    37      |    2 |   67 |   103 |   43 | Unit is inoperative                                     |
+    38      |      |      |       |      |   For  SI  drives, the  controller has marked the drive |
+    39      |      |      |       |      |   inoperative  due  to  an  unrecoverable  error  in  a |
+    40      |      |      |       |      |   previous  level 2  exchange, the drive's C1  flag  is |
+    41      |      |      |       |      |   set, or  the  drive  has a duplicate unit identifier. |
+    42      |    4 |  131 |   203 |   83 | Duplicate unit number                                   |
+    43      |    8 |  259 |   403 |  103 | Unit disabled by field service or diagnostic.           |
+    44      |      |      |       |      |   For SI drives, the drive's DD flag is set.            |
+    45      |   16 |  515 |  1003 |  203 | Exclusive Use                                           |
+    46      +------+------+-------+------+---------------------------------------------------------+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-4
+                                                                                          11 June 1992
+
+
+
+     1                            Table B-2: Status Only Subcode Values (cont.)
+
+     2      +------+---------------------+---------------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |                                                         |
+     4      | code +------+-------+------+                 Status Subcode                          |
+     5      |      | Dec. |  Oct. | Hex. |                                                         |
+     6      +------+------+-------+------+---------------------------------------------------------+
+     7      | "Unit-Available" Subcode values:                                                     |
+     8      |    0 |    4 |     4 |    4 | Available                                               |
+     9      |    2 |   68 |   104 |   44 | "Bundled Shadowing" (see Note 5)                        |
+    10      |    4 |  132 |   204 |   84 | "Bundled Shadowing" (see Note 5)                        |
+    11      |   32 | 1028 |  2004 |  404 | Already In Use                                          |
+    12      +------+------+-------+------+---------------------------------------------------------+
+    13      | "Media Format Error" Subcode values -- see Table B-3                                 |
+    14      +------+------+-------+------+---------------------------------------------------------+
+    15      | "Write Protected" Subcode values:                                                    |
+    16      |    8 |  262 |   406 |  106 | Unit is Data Safety Write Protected                     |
+    17      |  128 | 4102 | 10006 | 1006 | Unit is Software Write Protected                        |
+    18      |  256 | 8198 | 20006 | 2006 | Unit is Hardware Write Protected                        |
+    19      +------+------+-------+------+---------------------------------------------------------+
+    20      | "Compare Error" Subcode values -- see Table B-4                                      |
+    21      +------+------+-------+------+---------------------------------------------------------+
+    22      | "Data Error" Subcode values -- see Table B-5                                         |
+    23      +------+------+-------+------+---------------------------------------------------------+
+    24      | "Host Buffer Access Error" Subcode values -- see Table B-6                           |
+    25      +------+------+-------+------+---------------------------------------------------------+
+    26      | "Controller Error" subcode values -- see Table B-7                                   |
+    27      +------+------+-------+------+---------------------------------------------------------+
+    28      | "Write History Entry Access Error" subcodes:                                         |
+    29      |    1 |   45 |    55 |   2D | Previous Allocation Failure                             |
+    30      |    2 |   77 |   115 |   4D | Allocation Failure Table Full                           |
+    31      |    8 |  269 |   415 |  10D | No such entry                                           |
+    32      +------+------+-------+------+---------------------------------------------------------+
+    33      | "Subcommand Error" subcode values -- see Table B-12                                  |
+    34      +------+------+-------+------+---------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-5
+                                                                                          11 June 1992
+
+
+     1                  Table B-3:  "Media Format Error" Status or Event Subcode Values
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      | many |     |        |      |*|*| "Data Error" accessing RCT or FCT.                  |
+     8      |      For  every "Data Error"  subcode  that  may be reported as a status code (i.e., |
+     9      |      for every  subcode  listed  in  Table B-5 that  has  an  asterisk in  the  "ST" |
+    10      |      column), the  exact  same  subcode   value may be returned  as a "Media  Format |
+    11      |      Error" subcode value.  These  subcodes  indicate  that a transfer to the unit's |
+    12      |      FCT or  RCT (using  the multicopy  structure read and  write algorithms) failed |
+    13      |      for the  specified reason.  The  subcode reports the error that occurred on the |
+    14      |      last  copy  of  the multicopy  structure.  For  example, the  subcode  value  7 |
+    15      |      (combined  code and subcode value of 229 decimal) indicates that the FCT or RCT |
+    16      |      is unreadable due to an Uncorrectable ECC Error.                                |
+    17      |                                                                                      |
+    18      |      In  addition  to  the subcode  values  copied from "Data Error" subcode values, |
+    19      |      the subcode values listed below also exist.                                     |
+    20      +------+------+-------+------+-+-+-----------------------------------------------------+
+    21      |    4 |  133 |   205 |   85 |-|-| "Bundled Shadowing" (see Note 5)                    |
+    22      +------+------+-------+------+-+-+-----------------------------------------------------+
+    23      |    5 |  165 |   245 |   A5 | |*| Disk isn't formatted with 512 byte sectors          |
+    24      |      |      |       |      | | |   The  disk's FCT  indicates that  it is  formatted |
+    25      |      |      |       |      | | |   with 576 byte  sectors, yet either the controller |
+    26      |      |      |       |      | | |   or the drive only supports 512 byte sectors.      |
+    27      +------+------+-------+------+-+-+-----------------------------------------------------+
+    28      |    6 |  197 |   305 |   C5 | |*| Disk isn't formatted or FCT corrupted               |
+    29      |      |      |       |      | | |   The disk's FCT does not indicate that the disk is |
+    30      |      |      |       |      | | |   formatted with either 512 or 576 byte sectors.    |
+    31      +------+------+-------+------+-+-+-----------------------------------------------------+
+    32      |    8 |  261 |   405 |  105 |*|*| RCT corrupted.                                      |
+    33      |      |      |       |      | | |   The RCT  search algorithm  encountered an invalid |
+    34      |      |      |       |      | | |   RCT  entry.  This  subcode  may  be  returned  by |
+    35      |      |      |       |      | | |   replacement  (searching   the  RCT   for  a   new |
+    36      |      |      |       |      | | |   replacement block), by revectoring (searching the |
+    37      |      |      |       |      | | |   RCT  to  locate the replacement  block for  a bad |
+    38      |      |      |       |      | | |   block), and when a unit is brought "Unit-Online." |
+    39      +------+------+-------+------+-+-+-----------------------------------------------------+
+    40      |    9 |  293 |   445 |  125 |*| | No replacement block available.                     |
+    41      |      |      |       |      | | |   Replacement  was attempted for a bad block, but a |
+    42      |      |      |       |      | | |   replacement block  could not  be allocated (i.e., |
+    43      |      |      |       |      | | |   the  volume's RCT is  full).  This subcode may be |
+    44      |      |      |       |      | | |   returned  both during actual replacement and when |
+    45      |      |      |       |      | | |   an interrupted  replacement is  completed as part |
+    46      |      |      |       |      | | |   of bringing a unit "Unit-Online."                 |
+    47      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-6
+                                                                                          11 June 1992
+
+
+
+     1               Table B-3: "Media Format Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |   10 |  325 |   505 |  145 |*| | No multicopy protection.                            |
+     8      |      |      |       |      | | |   All  but  one  copy  of  a  block  in a multicopy |
+     9      |      |      |       |      | | |   structure are bad.  This is a  warning  that  the |
+    10      |      |      |       |      | | |   disk  should  be  reformatted  or replaced at the |
+    11      |      |      |       |      | | |   earliest convenient time.                         |
+    12      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+    13                     Table B-4:  "Compare Error" Status or Event Subcode Values
+
+    14      +------+---------------------+-+-+-----------------------------------------------------+
+    15      | Sub- |   Code + Subcode    |E|S|                                                     |
+    16      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    17      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    18      +------+------+-------+------+-+-+-----------------------------------------------------+
+    19      |    0 |    7 |    7  |    7 |*|*| Subcodes  are  not  used.  Note: the  compare error |
+    20      |      |      |       |      | | | code  is only used  as an event code when the error |
+    21      |      |      |       |      | | | occurs   during   a read-compare  or  write-compare |
+    22      |      |      |       |      | | | operation.                                          |
+    23      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+    24                      Table B-5:  "Data Error" Status or Event Subcode Values
+
+    25      +------+---------------------+-+-+-----------------------------------------------------+
+    26      | Sub- |   Code + Subcode    |E|S|                                                     |
+    27      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    28      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    29      +------+------+-------+------+-+-+-----------------------------------------------------+
+    30      |    0 |    8 |    10 |    8 | |*| Sector was written with "Force Error" modifier      |
+    31      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-7
+                                                                                          11 June 1992
+
+
+
+     1                   Table B-5: "Data Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    2 |   72 |   110 |   48 |*|*| Invalid header.                                     |
+     8      |      |      |       |      | | |   The  subsystem read  an invalid  or  inconsistent |
+     9      |      |      |       |      | | |   header for the requested sector.  For recoverable |
+    10      |      |      |       |      | | |   errors,  this code  implies that  a retry  of the |
+    11      |      |      |       |      | | |   transfer read  a valid header.  For unrecoverable |
+    12      |      |      |       |      | | |   errors,  this code  implies  that  the  subsystem |
+    13      |      |      |       |      | | |   attempted  tertiary  revectoring  and  determined |
+    14      |      |      |       |      | | |   that  the  requested  sector  is  not  revectored |
+    15      |      |      |       |      | | |   (i.e.,  the RCT indicates  that the sector is not |
+    16      |      |      |       |      | | |   revectored).  Causes of an invalid header include |
+    17      |      |      |       |      | | |   header  missync,  header   sync  timeout,  and  a |
+    18      |      |      |       |      | | |   garbage (inconsistent) header.                    |
+    19      +------+------+-------+------+-+-+-----------------------------------------------------+
+    20      |    3 |  104 |   150 |   68 |*|*| Data Sync not found (Data Sync timeout).            |
+    21      +------+------+-------+------+-+-+-----------------------------------------------------+
+    22      |    4 |  136 |   210 |   88 |*| | Correctable error in ECC field.                     |
+    23      |      |      |       |      | | |   A transfer  encountered  a correctable  error  in |
+    24      |      |      |       |      | | |   which only  the ECC field was affected.  That is, |
+    25      |      |      |       |      | | |   all  data bits  were correct  but some portion of |
+    26      |      |      |       |      | | |   the ECC field was incorrect.  The severity of the |
+    27      |      |      |       |      | | |   error (i.e., the  number of symbols  in error) is |
+    28      |      |      |       |      | | |   unknown.  If the  number of  symbols in  error is |
+    29      |      |      |       |      | | |   known,  an "n Symbol ECC Error" subcode should be |
+    30      |      |      |       |      | | |   returned instead.                                 |
+    31      +------+------+-------+------+-+-+-----------------------------------------------------+
+    32  |   |    5 |  168 |   250 |   A8 |*| | Header Threshold Event                              |
+    33  |   |      |      |       |      | | |   This is not an error. Data correction or recovery |
+    34  |   |      |      |       |      | | |   was used in the header portion of a block at      |
+    35  |   |      |      |       |      | | |   levels within the range of normal device          |
+    36  |   |      |      |       |      | | |   operation. This subcode is typically used when    |
+    37  |   |      |      |       |      | | |   sufficient events are available to justify        |
+    38  |   |      |      |       |      | | |   reporting them to automated fault analysis tools. |
+    39  |   |      |      |       |      | | |   The information in the log message is unlikely to |
+    40  |   |      |      |       |      | | |   be useful for any  other purpose.                 |
+    41  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-8
+                                                                                          11 June 1992
+
+
+
+     1                   Table B-5: "Data Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7  |   |    6 |  200 |   310 |   C8 |*| | Data Threshold Event                                |
+     8  |   |      |      |       |      | | |   This is not an error. Data correction or recovery |
+     9  |   |      |      |       |      | | |   was used in the data portion of a block at levels |
+    10  |   |      |      |       |      | | |   within the range of normal device operation. This |
+    11  |   |      |      |       |      | | |   subcode is typically used when sufficient events  |
+    12  |   |      |      |       |      | | |   are available to justify reporting them to        |
+    13  |   |      |      |       |      | | |   automated fault analysis tools. The information in|
+    14  |   |      |      |       |      | | |   the log message is unlikely to be useful for any  |
+    15  |   |      |      |       |      | | |   other purpose.                                    |
+    16  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+    17      |    7 |  232 |   350 |   E8 |*|*| Uncorrectable ECC Error.                            |
+    18      |      |      |       |      | | |   A    transfer   without   the   "Suppress   Error |
+    19      |      |      |       |      | | |   Correction"  modifier  encountered  an  ECC error |
+    20      |      |      |       |      | | |   that  exceeds  the  correction capability  of the |
+    21      |      |      |       |      | | |   subsystem's  error  correction  algorithms, or  a |
+    22      |      |      |       |      | | |   transfer  with  the "Suppress  Error  Correction" |
+    23      |      |      |       |      | | |   modifier encountered an ECC error of any severity |
+    24      |      |      |       |      | | |   whatsoever.                                       |
+    25      +------+------+-------+------+-+-+-----------------------------------------------------+
+    26      |    8 |  264 |   410 |  108 |*| | One Symbol ECC Error.                               |
+    27      |    9 |  296 |   450 |  128 |*| | Two Symbol ECC Error.                               |
+    28      |   10 |  328 |   510 |  148 |*| | Three Symbol ECC Error.                             |
+    29      |   11 |  360 |   550 |  168 |*| | Four Symbol ECC Error.                              |
+    30      |   12 |  392 |   610 |  188 |*| | Five Symbol ECC Error.                              |
+    31      |   13 |  424 |   650 |  1A8 |*| | Six Symbol ECC Error.                               |
+    32      |   14 |  456 |   710 |  1C8 |*| | Seven Symbol ECC Error.                             |
+    33      |   15 |  488 |   750 |  1E8 |*| | Eight Symbol ECC Error.                             |
+    34      |   16 |  520 |  1010 |  208 |*| | Nine Symbol ECC Error.                              |
+    35      |   17 |  552 |  1050 |  228 |*| | Ten Symbol ECC Error.                               |
+    36      |   18 |  584 |  1110 |  248 |*| | Eleven Symbol ECC Error.                            |
+    37      |   19 |  616 |  1150 |  268 |*| | Twelve Symbol ECC Error.                            |
+    38      |   20 |  648 |  1210 |  288 |*| | Thirteen Symbol ECC Error.                          |
+    39      |   21 |  680 |  1250 |  2A8 |*| | Fourteen Symbol ECC Error.                          |
+    40      |   22 |  712 |  1310 |  2C8 |*| | Fifteen Symbol ECC Error.                           |
+    41      |      |      |       |      | | |   A transfer  encountered  a correctable  ECC error |
+    42      |      |      |       |      | | |   with  the  specified  number  of  ECC  symbols in |
+    43      |      |      |       |      | | |   error.  The  number of  symbols in  error roughly |
+    44      |      |      |       |      | | |   corresponds to the severity of the error.         |
+    45      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-9
+                                                                                          11 June 1992
+
+
+     1               Table B-6:  "Host Buffer Access Error" Status or Event Subcode Values
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    0 |    9 |    11 |    9 | |*| Host buffer access error, cause not available.      |
+     8      |      |      |       |      | | |   The controller was unable to access a host buffer |
+     9      |      |      |       |      | | |   to perform a transfer, but has no visibility into |
+    10      |      |      |       |      | | |   the cause of the error.                           |
+    11      +------+------+-------+------+-+-+-----------------------------------------------------+
+    12      |    1 |   41 |    51 |   29 | |*| Odd transfer address                                |
+    13      +------+------+-------+------+-+-+-----------------------------------------------------+
+    14      |    2 |   73 |   111 |   49 | |*| Odd byte count                                      |
+    15      +------+------+-------+------+-+-+-----------------------------------------------------+
+    16      |    3 |  105 |   151 |   69 |*|*| Nonexistent memory error                            |
+    17      +------+------+-------+------+-+-+-----------------------------------------------------+
+    18      |    4 |  137 |   211 |   89 |*|*| Host memory parity error                            |
+    19      +------+------+-------+------+-+-+-----------------------------------------------------+
+    20      |    5 |  169 |   251 |   A9 |*|*| Invalid page table entry                            |
+    21      |      |      |       |      | | |   (See the  Storage  Systems Port Specification for |
+    22      |      |      |       |      | | |    additional detail.)                              |
+    23      +------+------+-------+------+-+-+-----------------------------------------------------+
+    24      |    6 |  201 |   311 |   C9 |*|*| Invalid buffer name                                 |
+    25      |      |      |       |      | | |   The key in the buffer name does not match the key |
+    26      |      |      |       |      | | |   in the buffer descriptor, the V bit in the buffer |
+    27      |      |      |       |      | | |   descriptor is clear, or the index into the Buffer |
+    28      |      |      |       |      | | |   Descriptor  Table  is  too large. (See the BI VAX |
+    29      |      |      |       |      | | |   Port Specification for additional detail.)        |
+    30      +------+------+-------+------+-+-+-----------------------------------------------------+
+    31      |    7 |  233 |   351 |   E9 |*|*| Buffer length violation                             |
+    32      |      |      |       |      | | |   The  number  of  bytes  requested   in  the  MSCP |
+    33      |      |      |       |      | | |   command   exceeds  the buffer length as specified |
+    34      |      |      |       |      | | |   in   the  buffer  descriptor.  (See  the  BI  VAX |
+    35      |      |      |       |      | | |   Port Specification for additional detail.)        |
+    36      +------+------+-------+------+-+-+-----------------------------------------------------+
+    37      |    8 |  265 |   411 |  109 |*|*| Access control violation                            |
+    38      |      |      |       |      | | |   The   access   mode   specified   in  the  buffer |
+    39      |      |      |       |      | | |   descriptor   is  protected  against  by  the PROT |
+    40      |      |      |       |      | | |   field   in   the   PTE.  (See  the  BI  VAX  Port |
+    41      |      |      |       |      | | |   Specification for additional detail.)             |
+    42      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-10
+                                                                                          11 June 1992
+
+
+     1                   Table B-7:  "Controller Error" Status or Event Subcode Values
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    0 |   10 |    12 |    A |-|-| Reserved  for host  detected command  timeout error |
+     8      |      |      |       |      | | | logging.  This  subcode  is  never  reported  by  a |
+     9      |      |      |       |      | | | controller.                                         |
+    10      +------+------+-------+------+-+-+-----------------------------------------------------+
+    11      |    1 |   42 |    52 |   2A |*|*| SERDES overrun or underrun error.                   |
+    12      |      |      |       |      | | |   Either the  drive is too fast for the controller, |
+    13      |      |      |       |      | | |   or (more typically) a  controller hardware  fault |
+    14      |      |      |       |      | | |   has  prevented controller  microcode  from  being |
+    15      |      |      |       |      | | |   able to keep up with data transfer to or from the |
+    16      |      |      |       |      | | |   drive.                                            |
+    17      +------+------+-------+------+-+-+-----------------------------------------------------+
+    18      |    2 |   74 |   112 |   4A |*|*| EDC error.                                          |
+    19      |      |      |       |      | | |   The  sector was  read with correct or correctable |
+    20      |      |      |       |      | | |   ECC and an invalid  EDC.  There is most  likely a |
+    21      |      |      |       |      | | |   fault in  the ECC logic of either this controller |
+    22      |      |      |       |      | | |   or the controller that last wrote the sector.     |
+    23      +------+------+-------+------+-+-+-----------------------------------------------------+
+    24      |    3 |  106 |   152 |   6A |*|*| Software Logic error.                               |
+    25      |      |      |       |      | | |   Some  high  level  check detected  an  error that |
+    26      |      |      |       |      | | |   indicates a fault in the execution of code within |
+    27      |      |      |       |      | | |   the controller. For example, an inconsistent data |
+    28      |      |      |       |      | | |   structure  is  detected (a nonzero reserved field |
+    29      |      |      |       |      | | |   or a value in a field outside its valid range) or |
+    30      |      |      |       |      | | |   an error in the code  flow  occurs.   This  error |
+    31      |      |      |       |      | | |   almost   always   implies   the  existence  of  a |
+    32      |      |      |       |      | | |   microcode bug.                                    |
+    33      +------+------+-------+------+-+-+-----------------------------------------------------+
+    34      |    4 |  138 |   212 |   8A |*|*| Internal EDC error.                                 |
+    35      |      |      |       |      | | |   Some  low  level  check detected an  inconsistent |
+    36      |      |      |       |      | | |   data    structure.  For   example,  a   microcode |
+    37      |      |      |       |      | | |   implemented checksum or vertical parity (hardware |
+    38      |      |      |       |      | | |   parity  is  horizontal)  associated with internal |
+    39      |      |      |       |      | | |   sector data was inconsistent.  This error usually |
+    40      |      |      |       |      | | |   implies a fault in the memory addressing logic of |
+    41      |      |      |       |      | | |   one  or  more  of  the   controller's  processing |
+    42      |      |      |       |      | | |   elements.  It  may also  result from a double bit |
+    43      |      |      |       |      | | |   error   or  other  error  that  exceeds the error |
+    44      |      |      |       |      | | |   detection capability of the controller's hardware |
+    45      |      |      |       |      | | |   memory checking circuitry.                        |
+    46      +------+------+-------+------+-+-+-----------------------------------------------------+
+    47      |    5 |  170 |   252 |   AA |*|*| LESI Adaptor Card parity error on input (adaptor to |
+    48      |      |      |       |      | | | controller).                                        |
+    49      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-11
+                                                                                          11 June 1992
+
+
+
+     1                Table B-7: "Controller Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    6 |  202 |   312 |   CA |*|*| LESI   Adaptor   Card    parity   error  on  output |
+     8      |      |      |       |      | | | (controller to adaptor).                            |
+     9      +------+------+-------+------+-+-+-----------------------------------------------------+
+    10      |    7 |  234 |   352 |   EA |*|*| LESI Adaptor Card "cable in place" not asserted.    |
+    11      +------+------+-------+------+-+-+-----------------------------------------------------+
+    12      |    8 |  266 |   412 |  10A |*|*| Controller overrun or underrun.                     |
+    13      |      |      |       |      | | |   The   controller  attempted  to  perform too many |
+    14      |      |      |       |      | | |   concurrent transfers, causing one or more of them |
+    15      |      |      |       |      | | |   to fail due to a data overrun or underrun.        |
+    16      +------+------+-------+------+-+-+-----------------------------------------------------+
+    17      |    9 |  298 |   452 |  12A |*|*| Controller memory error.                            |
+    18      |      |      |       |      | | |   The controller  detected an  error in an internal |
+    19      |      |      |       |      | | |   memory, such  as  a parity error or nonresponding |
+    20      |      |      |       |      | | |   address.  This  subcode  only  applies  to errors |
+    21      |      |      |       |      | | |   that  do not  affect the controller's  ability to |
+    22      |      |      |       |      | | |   properly  generate end and error log messages, as |
+    23      |      |      |       |      | | |   errors  that  might  affect  end  and  error  log |
+    24      |      |      |       |      | | |   messages  are not  reported via MSCP.  Therefore, |
+    25      |      |      |       |      | | |   for most  controllers  this  subcode will only be |
+    26      |      |      |       |      | | |   returned for controller  memory errors in data or |
+    27      |      |      |       |      | | |   buffer    memory     and    noncritical   control |
+    28      |      |      |       |      | | |   structures.  If the  controller  has several such |
+    29      |      |      |       |      | | |   memories,  the   specific   memory   involved  is |
+    30      |      |      |       |      | | |   reported   as   part  of the error address in the |
+    31      |      |      |       |      | | |   error log message.                                |
+    32      +------+------+-------+------+-+-+-----------------------------------------------------+
+    33      |   10 |  330 |   512 |  14A |-|-| "Bundled Shadowing (see Note 5)                     |
+    34      +------+------+-------+------+-+-+-----------------------------------------------------+
+    35      |   11 |  362 |   552 |  16A |*|*| Drive Interface Hardware error.                     |
+    36      |      |      |       |      | | |   The controller detected an error that indicates a |
+    37      |      |      |       |      | | |   fault in the Drive Interface Hardware.            |
+    38      +------+------+-------+------+-+-+-----------------------------------------------------+
+    39      |   12 |  394 |   612 |  18A |*|*| Host Interface Hardware error.                      |
+    40      |      |      |       |      | | |   The controller detected an error that indicates a |
+    41      |      |      |       |      | | |   fault in the Host Interface Hardware.             |
+    42      +------+------+-------+------+-+-+-----------------------------------------------------+
+    43      |   13 |  426 |   652 |  1AA |*|*| Internal Bus error.                                 |
+    44      |      |      |       |      | | |   The controller detected an error that indicates a |
+    45      |      |      |       |      | | |   fault in the Internal Bus Hardware.               |
+    46      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-12
+                                                                                          11 June 1992
+
+
+
+     1                Table B-7: "Controller Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |   14 |  458 |   712 |  1CA |*|*| Policy Processor error.                             |
+     8      |      |      |       |      | | |   The controller detected an error that indicates a |
+     9      |      |      |       |      | | |   fault in the internal Policy Processor hardware.  |
+    10      +------+------+-------+------+-+-+-----------------------------------------------------+
+    11      |   15 |  490 |   752 |  1EA |*| | Fault Management Analysis event                     |
+    12      |      |      |       |      | | |  Some specific  fault management  threshold  within |
+    13      |      |      |       |      | | |  the  controller   has  been  exceeded.  The server |
+    14      |      |      |       |      | | |  is still operating with designed functionality and |
+    15      |      |      |       |      | | |  performance.   Further  analysis  is  required  to |
+    16      |      |      |       |      | | |  determine if  Field Service action is needed.  For |
+    17      |      |      |       |      | | |  example, a pool of spare memory pages has been two |
+    18      |      |      |       |      | | |  thirds consumed  to  map around bad  pages.   This |
+    19      |      |      |       |      | | |  event code would  have the informational flag  set |
+    20      |      |      |       |      | | |  and  the  Controller Dependent  Information  field |
+    21      |      |      |       |      | | |  would be used to further define the event.         |
+    22      +------+------+-------+------+-+-+-----------------------------------------------------+
+    23      |   16 |  522 |  1012 |  20A |*| | Self Test error.                                    |
+    24      |      |      |       |      | | |  The controller detected  an error  while executing |
+    25      |      |      |       |      | | |  an  internal  self test.   The  error detected may |
+    26      |      |      |       |      | | |  still allow continued operation of the server. For |
+    27      |      |      |       |      | | |  example, a  periodic test of an unused device port |
+    28      |      |      |       |      | | |  detects an error in that device port.  This  event |
+    29      |      |      |       |      | | |  code  would  use Controller Dependent  Information |
+    30      |      |      |       |      | | |  field to further define the event.                 |
+    31      +--------------------------------------------------------------------------------------+
+    32      |   20 |  650 |  1212 |  28A | |*| Remote Connection Lost                              |
+    33      +------+------+-------+------+-+-+-----------------------------------------------------+
+    34      |   21 |  682 |  1252 |  2AA | |*| Remote   Connection  Request  Failed,  insufficient |
+    35      |      |      |       |      | | | resources to request remote connection              |
+    36      +------+------+-------+------+-+-+-----------------------------------------------------+
+    37      |   22 |  714 |  1312 |  2CA |*|*| Remote  Connection  Request  Failed, subsystem port |
+    38      |      |      |       |      | | | address responded but subsystem system address does |
+    39      |      |      |       |      | | | not match                                           |
+    40      +------+------+-------+------+-+-+-----------------------------------------------------+
+    41      |   23 |  746 |  1352 |  2EA |*|*| Remote  Connection  Request  Failed, subsystem port |
+    42      |      |      |       |      | | | address responded but no MSCP server present        |
+    43      +------+------+-------+------+-+-+-----------------------------------------------------+
+    44      |   24 |  778 |  1412 |  30A |*|*| Remote  Connection  Request  Failed, subsystem port |
+    45      |      |      |       |      | | | address responded but no resources available        |
+    46      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-13
+                                                                                          11 June 1992
+
+
+
+     1                Table B-7: "Controller Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |   25 |  810 |  1452 |  32A |*|*| Remote  Connection  Request  Failed, subsystem port |
+     8      |      |      |       |      | | | address responded but no credits available          |
+     9      +------+------+-------+------+-+-+-----------------------------------------------------+
+    10      |   26 |  842 |  1512 |  34A |*|*| Remote  Connection Request Failed, no response from |
+    11      |      |      |       |      | | | subsystem port address                              |
+    12      +------+------+-------+------+-+-+-----------------------------------------------------+
+    13      |   27 |  874 |  1552 |  36A |*|*| Remote  Connection  Request  Failed, subsystem port |
+    14      |      |      |       |      | | | address responding but negative  acknowledge  retry |
+    15      |      |      |       |      | | | algorithm exhausted                                 |
+    16      +------+------+-------+------+-+-+-----------------------------------------------------+
+    17      |   28 |  906 |  1612 |  38A |*|*| Remote  Connection  Request  Failed, subsystem port |
+    18      |      |      |       |      | | | request timeout exceeded                            |
+    19      +------+------+-------+------+-+-+-----------------------------------------------------+
+    20      |   29 |  938 |  1652 |  3AA |*|*| Local   Connection   Request  Failed,  insufficient |
+    21      |      |      |       |      | | | resources to request local connection               |
+    22      +------+------+-------+------+-+-+-----------------------------------------------------+
+    23      |   30 |  970 |  1712 |  3CA |*|*| Local   Connection  Request  Failed, subsystem port |
+    24      |      |      |       |      | | | address responded but rejected with Disconnected    |
+    25      +------+------+-------+------+-+-+-----------------------------------------------------+
+    26  |   |   31 | 1002 |  1752 |  3EA |*| | Last Fail - Hardware                                |
+    27  |   |      |      |       |      | | |   The controller detected a fatal hardware error    |
+    28  |   |      |      |       |      | | |   that can not be resolved without bugcheck/reinit- |
+    29  |   |      |      |       |      | | |   ialization. After reinitialization the controller |
+    30  |   |      |      |       |      | | |   builds a last fail error log packet describing    |
+    31  |   |      |      |       |      | | |   the reason for the bugcheck.                      |
+    32  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+    33  |   |  32  | 1034 |  2012 |  40A |*| | Last Fail - Software                                |
+    34  |   |      |      |       |      | | |   The controller detected a fatal software error    |
+    35  |   |      |      |       |      | | |   that can not be resolved without bugcheck/re-     |
+    36  |   |      |      |       |      | | |   initialization. After reinitialization the        |
+    37  |   |      |      |       |      | | |   controller builds a last fail error log packet    |
+    38  |   |      |      |       |      | | |   describing the reason for the bugcheck.           |
+    39  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-14
+                                                                                          11 June 1992
+
+
+     1                      Table B-8:  "Drive Error" Status or Event Subcode Values
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    1 |   43 |    53 |   2B |*|*| Drive command time out.                             |
+     8      |      |      |       |      | | |   For  SI drives, the  controller's timeout expired |
+     9      |      |      |       |      | | |   for either a level 2 exchange or the assertion of |
+    10      |      |      |       |      | | |   Read/Write Ready after an Initiate Seek.          |
+    11      +------+------+-------+------+-+-+-----------------------------------------------------+
+    12      |    2 |   75 |   113 |   4B |*|*| Controller detected transmission error.             |
+    13      |      |      |       |      | | |   For SI drives, the controller detected an invalid |
+    14      |      |      |       |      | | |   framing  code  or a  checksum  error in a level 2 |
+    15      |      |      |       |      | | |   response  from  the  drive.  The UDA50 and UDA50A |
+    16      |      |      |       |      | | |   also  return this subcode for controller detected |
+    17      |      |      |       |      | | |   protocol errors.  All other SI controllers return |
+    18      |      |      |       |      | | |   subcode 10 for protocol errors.                   |
+    19      +------+------+-------+------+-+-+-----------------------------------------------------+
+    20      |    3 |  107 |   153 |   6B |*|*| Positioner error (misseek).                         |
+    21      |      |      |       |      | | |   The  drive  reported  that  a  seek operation was |
+    22      |      |      |       |      | | |   successful, but  the  controller  determined that |
+    23      |      |      |       |      | | |   the drive  had  positioned itself to an incorrect |
+    24      |      |      |       |      | | |   cylinder.                                         |
+    25      +------+------+-------+------+-+-+-----------------------------------------------------+
+    26      |    4 |  139 |   213 |   8B |*|*| Lost read/write ready during or between transfers.  |
+    27      |      |      |       |      | | |   For  SI drives, Read/Write ready was negated when |
+    28      |      |      |       |      | | |   the  controller  attempted to initiate a transfer |
+    29      |      |      |       |      | | |   or  at  the    completion   of  a  transfer,  and |
+    30      |      |      |       |      | | |   Read/Write  ready  had  previously  been asserted |
+    31      |      |      |       |      | | |   (indicating  completion  of  the preceding seek). |
+    32      |      |      |       |      | | |   This  usually  results   from  a  drive  detected |
+    33      |      |      |       |      | | |   transfer error, in which case an additional error |
+    34      |      |      |       |      | | |   log  message  may  be  generated  containing  the |
+    35      |      |      |       |      | | |   "Drive detected error" subcode.                   |
+    36      +------+---------------------+-+-+-----------------------------------------------------+
+    37      |    5 |  171 |   253 |   AB |*|*| Drive clock dropout.                                |
+    38      |      |      |       |      | | |   For  SI  drives, either  data  or state clock was |
+    39      |      |      |       |      | | |   missing  when  it should have been present.  This |
+    40      |      |      |       |      | | |   is usually detected by means of a timeout.        |
+    41      +------+------+-------+------+-+-+-----------------------------------------------------+
+    42      |    6 |  203 |   313 |   CB |*|*| Lost receiver ready for transfer.                   |
+    43      |      |      |       |      | | |   For  SI  drives, Receiver Ready  was negated when |
+    44      |      |      |       |      | | |   the controller  attempted to  initiate a transfer |
+    45      |      |      |       |      | | |   or  did   not  assert  at  the  completion  of  a |
+    46      |      |      |       |      | | |   transfer.  This  includes   all   cases  of   the |
+    47      |      |      |       |      | | |   controller's  timeout  expiring  for  a  transfer |
+    48      |      |      |       |      | | |   operation (level 1 real time command).            |
+    49      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-15
+                                                                                          11 June 1992
+
+
+
+     1                   Table B-8: "Drive Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    7 |  235 |   353 |   EB |*|*| Drive detected error.                               |
+     8      |      |      |       |      | | |   For  SI  drives, the  controller  received  a Get |
+     9      |      |      |       |      | | |   Status  or Unsuccessful response with the EL flag |
+    10      |      |      |       |      | | |   set,  or  the controller received such a response |
+    11      |      |      |       |      | | |   with  the  DR  flag  set  and it does not support |
+    12      |      |      |       |      | | |   automatic diagnosis for that drive type.          |
+    13      +------+------+-------+------+-+-+-----------------------------------------------------+
+    14      |    8 |  267 |   413 |  10B |*|*| Controller detected pulse or state parity error.    |
+    15      |      |      |       |      | | |   For  SI drives, the  controller detected  a pulse |
+    16      |      |      |       |      | | |   error  on  either  the  state or data line or the |
+    17      |      |      |       |      | | |   controller  detected  a  parity  error in a state |
+    18      |      |      |       |      | | |   frame.                                            |
+    19      +------+---------------------+-+-+-----------------------------------------------------+
+    20      |   10 |  331 |   513 |  14B |*|*| Controller detected protocol error.                 |
+    21      |      |      |       |      | | |   For  SI drives, a level 2 response from the drive |
+    22      |      |      |       |      | | |   had  correct  framing codes and checksum, but was |
+    23      |      |      |       |      | | |   not  a  valid  response within the constraints of |
+    24      |      |      |       |      | | |   the  SI  protocol.  That  is, the response had an |
+    25      |      |      |       |      | | |   invalid  opcode, was  an  improper length, or was |
+    26      |      |      |       |      | | |   not  a  possible  response  in the context of the |
+    27      |      |      |       |      | | |   exchange.                                         |
+    28      +------+------+-------+------+-+-+-----------------------------------------------------+
+    29      |   11 |  363 |   553 |  16B |*|*| Drive failed initialization.                        |
+    30      |      |      |       |      | | |   For  SI  drives, the  drive  clock did not resume |
+    31      |      |      |       |      | | |   following  a controller attempt to initialize the |
+    32      |      |      |       |      | | |   drive.  This implies that the drive encountered a |
+    33      |      |      |       |      | | |   fatal initialization error.                       |
+    34      +------+------+-------+------+-+-+-----------------------------------------------------+
+    35      |   12 |  395 |   613 |  18B |*|*| Drive ignored initialization.                       |
+    36      |      |      |       |      | | |   For  SI  drives, the  drive  clock  did not cease |
+    37      |      |      |       |      | | |   following  a controller attempt to initialize the |
+    38      |      |      |       |      | | |   drive.  This  implies  that  the  drive  did  not |
+    39      |      |      |       |      | | |   recognize the initialization attempt.             |
+    40      +------+------+-------+------+-+-+-----------------------------------------------------+
+    41      |   13 |  427 |   653 |  1AB |*|*| Receiver Ready collision.                           |
+    42      |      |      |       |      | | |   For SI drives, the controller attempted to assert |
+    43      |      |      |       |      | | |   its  Receiver  Ready (to  receive a response) and |
+    44      |      |      |       |      | | |   the drive's Receiver Ready was still asserted (to |
+    45      |      |      |       |      | | |   receive a command).                               |
+    46      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-16
+                                                                                          11 June 1992
+
+
+     1              Table B-9:  "Bad Block Replacement Completion" Event Only Subcode Values
+
+     2      +------+---------------------+---------------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |                                                         |
+     4      | code +------+-------+------+         Event Subcode                                   |
+     5      |      | Dec. |  Oct. | Hex. |                                                         |
+     6      +------+------+-------+------+---------------------------------------------------------+
+     7      |    0 |   20 |    24 |   14 | Bad block successfully replaced.                        |
+     8      +------+------+-------+------+---------------------------------------------------------+
+     9      |    1 |   52 |    64 |   34 | Block verified OK; not a bad block.                     |
+    10      +------+------+-------+------+---------------------------------------------------------+
+    11      |    2 |   84 |   124 |   54 | Replacement  failure;  REPLACE  command or its analogue |
+    12      |      |      |       |      | failed.                                                 |
+    13      +------+------+-------+------+---------------------------------------------------------+
+    14      |    3 |  116 |   164 |   74 | Replacement failure; inconsistent RCT.                  |
+    15      +------+------+-------+------+---------------------------------------------------------+
+    16      |    4 |  148 |   224 |   94 | Replacement failure; drive access failure.              |
+    17      |      |      |       |      |   One  or  more  transfers specified by the replacement |
+    18      |      |      |       |      |   algorithm failed.                                     |
+    19      +------+------+-------+------+---------------------------------------------------------+
+    20      |    5 |  180 |   264 |   B4 | Replacement failure; no replacement block available.    |
+    21      |      |      |       |      |   Replacement  was  attempted  for  a  bad block, but a |
+    22      |      |      |       |      |   replacement  block  could  not  be  allocated  (i.e., |
+    23      |      |      |       |      |   the volume's RCT is  full).                           |
+    24      +------+------+-------+------+---------------------------------------------------------+
+    25      |    6 |  212 |   324 |   D4 | Replacement failure; recursion failure.                 |
+    26      |      |      |       |      |   Two successive RBNs were bad.                         |
+    27      +------+------+-------+------+---------------------------------------------------------+
+
+
+
+    28                  Table B-10:  "Media Loader Error" Status or Event Subcode Values
+
+    29      +------+---------------------+-+-+-----------------------------------------------------+
+    30      | Sub- |   Code + Subcode    |E|S|                                                     |
+    31      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    32      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    33      +------+------+-------+------+-+-+-----------------------------------------------------+
+    34      |    1 |   55 |   67  |   37 |*|*| Loader command time out                             |
+    35      +------+------+-------+------+-+-+-----------------------------------------------------+
+    36      |    2 |   87 |  127  |   57 |*|*| Controller detected transmission error              |
+    37      +------+------+-------+------+-+-+-----------------------------------------------------+
+    38      |    3 |  119 |  167  |   77 |*|*| Controller detected protocol error                  |
+    39      +------+------+-------+------+-+-+-----------------------------------------------------+
+    40      |    4 |  151 |  227  |   97 |*|*| Loader detected error                               |
+    41      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-17
+                                                                                          11 June 1992
+
+
+     1                       Table B-11:  "Informational" Event Only Subcode Values
+
+     2      +------+---------------------+---------------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |                                                         |
+     4      | code +------+-------+------+         Event Subcode                                   |
+     5      |      | Dec. |  Oct. | Hex. |                                                         |
+     6      +------+------+-------+------+---------------------------------------------------------+
+     7      |   1  |   54 |   66  |   36 | Media Quality log                                       |
+     8      +------+------+-------+------+---------------------------------------------------------+
+     9      |   2  |   86 |  126  |   56 | Unload / Spin down statistics                           |
+    10      +------+------+-------+------+---------------------------------------------------------+
+    11      |   3  | 118  |  166  |  76  | Disk Copy Data Correlation                              |
+    12      +------+------+-------+------+---------------------------------------------------------+
+
+
+    13                   Table B-12:  "Subcommand Error" Status or Event Subcode Values
+
+    14      +------+---------------------+-+-+-----------------------------------------------------+
+    15      | Sub- |   Code + Subcode    |E|S|                                                     |
+    16      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    17      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    18      +------+------+-------+------+-+-+-----------------------------------------------------+
+    19      | 1024 |      Destination/source flag bit: 0=Destination, 1=Source                     |
+    20      +------+------+-------+------+-+-+-----------------------------------------------------+
+    21      |    1 |   62 |    76 |   3E |*|*| Destination -- command timed out                    |
+    22      +------+------+-------+------+-+-+-----------------------------------------------------+
+    23      | 1025 |32830 |100076 | 803E |*|*| Source -- command timed out                         |
+    24      +------+------+-------+------+-+-+-----------------------------------------------------+
+    25      |    2 |   94 |   136 |   5E |*|*| Destination -- inconsistent state                   |
+    26      +------+------+-------+------+-+-+-----------------------------------------------------+
+    27      | 1026 |32862 |100136 | 805E |*|*| Source -- inconsistent state                        |
+    28      +------+------+-------+------+-+-+-----------------------------------------------------+
+    29      |    4 |  158 |   236 |   9E |*|*| Destination -- unrecoverable error                  |
+    30      +------+------+-------+------+-+-+-----------------------------------------------------+
+    31      | 1028 |32926 |100236 | 809E |*|*| Source -- unrecoverable error                       |
+    32      +------+------+-------+------+---------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           STATUS AND EVENT CODE DEFINITIONS                                                 Page B-18
+                                                                                          11 June 1992
+
+
+     1  |                  Table B-13:  "Cache Error" Status or Event Subcode Values
+
+     2  |   +------+---------------------+-+-+-----------------------------------------------------+
+     3  |   | Sub- |   Code + Subcode    |E|S|                                                     |
+     4  |   | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5  |   |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+     7  |   | many |      |       |      |*|*| "Data Error"                                        |
+     8  |   |      For every "Data Error" subcode that may be reported as a status code (i.e., for |
+     9  |   |      every subcode listed in Table B-5, "Data Error" Status or Event Subcode Values, |
+    10  |   |      that has an asterisk in the "ST" column), the exact same subcode value may be   |
+    11  |   |      returned as a "Cache Error" subcode value. These subcodes indicate that a       |
+    12  |   |      transfer to the unit's cache failed for the specified reason. For example, the  |
+    13  |   |      subcode value 7 (combined code and subcode value of 249 decimal) indicates that |
+    14  |   |      the cache block accessed is unreadable due to an Uncorrectable ECC Error.       |
+    15  |   |                                                                                      |
+    16  |   |      In addition to the subcode values copied from "Data Error" subcode values, the  |
+    17  |   |      subcodes listed below also exist.                                               |
+    18  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+    19  |   |    4 |  153 |   231 |   99 |*| | Cache Disabled                                      |
+    20  |   |      |      |       |      | | |   The controller has determined that the ability of |
+    21  |   |      |      |       |      | | |   the cache to maintain data integrity is in        |
+    22  |   |      |      |       |      | | |   question. Therefore, the cache has been disabled. |
+    23  |   |      |      |       |      | | |   Note that before the cache is disabled, a cache   |
+    24  |   |      |      |       |      | | |   flush operation is performed.                     |
+    25  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+    26  |   |    5 |  185 |   271 |   B9 |*|*| Cache Detected Error                                |
+    27  |   |      |      |       |      | | |   For controller internal caches, the controller    |
+    28  |   |      |      |       |      | | |   received an unsuccessful response from the cache, |
+    29  |   |      |      |       |      | | |   indicating a hardware error.                      |
+    30  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+    31  |   |    6 |  217 |   331 |   D9 |*|*| Hardware Port Error                                 |
+    32  |   |      |      |       |      | | |   The controller received an unsuccessful response  |
+    33  |   |      |      |       |      | | |   from the device port, used as a cache, indicating |
+    34  |   |      |      |       |      | | |   a hardware error.                                 |
+    35  |   +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-1
+                                                                                          11 June 1992
+
+
+     1                                             APPENDIX C
+
+     2                            CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS
+
+
+
+     3                                                NOTES
+
+
+     4              o  Values  for  new  products  are  assigned  by   the   Storage   Systems
+     5                 Architecture  Group.   Requests for new assignments should be addressed
+     6                 to SSAG::SSAG.
+
+     7              o  Values for unannounced products are given in Appendix  G,  "Unannounced
+     8                 Products."
+
+
+
+     9                           Table C-1:  Controller and Unit "Class" Values
+
+    10                     +----------+----------------------------------------------+
+    11                     |Class Byte|                                              |
+    12                     | (decimal)|     Subsystem Type                           |
+    13                     +----------+----------------------------------------------+
+    14                     |     0    | reserved -- must not be assigned             |
+    15                     +----------+----------------------------------------------+
+    16                     |     1    | Mass storage controllers                     |
+    17                     +----------+----------------------------------------------+
+    18                     |     2    | Disk class devices -- DEC Standard 166 disks |
+    19                     +----------+----------------------------------------------+
+    20                     |     3    | Tape class devices                           |
+    21                     +----------+----------------------------------------------+
+    22                     |     4    | Disk class devices -- DEC Standard 144 disks |
+    23                     +----------+----------------------------------------------+
+    24                     |     5    | Loader class devices (see Note below)        |
+    25                     +----------+----------------------------------------------+
+    26                     |     6    | SCSI class devices                           |
+    27                     +----------+----------------------------------------------+
+    28                     | Note: The  Media  Loader  device  class  is  defined in |
+    29                     |       anticipation of the adoption of  a  formal  media |
+    30                     |       loader  architecture.   This  class  value  is in |
+    31                     |       current  use  as dictated by the temporary waiver |
+    32                     |       in Section E.6.                                   |
+    33                     +---------------------------------------------------------+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-2
+                                                                                          11 June 1992
+
+
+     1                         Table C-2:  Mass Storage Controller "Model" Values
+
+     2                     +--------------+------------------------------------------+
+     3                     |  Model Byte  |                                          |
+     4                     |  (decimal)   |                                          |
+     5                     | (see Note 1) |   Controller Type                        |
+     6                     +--------------+------------------------------------------+
+     7                     |       0      | reserved -- must not be assigned         |
+     8                     +--------------+------------------------------------------+
+     9                     |       1      | HSC50                                    |
+    10                     +--------------+------------------------------------------+
+    11                     |       2      | UDA50                                    |
+    12                     +--------------+------------------------------------------+
+    13                     |       3      | RC25 integrated controller               |
+    14                     +--------------+------------------------------------------+
+    15                     |       4      | VMS (software MSCP server)               |
+    16                     +--------------+------------------------------------------+
+    17                     |       5      | TU81 integrated controller               |
+    18                     +--------------+------------------------------------------+
+    19                     |       6      | UDA5OA                                   |
+    20                     +--------------+------------------------------------------+
+    21                     |       7      | RQDX1/RQDX2                              |
+    22                     +--------------+------------------------------------------+
+    23                     |       8      | TOPS 10/20 (software MSCP server)        |
+    24                     +--------------+------------------------------------------+
+    25                     |       9      | TQK50                                    |
+    26                     +--------------+------------------------------------------+
+    27                     |      10      | RUX50                                    |
+    28                     +--------------+------------------------------------------+
+    29                     |      11      | unassigned                               |
+    30                     +--------------+------------------------------------------+
+    31                     |      12      | KFBTA                                    |
+    32                     +--------------+------------------------------------------+
+    33                     |      13      | KDA50-Q                                  |
+    34                     +--------------+------------------------------------------+
+    35                     |      14      | TQK70                                    |
+    36                     +--------------+------------------------------------------+
+    37                     |      15      | RV20                                     |
+    38                     +--------------+------------------------------------------+
+    39                     |      16      | KRQ50                                    |
+    40                     +--------------+------------------------------------------+
+    41                     |      17      | unassigned                               |
+    42                     +--------------+------------------------------------------+
+    43                     |      18      | KDB50                                    |
+    44                     +--------------+------------------------------------------+
+    45                     |      19      | RQDX3                                    |
+    46                     +--------------+------------------------------------------+
+    47                     |      20      | see Appendix H, Table H-1                |
+    48                     +--------------+------------------------------------------+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-3
+                                                                                          11 June 1992
+
+
+
+     1                      Table C-2: Mass Storage Controller "Model" Values (cont.)
+
+     2                     +--------------+------------------------------------------+
+     3                     |  Model Byte  |                                          |
+     4                     |  (decimal)   |                                          |
+     5                     | (see Note 1) |   Controller Type                        |
+     6                     +--------------+------------------------------------------+
+     7                     |      21      | SSP/DSSI Disk (see Note 2)               |
+     8                     +--------------+------------------------------------------+
+     9                     |      22      | SSP/DSSI Tape (see Note 2)               |
+    10                     +--------------+------------------------------------------+
+    11                     |      23      | SSP/DSSI Disk and Tape(see Note 2)       |
+    12                     +--------------+------------------------------------------+
+    13                     |      24      | SSP/DSSI Other (see Note 2)              |
+    14                     +--------------+------------------------------------------+
+    15                     |      25      | TUK50                                    |
+    16                     +--------------+------------------------------------------+
+    17                     |      26      | unassigned                               |
+    18                     +--------------+------------------------------------------+
+    19                     |      27      | KDM70                                    |
+    20                     +--------------+------------------------------------------+
+    21                     |      28      | reserved                                 |
+    22                     +--------------+------------------------------------------+
+    23                     |      29      | TM32                                     |
+    24                     +--------------+------------------------------------------+
+    25  |                  |      30      | RQZX1 (See note 2)                       |
+    26  |                  +--------------+------------------------------------------+
+    27  |                  |      31      | KCM44 (Polecat)                          |
+    28                     +--------------+------------------------------------------+
+    29                     |      32      | HSC70                                    |
+    30                     +--------------+------------------------------------------+
+    31                     |      33      | HSC40                                    |
+    32                     +--------------+------------------------------------------+
+    33                     |      34      | HSC60                                    |
+    34                     +--------------+------------------------------------------+
+    35                     |      35      | HSC90                                    |
+    36                     +--------------+------------------------------------------+
+    37                     |    36 - 40   | unassigned                               |
+    38                     +--------------+------------------------------------------+
+    39                     |      41      | see Appendix H, Table H-1                |
+    40                     +--------------+------------------------------------------+
+    41                     |      42      | see Appendix H, Table H-1                |
+    42                     +--------------+------------------------------------------+
+    43                     |    43 - 64   | unassigned                               |
+    44                     +--------------+------------------------------------------+
+    45                     |      65      | TK50-DEBNT                               |
+    46                     +--------------+------------------------------------------+
+    47                     |      66      | TBK70                                    |
+    48                     +--------------+------------------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-4
+                                                                                          11 June 1992
+
+
+
+     1                      Table C-2: Mass Storage Controller "Model" Values (cont.)
+
+     2                     +--------------+------------------------------------------+
+     3                     |  Model Byte  |                                          |
+     4                     |  (decimal)   |                                          |
+     5                     | (see Note 1) |   Controller Type                        |
+     6                     +--------------+------------------------------------------+
+     7                     |    67 - 95   | unassigned                               |
+     8                     +--------------+------------------------------------------+
+     9                     |      96      | RF30                                     |
+    10                     +--------------+------------------------------------------+
+    11                     |      97      | RF71                                     |
+    12                     +--------------+------------------------------------------+
+    13                     |      98      | TF85                                     |
+    14                     +--------------+------------------------------------------+
+    15                     |      99      | TF70                                     |
+    16                     +--------------+------------------------------------------+
+    17                     |     100      | RF31                                     |
+    18                     +--------------+------------------------------------------+
+    19                     |     101      | RF72                                     |
+    20                     +--------------+------------------------------------------+
+    21                     |     102      | RF73                                     |
+    22                     +--------------+------------------------------------------+
+    23                     |     103      | TF837                                    |
+    24                     +--------------+------------------------------------------+
+    25                     |     104      | RF35                                     |
+    26                     +--------------+------------------------------------------+
+    27                     |     105      | see Appendix H, Table H-1                |
+    28                     +--------------+------------------------------------------+
+    29                     |  106 - 107   | unassigned                               |
+    30                     +--------------+------------------------------------------+
+    31  |                  |  108 - 109   | see Appendix H, Table H-1                |
+    32  |                  +--------------+------------------------------------------+
+    33  |                  |     110      | TF86                                     |
+    34                     +--------------+------------------------------------------+
+    35                     |     111      | unassigned                               |
+    36                     +--------------+------------------------------------------+
+    37                     |     112      | see Appendix H, Table H-1                |
+    38                     +--------------+------------------------------------------+
+    39                     |  113 - 127   | unassigned                               |
+    40                     +--------------+------------------------------------------+
+    41                     |  128 - 247   | reserved for future use                  |
+    42                     +--------------+------------------------------------------+
+    43                     |  248 - 250   | see Appendix H, Table H-1                |
+    44                     +--------------+------------------------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-5
+                                                                                          11 June 1992
+
+
+
+     1                      Table C-2: Mass Storage Controller "Model" Values (cont.)
+
+     2                     +--------------+------------------------------------------+
+     3                     |  Model Byte  |                                          |
+     4                     |  (decimal)   |                                          |
+     5                     | (see Note 1) |   Controller Type                        |
+     6                     +--------------+------------------------------------------+
+     7                     |  251 - 255   | unassigned                               |
+     8                     +--------------+------------------------------------------+
+     9                     | Notes: 1. The   preferred   method   for   assigning  a |
+    10                     |           controller  model  value  is according to the |
+    11                     |           port  protocol  used.  The  recommended model |
+    12                     |           ranges are as follows:                        |
+    13                     |                                                         |
+    14                     |              o 1-31    SSP                              |
+    15                     |              o 32-63   CI                               |
+    16                     |              o 64-95   BVP                              |
+    17                     |              o 96-127  DSSI                             |
+    18                     |              o 128-247 Reserved for future use          |
+    19                     |              o 248-255 Host Software Servers            |
+    20                     |                                                         |
+    21                     |        2.  The  model  values  that reference this note |
+    22                     |            identify the  generic type  of  a   SSP/DSSI |
+    23                     |            device   at  the  SSP  level.  Those  values |
+    24                     |            are  never  returned  at  the MSCP level but |
+    25                     |            appear here to ensure compatibility with the |
+    26                     |            SSP specification assignments.               |
+    27                     +---------------------------------------------------------+
+
+
+    28                           Table C-3:  Disk Drive/Media Identifier Values
+
+    29     +----------+---------+-------+---------------------------+--------------------------------+
+    30     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+    31     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+    32     |          |         |       |     octal     |    hex    |                                |
+    33     +----------+---------+-------+---------------+-----------+--------------------------------+
+    34     |     0    | reserved -- must not be assigned                                             |
+    35     +----------+---------+-------+---------------+-----------+--------------------------------+
+    36     |     1    |    DU   | RA80  | 022544,010120 | 2564,1050 | 121 MB, 14", fixed             |
+    37     +----------+---------+-------+---------------+-----------+--------------------------------+
+    38     |     2    |    DA   | RC25  | 020144,030031 | 2064,3019 | 26 MB, 8", removable           |
+    39     +----------+---------+-------+---------------+-----------+--------------------------------+
+    40     |     3    |    DA   | RCF25 | 020144,031431 | 2064,3319 | 26 MB, 8", fixed               |
+    41     +----------+---------+-------+---------------+-----------+--------------------------------+
+    42     |     4    |    DJ   | RA60  | 021244,010074 | 22A4,103C | 205 MB, 14", removable         |
+    43     +----------+---------+-------+---------------+-----------+--------------------------------+
+    44     |     5    |    DU   | RA81  | 022544,010121 | 2564,1051 | 456 MB, 14", fixed             |
+    45     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-6
+                                                                                          11 June 1992
+
+
+
+     1                        Table C-3: Disk Drive/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |     6    |    DU   | RD51  | 022544,040063 | 2564,4033 | 10 MB,  5.25",   fixed,   full |
+     8     |          |         |       |               |           | height                         |
+     9     +----------+---------+-------+---------------+-----------+--------------------------------+
+    10     |     7    |    DU   | RX50  | 022545,100062 | 2565,8032 | 400  KB,  5.25",  single-sided |
+    11     |          |         |       |               |           | 96  TPI  floppy,  full height, |
+    12     |          |         |       |               |           | dual drives (800 KB total)     |
+    13     +----------+---------+-------+---------------+-----------+--------------------------------+
+    14     |     8    |    DU   | RD52  | 022544,040064 | 2564,4034 | 33  MB,   5.25",  fixed,  full |
+    15     |          |         |       |               |           | height                         |
+    16     +----------+---------+-------+---------------+-----------+--------------------------------+
+    17     |     9    |    DU   | RD53  | 022544,040065 | 2564,4035 | 71  MB,  5.25",   fixed,  full |
+    18     |          |         |       |               |           | height                         |
+    19     +----------+---------+-------+---------------+-----------+--------------------------------+
+    20     |    10    |    DU   | RX33  | 022545,100041 | 2565,8021 | 1200  KB,  5.25", double-sided |
+    21     |          |         |       |               |           | 96  TPI  floppy,  half height, |
+    22     |          |         |       |               |           | (also accepts RX50 media)      |
+    23     +----------+---------+-------+---------------+-----------+--------------------------------+
+    24     |    11    |    DU   | RA82  | 022544,010122 | 2564,1052 | 622 MB, 14", fixed             |
+    25     +----------+---------+-------+---------------+-----------+--------------------------------+
+    26     |    12    |    DU   | RD31  | 022544,040037 | 2564,401F | 20  MB,  5.25",   fixed,  half |
+    27     |          |         |       |               |           | height                         |
+    28     +----------+---------+-------+---------------+-----------+--------------------------------+
+    29     |    13    |    DU   | RD54  | 022544,040066 | 2564,4036 | 160  MB,  5.25",  fixed,  full |
+    30     |          |         |       |               |           | height                         |
+    31     +----------+---------+-------+---------------+-----------+--------------------------------+
+    32     |    14    |    DU   | RRD50 | 022545,021062 | 2565,2232 | 500  MB,   4.75",   removable, |
+    33     |          |         |       |               |           | DAD (optical) format           |
+    34     +----------+---------+-------+---------------+-----------+--------------------------------+
+    35     |    15    | see Appendix H, Table H-2                                                    |
+    36     +----------+---------+-------+---------------+-----------+--------------------------------+
+    37     |    16    | unassigned                                                                   |
+    38     +----------+---------+-------+---------------+-----------+--------------------------------+
+    39     |    17    |    DU   | RX18  | 022545,100022 | 2565,8012 | 180  KB,  5.25",  single-sided |
+    40     |          |         |       |               |           | 96  TPI  floppy,  full height, |
+    41     |          |         |       |               |           | (aka RX180)                    |
+    42     +----------+---------+-------+---------------+-----------+--------------------------------+
+    43     |    18    |    DU   | RA70  | 022544,010106 | 2564,1046 | 280  MB,  5.25",  fixed,  full |
+    44     |          |         |       |               |           | height                         |
+    45     +----------+---------+-------+---------------+-----------+--------------------------------+
+    46     |    19    |    DU   | RA90  | 022544,010132 | 2564,105A | 1.216 GB, 9", fixed            |
+    47     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-7
+                                                                                          11 June 1992
+
+
+
+     1                        Table C-3: Disk Drive/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |    20    | see Appendix H, Table H-2                                                    |
+     8     +----------+---------+-------+---------------+-----------+--------------------------------+
+     9     |    21    |    DI   | RF30  | 021144,060036 | 2264,601E | 150 MB, 5.25" , fixed, half    |
+    10     |          |         |       |               |           | height (DSSI Disk Server)      |
+    11     +----------+---------+-------+---------------+-----------+--------------------------------+
+    12     |    22    |    DI   | RF71  | 021144,060107 | 2264,6047 | 400 MB, 5.25" , fixed, half    |
+    13     |          |         |       |               |           | height (DSSI Disk Server)      |
+    14     +----------+---------+-------+---------------+-----------+--------------------------------+
+    15     |  23 - 24 | see Appendix H, Table H-2                                                    |
+    16     +----------+---------+-------+---------------+-----------+--------------------------------+
+    17     |    25    |    DU   | ESE20 | 022513,031224 | 254B,3294 | 120 MB Solid State disk drive  |
+    18     |          |         |       |               |           | with data retention            |
+    19     +----------+---------+-------+---------------+-----------+--------------------------------+
+    20     |    26    |    DU   | RRD40 | 022545,021050 | 2565,2228 | 500  MB,   4.75",   removable, |
+    21     |          |         |       |               |           | DAD (optical) format           |
+    22     +----------+---------+-------+---------------+-----------+--------------------------------+
+    23     |    27    |    DI   | RF31  | 021144,060037 | 2264,601F | 381 MB, 5.25" fixed, half      |
+    24     |          |         |       |               |           | height (DSSI Disk Server)      |
+    25     +----------+---------+-------+---------------+-----------+--------------------------------+
+    26     |    28    |    DI   | RF72  | 021144,060110 | 2264,6048 | 1 GB, 5.25", fixed, full       |
+    27     |          |         |       |               |           | height (DSSI Disk Server)      |
+    28     +----------+---------+-------+---------------+-----------+--------------------------------+
+    29     |    29    |    DU   | RA92  | 022544,010134 | 2564,105C | 1.5 GB, 9", fixed              |
+    30     +----------+---------+-------+---------------+-----------+--------------------------------+
+    31     |    30    | unassigned                                                                   |
+    32     +----------+---------+-------+---------------+-----------+--------------------------------+
+    33     |    31    | see Appendix H, Table H-2                                                    |
+    34     +----------+---------+-------+---------------+-----------+--------------------------------+
+    35     |    32    | unassigned                                                                   |
+    36     +----------+---------+-------+---------------+-----------+--------------------------------+
+    37     |    33    |    DI   | RFH31 | 021144,062037 | 2264,641F | 190.5 MB, 5.25" fixed, half    |
+    38     |          |         |       |               |           | height (DSSI Disk Server)      |
+    39     +----------+---------+-------+---------------+-----------+--------------------------------+
+    40     |    34    |    DI   | RFH72 | 021144,062110 | 2264,6448 | 500 MB, 5.25", fixed, full     |
+    41     |          |         |       |               |           | height (DSSI Disk Server)      |
+    42     +----------+---------+-------+---------------+-----------+--------------------------------+
+    43     |    35    |    DI   | RF73  | 021144,060111 | 2264,6049 | 2 GB, 5.25", fixed             |
+    44     |          |         |       |               |           | (DSSI Disk Server)             |
+    45     +----------+---------+-------+---------------+-----------+--------------------------------+
+    46     |    36    |    DI   | RFH73 | 021144,062111 | 2264,6449 | 1 GB, 5.25", fixed             |
+    47     |          |         |       |               |           | (DSSI Disk Server)             |
+    48     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-8
+                                                                                          11 June 1992
+
+
+
+     1                        Table C-3: Disk Drive/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |    37    |    DU   | RA72  | 022544,010110 | 2564,1048 | 1.0 GB, 5.25", fixed           |
+     8     +----------+---------+-------+---------------+-----------+--------------------------------+
+     9     |  38 - 39 | unassigned                                                                   |
+    10     +----------+---------+-------+---------------+-----------+--------------------------------+
+    11     |    40    |    DU   | RA71  | 022544,010107 | 2564,1047 | 700 MB, 5.25", fixed           |
+    12     +----------+---------+-------+---------------+-----------+--------------------------------+
+    13     |    41    |    DI   | RFF31 | 021144,061437 | 2264,631F | 200 MB, 5.25", fixed, half     |
+    14     |          |         |       |               |           | height. (DSSI Disk Server)     |
+    15     +----------+---------+-------+---------------+-----------+--------------------------------+
+    16     |    42    |    DI   | RF35  | 021144,060043 | 2264,6023 | 852 MB, 3.5", fixed            |
+    17     |          |         |       |               |           | (DSSI Disk Server)             |
+    18     +----------+---------+-------+---------------+-----------+--------------------------------+
+    19     |    43    |    DI   | RFH35 | 021144,062043 | 2264,6423 | 426 MB,3.5", fixed             |
+    20     |          |         |       |               |           | (DSSI Disk Server)             |
+    21     +----------+---------+-------+---------------+-----------+--------------------------------+
+    22     |  44 - 46 | unassigned                                                                   |
+    23     +----------+---------+-------+---------------+-----------+--------------------------------+
+    24     |  47 - 57 | see Appendix H, Table H-2                                                    |
+    25     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+
+    26                           Table C-4:  Tape Drive/Media Identifier Values
+
+    27     +----------+---------+-------+---------------------------+--------------------------------+
+    28     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+    29     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+    30     |          |         |       |     octal     |    hex    |                                |
+    31     +----------+---------+-------+---------------+-----------+--------------------------------+
+    32     |     0    | reserved -- must not be assigned                                             |
+    33     +----------+---------+-------+---------------+-----------+--------------------------------+
+    34     |     1    |    MU   | TA78  | 066550,010116 | 6D68,104E | 9-track, PE/GCR modes, 125 ips |
+    35     |          |         |       |               |           | start/stop                     |
+    36     +----------+---------+-------+---------------+-----------+--------------------------------+
+    37     |     2    |    MU   | TU81  | 066551,050121 | 6D69,5051 | 9-track,  PE/GCR modes,  25/75 |
+    38     |          |         |       |               |           | ips streamer                   |
+    39     +----------+---------+-------+---------------+-----------+--------------------------------+
+    40     |     3    |    MU   | TK50  | 066550,130062 | 6D68,B032 | 94  MB,  Block Mode Cartridge; |
+    41     |          |         |       |               |           | emulates   9-track   operation |
+    42     |          |         |       |               |           | (except  read reverse), 75 ips |
+    43     |          |         |       |               |           | streamer                       |
+    44     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                       Page C-9
+                                                                                          11 June 1992
+
+
+
+     1                        Table C-4: Tape Drive/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |             Drive              |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |     4    |    MU   | TA81  | 066550,010121 | 6D68,1051 | STI version of the TU81        |
+     8     +----------+---------+-------+---------------+-----------+--------------------------------+
+     9     |     5    |    MU   | TA79  | 066550,010117 | 6D68,104F | 9-track, PE/GCR modes, 125 ips |
+    10     |          |         |       |               |           | start/stop                     |
+    11     +----------+---------+-------+---------------+-----------+--------------------------------+
+    12     |     6    | unassigned                                                                   |
+    13     +----------+---------+-------+---------------+-----------+--------------------------------+
+    14     |     7    |    MU   | TA90  | 066550,010132 | 6D68,105A | 200  MB,  Cartridge,  Modified |
+    15     |          |         |       |               |           | GCR  (IBM  3480  .5" Cartridge |
+    16     |          |         |       |               |           | Tape  Format), 79 ips streamer |
+    17     +----------+---------+-------+---------------+-----------+--------------------------------+
+    18     |     8    |    MU   | RV60  | 066545,060074 | 6D65,603C | 2  GB, 12.0", removable, Write |
+    19     |          |         |       |               |           | Once   (optical)  disk,  "tape |
+    20     |          |         |       |               |           | mode";     emulates    9-track |
+    21     |          |         |       |               |           | operation, machine loadable    |
+    22     +----------+---------+-------+---------------+-----------+--------------------------------+
+    23     |     9    | see Appendix H, Table H-3                                                    |
+    24     +----------+---------+-------+---------------+-----------+--------------------------------+
+    25     |    10    |    MI   | TF85  | 065150,060125 | 6A68,6055 | 2.6 GB .50 Cart. 100 ips,      |
+    26     |          |         |       |               |           | 800 KB/sec streamer. for DSSI. |
+    27     +----------+---------+-------+---------------+-----------+--------------------------------+
+    28     |    11    |    MI   | TK70  | 065150,130106 | 6A68,B046 | 300 MB,  Block Mode Cartridge; |
+    29     |          |         |       |               |           | emulates   9-track   operation |
+    30     |          |         |(TF70) |               |           | (except read reverse), 100 ips |
+    31     |          |         |       |               |           | streamer                       |
+    32     +----------+---------+-------+---------------+-----------+--------------------------------+
+    33     |    12    |    MU   | TA91  | 066550,010133 | 6D68,105B | 200MB Cartridge, Modified GCR, |
+    34     |          |         |       |               |           | (IBM 3490 .5" Cartridge Tape   |
+    35     |          |         |       |               |           | Format), 79 ips Streamer. Store|
+    36     |          |         |       |               |           | up to 1 GB with Compaction.    |
+    37     +----------+---------+-------+---------------+-----------+--------------------------------+
+    38     |    13    | unassigned                                                                   |
+    39     +----------+---------+-------+---------------+-----------+--------------------------------+
+    40     |    14    |    MU   | TK70  | 066550,130106 | 6D68,B046 | 300 MB,  Block Mode Cartridge; |
+    41     |          |         |       |               |           | emulates   9-track   operation |
+    42     |          |         |       |               |           | (except read reverse), 100 ips |
+    43     |          |         |       |               |           | streamer                       |
+    44     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                      Page C-10
+                                                                                          11 June 1992
+
+
+
+     1                        Table C-4: Tape Drive/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |             Drive              |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |    15    |    MU   | RV20  | 066545,060024 | 6D65,6014 | 2  GB, 12.0", removable, Write |
+     8     |          |         |       |               |           | Once   (optical)  disk,  "tape |
+     9     |          |         |       |               |           | mode";     emulates    9-track |
+    10     |          |         |       |               |           | operation                      |
+    11     +----------+---------+-------+---------------+-----------+--------------------------------+
+    12     |    16    |    MU   | TM32  | 066550,150040 | 6D68,D020 | Gapless   tape   controller /  |
+    13     |          |         |       |               |           | interface                      |
+    14     +----------+---------+-------+---------------+-----------+--------------------------------+
+    15  |  |    17    |    MU   | TA85  | 066550,010125 | 6D68,1055 | 2.6 GB .50 Cartridge with 7    |
+    16  |  |          |         |       |               |           | cartridge magazine, 100 ips    |
+    17  |  |          |         |       |               |           | streamer for KDM/HSC.          |
+    18  |  |          |         |       |               |           | STI version of the TF85        |
+    19     +----------+---------+-------+---------------+-----------+--------------------------------+
+    20     |    18    | see Appendix H, Table H-3                                                    |
+    21     +----------+---------+-------+---------------+-----------+--------------------------------+
+    22     |    19    | see Appendix H, Table H-3                                                    |
+    23     +----------+---------+-------+---------------+-----------+--------------------------------+
+    24     |    20    | see Appendix H, Table H-3                                                    |
+    25     +----------+---------+-------+---------------+-----------+--------------------------------+
+    26  |  |    21    |    MI   | TF86  | 065150,060126 | 6A68,6056 | 6.0 GB .50 Cart. 100 ips,      |
+    27  |  |          |         |       |               |           | 800 KB/sec streamer. for DSSI. |
+    28     +----------+---------+-------+---------------+-----------+--------------------------------+
+                               
+
+    29                             Table C-5:  Media Loader Identifier Values
+
+    30     +----------+---------+-------+---------------------------+--------------------------------+
+    31     |Model Byte|  Device | Media |   Media Type Identifier   |                                |
+    32     | (decimal)|Type Name| Name  +---------------+-----------+ Device (see Note below)        |
+    33     |          |         |       |     octal     |    hex    |                                |
+    34     +----------+---------+-------+---------------+-----------+--------------------------------+
+    35     |     0    | reserved -- must not be assigned                                             |
+    36     +----------+---------+-------+---------------+-----------+--------------------------------+
+    37     |     1    |  none   | none  |      none     |    none   | 64 cartridge jukebox           |
+    38     +----------+---------+-------+---------------+-----------+--------------------------------+
+    39     | Note: The Media Loader Identifier Values are defined in anticipation of the adoption of |
+    40     |       a formal media loader architecture.   These values are in current use as dictated |
+    41     |       by the temporary waiver in Section E.6.                                           |
+    42     +-----------------------------------------------------------------------------------------+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                      Page C-11
+                                                                                          11 June 1992
+
+
+     1                      Table C-6:  SCSI Storage Device/Media Identifier Values
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |     0    | reserved -- must not be assigned                                             |
+     8     +----------+---------+-------+---------------+-----------+--------------------------------+
+     9     |     1    |    DK   | RX23  | 021345,100027 | 22E5,8017 |                                |
+    10     +----------+---------+-------+---------------+-----------+--------------------------------+
+    11     |     2    |    DK   | RX26  | 021345,100032 | 22E5,801A |                                |
+    12     +----------+---------+-------+---------------+-----------+--------------------------------+
+    13     |     3    |    DK   | RX33  | 021345,100041 | 22E5,8021 |                                |
+    14     +----------+---------+-------+---------------+-----------+--------------------------------+
+    15     |     4    |    DK   | RZ22  | 021345,120026 | 22E5,A016 |                                |
+    16     +----------+---------+-------+---------------+-----------+--------------------------------+
+    17     |     5    |    DK   | RZ23  | 021345,120027 | 22E5,A017 |                                |
+    18     +----------+---------+-------+---------------+-----------+--------------------------------+
+    19     |     6    |    DK   | RZL23 | 021345,123027 | 22E5,A617 |                                |
+    20     +----------+---------+-------+---------------+-----------+--------------------------------+
+    21     |     7    |    DK   | RZ24  | 021345,120030 | 22E5,A018 |                                |
+    22     +----------+---------+-------+---------------+-----------+--------------------------------+
+    23     |     8    |    DK   | RZ25  | 021345,120031 | 22E5,A019 |                                |
+    24     +----------+---------+-------+---------------+-----------+--------------------------------+
+    25  |  |     9    |    DK   | RZL24 | 021345,123030 | 22E5,A618 |                                |
+    26     +----------+---------+-------+---------------+-----------+--------------------------------+
+    27     |    10    |   unassigned                                                                 |
+    28     +----------+---------+-------+---------------+-----------+--------------------------------+
+    29     |    11    | see Appendix H, Table H-4                                                    |
+    30     +----------+---------+-------+---------------+-----------+--------------------------------+
+    31     |   12-13  |   unassigned                                                                 |
+    32     +----------+---------+-------+---------------+-----------+--------------------------------+
+    33     |    14    |    DK   | RZ55  | 021345,120067 | 22E5,A037 |                                |
+    34     +----------+---------+-------+---------------+-----------+--------------------------------+
+    35     |    15    |    DK   | RZ56  | 021345,120070 | 22E5,A038 |                                |
+    36     +----------+---------+-------+---------------+-----------+--------------------------------+
+    37     |    16    |    DK   | RZ57  | 021345,120071 | 22E5,A039 |                                |
+    38     +----------+---------+-------+---------------+-----------+--------------------------------+
+    39     |    17    |    DK   | RZ58  | 021345,120072 | 22E5,A03A | tbs                            |
+    40     +----------+---------+-------+---------------+-----------+--------------------------------+
+    41     |  18-20   |   unassigned                                                                 |
+    42     +----------+---------+-------+---------------+-----------+--------------------------------+
+    43     |    21    |    DK   | RZ72  | 021345,120110 | 22E5,A048 |                                |
+    44     +----------+---------+-------+---------------+-----------+--------------------------------+
+    45     |    22    | see Appendix H, Table H-4                                                    |
+    46     +----------+---------+-------+---------------+-----------+--------------------------------+
+    47     |   23-49  |   unassigned                                                                 |
+    48     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                      Page C-12
+                                                                                          11 June 1992
+
+
+
+     1                   Table C-6: SCSI Storage Device/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |    50    | see Appendix H, Table H-4                                                    |
+     8     +----------+---------+-------+---------------+-----------+--------------------------------+
+     9     |  51-127  |   unassigned                                                                 |
+    10     +----------+---------+-------+---------------+-----------+--------------------------------+
+    11     |   128    |    MK   | TZ30  | 065351,120036 | 6AE9,A01E |                                |
+    12     +----------+---------+-------+---------------+-----------+--------------------------------+
+    13     |   129    |    MK   | TZ50  | 065351,120062 | 6AE9,A032 |                                |
+    14     +----------+---------+-------+---------------+-----------+--------------------------------+
+    15     |   130    |    MK   | TZ70  | 065351,120106 | 6AE9,A046 |                                |
+    16     +----------+---------+-------+---------------+-----------+--------------------------------+
+    17     |   131    |    MK   | TZ85  | 065351,120125 | 6AE9,A055 | TZ857.                         |
+    18     +----------+---------+-------+---------------+-----------+--------------------------------+
+    19  |  |   132    |    MK   | TZ86  | 065351,120126 | 6AE9,A056 | tbs                            |
+    20     +----------+---------+-------+---------------+-----------+--------------------------------+
+    21     | 133-137  |   unassigned                                                                 |
+    22     +----------+---------+-------+---------------+-----------+--------------------------------+
+    23     |   138    |    MK   | TLZ04 | 065350,146404 | 6AE8,CD04 |                                |
+    24     +----------+---------+-------+---------------+-----------+--------------------------------+
+    25     |   139    | see Appendix H, Table H-4                                                    |
+    26     +----------+---------+-------+---------------+-----------+--------------------------------+
+    27     |   140    |    MK   | TKZ08 | 065350,136410 | 6AE8,BD08 | Exabyte                        |
+    28     +----------+---------+-------+---------------+-----------+--------------------------------+
+    29     |   141    | see Appendix H, Table H-4                                                    |
+    30     +----------+---------+-------+---------------+-----------+--------------------------------+
+    31     |   142    | see Appendix H, Table H-4                                                    |
+    32     +----------+---------+-------+---------------+-----------+--------------------------------+
+    33  |  |   143    |    MK   | TKZ09 | 065350,136411 | 6AE8,BD09 | Exabyte-2                      |
+    34     +----------+---------+-------+---------------+-----------+--------------------------------+
+    35     |   144    |    MK   | TSZ05 | 065351,036405 | 6AE9,3D05 | 9 track 1600 bpi               |
+    36     +----------+---------+-------+---------------+-----------+--------------------------------+
+    37     |   145    |    MK   | TSZ07 | 065351,035411 | 6AE9,3D09 | 9 track 1600/6250 bpi          |
+    38     +----------+---------+-------+---------------+-----------+--------------------------------+
+    39     | 146-191  |   unassigned                                                                 |
+    40     +----------+---------+-------+---------------+-----------+--------------------------------+
+    41     |   192    |    DK   | RRD40 | 021345,021050 | 22E5,2228 |                                |
+    42     +----------+---------+-------+---------------+-----------+--------------------------------+
+    43     |   193    |    DK   | RRD42 | 021345,021052 | 22E5,222A |                                |
+    44     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           CONTROLLER, UNIT, AND MEDIA TYPE IDENTIFIERS                                      Page C-13
+                                                                                          11 June 1992
+
+
+
+     1                   Table C-6: SCSI Storage Device/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     | 194-255  |   unassigned                                                                 |
+     8     +----------+---------+-------+---------------+-----------+--------------------------------+
+     9     | Notes: 1. The   preferred   method   for   assigning  a model  value  is according to   |
+    10     |           the device type (i.e. disk, tape, or optical). The  recommended model byte    |
+    11     |           ranges are as follows:                                                        |
+    12     |                                                                                         |
+    13     |              o 1-127   Disk devices                                                     |
+    14     |              o 128-191 Tape devices                                                     |
+    15     |              o 192-255 Optical disk devices                                             |
+    16     |                                                                                         |
+    17     +-----------------------------------------------------------------------------------------+
+
+
+
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+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           BUFFER DESCRIPTOR FORMATS                                Page D-1
+                                                                11 June 1992
+
+
+     1                                APPENDIX D
+
+     2                         BUFFER DESCRIPTOR FORMATS
+
+
+     3     The information in this Appendix is NOT a formal  part  of  MSCP.
+     4     The  formal  specification  for  buffer  descriptors  is  in  the
+     5     individual   communications   mechanism   specifications.    This
+     6     information  is  summarized here in order to provide a convenient
+     7     reference for all buffer descriptors in a single document.
+
+     8     It  is  recommended  that  communications  mechanisms  adhere  as
+     9     closely  as possible to the following preferred or generic buffer
+    10     descriptor format.  Although not required by MSCP, similarity  of
+    11     buffer descriptor format can only simplify the design of host and
+    12     controller  port  drivers.   The  preferred  or  generic   buffer
+    13     descriptor format is as follows:
+
+    14                     31                             0 
+    15                     +-------------------------------+
+    16                     |   offset to start of buffer   |
+    17                     +-------------------------------+
+    18                     |          buffer name          |
+    19                     +-------------------------------+
+    20                     |     connection identifier     |
+    21                     +-------------------------------+
+
+    22     The "connection  identifier"  identifies  a  specific  connection
+    23     across  which  the  block  data transfer should take place.  Thus
+    24     this field identifies, in a  communications  mechanism  dependent
+    25     manner,  the  host  or  node  with which the transfer should take
+    26     place.   The  "buffer  name"  identifies,  in  a   communications
+    27     mechanism  dependent  manner,  the  buffer or region of memory on
+    28     that system to which the transfer should take place.  The "offset
+    29     to  start of buffer" is the byte offset from the beginning of the
+    30     named buffer to the point at which the transfer  should  actually
+    31     start.
+
+    32     The "connection identifier" allows for third party  transfers  --
+    33     that  is,  it  allows a host to request transfers to or from some
+    34     other host's memory.  The "buffer name" provides a logical rather
+    35     than  physical  identification  of  the  buffer, thus simplifying
+    36     virtual memory management and possibly  providing  a  consistency
+    37     check  that the buffer is valid.  The "offset to start of buffer"
+    38     simplifies certain aspects of third party transfers.  It allows a
+    39     controlling host to accept a transfer from some other host, break
+    40     the  transfer  up  into  segments,  and  forward   each   segment
+    41     independently to a controller.  Such segmentation of transfers is
+    42     especially useful  with  file  systems,  in  which  a  contiguous
+    43     transfer  within  a  file  may  require  transferring  physically
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           BUFFER DESCRIPTOR FORMATS                                Page D-2
+                                                                11 June 1992
+
+
+     1     noncontiguous portions of a disk.
+
+     2     The buffer descriptor used with the CI  communications  mechanism
+     3     uses   the  exact  format  illustrated  for  the  generic  buffer
+     4     descriptor.  The "offset to start of buffer"  is  indeed  a  byte
+     5     offset  and  the  "buffer  name"  is the CI block transfer buffer
+     6     name.   The  "connection  identifier"  is  the  controller's  SCA
+     7     connection  identifier  for the connection between the disk class
+     8     driver and the disk MSCP server.
+
+     9     There are two buffer descriptor formats used with the UNIBUS  and
+    10     QBUS  in  order  to  accommodate  two different transfer methods:
+    11     unmapped and mapped.
+
+    12     The format for unmapped transfers is as follows:
+
+    13                     31                             0 
+    14                     +-+-----+-----------------------+
+    15                     |0| chan|buffer physical address|
+    16                     +-+-----+-----------------------+
+    17                     |           reserved            |
+    18                     +-------------------------------+
+    19                     |           reserved            |
+    20                     +-------------------------------+
+
+    21     The format for mapped transfers is as follows:
+
+    22                     31                    9 8      0 
+    23                     +-+-----+-----------------------+
+    24                     |1| map register index | offset |
+    25                     +-+-----+-----------------------+
+    26                     |mbz|    map register base      |
+    27                     +-------------------------------+
+    28                     |           reserved            |
+    29                     +-------------------------------+
+
+    30     The following interpretation shows  that  the  unmapped  transfer
+    31     format  is  consistent with the generic buffer descriptor format.
+    32     The  UNIBUS  and  QBUS  are  inherently   single   host   busses.
+    33     Furthermore,  all  anticipated  controllers for these busses only
+    34     support a single device class.  Therefore there  can  only  be  a
+    35     single  connection  between the host and any controller.  Thus it
+    36     is reasonable to assign a fixed connection  identifier  of  zero.
+    37     Secondly,  since these busses are not used with any (bus visible)
+    38     form of virtual memory, there is no use  for  the  concept  of  a
+    39     logical  buffer  name.   Thus  it is reasonable to assign a fixed
+    40     buffer name of zero, denoting the entire address space  available
+    41     on  the  bus.   This  leaves  the offset field to denote the byte
+    42     offset from address zero, or merely the starting address  of  the
+    43     buffer.  In order to accommodate VAX-11/780 UBAs, it is necessary
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           BUFFER DESCRIPTOR FORMATS                                Page D-3
+                                                                11 June 1992
+
+
+     1     to include the UBA channel number so that controllers can request
+     2     UBA  purges.   This  has  been  placed  with  the bus address for
+     3     controller convenience.
+
+     4     On the other hand the mapped transfer format, with the  exception
+     5     of the fixed connection identifier of zero, deviates totally from
+     6     the generic buffer descriptor format.  See  the  Storage  Systems
+     7     Port   Specification   (SSP)   for  complete  details  on  mapped
+     8     transfers.
+
+
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+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-1
+                                                                11 June 1992
+
+
+     1                                APPENDIX E
+
+     2                          WAIVERS AND EXCEPTIONS
+
+
+
+     3     E.1  UDA50
+
+     4     E.1.1  Unit And Controller Revision Numbers
+
+     5     Waiver:
+
+     6         Early serial number UDA50  controllers  do  not  return  unit
+     7         revision  numbers  in  the  GET  UNIT  STATUS  end message or
+     8         controller   revision   numbers   in   the   SET   CONTROLLER
+     9         CHARACTERISTICS end message.
+
+
+    10     E.2  RC25
+
+    11     E.2.1  Error Log Message Ordering
+
+    12     Waiver:
+
+    13         Under certain circumstances the RC25 will  return  error  log
+    14         messages  after  the  corresponding  command's  end  message.
+    15         Section  5.9  "Error  Messages,"  requirement   #3   of   the
+    16         requirements  controllers  must  meet in order to not provide
+    17         error log sequence  numbers,  requires  that  all  error  log
+    18         messages  be  returned before the corresponding command's end
+    19         message.
+
+    20         The particular circumstances where the error log is  returned
+    21         after the corresponding command's end message are as follows:
+
+    22           1.  If the host  does  not  abide  by  the  credit  limit
+    23               supplied   by   the  controller,  and  posts  <credit
+    24               limit>+1 commands, and an event occurs which requires
+    25               the generation of an error log.
+
+    26           2.  If the host abides by the credit  limit  supplied  by
+    27               the  controller,  and  posts <credit limit> commands,
+    28               and two (or more)  events  occur  which  require  the
+    29               generation of two (or more) error logs.
+
+    30           3.  Sometimes,  if  while   processing   Immediate   type
+    31               commands  the  host posts <credit limit> commands and
+    32               an event occurs which requires the generation  of  an
+    33               error log.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-2
+           RC25                                                 11 June 1992
+
+
+     1     E.2.2  Data Error (subcode "Forced Error") Reporting
+
+     2     Waiver:
+
+     3         The RC25 reports Success (subcode "Normal") status instead of
+     4         Data  Error  (subcode "Forced Error") status upon termination
+     5         of the following:
+
+     6           1.  a  COMPARE  HOST  DATA  command  which  encounters  a
+     7               logical block containing a "Forced Error." 
+
+     8           2.  the compare  portion  of  a  "Compare,  Force  Error"
+     9               modified WRITE command.
+
+    10           3.  the compare portion of a "Force Error" modified WRITE
+    11               command  when the "Compare Write" unit characteristic
+    12               was enabled via an ONLINE or SET UNIT CHARACTERISTICS
+    13               command.
+
+
+    14                                    WARNING
+
+    15                 A discovery was made after  this  waiver  was
+    16                 approved  (and the RC25 already shipped) that
+    17                 the  erroneous  behavior   described   above,
+    18                 especially  with regard to the last two cases
+    19                 listed, adversely affected the  operation  of
+    20                 the  Host  Initiated  Bad  Block  Replacement
+    21                 (HIBBR) algorithm described in early versions
+    22                 of  the  Digital  Storage  Architecture  Disk
+    23                 Format (DSDF) specification.  One of the most
+    24                 harmful  consequences  is  that under certain
+    25                 conditions HIBBR  could  mark  all  available
+    26                 replacement  blocks contained on an RC25 disk
+    27                 as unusable (BAD), thereby rendering the disk
+    28                 essentially  unusable.   A brief description,
+    29                 paraphrasing DSDF V1.4.0, Section  7.5  "Host
+    30                 Initiated  Block  Replacement  Algorithm," of
+    31                 how this problem might occur is as follows:
+
+    32                   1.  HIBBR  determines   that   the   data
+    33                       obtained   from   the   block   being
+    34                       replaced were invalid, and  that  the
+    35                       block was in fact bad.
+    36                   2.  HIBBR  scans  the  RCT  to  obtain  a
+    37                       replacement  block.   If  RBN  found,
+    38                       proceed to Step 3, below.  If RBN not
+    39                       found proceed to Step 7, below.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-3
+           RC25                                                 11 June 1992
+
+
+     1                   3.  HIBBR issues  a  REPLACE  command  to
+     2                       revector the LBN to the RBN acquired.
+     3                   4.  HIBBR issues a "Force Error, Compare"
+     4                       modified   WRITE   command   to   the
+     5                       revectored LBN.
+     6                   5.  Assuming that no other  errors  occur
+     7                       during  the  WRITE,  the RC25 reports
+     8                       Success (subcode "Normal") status.
+     9                   6.  HIBBR assumes that the  RBN  was  bad
+    10                       because a Data Error (subcode "Forced
+    11                       Error") status was not returned.
+    12                   7.  HIBBR marks the RCT entry of the  RBN
+    13                       in   question   as   unusable,   then
+    14                       continues  processing  at   Step   2,
+    15                       above.
+    16                   8.  HIBBR issues a "Force Error" modified
+    17                       WRITE,  of  the saved data to the LBN
+    18                       that  was  being  replaced,  puts  an
+    19                       entry   in   the   host's  error  log
+    20                       indicating   that   the   replacement
+    21                       failed, cleans-up the RCT to indicate
+    22                       replacement is no longer in progress,
+    23                       and  takes  whatever  host  dependent
+    24                       action is appropriate  for  a  failed
+    25                       bad block replacement.
+
+    26                 Note that the DSDF HIBBR has since been ECOed
+    27                 to  eliminate the possibility of this problem
+    28                 (and numerous others) occurring.
+
+
+
+    29     E.2.3  Improper Duplicate Unit Number Processing
+
+    30     Waiver:
+
+    31         RC25 operation does not fully comply with all Duplicate  Unit
+    32         Number   processing  requirements.   The  specific  erroneous
+    33         behavior is described below in the form of  four  operational
+    34         cases.
+
+    35         The case descriptions assume two RC25 drives connected  in  a
+    36         master/slave  configuration.   Throughout  the discussion the
+    37         master drive is referred to as "M" and the slave as "S".
+
+    38          o  Case #1
+
+    39             Set-up Conditions:
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-4
+           RC25                                                 11 June 1992
+
+
+     1               1.  "M" drive spun up with 0/1 unit plug inserted
+     2               2.  "S" drive spun down with no unit plug (unknown to
+     3                   the controller)
+     4               3.  controller initialized and characteristics set
+     5               4.  "M" unit 0 and "M" unit 1 brought ONLINE to host
+     6               5.  duplicate  unit  number  condition   created   by
+     7                   inserting  0/1  unit  number  plug  into  "S" and
+     8                   attempting to spin it up
+
+
+     9             Improper operation:
+
+    10               Status code Success (subcode "Duplicate Unit Number")
+    11               is  not  reported in the end messages of all commands
+    12               which  require  it  when  a  duplicate  unit   number
+    13               condition exists and the affected unit is ONLINE to a
+    14               host.  The improper operation occurs on the following
+    15               commands:
+
+    16                >  WRITE
+    17                >  READ
+    18                >  ACCESS
+    19                >  COMPARE HOST DATA
+    20                >  ERASE
+    21                >  REPLACE
+    22                >  SET UNIT CHARACTERISTICS
+
+
+    23               The only command which reports the  status  correctly
+    24               is  the  GET UNIT STATUS command.  The failure occurs
+    25               on commands issued to both "M" unit 0 and "M" unit 1.
+
+    26          o  Case #2
+
+    27             Set-up Conditions:
+
+    28               1.  "M" unit 0 and "M" unit 1 ONLINE from Case #1
+    29               2.  AVAILABLE command issued to "M" unit 0
+
+
+    30             Improper operation:
+
+    31               "M" unit 0 enters the "Unit-Available" state  instead
+    32               of the "Unit-Offline" state as required.
+
+    33               An ONLINE command directed  to  "M"  unit  0,  issued
+    34               immediately  after  the  AVAILABLE  command,  reports
+    35               Success (subcode "Duplicate Unit Number") instead  of
+    36               Unit-Offline   (subcode   "Duplicate  Unit  Number").
+    37               Success (subcode "Duplicate Unit Number")  status  is
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-5
+           RC25                                                 11 June 1992
+
+
+     1               not valid for the ONLINE command.
+
+     2               In  addition,  "M"  unit  0   is   actually   brought
+     3               "Unit-Online"   and   subsequent   commands   execute
+     4               successfully, reporting  Success  (subcode  "Normal")
+     5               status  instead  of  being rejected with Unit-Offline
+     6               (subcode  "Duplicate  Unit  Number")  status.    That
+     7               improper operation occurs on the following commands:
+
+     8                >  GET UNIT STATUS
+     9                >  WRITE
+    10                >  READ
+    11                >  ACCESS
+    12                >  COMPARE HOST DATA
+    13                >  ERASE
+    14                >  REPLACE
+    15                >  SET UNIT CHARACTERISTICS
+
+
+    16          o  Case #3
+
+    17             Set-up Conditions:
+
+    18               1.  "M" unit 1 ONLINE from Case #3
+    19               2.  AVAILABLE command issued to "M" unit 1
+
+
+    20             Improper Operation:
+
+    21               Same as that described in Case #2 with one additional
+    22               anomaly:  the drive is not spun down as required.
+
+    23          o  Case #4
+
+    24             Set-up Conditions:
+
+    25               1.  controller initialized and  default  host  access
+    26                   timeout   allowed  to  expire  resulting  in  the
+    27                   controller  entering  the  "Controller-Available"
+    28                   state
+    29               2.  "M" drive spun down with 0/1 unit plug inserted
+    30               3.  "S" drive spun down with 0/1 unit plug inserted
+    31               4.  controller initialized and characteristics set
+
+
+    32             Improper operation:
+
+    33               Status code  Unit-Offline  (subcode  "Duplicate  Unit
+    34               Number")  is  not reported in the end messages of all
+    35               commands which  require  it  when  a  duplicate  unit
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-6
+           RC25                                                 11 June 1992
+
+
+     1               number  condition exists and the affected unit is not
+     2               ONLINE to a  host.   The  RC25  reports  status  code
+     3               Unit-Offline (subcode "Unit Disabled by Field Service
+     4               or  Internal  Diagnostic")  instead.   The   improper
+     5               operation occurs on the following commands:
+
+     6                >  ONLINE
+     7                >  GET UNIT STATUS
+     8                >  WRITE
+     9                >  READ
+    10                >  ACCESS
+    11                >  COMPARE HOST DATA
+    12                >  ERASE
+    13                >  REPLACE
+    14                >  SET UNIT CHARACTERISTICS
+
+
+    15               The failure occurs on commands  issued  to  both  "M"
+    16               unit 0 and "M" unit 1.
+
+
+
+    17     E.3  HSC And VAX/VMS
+
+
+    18     E.3.1  "Bundled" Shadowing
+
+    19     Temporary Implementation:
+
+    20         "Bundled Shadowing" is  a  VAX/VMS-HSC  (models  50  and  70)
+    21         specific  temporary  implementation.  A more general approach
+    22         to shadowing, referred to as "Unbundled Shadowing," has  been
+    23         proposed.    The   "Unbundled   Shadowing"  specification  is
+    24         currently  in  a  stable  state,   lacking   only   a   first
+    25         implementation before becoming an approved ECO to MSCP.  When
+    26         "Unbundled  Shadowing"  is  finally  approved  the  temporary
+    27         "Bundled  Shadowing"  implementation  will be eliminated in a
+    28         timely fashion.
+
+    29     E.3.1.1  Shadowing Concepts
+
+    30     E.3.1.1.1  Shadowing Functionality
+
+    31     Shadowing is an  optional  feature  of  MSCP  for  enhancing  the
+    32     availability  of  critical data by the duplication of the data on
+    33     two or more compatible disk volumes so that the data  will  still
+    34     be   available  if  one  of  the  volumes  becomes  inaccessible.
+    35     Furthermore, data which becomes  unreadable  due  to  degradation
+    36     over  a  period  of time may be repaired by reading the data from
+    37     another volume and restoring the data to the failing volume.  The
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-7
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     individual   volumes   containing   the  duplicate  data  may  be
+     2     considered to be members of a 'virtual' volume, which is known as
+     3     the shadow set.
+
+     4     Note that shadowing may decrease the write performance, since new
+     5     data  must  be  written  to all members of the shadow set.  On an
+     6     optimizing  controller  however,  shadowing  may   enhance   read
+     7     performance,  as  the  data  may  be  read from any member of the
+     8     shadow set,  and  the  controller  may  therefore  optimize  read
+     9     requests across the various members.
+
+    10     Conceptually, shadowing is implemented by inserting  a  shadowing
+    11     layer  between  the  gatekeeper  and bad block replacement layers
+    12     (note  that  controller  initiated  bad  block   replacement   is
+    13     mandatory  for  shadowing).   The  shadowing layer recognizes and
+    14     manages shadow sets by issuing MSCP requests  to  the  bad  block
+    15     replacement  layer.   This  section  describes  the concepts that
+    16     shadowing adds to MSCP,  the  changes  to  MSCP  control  message
+    17     formats,  and  the  algorithms used by the shadowing layer.  Note
+    18     that the  actual  implementation  of  shadowing  need  not  be  a
+    19     distinct  layer  nor use the exact algorithms given here, as long
+    20     as all class driver visible results are identical.
+
+    21                                       Class Driver
+    22                                            A
+    23                                            |
+    24                                            V
+    25                         / +----------------------------------+
+    26                        /  |            Gatekeeper            |
+    27                       /   +----------------------------------+
+    28                      /    |            Shadowing             |
+    29               MSCP server +----------------------------------+
+    30                      \    | Controller Bad Block Replacement |
+    31                       \   +----------------------------------+
+    32                        \  |            Basic MSCP            |
+    33                         \ +----------------------------------+
+
+
+    34     E.3.1.1.2  Invalid Commands
+
+    35     Basic MSCP allows considerable flexibility in how  extensively  a
+    36     server  checks  for  invalid  commands.  For example, if the byte
+    37     count in  a  transfer  command  is  too  large,  the  server  may
+    38     optionally  perform  the  transfer  up  to the maximum legal byte
+    39     count.   As  another  example,  checking  that  reserved   fields
+    40     actually contain zero is usually optional.
+
+    41     This flexibility can cause problems with shadowing.   Consider  a
+    42     write operation on a shadow set where the byte count is too large
+    43     or some  other  field  is  invalid.   The  shadowing  layer  will
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-8
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     decompose  the  write  operations  into  one  subcommand for each
+     2     member of the shadow set and, if  the  members  are  serviced  by
+     3     different  basic  MSCP  servers, one server may choose to perform
+     4     the transfer while another server may choose to not  perform  it.
+     5     The  net  result  of this series of events is that the shadow set
+     6     has become inconsistent, which is unacceptable.
+
+     7     For this reason, the shadowing layer may not just blindly reissue
+     8     commands  that  it  receives  as  subcommands,  but instead, must
+     9     guarantee that all  subcommands  that  it  issues  to  the  lower
+    10     layer(s)  are  valid.   In  other words, the shadowing layer must
+    11     perform all command validity checking itself, without trusting to
+    12     any  checking  which  may be performed by the lower layers.  Note
+    13     that if the shadowing layer  has  knowledge  of  the  basic  MSCP
+    14     layer(s) below it (as will generally be the case in an integrated
+    15     controller), and it can guarantee  that  all  of  the  individual
+    16     servers   will  treat  the  the  command  identically,  then  the
+    17     shadowing  layer  may  pass   the   responsibility   of   command
+    18     verification to the individual servers.
+
+    19     If this is not the case, the shadowing layer must use one of  two
+    20     approaches.   The  first  is  that it may explicitly check that a
+    21     reserved field is zero before it begins processing  the  command,
+    22     and  reject  the  command if the field is nonzero.  The second is
+    23     that it may totally ignore the contents of the reserved field for
+    24     its own internal processing, and explicitly zero the field in any
+    25     copies of the command  that  it  issues  as  subcommands.   These
+    26     correspond  to  the  options allowed for reserved fields in basic
+    27     MSCP, namely rejecting the command or being totally unaffected by
+    28     the field's contents.
+
+    29     For  nonreserved  fields,  the  shadowing  layer  generally  must
+    30     validate  the  field before it begins processing the command, and
+    31     reject the command if the field's contents are invalid.  The only
+    32     exception  to  this  general rule is the byte count in a transfer
+    33     command.  In this case, the shadowing layer must still check that
+    34     the  byte count is valid, however if the byte count is too large,
+    35     the  layer  may  either  reject  the  command  before   beginning
+    36     processing  or  process  the command up to the maximum legal byte
+    37     count.  The latter case  effectively  means  that  the  shadowing
+    38     layer  substitutes  the  maximum  legal byte count for the actual
+    39     byte count before processing the command.
+
+    40     Since the shadowing layer guarantees that it will never issue  an
+    41     invalid  command  to  a lower layer, any "Invalid Command" errors
+    42     returned by a lower layer are internal controller errors and must
+    43     be  reported  as  such.   The shadowing layer translates "Invalid
+    44     Command" status codes to a "Controller Error" status code  before
+    45     returning such errors to higher layers.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                   Page E-9
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     E.3.1.1.3  Virtual And Member Units
+
+     2     The shadow set itself exists as a virtual unit distinct from  any
+     3     physical  unit,  that is, the shadow set itself has a unit number
+     4     and context that is distinct from the unit number and context  of
+     5     any  member  of the shadow set.  The shadow set -- i.e., its unit
+     6     number   --   may   be   "Unit-Offline,"   "Unit-Available,"   or
+     7     "Unit-Online,"  in  a  like manner to a real unit.  Class drivers
+     8     access the shadow set with commands that specify the shadow set's
+     9     unit  number.   Although  the  full  16  bit unit number range is
+    10     available for virtual units, it is suggested that numbers outside
+    11     the  SDI  range  (i.e., greater than 4095) be utilized to prevent
+    12     possible operator confusion.
+
+    13     The shadowing layer presents both the individual member units  in
+    14     the shadow set as well as the virtual unit itself to upper layers
+    15     or class drivers for access.  It provides full read/write  access
+    16     to both the shadow set itself and to the member units.  It relies
+    17     upon the host class driver in combination with the host shadowing
+    18     layer  to  take appropriate actions to ensure shadow set internal
+    19     consistency.
+
+    20     E.3.1.1.4  Shadow Member Compatibility
+
+    21     Although it is conceptually possible for a shadow set to  consist
+    22     of  a  mixture  of  physical  disk  types,  the overhead could be
+    23     significant, and for this reason all members of a shadow set must
+    24     be  compatible.   Two  disk volumes are compatible, and therefore
+    25     allowable as members of the same shadow set, if and only  if  the
+    26     following geometry parameters are identical:
+
+    27          1.  Unit size
+
+    28          2.  Track size
+
+    29          3.  Group size
+
+    30          4.  Cylinder size
+
+    31          5.  Hardware/software write protect status
+
+    32          6.  Sector size (512/576)
+
+    33          7.  Media type
+
+    34     Any attempt to create a shadow set  with  members  which  do  not
+    35     conform  to the above requirements will be rejected with a status
+    36     of  "Media  Format  Error,"  subcode   of   "Characteristics   Or
+    37     Protection   Mismatch   For  Shadow  Member."   Note  that  these
+    38     parameters, which are returned in the end  messages  of  the  GET
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-10
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     UNIT  STATUS,  ONLINE, and SET UNIT CHARACTERISTICS commands, are
+     2     characteristic of a specific disk volume, and that it is possible
+     3     for  a  single  disk  drive to accept different disk volumes that
+     4     have different geometries.
+
+     5     E.3.1.1.4.1  Hardware Write Protected Shadow Sets
+
+     6     When there is no intention to write to a shadow set,  the  shadow
+     7     set  should  be  software  write  protected,  not  hardware write
+     8     protected.  Hardware write protection  of  a  shadow  set  is  an
+     9     anachronism.   Hardware write protection of a shadow set prevents
+    10     both bad block replacement and shadow set data repair operations.
+
+    11     Because   hardware   write   protection   prohibits   bad   block
+    12     replacements,  blocks  which degrade over time will produce "Data
+    13     error" subcode values other than "Forced Error."  Such errors are
+    14     not   repairable,  and  shadow  set  members  on  which  a  block
+    15     spontaneously becomes unreadable will be removed from the  shadow
+    16     set.
+
+    17     Since spontaneous decay of a block cannot produce a forced error,
+    18     all  blocks  containing  forced  errors  must  be  present due to
+    19     explicit host action.   Therefore,  there  is  no  need  to  read
+    20     additional  members of a hardware write protected shadow set when
+    21     one member has a forced error.  All members  of  the  shadow  set
+    22     contain  a  forced  error  because  they  were identical when the
+    23     shadow set was formed (an implicit assumption the shadowing layer
+    24     must make because no copy operations could be performed).
+
+    25     The only possible use for a hardware write protected  shadow  set
+    26     is  during  an attempt to recover data from a shadow set in which
+    27     all the members  have  suffered  sufficient  data  corruption  to
+    28     require  their mounting in a hardware write protected mode, (e.g.
+    29     all member RCTs  are  invalid).   In  such  a  case,  the  normal
+    30     shadowing   layer   algorithms  are  insufficient  to  provide  a
+    31     reasonable probability of recovering  the  data.   Therefore,  an
+    32     intelligent  MSCP controller may choose to implement a shadow set
+    33     data recovery mode which is activated  by  the  presence  of  the
+    34     "Ignore  Media Format Errors" modifier on an ONLINE command for a
+    35     virtual unit.
+
+    36     If supported, "Ignore Media Format Errors"  modifier  places  the
+    37     shadow  set in a special data recovery state.  In this state, any
+    38     error detected while reading one shadow set member will result in
+    39     an  attempt to read another shadow set member.  If all shadow set
+    40     members produce an error, the last error received along with  any
+    41     data transferred will be returned to the host.  Member units will
+    42     be removed from the shadow set only upon  explicit  request  from
+    43     the host.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-11
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     The "Ignore Media Format Errors" modifier  is  illegal  when  the
+     2     shadow set is not hardware write protected or when the shadow set
+     3     is online to another connection.   In  these  cases,  the  ONLINE
+     4     command will produce a status of "Invalid Command" with a subcode
+     5     which points to the modifiers field.  If the controller does  not
+     6     support  the  special  data recovery mode, an ONLINE command with
+     7     the "Ignore Media Format Errors" modifier will be rejected with a
+     8     status  of  "Invalid  Command"  and  a  subcode  pointing  to the
+     9     modifiers field.
+
+    10     E.3.1.1.5  Shadow Set Management
+
+    11     This section provides an overview of shadow set  management  from
+    12     the  perspective  of  both  the  host  and  the  controller, with
+    13     numerous details omitted to allow a general understanding of  the
+    14     principles  without adding unnecessary confusion.  Later sections
+    15     of this specification deal with the actual specifics  in  a  more
+    16     rigorous fashion.
+
+    17     The mechanism for managing the membership of a shadow set is  via
+    18     the  ONLINE,  SET  UNIT  CHARACTERISTICS, and AVAILABLE commands.
+    19     Member units are added to the shadow set one at a time, with  the
+    20     first  member also performing an implicit creation of the virtual
+    21     unit (effectively causing the virtual  unit  to  transition  from
+    22     "Unit-Offline"  to  "Unit-Available").   If the member unit to be
+    23     added is "Unit-Available" to the issuing host, it must  be  added
+    24     to  the  shadow  set  via  the  ONLINE  command,  while  if it is
+    25     "Unit-Online" to the issuing host, it must be added to the set by
+    26     the SET UNIT CHARACTERISTICS command.
+
+    27     When adding additional members to the  shadow  set,  a  host  may
+    28     specify  that  all  data from the virtual unit is to be copied to
+    29     the newly added member (allowing an uninitialized  volume  to  be
+    30     consistent  with  the  shadow  set).   During  the process of the
+    31     controller copying this data, normal host I/O to all units except
+    32     the  newly added member is unaffected, with the controller having
+    33     the responsibility for correctly segmenting the host I/O and  the
+    34     internal disk to disk copy operation.
+
+    35     The state of the virtual unit relative to the issuing  host  will
+    36     remain  unchanged  by  these  operations  (other than the initial
+    37     transition  from  "Unit-Offline"  to  "Unit-Available"),  so  the
+    38     issuing  host  must also issue an ONLINE command addressed to the
+    39     virtual unit itself in order to access it with commands requiring
+    40     the  unit  to  be  in a "Unit-Online" state.  If a host issues an
+    41     ONLINE command to a virtual unit with  no  members,  it  will  be
+    42     rejected  with  a  status  of  "Unit-Available,"  subcode  of "No
+    43     Members In Shadow Set." 
+
+    44     Once a host brings a  virtual  unit  online,  it  may  still  add
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-12
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     members  with  ONLINE  or  SET  UNIT  CHARACTERISTICS commands as
+     2     appropriate for the specific physical unit.  As previously noted,
+     3     such additions will not alter the unit state of the virtual unit;
+     4     i.e., it will remain "Unit-Online." 
+
+     5     Removal of a member from a shadow set  (other  than  asynchronous
+     6     error  states)  is  accomplished by the host issuing an AVAILABLE
+     7     command to the specific member  it  is  desired  to  remove.   On
+     8     receipt  of this command, the controller will remove the affected
+     9     member  from  the  shadow   set,   place   that   member   in   a
+    10     "Unit-Available"  state,  and  place  the  shadow set itself in a
+    11     "Unit-Available" state.  All other members of the shadow set will
+    12     remain in a "Unit-Online" state.
+
+    13     If an AVAILABLE command is addressed to the virtual unit  itself,
+    14     the  shadow  set  is  placed in a "Unit-Available" state, and all
+    15     members remain in a state of "Unit-Online."  The host may issue a
+    16     modifier  which  removes all members from the shadow set, placing
+    17     the virtual unit in a "Unit-Offline" state.
+
+    18     Error states within shadowing may be broken  into  three  generic
+    19     cases:
+
+    20          1.  Repairable (recoverable) error - A repairable  error  is
+    21              defined  as  being a data error with a subcode of forced
+    22              EDC.  Note that although this is  usually  generated  is
+    23              the last step of the bad block replacement layer, it may
+    24              also be present due to explicit action by the host.  The
+    25              controller is unable to distinguish these two cases, and
+    26              will proceed with the repair process as detailed below.
+
+    27          2.  Unrecoverable  (unrepairable)  error  -  This  class  of
+    28              errors encompasses all drive errors which the controller
+    29              is unable to recover from  after  its  retry  algorithms
+    30              have been exhausted.
+
+    31          3.  Controller errors - This class of  errors  includes  not
+    32              only   those   errors  currently  defined  in  the  MSCP
+    33              specification,  but  also  any  invalid  command  errors
+    34              returned to the shadowing layer by the MSCP layer.
+
+
+    35     If a forced EDC error (e.g., a repairable error) is detected on a
+    36     member of a shadow set while reading, the controller will attempt
+    37     to repair the  data  by  reading  the  equivalent  data  from  an
+    38     alternate member of the set, writing the data to the member which
+    39     failed, and then returning the (corrected) data to the host.   If
+    40     a  member  is  determined to be unrepairable, the affected member
+    41     will be removed from the set, and the virtual unit will be placed
+    42     in a "Unit-Available" state.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-13
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     If an unrecoverable  error  occurs  on  a  member  which  is  not
+     2     associated  with  a  read  request,  the  failing  member will be
+     3     removed from the shadow set, and the shadow set will be placed in
+     4     a "Unit-Available" state.
+
+     5     E.3.1.1.6  MSCP Command Changes
+
+     6     One of the goals of shadowing is to attempt  to  make  a  virtual
+     7     unit  present  much  the  same appearance to a host as a physical
+     8     unit.  For this reason, no new MSCP commands have been added  for
+     9     shadowing,  and  only  those  commands  which  are  involved with
+    10     creation and dissolution  of  a  shadow  set  have  new  features
+    11     (although  transfer  commands  now have a new status code to deal
+    12     with errors detected during shadow set operations).
+
+    13     In the descriptions below, the terms "host" and "connection"  are
+    14     used  interchangeably.   Unless  otherwise explicitly stated, the
+    15     descriptions in Sections E.3.1.1.6.1 through  E.3.1.1.6.10  refer
+    16     to single host operation.
+
+    17     E.3.1.1.6.1  ONLINE Command Changes
+
+    18     The ONLINE command is issued by a host  when  it  is  desired  to
+    19     bring  a physical unit which is currently "Unit-Available" to the
+    20     issuing host into a shadow set.  If the shadow set already exists
+    21     at  the time of the ONLINE command, the physical unit is added to
+    22     the set.  If the shadow unit is nonexistent at the  time  of  the
+    23     command,  the virtual unit representing the shadow set is created
+    24     as part of the online process.  As part of the addition  process,
+    25     the  host  may  optionally  specify  that all data on the virtual
+    26     volume be copied to the newly added member, and if this option is
+    27     chosen,  may  also specify that a single definable portion of the
+    28     virtual volume be excluded from the copy operation.  As mentioned
+    29     previously,  the  state of the virtual unit relative to all hosts
+    30     remains unaltered by the addition of a new  member  to  the  set,
+    31     other   than  the  possible  transition  from  "Unit-Offline"  to
+    32     "Unit-Available" if the virtual unit did not previously exist.
+
+    33     If  the  unit  to  which  the  command  is  issued  is  currently
+    34     "Unit-Online"  as  a  nonmember  to any host, the command will be
+    35     rejected with a status of "Illegal  Command,"  with  the  subcode
+    36     pointing  to  the  modifiers field.  In this case, the command is
+    37     rejected without changing the unit state of any unit.
+
+    38     E.3.1.1.6.1.1  Changes in the ONLINE Command Syntax
+
+    39     Volume shadowing introduces the following changes to  the  syntax
+    40     of the ONLINE command:
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-14
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1          1.  Shadow Unit Specified - This modifier is applied  to  an
+     2              ONLINE  command addressed to a physical unit to indicate
+     3              to the controller that the remaining fields  are  valid.
+     4              If  this  modifier  is  clear,  all the remaining fields
+     5              detailed below are undefined, while if it  is  set,  all
+     6              fields  must  be examined to determine their validity as
+     7              detailed below.
+
+     8          2.  Shadow Unit - This field  refers  to  the  virtual  unit
+     9              number to which the new member is to be added.  It is 16
+    10              bits in length and may be fully utilized.  Four possible
+    11              states  exist  with  reference to the passed shadow unit
+    12              number:
+
+    13              a.  The specified shadow unit number is already known to
+    14                  the  controller  as  an existing virtual unit number
+    15                  with known characteristics.  Note this includes  the
+    16                  possibility  that  the  unit has no members but that
+    17                  the characteristics were previously defined  by  the
+    18                  addition   of   one   or  more  members  which  were
+    19                  subsequently removed.  The virtual unit will  be  in
+    20                  either  the "Unit-Online" or "Unit-Available" state.
+    21                  In this instance, the newly added  member  is  added
+    22                  into   the   already   existing   shadow  set  (upon
+    23                  successful completion of the  ONLINE  command),  and
+    24                  the state of the virtual unit remains unchanged with
+    25                  respect to the host.
+
+    26              b.  The specified shadow unit number is already known to
+    27                  the  controller  as  an existing virtual unit number
+    28                  with unknown characteristics.  The virtual unit will
+    29                  be  in  the  "Unit-Offline"  state,  substate of "No
+    30                  Volume Mounted."  In this instance, the virtual unit
+    31                  will  be  assigned  the characteristics of the newly
+    32                  added member.  The virtual unit  is  placed  in  the
+    33                  "Unit-Available"  state  and  an AVAILABLE ATTENTION
+    34                  message will be sent  notifying  all  hosts  of  the
+    35                  existence of a new unit.
+
+    36              c.  The specified shadow unit number is unknown  to  the
+    37                  controller.   In this instance, the controller will,
+    38                  during execution of the ONLINE command, 'create' the
+    39                  virtual  unit  and  assign it the characteristics of
+    40                  the newly added member.  The virtual unit is  placed
+    41                  in   a   "Unit-Available"  state  and  an  AVAILABLE
+    42                  ATTENTION message will be sent notifying  all  hosts
+    43                  of  the  existence  of  a  new  unit.  If the ONLINE
+    44                  command fails after the virtual  unit  is  'created'
+    45                  but  before  the  characteristics  are  assigned, no
+    46                  AVAILABLE ATTENTION message will  be  sent  and  the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-15
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1                  virtual  unit  will  be  left  in the "Unit-Offline"
+     2                  state, substate of "No Volume Mounted." 
+
+     3              d.  The specified shadow unit number  is  known  to  the
+     4                  controller  as an existing physical unit number.  In
+     5                  this  instance,  the  controller  must  reject   the
+     6                  attempted  ONLINE  command with a status of "Invalid
+     7                  Command," with the subcode pointing to  the  "Shadow
+     8                  Unit" field.
+
+     9          3.  Copy Mode - This field is used by the host  to  indicate
+    10              how the entire LBN space on the virtual volume should be
+    11              copied to the newly added shadow set member as  part  of
+    12              the online process.
+
+    13              The virtual unit and the newly added member must  be  in
+    14              the  same  state  with reference to write protection, or
+    15              the command will be rejected with  a  status  of  "Media
+    16              Format Error," subcode of "Characteristics or Protection
+    17              Mismatch For Shadow Member." 
+
+    18              The "Copy Mode" field itself is settable at  the  option
+    19              of  the  host,  and  may  currently  take  one  of three
+    20              distinct values.  Symbolic names for  these  values  are
+    21              defined  in  Table "Bundled Shadowing" A-a, "Shadow Copy
+    22              Mode Operation Codes."  The values are:
+
+    23              a.  ZERO, indicating that no copy  operation  is  to  be
+    24                  performed.
+
+    25              b.  ONE,  indicating  that  a  FULL  copy  operation  is
+    26                  performed.  The controller will copy all LBNs in the
+    27                  host area from the virtual unit to the  newly  added
+    28                  shadow  member  (Note  that  the term 'virtual unit'
+    29                  implies the entire existing shadow  set  membership,
+    30                  so  the  controller  may  elect  to  divide the READ
+    31                  requests among the various membership).
+
+    32              c.  TWO, indicating that a MERGE copy operation is to be
+    33                  performed.   This  operation  is  identical  to  the
+    34                  NORMAL  copy  operation  with  one  exception.   If,
+    35                  during the copy operation, all members of the shadow
+    36                  set return  a  forced  error,  the  controller  will
+    37                  attempt  to  read  the  same  LBN on the newly added
+    38                  member, and if successful, will copy  this  data  to
+    39                  all  members  of the shadow set.  If the data on the
+    40                  newly  added  member  also  has   a   forced   error
+    41                  indicator,  the  controller  will merely insure that
+    42                  all members of the shadow set and  the  newly  added
+    43                  member  contain consistent data, flagging the LBN on
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-16
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1                  all members with the forced error  indicator.   This
+     2                  "backwards  copy" operation encompasses ONLY the LBN
+     3                  in error, with the remainder of the  copy  operation
+     4                  reverting  to  the  normal mode (unless other errors
+     5                  are detected).
+
+     6                  The presence of a shadow set  member  in  the  merge
+     7                  copy  mode  also  effects  the  processing  of  READ
+     8                  transfer requests.  See  Section  E.3.1.1.6.1.4  for
+     9                  details of this effect.
+
+    10                  Note that although this mode of operation may result
+    11                  in  "repairing"  an LBN on the shadow set by copying
+    12                  data from the newly added member, it should only  be
+    13                  invoked   when   the  host  has  a  high  degree  of
+    14                  confidence that the member to be added differs  from
+    15                  the  rest of the shadow set only by write operations
+    16                  in progress when the shadow set  was  "accidentally"
+    17                  dissolved.
+
+    18              All other values are undefined  and  will  result  in  a
+    19              returned  status  of  "Invalid Command" with the subcode
+    20              pointing to this  field.   In  all  but  one  case,  the
+    21              command will be rejected without changing the unit state
+    22              of any unit.  If the unit field specifies a  unit  which
+    23              is already online to the issuing host as a nonmember and
+    24              the command is SET UNIT CHARACTERISTICS, the unit  state
+    25              will be changed to "Unit-Available." 
+
+    26              The presence of  a  nonzero  copy  mode  field  requires
+    27              special   handling   of  shadow  sets  which  are  write
+    28              protected.  If the virtual unit is in a  hardware  write
+    29              protect  state, then the ONLINE command will be rejected
+    30              with a status of "Illegal  Command,"  with  the  subcode
+    31              pointing  to the "Copy Mode" field.  If the virtual unit
+    32              is software write protected, then the  write  protection
+    33              will  be  disabled  on  the  newly  added member for the
+    34              duration of the copy operation, and re-enabled when  the
+    35              copy is completed.
+
+    36          4.  Excluded LBN Block Address - This field may be  used  by
+    37              the  host  in  conjunction with the "Copy Mode" field to
+    38              indicate that a specific portion of the  virtual  volume
+    39              is  not  to  be  copied to the newly added volume.  This
+    40              field is valid only if both the "Copy  Mode"  field  and
+    41              the   "Excluded  LBN  Count"  fields  are  nonzero.   In
+    42              addition, if this field is valid  it  must  represent  a
+    43              host  accessible  LBN,  or  the command will be rejected
+    44              with a status of "Invalid  Command,"  with  the  subcode
+    45              pointing  to  the  offset  of this field.  If either the
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-17
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1              "Copy Mode" or "Excluded LBN Count" fields are zero, the
+     2              contents of this field are undefined.
+
+     3          5.  Excluded LBN Count - This field may be used by the  host
+     4              in  conjunction  with  the "Copy Mode" and "Excluded LBN
+     5              Block  Address"  fields  to  indicate  that  a  specific
+     6              portion of the virtual volume is not to be copied to the
+     7              newly added volume.  This field is  valid  only  if  the
+     8              "Copy  Mode"  field  is  nonzero.   In addition, if this
+     9              field is nonzero, the contents of this  field  added  to
+    10              the  "Excluded LBN Block Address" must represent a range
+    11              of host accessible LBNs, or the command will be rejected
+    12              with  a  status  of  "Invalid Command," with the subcode
+    13              pointing to the offset of this field.  If this field  is
+    14              zero,  the  contents of the "Excluded LBN Block Address"
+    15              field is undefined, and no LBN's will be  excluded  from
+    16              the copy operation.
+
+    17     E.3.1.1.6.1.2  Changes Which Enforce Shadow Set Membership Rules
+
+    18     The  ONLINE  command  modified  by  "Shadow  Unit  Specified"  is
+    19     responsible  for enforcing all shadow set membership rules.  Some
+    20     of these rules are listed in Section E.3.1.1.4.  Other  of  these
+    21     rules are detailed in the paragraphs which follow.
+
+    22     A shadow set may consist of only physical volumes, e.g.,  a  host
+    23     may  not  create  a shadow set and then attempt to create another
+    24     shadow set specifying the virtual unit number of the first set as
+    25     a  member  of  the  second  set.   If  a  host  attempts  such an
+    26     operation, the controller will reject the command with  a  status
+    27     of  "Invalid  Command,"  with  the subcode pointing to the "Unit"
+    28     field.  In this case, the command is  rejected  without  changing
+    29     the unit state of any unit.
+
+    30     A physical unit may belong to one and only one shadow  set.   Any
+    31     attempt by a host to add a physical unit which is a member of one
+    32     set to another set will be rejected with  a  status  of  "Invalid
+    33     Command"  with  the  subcode pointing to the "Shadow Unit" field.
+    34     In this case, the command is rejected without changing  the  unit
+    35     state of any unit.
+
+    36     A physical unit cannot be "Unit-Online" both as a member and as a
+    37     nonmember.   Any  attempt  by  a  host  to  bring a unit which is
+    38     already a member of a shadow set online as a  nonmember  will  be
+    39     rejected  with  a  status  of  "Invalid Command" with the subcode
+    40     pointing to the "Shadow Unit" field.  In this case,  the  command
+    41     is rejected without changing the unit state of any unit.
+
+    42     If any of the unit  characteristics  of  a  member  proposed  for
+    43     addition  to  a  shadow  set  fail  to match those of the already
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-18
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     existing shadow set, the ONLINE command will be rejected  with  a
+     2     status  of  "Media  Format Error," subcode of "Characteristics Or
+     3     Protection Mismatch  For  Shadow  Member."   In  this  case,  the
+     4     physical  unit is placed in the "Unit-Available" state.  The unit
+     5     state of the virtual unit remains unchanged.
+
+     6     E.3.1.1.6.1.3  Changes in Sequential Gatekeeping
+
+     7     Since a copy operation  takes  an  extended  period  of  time  to
+     8     complete,  the Sequential command operation of the ONLINE command
+     9     is modified somewhat, and is detailed as follows:
+
+    10          1.  The  ONLINE  command  is  deferred  by  the   sequential
+    11              gatekeeper  until  such time as all outstanding commands
+    12              addressed to both the shadow unit and the physical  unit
+    13              in question known to the controller have completed.
+
+    14          2.  The ONLINE command is then performed on the  controller,
+    15              with  all  commands  to  both  the  shadow  unit and the
+    16              physical unit (except Immediate commands) being deferred
+    17              until the noncopy portion has completed.
+
+    18          3.  When the copy portion of the command  starts,  the  gate
+    19              imposed  by  the  sequential  nature  of  the  ONLINE is
+    20              lifted, and all commands directed to the shadow unit may
+    21              proceed  in  parallel  with  the  copy  operation.   Any
+    22              commands (except Immediate  commands)  directed  to  the
+    23              physical  unit prior to completion of the copy operation
+    24              will be rejected with  a  status  of  "Unit  Available,"
+    25              subcode of "Copy In Progress." 
+
+    26              The copy operation will not be affected by any  commands
+    27              directed to the virtual unit with two exceptions:
+
+    28              a.  If an AVAILABLE command is addressed to the  virtual
+    29                  unit  which  would  cause the shadow set to dissolve
+    30                  (see Section E.3.1.1.6.5), the copy  operation  will
+    31                  be  aborted  with a status of "Shadow Set Status Has
+    32                  Changed." 
+
+    33              b.  If the host issues a SET UNIT CHARACTERISTICS to the
+    34                  virtual unit which would cause the virtual unit (and
+    35                  all member units) to become software write protected
+    36                  (see  Section  E.3.1.1.6.3), the member to which the
+    37                  copy is being done will only  be  'logically'  write
+    38                  protected  until  completion  of  the copy (at which
+    39                  time it will become physically write protected).  In
+    40                  other  words, all host attempts to write the virtual
+    41                  unit must be  rejected  by  the  controller  with  a
+    42                  status  of  "Write Protected" even though the member
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-19
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1                  unit is  still  physically  write  enabled  for  the
+     2                  duration  of  the  copy  operation.   Note  that any
+     3                  attempt to directly write the member  unit  will  be
+     4                  rejected  with a status of "Unit-Available," subcode
+     5                  of "Copy In Progress." 
+
+     6     E.3.1.1.6.1.4  Performing Shadow Copy Operations
+
+     7     Controllers may optionally allow concurrent addition of more than
+     8     one  shadow  set  member  requiring  a shadow copy.  Whenever the
+     9     controller's  limit  on  concurrent  shadow  copy  operations  is
+    10     reached,  further  attempts  to  add  members  with  shadow  copy
+    11     operations must be rejected with a  status  of  "Unit-Available,"
+    12     subcode of "Copy In Progress." 
+
+    13     At any given point in time, a  shadow  set  may  be  composed  of
+    14     members in one of three states:
+
+    15          1.  Full-fledged member, no copy in progress.
+    16          2.  New member, target of NORMAL copy.
+    17          3.  New member, target of MERGE copy.
+
+    18     Because multiple  shadow  copy  operations  may  be  in  progress
+    19     concurrently, as few as zero units and as many units as permitted
+    20     by the controller may be in  each  of  these  states.   Note:   a
+    21     shadow  set  containing zero full-fledged members but one or more
+    22     copy members of either type is  a  short-lived  degenerate  case.
+    23     Based  upon  the  copy  algorithms  detailed  below,  lack of any
+    24     full-fledged members should quickly produce the expulsion of  all
+    25     copy members.  The remainder of this section will refer to shadow
+    26     set member in the various states as  full-members,  copy-members,
+    27     and merge-members, respectively.
+
+    28     Due to the sequential  gatekeeping  rules  described  in  Section
+    29     E.3.1.1.6.1.3, write operations directed to the shadow set may be
+    30     executed  concurrently  with   shadow   copy   operations.    The
+    31     controller is responsible for insuring the coordination of writes
+    32     to the newly added member in such a manner as  to  insure  shadow
+    33     set  consistency.   Note:   this  means that ALL write operations
+    34     must be propagated to  ALL  merge-members.   Depending  upon  the
+    35     exact shadow copy implementation, write operations may or may not
+    36     need to be propagated to one or to all copy-members.
+
+    37     Also, due  to  the  sequential  gatekeeping  rules  described  in
+    38     Section E.3.1.1.6.1.3, read operations directed to the shadow set
+    39     may be executed concurrently with shadow  copy  operations.   The
+    40     controller  is responsible for insuring that the data read is NOT
+    41     CHANGED by a copy operation, i.e., that an LBN read before it  is
+    42     copied  does  not differ from when it is read after it is copied.
+    43     This possibility can occur if a merge copy  operation  is  taking
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-20
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     place since the data which ultimately ends up in a particular LBN
+     2     can be taken from one of two or more members  which  MAY  contain
+     3     different  data  for  that LBN.  This implies that LBNs which are
+     4     read from a shadow set which is undergoing a  shadow  merge  copy
+     5     operation  MUST  have  the  copy  operation(s)  performed on them
+     6     before they are read.   Depending  upon  the  exact  shadow  copy
+     7     implementation,  this  shadow  copy  operation  may or may not be
+     8     performed redundantly.
+
+     9     Write operations which fall completely within an excluded  region
+    10     must  NEVER be propagated to either copy-members or merge-members
+    11     for which a excluded region has been specified.   Obviously,  any
+    12     shadow  copy  operation must not copy the excluded region, if one
+    13     has been legally specified.  The results of  writing  a  multiple
+    14     block  region  some  of which lies within the excluded region and
+    15     some of which lies outside the excluded region are unpredictable.
+    16     Hosts  must  confine  their  shadow set write operations to being
+    17     either entirely inside or entirely outside the excluded region.
+
+    18     If the connection between the host  class  driver  and  the  MSCP
+    19     shadowing  server is broken during the copy operation or an ABORT
+    20     command is received,  the  controller  must  terminate  the  copy
+    21     operation within the controller timeout interval.
+
+    22     All types of shadow copy operation treat the set of  full-members
+    23     as the primary source of copy data.  Whenever copy data cannot be
+    24     successfully read from  the  set  of  full-members,  the  set  of
+    25     merge-members  is  used as a secondary source of data.  A failure
+    26     to locate source copy data occurs  only  when  both  the  set  of
+    27     full-members  and  the  set of merge-members have been exhausted.
+    28     All errors detected during attempts to acquire source  copy  data
+    29     are  subject to read repair (exactly as if they occurred during a
+    30     normal read operation).  In addition, any read  repair  operation
+    31     which  repairs using data obtained from a merge-member must write
+    32     both all full-members and all merge-members.
+
+    33     As with normal read and write operations, any unrecoverable error
+    34     is  cause  for removing the failing unit from the shadow set.  If
+    35     the failing unit is a full-member, the resulting shadow set state
+    36     change places the virtual unit in the "Unit-Available" state, but
+    37     the shadow copy operation continues (if possible).  Note:  when a
+    38     full-member  is  failed  out  of  a  shadow  set  due to an error
+    39     detected during a shadow copy operation,  the  host  receives  no
+    40     notification  other  than  rejection  of  future  commands with a
+    41     "Unit-Available"  status.   If  the  failing  unit  is  either  a
+    42     copy-member  or  a  merge-member, the ONLINE command is completed
+    43     with a status of "Shadow Set Status Has Changed," and the virtual
+    44     unit  will  be  placed in a "Unit-Available" state.  In the event
+    45     that source copy data cannot be located (i.e.  no full-member  or
+    46     merge-member is "readable"), the ONLINE command is completed with
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-21
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     a status of "Shadow Set Status Has Changed."  This is  equivalent
+     2     to having the shadow set membership reduced to zero volumes.
+
+     3     Whenever all full-members  and  merge-members  contain  a  forced
+     4     error  block,  a forced error block is written to the shadow copy
+     5     target.   The  controller  will  select  the  contents   of   one
+     6     full-member  copy  of  the  block  and insure that the equivalent
+     7     block in all shadow set  members  including  the  target  of  the
+     8     shadow copy contains that data with the forced error flag set.
+
+     9     E.3.1.1.6.1.5  Restrictions and Miscellaneous Notes
+
+    10     Since the controller is responsible for maintaining the integrity
+    11     of  the  shadow  set, it cannot allow the addition of members for
+    12     which data reliability cannot be maintained.   For  this  reason,
+    13     the  "Allow  Self  Destruction"  and  "Ignore Media Format Error"
+    14     command modifiers are not allowed.  If they  are  specified,  the
+    15     ONLINE  command  will  be  rejected  with  a  status  of "Invalid
+    16     Command," with the subcode pointing to the offset of the modifier
+    17     field.
+
+    18     If in the  process  of  bringing  a  member  unit  online  it  is
+    19     discovered  that  the  RCT  is  invalid  or  that  an  incomplete
+    20     replacement cannot be completed,  which  results  in  the  "Write
+    21     Protect  (data  safety)"  unit flag being set, the ONLINE command
+    22     will be rejected with a status of "Media Format  Error,"  subcode
+    23     of "Characteristics Or Protection Mismatch For Shadow Member." 
+
+    24     E.3.1.1.6.2  ONLINE End Message Changes
+
+    25     This following changes are made to the ONLINE command end message
+    26     for shadowing:
+
+    27          1.  Two additional unit flags are  defined  for  the  ONLINE
+    28              command end message:
+
+    29              1.  Shadow Set Master - This flag will  be  set  in  the
+    30                  unit  flags  field  by  the controller if the "Unit"
+    31                  field of the command references a virtual unit.
+
+    32              2.  Shadow Set Member - This flag will  be  set  in  the
+    33                  unit  flags  field  by  the controller if the "Unit"
+    34                  field of the command references a unit  which  is  a
+    35                  member  of  a shadow set including members which are
+    36                  the target of a shadow copy operation.
+
+
+    37          2.  Two additional fields are defined  for  the  ONLINE  end
+    38              message:
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-22
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1              1.  Shadow Status - If the "Shadow Set Master" unit flag
+     2                  is set (which will only be the case when referencing
+     3                  the virtual volume itself),  this  field  represents
+     4                  the  total  number of members (excluding the virtual
+     5                  volume) which are  currently  "Unit-Online"  in  the
+     6                  shadow  set.  Note that this number will include all
+     7                  members on which copy operations  are  currently  in
+     8                  progress.
+
+     9                  If the "Shadow Set Member" unit flag  is  set,  this
+    10                  field  will  contain  the  copy  mode for this unit.
+    11                  Note that the values are identical to the copy  mode
+    12                  values  permitted  in  the  "Copy Mode" field of the
+    13                  ONLINE and SET  UNIT  CHARACTERISTICS  commands.   A
+    14                  value of ZERO indicates that no copy operation is in
+    15                  progress.
+
+    16                  If both the "Shadow  Set  Master"  and  "Shadow  Set
+    17                  Member"   unit   flags  are  clear,  this  field  is
+    18                  undefined.
+
+    19              2.  Shadow Unit - This field may  take  on  one  of  two
+    20                  distinct values:
+
+    21                  1.  If "Shadow Set Master" is set (which  will  only
+    22                      be   the  case  when  the  original  command  is
+    23                      addressed to the virtual unit), then this  field
+    24                      represents the lowest unit number of the members
+    25                      in the shadow set.
+
+    26                  2.  If "Shadow Set Member" is set (which will be the
+    27                      case when the original command is addressed to a
+    28                      member of  the  shadow  set),  then  this  field
+    29                      represents the virtual unit number of the shadow
+    30                      set of which this unit is a member.
+
+
+    31                  If both the "Shadow  Set  Master"  and  "Shadow  Set
+    32                  Member"   unit   flags  are  clear,  this  field  is
+    33                  undefined.
+
+
+    34          3.  Although no architectural limit is placed on the minimum
+    35              or  maximum  number  of  shadow  sets  a controller must
+    36              support or the number of members  in  a  shadow  set,  a
+    37              controller  may elect to restrict the maximum number due
+    38              to  internal  resource  limitations.   If  a  controller
+    39              elects to reject an operation which would add members to
+    40              the shadow set strictly  because  of  internal  resource
+    41              limitations,  the  controller  must indicate this with a
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-23
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1              returned  status  of  "Controller  Error,"  subcode   of
+     2              "Insufficient Resources." 
+
+     3          4.  If the "Shadow Set Master" unit flag  is  set  (implying
+     4              that the unit is the virtual volume), then the following
+     5              additional changes exist:
+
+     6              1.  The "Multiunit Code" field is zero.
+
+     7              2.  The "Unit Identifier" field is undefined.
+
+     8              3.  The "Media Type Identifier" field contains the media
+     9                  type of the lowest member unit.
+
+    10              4.  The "Volume Serial Number" field contains zeroes.
+
+
+
+    11     E.3.1.1.6.3  SET UNIT CHARACTERISTICS Command Changes
+
+    12     The SET UNIT CHARACTERISTICS command exists as a way for  a  host
+    13     to  bring  a  unit  which is already "Unit-Online" to the issuing
+    14     host  into  a  shadow   set.    If   the   unit   has   different
+    15     characteristics  than  those  of  the  shadow  set  (which  would
+    16     normally be grounds for rejecting its entry into the shadow set),
+    17     this  command  allows  the characteristics to be changed to be in
+    18     conformance with the shadow set  characteristics  as  a  part  of
+    19     bringing  it  into  the  shadow set.  The changes to the SET UNIT
+    20     CHARACTERISTICS command are identical  to  those  of  the  ONLINE
+    21     command, with only three additions.
+
+    22     If a SET UNIT CHARACTERISTICS command is addressed to a  physical
+    23     unit  and the "Shadow" modifier is set (to bring it into a shadow
+    24     set), the connection issuing the command MUST  be  the  only  one
+    25     which  has  it  "Unit-Online."  If it is not, the command will be
+    26     rejected with a status of "Invalid  Command,"  with  the  subcode
+    27     pointing  to  the  "Unit"  field.  The unit will be placed in the
+    28     "Unit-Available" state relative to the issuing  connection.   The
+    29     state of the referenced virtual unit remains unchanged.
+
+    30     If a SET UNIT CHARACTERISTICS command is addressed to the virtual
+    31     unit,  the  successful completion of this command will modify not
+    32     only the virtual unit itself, but also all member units.
+
+    33     If a SET UNIT CHARACTERISTICS command is addressed to a member of
+    34     a  shadow set, it may not alter the characteristics of the member
+    35     in such a  manner  as  to  violate  the  requirements  set  forth
+    36     previously  for  shadow  set  membership  compatibility (hence, a
+    37     successful SET UNIT CHARACTERISTICS command addressed to a member
+    38     of  a  shadow  set  is nothing more than a Sequential NOP).  If a
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-24
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     host attempts to modify a  member's  characteristics  in  such  a
+     2     manner  as  to  make  it  incompatible  with  the shadow set, the
+     3     command will be rejected with a status of "Invalid Command," with
+     4     the  subcode  pointing  to  the  field in error.  In this special
+     5     case, the unit will not be placed in the  "Unit-Available"  state
+     6     but  will  remain in the "Unit-Online" state and the state of the
+     7     referenced virtual unit will also remain unchanged.
+
+     8     E.3.1.1.6.4  SET UNIT CHARACTERISTICS End Message Changes
+
+     9     The  SET  UNIT  CHARACTERISTICS  end  message  modifications  are
+    10     identical to those for the ONLINE end message.
+
+    11     E.3.1.1.6.5  AVAILABLE Command Changes
+
+    12     The AVAILABLE command is the mechanism  whereby  members  may  be
+    13     removed  from  a  shadow  set,  or  the  shadow set itself may be
+    14     dissolved.   The  actual  function  performed  by  the  AVAILABLE
+    15     command  depends  on whether the command is addressed to a member
+    16     of a shadow set, or the virtual unit itself.
+
+    17          1.  If the command is addressed to a member of a shadow set,
+    18              that  member  will  transition  from  "Unit-Online" to a
+    19              state of "Unit-Available," and will be removed from  the
+    20              shadow set, with the shadow set itself being placed in a
+    21              "Unit-Available" state.  This is  the  means  whereby  a
+    22              host   may  remove  members  from  a  shadow  set  while
+    23              preserving the shadow set.  Note that it is possible for
+    24              a  host  to  remove  all members from a shadow set while
+    25              preserving the virtual volume via this mechanism.
+
+    26          2.  If the command is addressed to  the  virtual  unit,  all
+    27              members  will  remain  "Unit-Online," and the shadow set
+    28              will transition to a  "Unit-Available"  state.   If  the
+    29              "Spin-down"  modifier  is  present, the end message will
+    30              contain a status of  "Success,"  subcode  of  "Spin-down
+    31              Ignored."   This is the means whereby a host may place a
+    32              virtual  unit  in  the  "Unit-Available"   state   while
+    33              preserving the shadow set.
+
+    34          3.  If the command is addressed to a virtual  unit  and  the
+    35              "Dissolve"  modifier is specified, all member units will
+    36              be placed  in  a  state  of  "Unit-Available,"  and  the
+    37              virtual unit will be placed in a state of "Unit-Offline"
+    38              (i.e., it will disappear).  If the "Spin-down"  modifier
+    39              is also specified, all member units will be spun down as
+    40              part of the transition to  the  "Unit-Available"  state.
+    41              This  is the means whereby a host may totally dissolve a
+    42              shadow set.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-25
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     E.3.1.1.6.6  GET UNIT STATUS Command End Message Changes
+
+     2     The end message of a GET UNIT STATUS command has  two  additional
+     3     fields  defined, "Shadow Unit" and "Shadow Status."  These fields
+     4     function  in  an  identical  fashion  to  the  fields   described
+     5     previously for the ONLINE command end message.
+
+     6     In addition, the GET UNIT STATUS end message  also  contains  the
+     7     additional  unit  flags  "Shadow  Set  Master"  and  "Shadow  Set
+     8     Member," which also operate in  the  same  manner  as  previously
+     9     described.
+
+    10     If the "Shadow Set Master" flag is set in the "Unit Flags"  field
+    11     (implying that the unit is a virtual unit) and there are members,
+    12     then the following changes exist:
+
+    13          1.  The "Multiunit Code" is zero.
+
+    14          2.  The "Unit Identifier" field is undefined.
+
+    15          3.  The "Media Type Identifier" contains the media  type  of
+    16              the lowest member unit.
+
+    17          4.  The  hardware  and  software  microcode  version  number
+    18              fields ("Usvrsn" and "Uhvrsn") are undefined.
+
+    19          5.  The "RCT Size" field is zero.
+
+    20          6.  The "RBNs" field contains  the  replacement  blocks  per
+    21              track for the lowest member unit.
+
+    22          7.  The "RCT Copies" field is zero.
+
+    23     If the "Shadow Set Master" flag is set in the "Unit Flags"  field
+    24     and  there  are  no members ("Shadow Status" IS zero), all fields
+    25     after the "Unit Flags" field will be zero.
+
+    26     E.3.1.1.6.7  DETERMINE ACCESS PATHS Command Changes
+
+    27     If a DETERMINE ACCESS PATHS command is  addressed  to  a  virtual
+    28     unit,  the  controller  will treat the command as a NOP, but must
+    29     follow the rules for sequential  command  gatekeeping.   If  this
+    30     command  is  addressed  to  a  member  of the shadow set, it will
+    31     operate as if the addressed unit were not a member of the  shadow
+    32     set.
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-26
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     E.3.1.1.6.8  COMPARE CONTROLLER DATA Command Changes
+
+     2     If a COMPARE CONTROLLER DATA COMMAND is addressed to  a  physical
+     3     unit, it functions in a manner unchanged from that of basic MSCP.
+
+     4     If this command is addressed  to  a  virtual  unit  however,  the
+     5     controller will interpret it to mean "Compare data on all members
+     6     of the shadow  set  as  specified  by  the  LBN  and  byte  count
+     7     fields."  The controller will perform a read-compare operation on
+     8     the specified range on all members of the shadow  set,  comparing
+     9     the  data  from  all members.  Six results are possible from this
+    10     operation:
+
+    11          1.  If all data  compares,  the  controller  will  return  a
+    12              status of "Success." 
+
+    13          2.  If the data differs between members, the controller will
+    14              return a status of "Compare Error." 
+
+    15          3.  If a forced error is detected on one unit with good (and
+    16              equivalent) data on all other units, the controller must
+    17              repair  the  forced  error  as   detailed   in   Section
+    18              E.3.1.1.6.9.1.
+
+    19          4.  If forced errors are detected on all units, and the data
+    20              in  the sectors containing the forced error is identical
+    21              on all units, the controller  will  treat  the  data  as
+    22              equal, and proceed with the remainder of the command.
+
+    23          5.  If forced errors are detected on all units, and the data
+    24              in  the  sectors  containing  the  forced  error  is NOT
+    25              identical on all units, the  controller  will  return  a
+    26              status of "Compare Error." 
+
+    27          6.  If an unrecoverable error occurs on a member during  the
+    28              compare operation, the command will be terminated with a
+    29              status of "Shadow Set Status Has Changed." 
+
+    30     It is important for a host to  realize  that  a  write  operation
+    31     directed  to  an  area  on  which  the  controller  is  currently
+    32     performing a compare operation may result in a returned status of
+    33     "Compare Error" (see Section E.3.1.1.6.9.4).
+
+    34     E.3.1.1.6.9  Transfer Commands
+
+    35     The shadowing layer passes transfer operations on  nonshadow  set
+    36     units  through  to  the basic MSCP layer unaltered, and similarly
+    37     returns their end messages unaltered.
+
+    38     Although not recommended, it is perfectly legal  for  a  host  to
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-27
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     issue  transfer commands to an individual member of a shadow set.
+     2     It must be remembered that if a write is  issued  to  a  specific
+     3     member  and not to the other members of the set, the volumes will
+     4     no longer be consistent.
+
+     5     Since the issuance of a transfer command to a specific member  of
+     6     the set implies that the host is only interested in that specific
+     7     unit, the controller will  interpret  this  command  to  have  an
+     8     implied  "suppress  shadowing"  action.   Failure of the transfer
+     9     command will NOT result in an attempt by the controller to repair
+    10     the  data  in the affected block(s) by replacing it with the data
+    11     from other members of the shadow set, but will instead result  in
+    12     the  end  packet  being returned to the host with the appropriate
+    13     error status.  The member on which the error occurred will NOT be
+    14     removed  from  the  shadow  set (unless the controller determines
+    15     that the unit is inoperative), and the shadow set membership will
+    16     continue as it was prior to the command.  It should be noted that
+    17     issuance of a command to an individual member of a shadow set  is
+    18     only  legal  if the issuing host has previously brought that unit
+    19     online with the ONLINE command.
+
+    20     As implied by the statements about  the  changes  in  the  ending
+    21     status  for  the  GET UNIT STATUS command, a shadow unit does not
+    22     have an RCT.  Thus, transfer commands  which  attempt  to  access
+    23     LBNs beyond the "host LBN area" will be rejected with a status of
+    24     "Invalid Command," with a subcode pointing to either the "Logical
+    25     Block Number" or the "Byte Count" depending upon their values.
+
+    26     E.3.1.1.6.9.1  READ Operations
+
+    27     Read operations encompass those MSCP commands which  obtain  data
+    28     from  the  virtual  unit, and (may) return that data to the host.
+    29     The read commands  are  ACCESS,  COMPARE  HOST  DATA,  and  READ.
+    30     (COMPARE  CONTROLLER  DATA  is  a special READ operation, and has
+    31     been detailed in Section E.3.1.1.6.8).
+
+    32     The shadowing layer performs a shadow read operation as  follows.
+    33     For each block of the transfer, the layer chooses a member of the
+    34     shadow set and attempts the transfer on that unit  (note  that  a
+    35     layer  may  elect to either perform the entire read on one member
+    36     or divide that read among  several  members).   If  that  attempt
+    37     succeeds,  then that block of the transfer is completed.  If that
+    38     attempt fails, then either that block of the  transfer  fails  or
+    39     that  block  must  be  repaired,  depending  on  the cause of the
+    40     failure as indicated by its status code and  the  hardware  write
+    41     protect status of the shadow set.
+
+    42     For hardware write protected shadow sets, if the status  code  is
+    43     "Data  Error,"  subcode of "Forced Error," the transfer completes
+    44     with that status.  The data obtained at that point  is  the  data
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-28
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     returned.   If  any  other  error  is  encountered, the member is
+     2     removed from the shadow set and the status  returned  is  "Shadow
+     3     Set Status Has Changed." 
+
+     4     For shadow sets that are not hardware write protected, the status
+     5     code "Data Error," subcode of "Forced Error," is considered to be
+     6     repairable.  Any other error is considered to  be  nonrepairable,
+     7     and  the  layer  will  not attempt to repair it, but will instead
+     8     remove the unit in accordance with Section E.3.1.1.6.10.
+
+     9     Note that the layer must attempt to repair  forced  errors,  even
+    10     though  they  may have been caused by deliberate host action.  If
+    11     the forced error was caused by deliberate host action,  then  the
+    12     repair  attempt  is  pointless  as  it  is  guaranteed  to  fail.
+    13     However, there is no way for the  layer  to  distinguish  between
+    14     forced errors due to deliberate host action and forced errors due
+    15     to a bad block  replacement  operation.   Forced  errors  due  to
+    16     replacements can be repaired, therefore the layer must attempt to
+    17     repair all forced errors.  If  all  members  of  the  shadow  set
+    18     contain  a forced error, the controller will report that fact via
+    19     the end packet with a status of "Data Error," subcode of  "Forced
+    20     Error."  Note that the virtual unit will remain "Unit-Online," as
+    21     will all shadow set members.  This may be  considered  a  special
+    22     case  of  error  handling in that the normal mechanism of placing
+    23     the virtual unit in a "Unit-Available" state is bypassed.
+
+    24     The repair process consists  of  two  phases.   The  first  phase
+    25     searches  the  other  members  of the shadow set to find a member
+    26     from which the data can be successfully obtained.  That  is,  the
+    27     layer tries to read the block on successive members of the shadow
+    28     set until it is read without error.  The second phase writes  the
+    29     data  obtained by the first phase out to all the members on which
+    30     the read failed, including the original member.   Note  that  the
+    31     repair  process  is  always  performed  on a specific, individual
+    32     block.  Repairable errors on several blocks  of  a  transfer  are
+    33     handled  as several distinct repair operations, each of which can
+    34     succeed or fail independently of the others.
+
+    35     To accomplish the first phase of the repair  process,  the  layer
+    36     first initializes a list of the members needing repair to contain
+    37     just the one member on which the transfer  was  originally  tried
+    38     (and  failed).   The layer chooses a member of the shadow set not
+    39     in the list of members needing repair and attempts  to  read  the
+    40     block's   data  from  it  with  a  read  operation.   If  another
+    41     repairable error occurs on that read attempt, then that member is
+    42     added  to  the  list of members needing repair, the layer chooses
+    43     another member of the  shadow  set,  and  attempts  to  read  the
+    44     block's   data  from  it.   This  continues  until  the  data  is
+    45     successfully read from some member or every member of the  shadow
+    46     set is listed as needing repair.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-29
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     If an unrepairable error is detected on a  member  while  reading
+     2     data  for  the  repair operation, that member is removed from the
+     3     shadow set, the shadow set is placed in a "Unit-Available" state,
+     4     the  original  command  (i.e.,  the  one  which caused the repair
+     5     operation to be initiated) is returned with a status  of  "Shadow
+     6     Set  Status  has  Changed,"  and the attempt to find an alternate
+     7     unit  to  read  from  continues.   Note  that  detection  of   an
+     8     unrepairable error does NOT terminate the repair operation (which
+     9     must continue to insure shadow  set  compatibility),  but  merely
+    10     terminates the original read operation.
+
+    11     If a repairable error occurs on every member of the  shadow  set,
+    12     then  correct  data cannot be obtained for that block.  The block
+    13     is not repaired on any member and  that  block  of  the  original
+    14     transfer  command fails with a status of "Data Error," subcode of
+    15     "Forced Error."  The  controller  must  then  insure  shadow  set
+    16     consistency  by  writing  identical  data (copied from one of the
+    17     members) to all members in the shadow set with the  forced  error
+    18     indicator set.
+
+    19     If the data is successfully read from some member, the  shadowing
+    20     layer  enters  phase  two of the repair process.  The layer takes
+    21     the data and attempts to write it  to  all  the  members  needing
+    22     repair.   This  is  done  as  a write-compare operation (with the
+    23     controller disabling software write protect for the  duration  of
+    24     the repair as necessary).  The repair of a member succeeds if the
+    25     write-compare  to  that  member  succeeds,  and  fails   if   the
+    26     write-compare  fails.   If the write-compare operation fails, the
+    27     error is considered to be unrepairable and the following  actions
+    28     take place:
+
+    29          1.  The failed member(s) are placed in the  "Unit-Available"
+    30              state.
+
+    31          2.  Other members remain in the "Unit-Online" state.
+
+    32          3.  The   shadow   volume   itself   is   placed   in    the
+    33              "Unit-Available" state.
+
+    34          4.  An end message is returned for the READ operation,  with
+    35              a  failure  status  indicating  "Shadow  Set  Status Has
+    36              Changed." 
+
+
+    37     In addition to writing the data to the  members  needing  repair,
+    38     the  layer  also  uses  the data to satisfy the original transfer
+    39     command.  In effect, the layer  retries  the  one  block  of  the
+    40     transfer  command  on  the  member  from  which  it  successfully
+    41     obtained the data.  It can either actually retry the transfer  on
+    42     that  member,  retry the transfer on some other member after that
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-30
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     member has been  successfully  repaired  (the  write-compare  has
+     2     succeeded), retry the entire transfer, or accomplish the original
+     3     transfer directly using the data it has read.
+
+     4     The layer must initiate another new repair sequence if the  retry
+     5     of  the original transfer fails with a repairable error.  This is
+     6     necessary to recover  from  such  errors  as  a  member  becoming
+     7     inaccessible in the middle of a repair sequence.
+
+     8     The shadowing layer must not allow  the  repair  sequence  to  be
+     9     aborted.  That is, once a repairable error has been detected, the
+    10     layer must complete the repair sequence regardless of whether  or
+    11     not  it  receives an ABORT command for the original transfer.  It
+    12     may omit the original transfer's retry if the  original  transfer
+    13     is aborted, but the repair sequence itself -- the read-compare(s)
+    14     followed by the write-compare(s) -- must not be affected.
+
+    15     E.3.1.1.6.9.2  WRITE Operations
+
+    16     Write operations encompass those MSCP commands which modify  data
+    17     on the virtual unit, possibly requesting that data from the host.
+    18     The write commands are ERASE, FLUSH, and WRITE.
+
+    19     The shadowing layer performs a shadow write  operation  in  three
+    20     phases:
+
+    21          1.  Issue one copy of the  original  command  to  the  lower
+    22              layer  for  each member of the shadow set (these command
+    23              copies are called subcommands).
+
+    24          2.  Handle any errors  that  occur  on  the  subcommands  by
+    25              removing the affected members from the shadow set.
+
+    26          3.  Determine the appropriate completion status  and  report
+    27              it to the higher layer or class driver.
+
+    28     By definition, all members in the shadow set have  the  identical
+    29     write  protect  status  (both  hardware  and  software),  so  the
+    30     shadowing layer need not be concerned  about  the  write  protect
+    31     status  of  the individual members.  If all the members are write
+    32     enabled, the shadowing layer proceeds  to  the  first  phase  and
+    33     issues a copy of the original command to the lower layer for each
+    34     member of the shadow set.
+
+    35     The subcommands to the  individual  members  can  be  issued  and
+    36     processed  (executed)  in  any  order  or  concurrently.   As the
+    37     subcommands complete, the shadowing layer examines  their  status
+    38     codes and divides the subcommands and their associated shadow set
+    39     members into three classes:
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-31
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1          1.  Subcommands that succeed.  This class includes  commands
+     2              that complete with the following status codes:
+
+     3              1.  Success
+
+     4              2.  Data Error with subcode zero (Forced Error)
+
+     5              3.  Command Aborted
+
+     6              Note  that  a  forced  error  can  only   occur   on   a
+     7              write-compare   operation   with   the   "Forced  Error"
+     8              modifier.
+
+     9          2.  Subcommands that indicate that the transfer  failed  due
+    10              to  loss  of access to the transfer data in host memory.
+    11              All failures of this type have a major  status  code  of
+    12              "Host Buffer Access Error." 
+
+    13          3.  Subcommands that  indicate  that  the  transfer  failed.
+    14              This  class  includes  commands  that  complete with all
+    15              other status codes, including:
+
+    16              1.  Unit-Offline
+
+    17              2.  Unit-Available
+
+    18              3.  Controller Error
+
+    19              4.  Drive Error
+
+    20              5.  Media Format Error
+
+    21              6.  Compare Error
+
+    22              7.  Data Error with nonzero subcode
+
+    23              Note that an "Invalid Command" status code is  converted
+    24              to  and  treated  as  a  "Controller  Error,"  since the
+    25              shadowing layer guarantees that it will only issue valid
+    26              subcommands.
+
+    27     After all the subcommands  have  been  completed,  the  shadowing
+    28     layer  examines  classes  1, 2, and 3 and returns the appropriate
+    29     status code as follows:
+
+    30          1.  If there are no members in classes  2  or  3,  then  the
+    31              transfer  has  succeeded  and  a  status of "Success" is
+    32              returned.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-32
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1          2.  If there are any members in class 2, then the shadow set
+     2              enters  host  write  failure  recovery  as  described in
+     3              Section E.3.1.1.6.9.3.  If there  are  also  members  in
+     4              class  3,  those  members  are ignored by the host write
+     5              failure  recovery  operation.   After  the  host   write
+     6              failure  recovery  is  completed,  shadow set members in
+     7              class 3 are removed from the  shadow  set  as  described
+     8              below.  The status returned in the end message should be
+     9              the "Host Buffer Access Error" status and substatus that
+    10              placed at least one of the shadow set members into class
+    11              2.
+
+    12              The presence of a shadow set member in class 2 does  not
+    13              affect  the  shadow  set  membership.   In this respect,
+    14              classes 1 and 2 are identical.
+
+    15          3.  If there are no members in class 2 and some  members  in
+    16              class 3, then:
+
+    17              1.  All class 1 member(s) remain  in  the  "Unit-Online"
+    18                  state.
+
+    19              2.  All  class   3   member(s)   are   placed   in   the
+    20                  "Unit-Available" state.
+
+    21              3.  The  shadow  volume  itself   is   placed   in   the
+    22                  "Unit-Available" state.
+
+    23              4.  An end message is returned for the write  operation,
+    24                  with  a failure status indicating "Shadow Set Status
+    25                  Has Changed." 
+
+
+    26     E.3.1.1.6.9.3  Host Write Failure Recovery
+
+    27     Any write command that fails due  to  inability  to  access  host
+    28     memory  must  initiate  a  recovery  sequence  performed  by  the
+    29     shadowing layer.  This recovery sequence has several objectives:
+
+    30          1.  over  the  region  affected  by  the  write   operation,
+    31              ensuring  that  all  shadow set members contain the same
+    32              data.
+
+    33          2.  within the constraints imposed by  inability  to  access
+    34              the  data  buffer  described  by  the  write  operation,
+    35              preserving  previously  written  user  data.   In   many
+    36              respects,  this objective is identical to the objectives
+    37              of a merge copy operation.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-33
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1          3.  if at least one shadow set member has been fully written
+     2              by  the  write  operation, cause the data stored on that
+     3              member to be propagated  to  the  remaining  shadow  set
+     4              members.
+
+     5     The remainder of this section  describes  the  actions  that  the
+     6     shadowing layer must perform during a host write failure recovery
+     7     process.  This description  assumes  a  shadowing  implementation
+     8     that  contains  a separate shadowing layer.  Note that the actual
+     9     implementation of host write failure recovery processing need not
+    10     be  done  in  a  distinct layer nor use the exact algorithm given
+    11     here, as long as all class driver visible results are identical.
+
+    12     The host write failure recovery  process  is  an  error  recovery
+    13     operation  and  is  performed as a part of the write command that
+    14     failed.  Within this context, the  host  write  failure  recovery
+    15     process  is  a  non-Sequential  MSCP operation.  Also within this
+    16     context, the host  write  failure  recovery  process  blocks  any
+    17     Sequential  commands  received  after  the original write command
+    18     began execution  but  before  the  host  write  failure  recovery
+    19     process  completes.  Any Sequential commands received during this
+    20     time cannot begin execution until after the  host  write  failure
+    21     recovery process completes.
+
+    22     Although  the  description  above  exactly   matches   the   MSCP
+    23     definition  of a non-Sequential command's effects on a Sequential
+    24     command, it is repeated here for emphasis.  This feature of  host
+    25     write   failure   recovery   processing   is   critical  to  host
+    26     synchronization of I/O requests to the disk  region  affected  by
+    27     the host write failure recovery process.
+
+    28     The host write  failure  recovery  process  is  composed  of  the
+    29     following steps:
+
+    30          1.  Starting at the specified LBN  and  proceeding  for  the
+    31              specified  number  of  bytes, the contents of the shadow
+    32              set are read.  This read proceeds according to the rules
+    33              for   reading   a   shadow   set  described  in  Section
+    34              E.3.1.1.6.9.1.  If at least one shadow  set  member  has
+    35              been   fully   written   by  the  write  operation,  the
+    36              controller may at its option use a fully written  member
+    37              as the preferred source for the read data.
+
+    38          2.  If the read operation results in  the  removal  of  some
+    39              shadow  set  members,  that  removal  is postponed until
+    40              write failure processing finishes.  Whenever the  shadow
+    41              set  contains  zero  or  one  operational members, write
+    42              failure recovery processing finishes immediately.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-34
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1          3.  After the read has finished, the data that  it  read  is
+     2              written  back  the  shadow  set virtual unit.  Note this
+     3              operation cannot generate any class 2 shadow set members
+     4              (as  defined in Section E.3.1.1.6.9.2) because no access
+     5              to host memory is required.
+
+
+     6     E.3.1.1.6.9.4  Read/Write Concurrency
+
+     7     It is important for  the  host  to  be  aware  that  if  multiple
+     8     transfer  operations  are outstanding, the controller provides no
+     9     guarantee  of  the  order  of  the  transfer   execution   and/or
+    10     completion.   This restriction may result in any of the following
+    11     possible inconsistencies:
+
+    12          1.  If both a read and write command are outstanding to  the
+    13              same  LBN,  although  the final data on the disk will be
+    14              the specified write data, the  data  returned  from  the
+    15              read command may consist of the original data before the
+    16              write,  the  new  data  after  the  write,  or  even   a
+    17              combination   of   both  (if  the  transfer  encompasses
+    18              multiple LBN's).
+
+    19          2.  If two or more write commands  are  outstanding  to  the
+    20              same  LBN, the final data on the disk may consist of the
+    21              first write, the second write, or a combination of  both
+    22              (if the transfer encompasses multiple LBN's).
+
+    23          3.  When a shadow set consists of more than one member,  the
+    24              issuance  of multiple write commands to the same LBN may
+    25              result in the above mentioned inconsistencies, with  the
+    26              added  complication  of  varying combinations across the
+    27              individual members of the shadow set.
+
+    28          4.  When a shadow set consists of more than one member,  the
+    29              issuance  of  interleaved read and write commands to the
+    30              same LBN which result in the  controller  attempting  to
+    31              repair the data from the read may result in inconsistent
+    32              data across the individual members of the shadow set.
+
+    33     The above problems may be avoided if the host(s) provide internal
+    34     gatekeeping  on  write  commands  to  an  individual  LBN.   This
+    35     gatekeeping may be summarized as "No write command may be  issued
+    36     which  encompasses an LBN if any other I/O operation is currently
+    37     outstanding which also encompasses that same LBN." 
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-35
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     E.3.1.1.6.10  Shadow Set Membership Changes
+
+     2     If a member  is  removed  from  the  shadow  set  by  either  the
+     3     controller  (e.g.,  as  a  result  of an unrecoverable error on a
+     4     drive), an operator action (e.g.,  changing  the  hardware  write
+     5     protect  status),  or the host (e.g., as a result of an AVAILABLE
+     6     command),   the   affected   member   will   be   placed   in   a
+     7     "Unit-Available" or "Unit-Offline" state, depending on the nature
+     8     of the  change,  and  the  virtual  unit  will  be  placed  in  a
+     9     "Unit-Available" state.
+
+    10     As a result of this change, all commands known to the  controller
+    11     which  are  directed to the affected virtual unit and require the
+    12     virtual unit to be in a "Unit-Online" state  will  be  terminated
+    13     with  a  status  code  of  "Shadow  Set Status Has Changed."  Any
+    14     commands received by the controller  subsequent  to  this  change
+    15     directed to the virtual unit which require the virtual unit to be
+    16     in a "Unit-Online" state  will  be  rejected  with  a  status  of
+    17     "Unit-Available." 
+
+    18     E.3.1.1.6.11  Multihost Access
+
+    19     A concept which is unique in the multihost environment is that of
+    20     "ownership"  and  "online  counts."  The controller must maintain
+    21     knowledge of all hosts which have brought a specific unit online,
+    22     and  also  know  which  hosts have which units online.  A command
+    23     which is addressed to a specific unit will only succeed  if  that
+    24     unit  has previously been brought online by the issuing host.  At
+    25     the  same  time,  the  knowledge  of  "online  hosts"   must   be
+    26     maintained.
+
+    27     Although the higher level management of the  shadow  set  is  the
+    28     responsibility  of  the host, many implicit actions are performed
+    29     by  the  controller  on  behalf  of  the  hosts  in  a  multihost
+    30     environment.   The  sections  which  follow describe the implicit
+    31     actions performed by a controller in a multihost environment.
+
+    32     E.3.1.1.6.11.1  Member Unit Actions
+
+    33     When an AVAILABLE command is issued by a host which currently has
+    34     the  designated unit "Unit-Online," the unit will transition to a
+    35     "Unit-Available" state relative to the  issuing  host,  but  will
+    36     remain  "Unit-Online"  to  all other hosts.  The unit itself will
+    37     not transition  to  a  "Unit-Available"  state  relative  to  the
+    38     controller  until  such  time  as  ALL  hosts which have the unit
+    39     online issue an AVAILABLE command  or  the  "All  Class  Drivers"
+    40     modifier  is  used.  When no host has the unit "Unit-Online," the
+    41     unit will be removed from the shadow set, and  the  virtual  unit
+    42     will  be placed in a "Unit-Available" state (since the membership
+    43     count has changed).  In this case, future  commands  directed  to
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-36
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     the   virtual   unit   would   be   rejected  with  a  status  of
+     2     "Unit-Available" rather than "Shadow  Set  Status  Has  Changed,"
+     3     since  the  sequential  gatekeeping  mechanism would preclude the
+     4     possibility of any outstanding commands when the (last) AVAILABLE
+     5     message is processed.
+
+     6     Note that if only one host had the specific unit ONLINE, then  if
+     7     that  host issued an AVAILABLE command, the unit would be removed
+     8     from the shadow set and the virtual unit would  be  placed  in  a
+     9     "Unit-Available"  state.   This  will  happen  regardless  of the
+    10     number of hosts which have the shadow set virtual unit ONLINE.
+
+    11     When processing an ONLINE command with the shadow unit  specified
+    12     modifier, the controller must first check to see if the specified
+    13     virtual unit has already been created by another  host.   If  the
+    14     virtual  unit  is  not  in  existence,  it will be created and an
+    15     AVAILABLE ATTENTION message will be sent to all  other  hosts  in
+    16     accordance  with  Section  5.8.2.   If  the  virtual unit already
+    17     exists and the ONLINE unit flags are identical,  several  actions
+    18     will be taken by the controller as follows:
+
+    19          1.  The shadow set membership will be updated  as  necessary
+    20              to reflect the addition of the new member.  Note that if
+    21              the member which the requesting host added was already a
+    22              member  of  the shadow set, this step is nugatory (other
+    23              than updating the knowledge that the requesting host now
+    24              has this unit in a "Unit-Online" state).
+
+    25          2.  The requesting host is informed of the total  membership
+    26              by   the   mechanism   of   the   ONLINE  (or  SET  UNIT
+    27              CHARACTERISTICS) end message.
+
+    28          3.  Any future requests to the virtual unit by any host will
+    29              be  addressed  to  the  shadow set as a whole, including
+    30              both the new and any pre-existing members.
+
+    31     The implication of the above items is that if  a  shadow  set  is
+    32     extended  by  one  host, other hosts will have implicit access to
+    33     the new member via the normal shadowing mechanisms, but will  not
+    34     be  explicitly informed of the new members by the controller.  At
+    35     the same time, only those hosts which have explicitly  issued  an
+    36     ONLINE  command  to  the  unit will have access to the individual
+    37     unit.  Finally, when those hosts  which  have  "privately"  added
+    38     units to a shadow set crash or issue AVAILABLE commands for those
+    39     units, they will be removed from the shadow set.
+
+    40     In a multihost environment, it is expected that several hosts may
+    41     desire  to bring members of a shadow set online relative to their
+    42     connections.  Therefore, commands  which  would  produce  results
+    43     identical  with  the  current  state  for  a specified "Unit" and
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-37
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     "Shadow Unit" are allowed and are treated as redundant except for
+     2     changing the unit state relative to the connection.
+
+     3     As detailed previously, a unit may not be "Unit-Online"  in  both
+     4     shadowed and nonshadowed mode simultaneously.  The implication of
+     5     this in a multihost environment is that the individual hosts must
+     6     coordinate their activity when bringing a unit into a shadow set.
+
+     7     E.3.1.1.6.11.2  Shadow Set Virtual Unit Actions
+
+     8     As noted above, the membership of a shadow set is  the  union  of
+     9     all  units  added  to  the  shadow  set  by  all hosts.  Any host
+    10     accessing the shadow set effects all  units  in  the  shadow  set
+    11     whether that host added said units or not.
+
+    12     When a host issues an AVAILABLE command to the shadow  set  as  a
+    13     whole,  that  shadow  set  will  transition to a "Unit-Available"
+    14     state  relative  to  the  requesting   host   but   will   remain
+    15     "Unit-Online"  to  all  hosts which have not issued the AVAILABLE
+    16     command (unless the "All Class Drivers"  modifier  is  supplied).
+    17     When  multiple  hosts  have the shadow set online, the "Dissolve"
+    18     modifier will make all shadow set members which the issuing  host
+    19     has  online  available  with  respect  to  that host.  If a given
+    20     shadow  set  member  was  online  only  to  that  host,  it  will
+    21     transition   into   the   "Unit-Available"   state  and,  if  the
+    22     "Spin-down" modifier was specified, it will spin down.  When ONLY
+    23     one  host  has  the  shadow  set  and  its  members  online,  the
+    24     "Dissolve" modifier will remove all members,  place  the  virtual
+    25     unit in the "Unit-Offline" state and, if the "Spin-down" modifier
+    26     is specified, all members will be spun down.
+
+    27     When a shadow set membership change is detected, the virtual unit
+    28     is  placed  in  the  "Unit-Available"  state  with respect to all
+    29     hosts.  To regain access to  the  virtual  unit  each  host  must
+    30     individually  issue  an  ONLINE  command addressed to the virtual
+    31     unit.  The first host  to  issue  an  ONLINE  command  makes  the
+    32     virtual unit "Unit-Online" only with respect to itself.  The unit
+    33     remains in the  "Unit-Available"  state  with  respect  to  other
+    34     hosts.
+
+    35     When a host issues an  AVAILABLE  command  with  the  "All  Class
+    36     Drivers"  modifier  to the shadow set as a whole, that shadow set
+    37     will transition to the "Unit-Available"  state  relative  to  all
+    38     hosts.   This  action  will be carried out even if the shadow set
+    39     was "Unit-Available" to the  issuing  host.   If  the  "Dissolve"
+    40     modifier  is  specified  in addition, all members will be removed
+    41     from the shadow set and the virtual unit will be  placed  in  the
+    42     "Unit-Offline"   state   and,  if  the  "Spin-down"  modifier  is
+    43     specified, all members will be spun down.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-38
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     E.3.1.1.6.11.3  Shadow Copy Actions
+
+     2     For any given physical unit, only one host may have a shadow copy
+     3     operation  in  progress.  Subsequent ONLINE commands requesting a
+     4     shadow  copy  operation  will  be  rejected  with  a  status   of
+     5     "Unit-Available," subcode of "Copy in Progress." 
+
+     6     If an ONLINE command which requests  some  form  of  shadow  copy
+     7     operation  is  received for a unit which was added without a copy
+     8     operation or which has  completed  a  previously  requested  copy
+     9     operation,  the  copy  mode field of the command will be ignored.
+    10     The unit will be  added  to  the  shadow  set  without  any  copy
+    11     operation being performed.
+
+    12     If an ONLINE command which requests  some  form  of  shadow  copy
+    13     operation  is  received for a unit which is the first member of a
+    14     shadow set, the copy mode field of the command will  be  ignored.
+    15     The  unit  will  be  added  to  the  shadow  set without any copy
+    16     operation being performed.
+
+    17     E.3.1.1.6.11.4  Get Unit Status Actions
+
+    18     The GET UNIT STATUS command will return the "shadow  set  master"
+    19     and  "shadow  set  member"  unit flags, and the Shadow Status and
+    20     Shadow Unit fields correctly regardless of  whether  on  not  the
+    21     specified  physical or virtual unit is "Unit-Online" with respect
+    22     to the host issuing the command.  If one host forms a shadow  set
+    23     composed  of  two  units, another host can use GET UNIT STATUS to
+    24     determine the status of either the virtual  unit  or  the  member
+    25     units  without bringing any of them "Unit-Online" with respect to
+    26     itself.
+
+    27     E.3.1.1.6.12  Error Logs
+
+    28     Any drive specific error log messages generated as  a  result  of
+    29     shadowing operations will be in accordance with Section 5.9.  The
+    30     command reference number of the  message  will  be  that  of  the
+    31     original  command  reference  number directed to the virtual unit
+    32     (as opposed to the one issued by the shadowing layer directed  to
+    33     the  MSCP layer), and the drive specific information will be that
+    34     of the drive which had the error.  The  restriction  limiting  to
+    35     three  the  number  of error logs generated for a single error is
+    36     extended in the shadowing case to a physical drive  basis  --  if
+    37     errors occur on all members of an 'n' member shadow set, then the
+    38     controller must limit its error log messages  to  three  messages
+    39     per drive, giving a maximum of 'n' times three messages.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-39
+           HSC And VAX/VMS                                      11 June 1992
+
+
+     1     E.3.1.1.7  "Bundled Shadowing" Definitions
+
+     2     The following  section  contains  specific  modifier,  flag,  and
+     3     offset  information.  The tables which follow contain the command
+     4     modifier,  controller  and  unit   flag,   and   message   offset
+     5     definitions  specific  to  "Bundled  Shadowing."   The tables are
+     6     formatted in the same manner as their Appendix A, "Opcode,  Flag,
+     7     and Offset Definitions" counterparts to ease cross-referencing.
+
+     8     In addition, Table "Bundled Shadowing"  A-a,  "Shadow  Copy  Mode
+     9     Operation   Codes"   contains   other   definitions  specific  to
+    10     Shadowing.  However, it has no Appendix  A,  "Opcode,  Flag,  and
+    11     Offset Definitions" counterpart.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-40
+           HSC And VAX/VMS                                                                        11 June 1992
+
+
+     1                              Table "Bundled Shadowing" A-2: Command Modifiers
+
+     2         +------+--------------+--------------------------+----------------------------------------+
+     3         | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4         |Number+-------+------+--------+-----------------+     Command Modifier                   |
+     5         |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+     6         +------+-------+------+--------+-----------------+----------------------------------------+
+     7         | AVAILABLE Command Modifiers:                   |                                        |
+     8         |   4  |    20 |   10 | MD.DIS | MSCP$x_MD_DSOLV | Dissolve Shadow Set                    |
+     9         +------+-------+------+--------+-----------------+----------------------------------------+
+    10         | ONLINE and SET UNIT CHARACTERISTICS Command Modifiers:                                  |
+    11         |   4  |    20 |   10 | MD.SHD | MSCP$x_MD_SHDSP | Shadow Unit Specified                  |
+    12         +------+-------+------+--------+-----------------+----------------------------------------+
+    13         | Note: The  "x"  in  a  32  bit  mnemonic  for  a  bit flag will be "V" if the symbol is |
+    14         |       defined as a bit number (offset) or an "M" if defined as a mask.                  |
+    15         +-----------------------------------------------------------------------------------------+
+
+
+    16                               Table "Bundled Shadowing" A-4: Controller Flags
+
+    17         +------+--------------+--------------------------+----------------------------------------+
+    18         | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    19         |Number+-------+------+--------------------------+     Controller Flag                    |
+    20         |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+    21         +------+-------+------+--------+-----------------+----------------------------------------+
+    22         |   1  |     2 |    2 | CF.SHD | MSCP$x_CF_SHADW | Shadowing                              |
+    23         +------+-------+------+--------+-----------------+----------------------------------------+
+    24         | Note: The  "x"  in  a  32  bit  mnemonic  for  a  bit flag will be "V" if the symbol is |
+    25         |       defined as a bit number (offset) or an "M" if defined as a mask.                  |
+    26         +-----------------------------------------------------------------------------------------+
+
+
+    27                                  Table "Bundled Shadowing" A-5: Unit Flags
+
+    28         +------+--------------+--------------------------+----------------------------------------+
+    29         | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    30         |Number+-------+------+--------+-----------------+     Unit Flag                          |
+    31         |      | Octal | Hex. | 16 bit |32 bit (see note)|                                        |
+    32         +------+-------+------+--------+-----------------+----------------------------------------+
+    33         |  14  | 40000 | 4000 | UF.MEM | MSCP$x_UF_SSMEM | Shadow Set Member                      |
+    34         +------+-------+------+--------+-----------------+----------------------------------------+
+    35         |   9  |  1000 |  200 | UF.MST | MSCP$x_UF_SSMST | Shadow Set Master                      |
+    36         +------+-------+------+--------+-----------------+----------------------------------------+
+    37         | Note: The  "x"  in  a  32  bit  mnemonic  for  a  bit flag will be "V" if the symbol is |
+    38         |       defined as a bit number (offset) or an "M" if defined as a mask.                  |
+    39         +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-41
+           HSC And VAX/VMS                                                                        11 June 1992
+
+
+     1                           Table "Bundled Shadowing" A-6: Command Message Offsets
+
+     2         +--------------------+--------------------------+-------+---------------------------------+
+     3         |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4         +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5         | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6         +------+------+------+--------+-----------------+-------+---------------------------------+
+     7         | ONLINE and SET UNIT CHARACTERISTICS Command Message Offsets:                            |
+     8         |  20  |  24  |  14  | P.EXBA | MSCP$L_EXCL_LBA |   4   | Excluded LBN Address            |
+     9         |  24  |  30  |  18  | P.EXBC | MSCP$W_EXCL_LBC |   2   | Excluded LBN Count              |
+    10         |  32  |  40  |  20  | P.SHUN | MSCP$W_SHDW_UNT |   2   | Shadow unit                     |
+    11         |  34  |  42  |  22  | P.CPMD | MSCP$W_COPY_MOD |   2   | Copy mode  (see  Table "Bundled |
+    12         |      |      |      |        |                 |       | Shadowing" A-a)                 |
+    13         +------+------+------+--------+-----------------+-------+---------------------------------+
+    14         | Note: An asterisk (*) in the field size indicates that the size varies.                 |
+    15         +-----------------------------------------------------------------------------------------+
+
+
+    16                      Table "Bundled Shadowing" A-7: End and Attention Message Offsets
+
+    17         +--------------------+--------------------------+-------+---------------------------------+
+    18         |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+    19         +------+------+------+--------+-----------------+ Size  | Field Description               |
+    20         | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+    21         +------+------+------+--------+-----------------+-------+---------------------------------+
+    22         | GET UNIT STATUS End Message Offsets:          |       |                                 |
+    23         |  32  |  40  |  20  | P.SHUN | MSCP$W_SHDW_UNT |   2   | Shadow unit                     |
+    24         |  34  |  42  |  22  | P.SHST | MSCP$W_SHDW_STS |   2   | Shadow   status    (see   Table |
+    25         |      |      |      |        |                 |       | "Bundled Shadowing" A-a)        |
+    26         +------+------+------+--------+-----------------+-------+---------------------------------+
+    27         | ONLINE and SET UNIT CHARACTERISTICS End Message offsets:                                |
+    28         |  32  |  40  |  20  | P.SHUN | MSCP$W_SHDW_UNT |   2   | Shadow unit                     |
+    29         |  34  |  42  |  22  | P.SHST | MSCP$W_SHDW_STS |   2   | Shadow   status    (see   Table |
+    30         |      |      |      |        |                 |       | "Bundled Shadowing" A-a)        |
+    31         +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-42
+           HSC And VAX/VMS                                                                        11 June 1992
+
+
+     1                       Table "Bundled Shadowing" A-a: Shadow Copy Mode Operation Codes
+
+     2         +--------------------+--------------------------+-----------------------------------------+
+     3         |    Opcode Value    |   Preferred Mnemonics    |                                         |
+     4         +------+------+------+--------+-----------------+   Operation                             |
+     5         | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+     6         +------+------+------+--------+-----------------+-----------------------------------------+
+     7         |   0  |   0  |   0  | CM.NOC | MSCP$K_CM_NOCPY | No Copy                                 |
+     8         +------+------+------+--------+-----------------+-----------------------------------------+
+     9         |   1  |   1  |   1  | CM.NCP | MSCP$K_CM_COPY  | Normal Copy                             |
+    10         +------+------+------+--------+-----------------+-----------------------------------------+
+    11         |   2  |   2  |   2  | CM.MCP | MSCP$K_CM_MGCPY | Merge Copy                              |
+    12         +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+
+
+
+    13     E.3.1.1.8  "Bundled Shadowing" Specific Status Code Definitions
+
+    14     The following tables contain the status code definitions  specific  to  "Bundled  Shadowing."   The
+    15     tables  are  formatted  in the same manner as their Appendix B, "Status and Event Code Definitions"
+    16     counterparts to ease cross-referencing.
+
+    17                            Table "Bundled Shadowing" B-1: Status and Event Codes
+
+    18         +--------------------+--------------------------+-----------------------------------------+
+    19         |       Value        |   Preferred Mnemonics    |                                         |
+    20         +------+------+------+--------+-----------------+     Status or Event Code                |
+    21         | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+    22         +------+------+------+--------+-----------------+-----------------------------------------+
+    23         |   12 |   14 |    C | ST.SSS | MSCP$K_ST_SHST  | Shadow Set Status Has Changed           |
+    24         +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-43
+           HSC And VAX/VMS                                                                        11 June 1992
+
+
+     1                          Table "Bundled Shadowing" B-2: Status Only Subcode Values
+
+     2         +------+---------------------+------------------------------------------------------------+
+     3         | Sub- |   Code + Subcode    |                                                            |
+     4         | code +------+-------+------+                 Status Subcode                             |
+     5         |      | Dec. |  Oct. | Hex. |                                                            |
+     6         +------+------+-------+------+------------------------------------------------------------+
+     7         | "Unit-Available" Subcode values:                                                        |
+     8         |    2 |   68 |   104 |   44 | Shadow Set Copy In Progress                                |
+     9         |      |      |       |      |   This  subcode  is  returned while a copy operation is in |
+    10         |      |      |       |      |   progress  if  any  of the following circumstances occur: |
+    11         |      |      |       |      |   1) any command is issued directly to the copy recipient; |
+    12         |      |      |       |      |   2) the controller dependent number of copy operations in |
+    13         |      |      |       |      |   progress  limit  has already been reached; or 3) another |
+    14         |      |      |       |      |   host has already initiated a copy operation to the unit. |
+    15         |      |      |       |      |   Note  that  this bit may also be returned in the "Shadow |
+    16         |      |      |       |      |   Status"   field   of  a  GET  UNIT  CHARACTERISTICS  end |
+    17         |      |      |       |      |   message  if  the  command  is  directed  to a shadow set |
+    18         |      |      |       |      |   member on which a copy is in progress.                   |
+    19         |    4 |  132 |   204 |   84 | No Members In Shadow Set                                   |
+    20         |      |      |       |      |   An  ONLINE  command was addressed to the virtual unit of |
+    21         |      |      |       |      |   an  existing  shadow  set  from  which  all members have |
+    22         |      |      |       |      |   been removed.                                            |
+    23         +------+------+-------+------+------------------------------------------------------------+
+    24         | "Shadow Set Status Has Changed" Subcode values:                                         |
+    25         |    0 |   12 |    14 |    C | Subcodes are not used.                                     |
+    26         |      |      |       |      |   In general this status code is returned as the result of |
+    27         |      |      |       |      |   a shadow set member being removed from a shadow set. See |
+    28         |      |      |       |      |   specific    references   in   the   "Bundled  Shadowing" |
+    29         |      |      |       |      |   specification  for  details  of particular circumstances |
+    30         |      |      |       |      |   under which this status code is returned.                |
+    31         +------+------+-------+------+------------------------------------------------------------+
+
+
+    32             Table "Bundled Shadowing" B-3: "Media Format Error" Status or Event Subcode Values
+
+    33         +------+---------------------+-+-+--------------------------------------------------------+
+    34         | Sub- |   Code + Subcode    |E|S|                                                        |
+    35         | code +------+-------+------+V|T|     Status or Event Subcode                            |
+    36         |      | Dec. |  Oct. | Hex. | | |                                                        |
+    37         +------+------+-------+------+-+-+--------------------------------------------------------+
+    38         |    4 |  133 |   205 |   85 | |*| Characteristics  Or  Protection  Mismatch  For  Shadow |
+    39         |      |      |       |      | | | Member                                                 |
+    40         |      |      |       |      | | |   The  geometry  or  write protect state of a physical |
+    41         |      |      |       |      | | |   unit  being  added  to  a  shadow set does not match |
+    42         |      |      |       |      | | |   that  of the shadow set or the physical unit's Write |
+    43         |      |      |       |      | | |   Protect (data safety) unit flag is set.              |
+    44         +------+------+-------+------+-+-+--------------------------------------------------------+
+
+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-44
+           HSC And VAX/VMS                                                                        11 June 1992
+
+
+     1              Table "Bundled Shadowing" B-7: "Controller Error" Status or Event Subcode Values
+
+     2         +------+---------------------+-+-+--------------------------------------------------------+
+     3         | Sub- |   Code + Subcode    |E|S|                                                        |
+     4         | code +------+-------+------+V|T|     Status or Event Subcode                            |
+     5         |      | Dec. |  Oct. | Hex. | | |                                                        |
+     6         +------+------+-------+------+-+-+--------------------------------------------------------+
+     7         |   10 |  330 |   512 |  14A | |*| Insufficient Resources                                 |
+     8         |      |      |       |      | | |   The  controller  is  unable  to  honor  a request to |
+     9         |      |      |       |      | | |   create  a  shadow set or to add an additional member |
+    10         |      |      |       |      | | |   to an existing shadow set, due to a lack of internal |
+    11         |      |      |       |      | | |   resources  to support the new entity. Note that this |
+    12         |      |      |       |      | | |   subcode must not be used for any other purpose.      |
+    13         +------+------+-------+------+-+-+--------------------------------------------------------+
+
+
+
+
+
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+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-45
+           TU81                                                 11 June 1992
+
+
+     1     E.4  TU81
+
+     2     E.4.1  "Enable Miscellaneous Error  Log  Messages"  Modifier  Not
+     3            Fully Supported
+
+     4     Waiver:
+
+     5         The TU81 does not fully  support  the  "Enable  Miscellaneous
+     6         Error   Log   Messages"   modifier   to  the  SET  CONTROLLER
+     7         CHARACTERISTICS  command  as   required   by   Section   5.6,
+     8         "Controller  Flags."   If the modifier is set by the host the
+     9         TU81 merely ignores it  and  returns  it  clear  in  the  end
+    10         message of the SET CONTROLLER CHARACTERISTICS command.
+
+
+    11     E.5  VAX/VMS
+
+    12     E.5.1  Bad Block Replacement Attempt Error Log Usage
+
+    13     Usage Note:
+
+    14         VAX/VMS implements usage of the BAD BLOCK REPLACEMENT ATTEMPT
+    15         error   log  message  format  for  host-initiated  bad  block
+    16         replacement operations performed by VMS.  Error log  messages
+    17         so  generated supply all message fields in conformance to the
+    18         MSCP  specification  with  the  exception  of  the  "sequence
+    19         number"  field.   In  VMS  generated  error log messages, the
+    20         "sequence number" field has all bits set (numeric value -1).
+
+    21     E.6  RV80
+
+    22     E.6.1  Media Loader Error Logs
+
+    23     Waiver:
+
+    24     Add or modify the following sections of the MSCP specification to
+    25     include  the  means  by  which  an  MSCP  or TMSCP controller can
+    26     generate Media Loader Error Logs:
+
+    27         o  Modify section 4.17    "Controller and Unit Identifiers"
+    28         o  Add new section        "Media Loader Error Log"
+    29         o  Modify table A-8       "Error Log Message Offsets
+    30         o  Modify table A-9       "Error Log Message Format Codes"
+    31         o  Modify table B-1       "Status and Event Codes"
+    32         o  Add table B-10         "Media Loader Error Status and
+    33                                    Event Subcode Values"
+    34         o  Modify table C-1       "Controller and Unit Class Values"
+    35         o  Add table C-5          "Media Loader Values"
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-46
+           RV80                                                 11 June 1992
+
+
+     1     Impact:
+
+     2     Host error log report generators must be modified to  respond  to
+     3     the  new  format  appropriately.   If  the error report generator
+     4     modification is not implemented for VMS Version 5.0, the  Ptolemy
+     5     Optical  Library  schedule will be delayed until the next version
+     6     release of VMS.
+
+     7     This ECO does *not* request that any operating system respond  to
+     8     a media loader as a new device, just to interpret its error logs.
+
+     9     E.6.1.1  Controller And Unit Identifiers
+
+    10     Section 4.17, "Controller and Unit Identifiers," is  modified  to
+    11     read as follows:
+
+    12     4.17 Controller, Media Loader, and Unit Identifiers
+
+    13     MSCP requires that all controllers,  media  loaders,  and  drives
+    14     have  unique  identifiers,  called  controller identifiers, media
+    15     loader identifiers, and unit identifiers.  The structure of these
+    16     identifiers is as follows:
+
+    17                     31                             0 
+    18                     +-------------------------------+
+    19                     |      unique device number     |
+    20                     +-------+-------+               +
+    21                     | class | model |               |
+    22                     +-------+-------+---------------+
+
+    23     The  "class"  byte  identifies  the  type  of  the  subsystem  --
+    24     controller,  media  loader,  disk  drive,  etc.  The "model" byte
+    25     identifies the exact model of the  subsystem  within  its  class.
+    26     All  valid  class  and model codes are nonzero, implying that all
+    27     valid identifiers are nonzero.  The "unique device number"  field
+    28     must  uniquely identify the device among all devices of that same
+    29     class and model.  The device serial number could be used  as  the
+    30     "unique  device  number,"  though that isn't required.  Currently
+    31     defined values for the "class" and "model" bytes  are  listed  in
+    32     Appendix  C,  "Controller,  Unit,  and  Media Type Identifiers." 
+    33     Table C-1, "Controller and  Unit  'Class'  Values"  contains  the
+    34     "class" values for controllers, media loaders, and drives.  Table
+    35     C-2,  "Mass  Storage  Controller  'Model'  Values"  contains  the
+    36     "model"  values  for both disk and tape controllers.  Tables C-3,
+    37     "Disk Drive/Media  Identifier  Values,"  C-4,  "Tape  Drive/Media
+    38     Identifier  Values,"  and  C-5, "Media Loader Identifier Values,"
+    39     contain the "model" values for  disk  drives,  tape  drives,  and
+    40     media  loaders,  respectively.   Values  for  new devices must be
+    41     added to those tables via an ECO to  this  specification  as  new
+    42     products  are  developed.  Note that different units of multiunit
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-47
+           RV80                                                 11 June 1992
+
+
+     1     drives are distinguished by having different "model"  bytes;  the
+     2     "class"   and   "unique   device  number"  fields  are  typically
+     3     identical.  Note also that all MSCP servers for the  same  device
+     4     class  within the same controller must return the same controller
+     5     identifier.
+
+     6     As previously stated, MSCP  requires  that  controller  and  unit
+     7     identifiers  be  unique  across  all devices accessible via MSCP.
+     8     This clearly cannot be checked by controllers.  Controllers  can,
+     9     however,  enforce  unique  unit identifiers across the units that
+    10     are attached to themselves.  This is  done  using  the  following
+    11     algorithms:
+
+    12          1.  Controllers should detect and respond to duplicate  unit
+    13              identifiers  across all units whose unit identifiers the
+    14              controller can obtain, including all  units  that  would
+    15              otherwise   be   "Unit-Online"   or   "Unit-Available." 
+    16              Detection of a duplicate unit identifier on one unit  of
+    17              a  multiunit  drive  is  treated  as  a  duplicate  unit
+    18              identifier condition on all other units that  share  one
+    19              or more of the following components with the unit having
+    20              the duplicate unit identifier:
+
+    21              a.  A unit number select mechanism.
+
+    22              b.  A Run/Stop or Load/Unload switch.
+
+    23              c.  A spindle or other mechanical components.
+
+    24              Note  that  duplicate  unit  identifiers  are   detected
+    25              regardless   of  the  state  of  a  unit's  Run/Stop  or
+    26              Load/Unload switch.
+
+    27          2.  Whenever a controller becomes aware of a duplicate  unit
+    28              identifier, it immediately spins-down all units with the
+    29              duplicate  identifier  and   forces   them   to   remain
+    30              spun-down.    The   controller   spins-down   the  units
+    31              regardless  of  their  current  state.   The  controller
+    32              typically  forces  them  to remain spun-down by spinning
+    33              them down again whenever an operator spins them up.
+
+    34          3.  The   controller    reports    "Unit-Offline    (subcode
+    35              'Inoperative')" status for all units with duplicate unit
+    36              identifiers.  In addition, if the unit might potentially
+    37              be  connected  to  another  controller,  the  controller
+    38              should  flag  the  presence  of   the   duplicate   unit
+    39              identifier  in  the  drive.   Other controllers, if any,
+    40              must  check  this  flag  and  also  treat  the  unit  as
+    41              "Unit-Offline" if the flag is set.
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-48
+           RV80                                                 11 June 1992
+
+
+     1     Whether or not a controller does in fact check for duplicate unit
+     2     identifiers  is controller dependent.  Note that a duplicate unit
+     3     identifier is a drastic failure,  indicative  of  some  otherwise
+     4     undiagnosed hardware malfunction.
+
+     5     E.6.1.2  Media Loader Error Log Format
+
+     6     The  following  section  is  added  after   5.9.8,   "BAD   BLOCK
+     7     REPLACEMENT ATTEMPT Error Log Format":
+
+     8     5.n.m MEDIA LOADER ERRORS Error Log Format
+
+     9     The following format is used to report errors that  occur  during
+    10     media loader operations.
+
+    11                     31                             0 
+    12                     +-------------------------------+
+    13                     |   command reference number    |
+    14                     +---------------+---------------+
+    15                     |sequence number|  unit number  |
+    16                     +---------------+-------+-------+
+    17                     |  event code   | flags | format|
+    18                     +---------------+-------+-------+
+    19                     |                               |
+    20                     +---  controller identifier  ---+
+    21                     |                               |
+    22                     +---------------+-------+-------+
+    23                     |multiunit code |chvrsn | csvrsn|
+    24                     +---------------+-------+-------+
+    25                     |                               |
+    26                     +---     unit identifier     ---+
+    27                     |                               |
+    28                     +-------+-------+-------+-------+
+    29                     |   reserved    | uhvrsn| usvrsn|
+    30                     +-------+-------+-------+-------+
+    31                     |                               |
+    32                     +--- media loader identifier ---+
+    33                     |                               |
+    34                     +-------+-------+-------+-------+
+    35                     | ml unit number|mlhvrsn|mlsvrsn|
+    36                     +-------+-------+-------+-------+
+    37                     |                               |
+    38                     /   controller or media loader  /
+    39                     /     dependent information     /
+    40                     |                               |
+    41                     +-------------------------------+
+
+    42     The fields in the MEDIA LOADER ERROR error  log  message  are  as
+    43     follows  (unit  number,  multiunit code, unit identifier, usvrsn,
+    44     and uhvrsn refer to the disk or tape unit that is the  target  of
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-49
+           RV80                                                 11 June 1992
+
+
+     1     the media loader operation):
+
+     2         command reference number
+     3         unit number
+     4         sequence number
+     5         format
+     6         flags
+     7         event code
+     8         csvrsn
+     9         chvrsn
+    10         multiunit code
+    11         unit identifier
+    12         usvrsn
+    13         uhvrsn
+
+    14             See Section 5.9.2, "Generic Error Log Message Format."
+
+    15         controller identifier
+
+    16             Uniquely identifies  the  controller  among  all  devices
+    17             accessible  via  MSCP.  See Section 4.17, "Controller and
+    18             Unit Identifiers".  If zero, the error log was  generated
+    19             by host software and not by a controller.
+
+    20         media loader identifier
+
+    21             Uniquely identifies the media loader  among  all  devices
+    22             accessible  via  MSCP.  See Section 4.17, "Controller and
+    23             Unit Identifiers." This field is only present for  errors
+    24             that relate to a specific media loader.
+
+    25         mlsvrsn
+
+    26             The  media  loader's  software,  firmware,  or  microcode
+    27             revision   number.    Always   zero   if   the  product's
+    28             development and  maintainability  groups  determine  that
+    29             there  is  no  benefit  from  supporting machine readable
+    30             revision numbers.
+
+    31         mlhvrsn
+
+    32             The media loader's hardware revision number.  Always zero
+    33             if  the  product's development and maintainability groups
+    34             determine  that  there  is  no  benefit  from  supporting
+    35             machine readable revision numbers.
+
+    36         ml unit number
+
+    37             The unit number of the media loader to  which  the  error
+    38             log  message  relates, or zero if either the message does
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                  Page E-50
+           RV80                                                 11 June 1992
+
+
+     1             not relate to a specific unit or if the media loader does
+     2             not support unit numbers.
+
+     3         controller or media loader dependent information
+
+     4             A variable (controller or media loader dependent)  amount
+     5             of  information.   The  length  of  this  information  is
+     6             implied by the total length of  the  error  log  message,
+     7             passed  to  the  class  driver  by the port driver.  This
+     8             information will typically not be  interpreted  by  error
+     9             log  formatting  programs,  instead  being  printed  as a
+    10             series of values.
+
+
+
+
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+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-51
+           RV80                                                                                   11 June 1992
+
+
+     1     E.6.1.3  Error Log Message Offsets
+
+     2     Table A-8, "Error Log Message Offsets" is modified to appear as follows:
+
+
+     3                                    Table A-8:  Error Log Message Offsets
+
+     4         +--------------------+--------------------------+-------+---------------------------------+
+     5         |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     6         +------+------+------+--------+-----------------+ Size  | Field Description               |
+     7         | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     8         +------+------+------+--------+-----------------+-------+---------------------------------+
+     9         |                                            .                                            |
+    10         |                                            .                                            |
+    11         |                                            .                                            |
+    12         +------+------+------+--------+-----------------+-------+---------------------------------+
+    13         | Bad Block Replacement Attempt Error Log Message Offsets:                                |
+    14         |  34  |  42  |  22  | L.RPFL | MSLG$W_RPL_FLGS |   2   | Replace Flags (see Table A-11)  |
+    15         |  36  |  44  |  24  | L.VSER | MSLG$L_VOL_SER  |   4   | Volume serial number            |
+    16         |  40  |  50  |  28  | L.LBN  | MSLG$L_BAD_LBN  |   4   | Bad LBN                         |
+    17         |  44  |  54  |  2C  | L.ORBN | MSLG$L_OLD_RBN  |   4   | Old RBN                         |
+    18         |  48  |  60  |  30  | L.NRBN | MSLG$L_NEW_RBN  |   4   | New RBN                         |
+    19         |  52  |  64  |  34  | L.RPEV | MSLG$W_CAUSE    |   2   | Cause (an event code)           |
+    20         +------+------+------+--------+-----------------+-------+---------------------------------+
+    21         | Media Loader Errors Error Log Message Offsets:                                          |
+    22         |  36  |  44  |  24  | L.MLID | MSLG$Q_ML_ID    |   8   | Media loader identifier         |
+    23         |  44  |  54  |  2C  | L.LSVR | MSLG$B_ML_SVR   |   1   | Media loader software version   |
+    24         |  45  |  55  |  2D  | L.LHVR | MSLG$B_ML_HVR   |   1   | Media loader hardware version   |
+    25         |  46  |  56  |  2E  | L.MLUN | MSLG$W_ML_UNIT  |   2   | Media loader unit number        |
+    26         |  48  |  60  |  30  |        |                 |   *   | Controller or media loader      |
+    27         |      |      |      |        |                 |       | dependent information (optional)|
+    28         +------+------+------+--------+-----------------+-------+---------------------------------+
+    29         | Note:  An asterisk (*) in the field size indicates that the size varies.                |
+    30         +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-52
+           RV80                                                                                   11 June 1992
+
+
+     1     E.6.1.4  Error Log Message Format Codes
+
+     2     Table A-9, "Error Log Message Format Codes" is modified to appear as follows:
+
+
+     3                                 Table A-9:  Error Log Message Format Codes
+
+     4         +--------------------+--------------------------+-----------------------------------------+
+     5         |    Format Code     |   Preferred Mnemonics    |                                         |
+     6         +------+------+------+--------+-----------------+   Format Description                    |
+     7         | Dec. | Oct. | Hex. | 16 bit |      32 bit     |                                         |
+     8         +------+------+------+--------+-----------------+-----------------------------------------+
+     9         |   0  |   0  |   0  | FM.CNT | MSLG$K_CNT_ERR  | Controller Errors                       |
+    10         +------+------+------+--------+-----------------+-----------------------------------------+
+    11         |   1  |   1  |   1  | FM.BAD | MSLG$K_BUS_ADDR | Memory Errors  with  (Host/Controller)  |
+    12         |      |      |      |        |                 | Memory Address                          |
+    13         +------+------+------+--------+-----------------+-----------------------------------------+
+    14         |   2  |   2  |   2  | FM.DSK | MSLG$K_DISK_TRN | Disk Transfer Errors                    |
+    15         +------+------+------+--------+-----------------+-----------------------------------------+
+    16         |   3  |   3  |   3  | FM.SDI | MSLG$K_SDI      | SDI Errors                              |
+    17         +------+------+------+--------+-----------------+-----------------------------------------+
+    18         |   4  |   4  |   4  | FM.SDE | MSLG$K_SML_DSK  | Small Disk Errors                       |
+    19         +------+------+------+--------+-----------------+-----------------------------------------+
+    20         |   9  |  11  |   9  | FM.RPL | MSLG$K_REPLACE  | Bad Block Replacement Attempt           |
+    21         +------+------+------+--------+-----------------+-----------------------------------------+
+    22         |  10  |  12  |   A  | FM.LDR | MSLG$K_LDR_ERR  | Media Loader Errors (see Note below)    |
+    23         +------+------+------+--------+-----------------+-----------------------------------------+
+
+    24     E.6.1.5  Status And Event Codes
+
+    25     Table B-1, "Status and Event Codes" is modified to appear as follows:
+
+
+    26                                     Table B-1:  Status and Event Codes
+
+    27         +--------------------+--------------------------+-----------------------------------------+
+    28         |       Value        |   Preferred Mnemonics    |                                         |
+    29         +------+------+------+--------+-----------------+     Status or Event Code                |
+    30         | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+    31         +------+------+------+--------+-----------------+-----------------------------------------+
+    32         |                                   .                                                     |
+    33         |                                   .                                                     |
+    34         |                                   .                                                     |
+    35         +------+------+------+--------+-----------------+-----------------------------------------+
+    36         |   20 |  24  |  14  | ST.BBR | MSCP$K_ST_BBR   | Bad Block Replacement Completion        |
+    37         +------+------+------+--------+-----------------+-----------------------------------------+
+    38         |   21 |  25  |  15  | ST.LDR | MSCP$K_ST_LOADR | Media Loader Error                      |
+    39         +------+------+------+--------+-----------------+-----------------------------------------+
+    40         |   31 |  37  |  1F  | ST.DIA | MSCP$K_ST_DIAG  | Message from an internal diagnostic     |
+    41         +------+------+------+--------+-----------------+-----------------------------------------+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-53
+           RV80                                                                                   11 June 1992
+
+
+     1     E.6.1.6  Media Loader Error Status Or Event Subcodes
+
+     2     Table B-10, "Media Loader Error" Status or Event Subcodes, is added  to  Appendix  B,  "Status  and
+     3     Event Code Definitions" and appears as follows:
+
+
+     4                         Table B-10:  "Media Loader Error" Status or Event Subcodes
+
+     5          +------+---------------------+-+-+-----------------------------------------------------+
+     6          | Sub- |   Code + Subcode    |E|S|                                                     |
+     7          | code +------+-------+------+V|T|     Status or Event Subcode (see Note below)        |
+     8          |      | Dec. |  Oct. | Hex. | | |                                                     |
+     9          +------+------+-------+------+-+-+-----------------------------------------------------+
+    10          |    1 |   55 |   67  |   37 |*|*| Loader command time out                             |
+    11          +------+------+-------+------+-+-+-----------------------------------------------------+
+    12          |    2 |   87 |  127  |   57 |*|*| Controller detected transmission error              |
+    13          +------+------+-------+------+-+-+-----------------------------------------------------+
+    14          |    3 |  119 |  167  |   77 |*|*| Controller detected protocol error                  |
+    15          +------+------+-------+------+-+-+-----------------------------------------------------+
+    16          |    4 |  151 |  227  |   97 |*|*| Loader detected error                               |
+    17          +------+------+-------+------+-+-+-----------------------------------------------------+
+
+    18     E.6.1.7  Controller And Unit Identifier Class Values
+
+    19     Table C-1, "Controller and Unit Identifier Class Values" is modified to appear as follows:
+
+
+    20                         Table C-1:   Controller and Unit Identifier "Class" Values
+
+    21                         +----------+----------------------------------------------+
+    22                         |Class Byte|                                              |
+    23                         | (decimal)|     Subsystem Type                           |
+    24                         +----------+----------------------------------------------+
+    25                         |     0    | reserved -- must not be assigned             |
+    26                         +----------+----------------------------------------------+
+    27                         |     1    | Mass storage controllers                     |
+    28                         +----------+----------------------------------------------+
+    29                         |     2    | Disk class devices -- DEC Standard 166 disks |
+    30                         +----------+----------------------------------------------+
+    31                         |     3    | Tape class devices                           |
+    32                         +----------+----------------------------------------------+
+    33                         |     4    | Disk class devices -- DEC Standard 144 disks |
+    34                         +----------+----------------------------------------------+
+    35                         |     5    | Loader class devices                         |
+    36                         +----------+----------------------------------------------+
+
+
+
+
+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                          Confidential And Proprietary
+           Mass Storage Control Protocol                                                         Version 2.4.0
+           WAIVERS AND EXCEPTIONS                                                                    Page E-54
+           RV80                                                                                   11 June 1992
+
+
+     1     E.6.1.8  Media Loader Identifier Values
+
+     2     Table C-5 is added to Appendix C, "Controller, Unit, and Media Type  Identifiers"  and  appears  as
+     3     follows:
+
+
+     4                                 Table C-5:   Media Loader Identifier Values
+
+     5         +----------+---------+-------+---------------------------+--------------------------------+
+     6         |Model Byte|  Device | Media |   Media Type Identifier   |                                |
+     7         | (decimal)|Type Name| Name  |     octal     |    hex    |     Device                     |
+     8         +----------+---------+-------+---------------+-----------+--------------------------------+
+     9         |     0    |  Reserved -- must not be assigned                                            |
+    10         +----------+---------+-------+---------------+-----------+--------------------------------+
+    11         |     1    |  none   | none  |     none      |   none    | 64 cartridge jukebox           |
+    12         +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
+
+
+
+
+
+                                                            
+                                             *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           PORT ADDRESS AND SYSTEM ADDRESS FORMATS                  Page F-1
+                                                                11 June 1992
+
+
+     1                                APPENDIX F
+
+     2                  PORT ADDRESS AND SYSTEM ADDRESS FORMATS
+
+
+     3     F.1  Port Address Formats
+
+     4     The format of the "storage subsystem port address"  field  varies
+     5     depending  on the communications mechanism being used.  The field
+     6     format  for  the  currently  defined  (multihost)  communications
+     7     mechanism is described in the subsection that follows.
+
+     8     F.1.1  CI Port Address
+
+     9     The format of the "storage subsystem  port  address"  field  when
+    10     used with the Computer Interconnect (CI) communications mechanism
+    11     is as follows:
+
+    12                     31                             0 
+    13                     +---------------+---------------+
+    14                     |               |  xport_addr   |
+    15                     +               +---------------+
+    16                     |              mbz              |
+    17                     +-------------------------------+
+
+
+    18         xport_addr
+
+    19             16-bit  CI   transport   address.    See   the   Computer
+    20             Interconnect   Specification   (DEC   STD  161-0)  for  a
+    21             description of the internal structure of this field.
+
+    22         mbz
+
+    23             This portion of the field Must Be Zero.
+
+    24     Note  that  the  Digital  Storage  Systems  Interconnect   (DSSI)
+    25     communications  mechanism uses the same port addressing scheme as
+    26     the CI.
+
+    27     F.2  System Address Formats
+
+    28     The format of the "storage subsystem system address" field varies
+    29     depending  on  the communications protocol being used.  The field
+    30     format  for  the  currently  defined  (multihost)  communications
+    31     protocol is described in the subsection that follows.
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           PORT ADDRESS AND SYSTEM ADDRESS FORMATS                  Page F-2
+           System Address Formats                               11 June 1992
+
+
+     1     F.2.1  SCA System Address
+
+     2     The format of the "storage subsystem system address"  field  when
+     3     used   with   the   Systems   Communications  Architecture  (SCA)
+     4     communications protocol is as follows:
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |        system address         |
+     8                     +---------------+               +
+     9                     |      mbz      |               |
+    10                     +---------------+---------------+
+
+
+    11         system address
+
+    12             48-bit system address.  See  the  Systems  Communications
+    13             Architecture  Specification  for  a  description  of  the
+    14             internal structure of this field.
+
+    15         mbz
+
+    16             This portion of the field Must Be Zero.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-1
+                                                                11 June 1992
+
+
+     1                                APPENDIX G
+
+     2                             REVISION HISTORY
+
+
+     3  |  The following changes were made from MSCP Version 2.3.0  to  MSCP
+     4  |  Version 2.4.0:
+
+     5  |      The following approved ECOs to V2.1.1 were incorporated:
+
+
+     6  |       o  MSCP21-26:    Vail    event    code/subcodes    (inpart).
+     7  |          Supporting  Device  Event  subcodes and code Drive Error,
+     8  |          subcode Integrated Drive Error not included.
+
+     9  |       o  MSCP23-3:  Yamle
+
+    10  |       o  MSCP23-4:  Performance Assist/Support Write Log
+
+
+    11  |      The following editorial changes/corrections were made:
+
+    12  |      1.  Unit Names removed.  (ECO MSCP22-8)
+
+    13  |      2.  Combined the subsections Status  codes  and  Event  codes
+    14  |          into one section titled Status and Event Codes.
+
+    15  |      3.  Corrected an error in the byte count modifer section  for
+    16  |          tape class device response to an invalid byte count.
+
+    17  |      4.  Added information to chapter 7 on releasing of  exclusive
+    18  |          access.  This information was from eco MSCP12-34.
+
+
+    19     The following changes were made from MSCP Version 2.2.0  to  MSCP
+    20     Version 2.3.0:
+
+    21         This appendix changed from Appendix F to Appendix G.   A  new
+    22         Appendix  F  (Port  Address  and  System Address Formats) was
+    23         added.  Appendix G (Unannounced Products) has been renamed to
+    24         Appendix H.
+
+    25         The following approved ECOs to V2.1.1 were incorporated:
+
+
+    26          o  MSCP21-22:  HBVS Host Based Volume shadowing
+
+    27          o  MSCP22-1:  Caching Support
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-2
+                                                                11 June 1992
+
+
+     1          o  MSCP22-2:  DISPLAY Command Category
+
+     2          o  MSCP22-5:  Write History Management
+
+     3          o  MSCP22-7:  Error Log Message Size Limits
+
+     4          o  MSCP22-8:  Vail Unit Naming
+
+     5          o  MSCP22-9:  Vail Multiple Spindle Units
+
+
+     6         Several editorial changes/corrections/additions were made and
+     7         many  new device identifiers and controller model values were
+     8         added.
+
+     9     The following changes were made from MSCP Version 2.1.1  to  MSCP
+    10     Version 2.2.0:
+
+    11         The following approved ECOs to V2.1.1 were incorporated:
+
+
+    12          o  MSCP21-7:  HBAE on ACCESS and ERASE
+
+    13          o  MSCP21-8:  Information Logging Event Code
+
+    14          o  MSCP21-9:  Information Error Log Flag
+
+    15          o  MSCP21-11:  New DISPLAY Command
+
+    16          o  MSCP21-14:  New Event Codes/Subcodes
+
+    17          o  MSCP21-15:  Command Opcode Encoding
+
+    18          o  MSCP21-16:  Controller Model Values
+
+
+    19         Several editorial changes/corrections/additions were made and
+    20         many  new device identifiers and controller model values were
+    21         added.
+
+    22     The following changes were made from MSCP  Version  2.1  to  MSCP
+    23     Version 2.1.1:
+
+    24         The following approved ECOs to V2.1 were incorporated:
+
+    25          o  MSCP21-1:  Controller Model Values
+
+    26          o  MSCP21-4:  Allocation Class
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-3
+                                                                11 June 1992
+
+
+     1          o  MSCP21-5:  DSSI Device Type Names
+
+
+     2     The following changes were made from MSCP  Version  2.1  to  MSCP
+     3     Version 2.1.1:
+
+     4         All change bars were removed to make Version 2.1 official.
+
+     5     The following changes were made  from  MSCP  Version  2.0.2  (aka
+     6     Version 1.3 Update #2) to MSCP Version 2.1:
+
+     7         The following approved ECOs to V2.0 were incorporated:
+
+     8          o  MSCP13-10 Format Command Revisited
+
+     9          o  MSCP13-11 Change RF70 to RF71
+
+    10          o  MSCP13-12 Add TA90 to Appendix C
+
+    11          o  MSCP13-13 Change RV80 to RV20
+
+    12          o  MSCP13-14 Add RV60 to Appendix C
+
+    13          o  MSCP13-15 Add SVS00 to Appendix C
+
+    14         The following editorial changes/corrections were made:
+
+    15         1.  Added RD33 to Appendix C
+
+    16         2.  Added three definitions to Chapter 2, "Terminology":
+
+    17              -  May
+
+    18              -  Shall
+
+    19              -  Should
+
+
+    20         3.  Applied the new terminology  to  the  affected  sentences
+    21             throughout the document
+
+    22         4.  Made Appendix E acceptable for non-DEC distribution:
+
+    23              -  Removed the initial NOTE
+
+    24              -  Removed all justification sections
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-4
+                                                                11 June 1992
+
+
+     1         5.  Changed Sections 4.16 and 4.17 and the  opening  NOTE  in
+     2             Appendix C to indicate the new method for obtaining model
+     3             values for new devices
+
+     4         6.  Created Appendix G, "Unannounced Products," to  hold  the
+     5             values  for  all  products that have been assigned values
+     6             but have not  yet  been  announced.   Three  tables  were
+     7             created,  one  for  controller model values, one for disk
+     8             drive/media  identifier  values,   and   one   for   tape
+     9             drive/media identifier values
+
+    10         7.  Removed all unannounced product names  from  Tables  C-2,
+    11             C-3,  and  C-4,  put  them  in  the appropriate tables in
+    12             Appendix G, and left pointers to the tables in  place  of
+    13             the product specifics
+
+    14         8.  Added revision histories for the  two  updates  prior  to
+    15             this one
+
+    16         9.  Indicated the new version numbers in the revision history
+
+
+    17     The following changes were made  from  MSCP  Version  2.0.1  (aka
+    18     Version  1.3  Update  #1)  to MSCP Version 2.0.2 (aka Version 1.3
+    19     Update #2):
+
+    20         The following approved ECOs to V2.0 were incorporated:
+
+    21          o  MSCP13-7 Media Loader Error Logs
+
+    22          o  MSCP13-8 Host Responsibilities at ONLINE with w.r.t.  BBR
+
+    23          o  MSCP13-9 Addition of "No Multicopy Protection" subcode
+
+    24         The following editorial changes/corrections were made:
+
+    25         1.  Added ACCESS NONVOLATILE MEMORY to the list  of  commands
+    26             in Note 3 of Table A-2
+
+    27         2.  Returned "Clear Serious Exception (ignored for disk class
+    28             devices)"   modifier   to  the  command  descriptions  of
+    29             commands for which it is allowed
+
+
+    30     The following changes were made from MSCP  Version  2.0  to  MSCP
+    31     Version 2.0.1 (aka Version 1.3 Update #1):
+
+    32         The following approved ECOs to V2.0 were incorporated:
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-5
+                                                                11 June 1992
+
+
+     1          o  MSCP13-1 Change MSCP "Exclusive Access" Unit Flag
+
+     2          o  MSCP13-3 Assignment of RQDX4 Controller and RX35 Drive
+
+     3          o  MSCP13-4 Assignment of RD32 Drive Model Value
+
+     4          o  MSCP13-5 Assignment of UQSSP/DSSI Generic Device Types
+
+     5          o  MSCP13-6 Assignment of RF30 and RF70 Drive Model Values
+
+
+     6     The following changes were made from MSCP  Version  1.2  to  MSCP
+     7     Version 2.0 (aka Version 1.3):
+
+     8         The following approved ECOs to V1.2 were incorporated:
+
+     9          o  MSCP12-1 Geometry Valid Only When Online
+    10          o  MSCP12-2 Error Log Sequence Numbers
+    11          o  MSCP12-9 Reserve OPCODE Zero
+    12          o  MSCP12-10 Revised Appendix B
+    13          o  MSCP12-11 Revised Appendix C
+    14          o  MSCP12-12 RCT Access Byte Count Clarification
+    15          o  MSCP12-16 Error Log Format Generalization
+    16          o  MSCP12-17 Expeditious Abort
+    17          o  MSCP12-18 Revised Appendix B for BI
+    18          o  MSCP12-19 Maximum Error Log Size
+    19          o  MSCP12-20 RC25 Waiver Requests
+    20          o  MSCP12-22 Revised Appendix C for RX Drives
+    21          o  MSCP12-25 Revised Appendix C for QDAs
+    22          o  MSCP12-26 Allocation Class Waiver (HSC/VMS)
+    23          o  MSCP12-27 Read Only Device Write Protect
+    24          o  MSCP12-30 Multihost Functionality
+    25          o  MSCP12-32 Maximum Byte Count
+    26          o  MSCP12-33 Controller Initiated Bad Block Replacement
+    27          o  MSCP12-34 Exclusive Access (Multihost)
+    28          o  MSCP12-35 Access Non-volatile Memory Command
+    29          o  MSCP12-37 Host BBR Error Log Usage
+    30          o  MSCP12-38 Host BBR RCT Full Error Code
+    31          o  MSCP12-39 TU81 Waiver Request
+    32          o  MSCP12-44 Newly Revised Appendix C
+    33          o  MSCP12-46 Appendix C Assignment
+    34          o  MSCP12-51 Zero Geometry Illegal
+    35          o  MSCP12-53 Diagnostic Opcodes
+    36          o  MSCP12-55 MSCP12-26 Time Extension
+    37          o  MSCP12-56 Multihost Flag Definition
+    38          o  MSCP12-57 Minor Errors in ECOs
+    39          o  MSCP12-59 Appendix C, Correct KDB50 Model Number
+    40          o  MSCP12-62 Appendix C Update
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-6
+                                                                11 June 1992
+
+
+     1          o  MSCP12-63 Appendix B Update
+     2          o  MSCP12-65 Appendix C Update  -  Add  TK70  and  MSCP12-62
+     3             Amendments
+     4          o  MSCP12-72 Revised Bundled Shadowing
+     5          o  MSCP12-78 Appendix C Update - Add TA79 (as amended)
+     6          o  MSCP12-79   "Exclusive   Access"   Unit   Flag   Conflict
+     7             Resolution
+
+     8         The  following  editorial  changes  were  performed  (on  the
+     9         original and ECO text):
+
+    10         1.  Spelling errors corrected.  Including removal of "hyphen"
+    11             from  common  prefixes  (e.g.,  non, re, sub, etc.) where
+    12             appropriate.
+
+    13         2.  Punctuation errors corrected.
+
+    14         3.  Rewording of text for the sake of clarity.
+
+    15         4.  Removal of redundant words, phrases, or statements.
+
+    16         5.  Addition of numerous section references.
+
+    17         6.  Addition of  "Multiaccessed  Drives"  description.   This
+    18             description  was  referenced  but  not  included  in  the
+    19             original document.
+
+    20         7.  All references to Shadowing and Data Caching were removed
+    21             from  the  original  text; working on the assumption that
+    22             when they  are  formally  defined  they  will  appear  as
+    23             separate  chapters  in  the  document.  Flags and message
+    24             offsets already defined for their  use  were  changed  to
+    25             "allocated  for  future  use"  and a description of their
+    26             treatment until such definition occurs was provided.
+
+    27         8.  All references to "Replacement and  Caching  Table"  were
+    28             changed to "Replacement Control Table."
+
+    29         9.  All references to "DEC Standard Disk Format" were changed
+    30             to    "Digital    Storage    Architecture   Disk   Format
+    31             Specification (DSDF)."
+
+    32        10.  Chapter  5  was  retitled  from  "MSCP  CONTROL   MESSAGE
+    33             FORMATS" to "MSCP CONTROL MESSAGES."  The contents of the
+    34             chapter underwent a major restructuring:
+
+    35             a.  The original lead-in section "Generic Control Message
+    36                 Format"  was  made  truly generic.  The types of MSCP
+    37                 control  messages  were  identified:   COMMAND,  END,
+    38                 ATTENTION, and ERROR LOG messages.
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-7
+                                                                11 June 1992
+
+
+     1             b.  The "command message" specific information  contained
+     2                 in  the  original lead-in was moved to a new section,
+     3                 "Command Messages," which describes a generic command
+     4                 message.
+
+     5             c.  The  original  "Transfer  Command   Message   Format"
+     6                 (without generic information) and "Command Modifiers"
+     7                 sections became subsections of the "Command Messages"
+     8                 section.
+
+     9             d.  The  original  "End  Message  Format"   section   was
+    10                 retitled   "End   Messages"  and  all  references  to
+    11                 transfer command fields were  removed.   The  section
+    12                 now describes a generic end message.
+
+    13             e.  The  transfer   command   field   descriptions   were
+    14                 incorporated  into  a  subsection  of  "End Messages"
+    15                 called:  "Transfer Command End Message Format."  That
+    16                 section,  as  its  name  implies, describes a generic
+    17                 transfer command end message.
+
+    18             f.  The  original  "Status  Codes"   section   became   a
+    19                 subsection of "End Messages." 
+
+    20             g.  The various descriptions of  "Invalid  Commands"  and
+    21                 "Invalid  Command  End Messages" originally contained
+    22                 in Chapter 6 and Chapter 10  were  consolidated  into
+    23                 one  concise  description,  which  is  located in the
+    24                 "Invalid Command" description of the  "Status  Codes"
+    25                 subsection.   The  original  text  was  removed  from
+    26                 Chapter 6, Chapter 10 was totally eliminated.
+
+    27             h.  The descriptions  of  "Controller  Flags"  originally
+    28                 contained  in  Sections 5.8 and 6.2 were consolidated
+    29                 into a single section:  "Controller Flags."
+
+    30             i.  The descriptions of "Unit Flags" originally contained
+    31                 in  Sections  5.7  and  6.1  were consolidated into a
+    32                 single section:  "Unit Flags."
+
+    33             j.  A new section, "Attention Messages," was added.  That
+    34                 section  describes  a generic attention message.  The
+    35                 attention message  formats  originally  contained  in
+    36                 Chapter  6  were  moved  to  Chapter  5 and appear as
+    37                 subsections of "Attention Messages."
+
+    38             k.  A new section, "Error Log Messages," was added.  That
+    39                 section consists of what was originally in Chapter 8.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-8
+                                                                11 June 1992
+
+
+     1        11.  Chapter 6 was retitled from "MINIMAL DISK MSCP SUBSET" to
+     2             "MINIMAL  MSCP COMMAND SET."  As implied by the change in
+     3             title the chapter is now devoted to describing  the  MSCP
+     4             command  set  rather  than defining the minimal MSCP disk
+     5             controller.  The "Overview" section  indicates  that  the
+     6             commands  described in the chapter apply only to a single
+     7             host disk controller which does  not  provide  controller
+     8             initiated  bad  block replacement (CIBBR).  References to
+     9             multihost and CIBBR operation were removed.  As mentioned
+    10             earlier  references  to  Shadowing  and Caching were also
+    11             removed,  including  the  descriptions  of  the   COMPARE
+    12             CONTROLLER  DATA and FLUSH commands.  A subsection of the
+    13             "Overview", "No-operation Commands," describes how  those
+    14             commands   are   to  be  handled.   The  "Overview"  also
+    15             describes the reformatting  of  the  command  description
+    16             sections  into  subsections.  Each component of a command
+    17             description is now contained in  a  separate  subsection:
+    18             command   message   format,   end   message  format,  and
+    19             description.  The reformatting  allows  for  more  direct
+    20             references to the specific area of a command description.
+
+    21        12.  Chapter 7 is devoted  to  describing  "Multihost  Support
+    22             Functionality."
+
+    23        13.  Chapter 8 is devoted to describing "Controller  Initiated
+    24             Bad Block Replacement."
+
+    25        14.  "Waivers  and  Exceptions"  now  appears  in  Appendix  E
+    26             instead  of  Chapter 9.  The purpose of that change is to
+    27             allow  the  addition  of   chapters   devoted   to   Data
+    28             Encryption/Decryption,  Caching,  and  Shadowing when the
+    29             time arises.  Note that "Bundled"  Shadowing  appears  in
+    30             Appendix   E   and   is   identified   as   a  "Temporary
+    31             Implementation."   Also,  there  was  no  opposition   to
+    32             tabling  Data Encryption/Decryption so it does not appear
+    33             anywhere.
+
+    34        15.  "Revision History" now appears in Appendix F.
+
+
+    35     The following changes were made from MSCP  Version  1.1  to  MSCP
+    36     Version 1.2:
+
+    37          1.  Editorial changes, primarily due to MSCP being  reviewed
+    38              by  the  technical documentation group.  As part of this
+    39              review  Chapter  1,  the  Introduction,  was  completely
+    40              rewritten.   Other  editorial  changes  are  due to some
+    41              reviewer's request for a clarification.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                         Page G-9
+                                                                11 June 1992
+
+
+     1          2.  The discussion of  Reserved  and  Undefined  fields  was
+     2              expanded and made into a separate section (Section 5.2).
+     3              This is in response to the frequent confusion concerning
+     4              this  topic  in  the  past.  Note that these changes are
+     5              purely editorial.
+
+     6          3.  Consistent terminology was adopted for the ways in which
+     7              execution  of  a  command can finish.  Thus a command is
+     8              now  said  to  be  completed,  aborted,  terminated,  or
+     9              rejected.   These  terms  are defined in section 4.5 and
+    10              used consistently throughout the document.
+
+    11          4.  Addition of the "controller dependent parameters"  field
+    12              to the SET CONTROLLER CHARACTERISTICS command.
+
+    13          5.  The "Disk Transfer Errors" error log message format  was
+    14              substantially  changed.  Formerly this message contained
+    15              "logical block number,"  "cylinder,"  "group,"  "track,"
+    16              and  "sector" fields.  These have all been replaced with
+    17              a single "header code" field, which the host's error log
+    18              analyzer can decode if it desires.
+
+    19          6.  The  "SDI  Errors"  error   log   message   format   was
+    20              substantially  changed.  Formerly this message contained
+    21              "logical block number," "cylinder," and "group"  fields.
+    22              These  have  been  replaced  with a single "header code"
+    23              field, which the host's error log analyzer can decode if
+    24              it desires.
+
+    25          7.  "EDC Error" has been eliminated  as  a  subcode  of  the
+    26              "Media  Format  Error"  and  "Data  Error" status codes.
+    27              Instead it is now a subcode of  the  "Controller  Error"
+    28              status  code.   This  reflects  the fact that EDC errors
+    29              should only be caused  by  faulty  controllers,  not  by
+    30              faulty  disks.   Any  disk error that might cause an EDC
+    31              error should also  cause  an  uncorrectable  ECC  error,
+    32              which will be reported instead of the EDC error.
+
+    33          8.  subcodes  of  the  "Drive  Error"   status   code   were
+    34              completely  revised  to  reflect what the UDA50 actually
+    35              returns.
+
+    36          9.  Additional identifiers  were  added  to  Appendix  C  to
+    37              reflect  all prospective MSCP devices that we know of at
+    38              this time.   In  addition,  the  Aztec  media  name  was
+    39              changed to reflect its new device name (RC25).
+
+    40         10.  An additional rule for MSCP servers was added to Section
+    41              3.4.   This  rule  can  be  summarized  as  saying  that
+    42              bootstraps have to work, even though they  won't  follow
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                        Page G-10
+                                                                11 June 1992
+
+
+     1              all  the  rules for "real" class drivers.  This rule has
+     2              always  existed  implicitly;  it  just  has  never  been
+     3              written down before.
+
+     4         11.  The RCT has been made  optional  for  certain  types  of
+     5              devices.   This is primarily aimed at the RX50 and other
+     6              low end disks.
+
+     7         12.  The discussion of forced errors was  expanded  to  state
+     8              that  the data must be recorded and preserved regardless
+     9              of  the  presence  of  a  forced  error  in   a   block.
+    10              Previously  this  was  a piece of folklore that everyone
+    11              "knew" to be true, as it was not formerly written down.
+
+    12         13.  Unit and controller revision numbers were added  to  the
+    13              GET  UNIT  STATUS and SET CONTROLLER CHARACTERISTICS end
+    14              messages.
+
+    15         14.  Statements were added that unit and controller  revision
+    16              numbers  need  not be returned if the groups responsible
+    17              for a product feel they are unnecessary.  This is merely
+    18              making  an  explicit  statement  of what has always been
+    19              implicitly true.
+
+    20         15.  The Invalid Command End Message format was  extended  to
+    21              return  a  controller  dependent  portion of the command
+    22              parameter fields.  Thus the entire message header can be
+    23              returned with only the opcode and modifiers fields lost,
+    24              hopefully simplifying error diagnosis.
+
+    25         16.  A waivers and exceptions chapter was  added  to  provide
+    26              "grandfather"  exceptions for products that have already
+    27              shipped, and therefore do not have  some  of  the  above
+    28              changes implemented.
+
+    29         17.  A clarifications chapter was added to record answers  to
+    30              frequently asked questions and clarifications that don't
+    31              fit in anywhere else.
+
+
+    32     The following changes were made from MSCP  Version  1.0  to  MSCP
+    33     Version 1.1:
+
+    34          1.  Editorial changes, including clarifications  in  various
+    35              sections.
+
+    36          2.  Addition  of  this  Appendix,  containing  the  Revision
+    37              History.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                        Page G-11
+                                                                11 June 1992
+
+
+     1          3.  Addition of Appendix D, Buffer Descriptor Formats.
+
+     2          4.  Addition of non-goal stating MSCP will  not  attempt  to
+     3              standardize  device  specific  error log information and
+     4              the interpretation of such information.
+
+     5          5.  Reduction of the  required  unit  number  range  from  0
+     6              through  255  to  0  through  251.   The purpose of this
+     7              change was to allow  a  specific  value  (all  ones)  to
+     8              indicate that no unit number is present.
+
+     9          6.  The majority of unit characteristics  will  be  provided
+    10              (to  hosts)  when  a unit is "Unit-Offline" but known to
+    11              exist.  This is necessary for generic device  allocation
+    12              --  previous  versions,  due  to an oversight, specified
+    13              that unit characteristics were not provided when a  unit
+    14              was "Unit-Offline." 
+
+    15          7.  As a necessary  side  effect  of  the  previous  change,
+    16              duplicate  unit  numbers  must be checked for units that
+    17              are  "Unit-Offline"   but   otherwise   known   to   the
+    18              controller.   The  reason  for  this  is that controller
+    19              needs a unique set of characteristics to  return,  which
+    20              is not necessarily possible when a duplicate unit number
+    21              exists.
+
+    22          8.  The  command  timeout  requirements  were  loosened  for
+    23              aborted  commands  on  single host controllers, allowing
+    24              the  controllers  to  timeout  in  various  pathological
+    25              situations.  If this occurs the command will effectively
+    26              be aborted by the host reinitializing the controller.
+
+    27          9.  The requirements for host access timeout timing accuracy
+    28              were  loosened,  so  that  controllers need not maintain
+    29              accurate  timeouts  for  an  idle  host  when  they  are
+    30              saturated with work from other hosts.
+
+    31         10.  Added the "Controller Initiated Bad  Block  Replacement"
+    32              controller flag.
+
+    33         11.  The maximum number of error log messages per  error  was
+    34              increased from 2 to 3.
+
+    35         12.  Waivers and exceptions relating to the BXV11, which  has
+    36              been canceled, were removed.
+
+    37         13.  Host identifiers and the "Alter Host Identifier" command
+    38              modifier were removed.
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Mass Storage Control Protocol                       Version 2.4.0
+           REVISION HISTORY                                        Page G-12
+                                                                11 June 1992
+
+
+     1         14.  The "Error Log Flags" field was renamed to be a  general
+     2              purpose "Device Dependent Parameters" field.
+
+     3         15.  subcodes of the "Success" status code were reassigned so
+     4              that  all subcodes would be unique bit flags.  I.e., the
+     5              same bit will not have different meanings  depending  on
+     6              the command.
+
+     7         16.  subcodes  of  the  "Unit-Offline"   status   code   were
+     8              reassigned to be bit flags, so that multiple reasons for
+     9              a device being "Unit-Offline"  could  be  reported.   In
+    10              addition, an additional (unique) subcode was defined for
+    11              the case of a unit being inoperative.
+
+    12         17.  subcodes of  the  "Write  Protected"  status  code  were
+    13              reassigned to be bit flags.
+
+    14         18.  Unit and media type identifiers were defined for  Aztec.
+    15              In addition, preliminary values were defined for various
+    16              products that are currently under development.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                                           
+                            *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           UNANNOUNCED PRODUCTS                                                               Page H-1
+                                                                                          11 June 1992
+
+
+     1                                             APPENDIX H
+
+     2                                        UNANNOUNCED PRODUCTS
+
+
+     3                         Table H-1:  Unannounced Controller "Model" Values
+
+     4                     +--------------+------------------------------------------+
+     5                     |  Model Byte  |                                          |
+     6                     |  (decimal)   |                                          |
+     7                     | (see Note 1) |   Controller Type                        |
+     8                     +--------------+------------------------------------------+
+     9                     |      20      | RQDX4  (CSS, Reading Special)            |
+    10                     +--------------+------------------------------------------+
+    11                     |      41      | HSC65                                    |
+    12                     +--------------+------------------------------------------+
+    13                     |      42      | HSC95                                    |
+    14                     +--------------+------------------------------------------+
+    15                     |     105      | EF51                                     |
+    16                     +--------------+------------------------------------------+
+    17                     |     108      | RF36                                     |
+    18                     +--------------+------------------------------------------+
+    19                     |     109      | RF74                                     |
+    20                     +--------------+------------------------------------------+
+    21                     |     112      | RF75                                     |
+    22                     +--------------+------------------------------------------+
+    23                     |     248      | ULTRIX-32                                |
+    24                     +--------------+------------------------------------------+
+    25                     |     249      | VAX SVS software emulator                |
+    26                     +--------------+------------------------------------------+
+    27                     |     250      | VAX SST Striping Driver                  |
+    28                     +--------------+------------------------------------------+
+    29                     | Note 1:   The   preferred   method   for   assigning  a |
+    30                     |           controller  model  value  is according to the |
+    31                     |           port   protocol   used. The recommended model |
+    32                     |           ranges are as follows:                        |
+    33                     |                                                         |
+    34                     |              o 1-31    SSP                              |
+    35                     |              o 32-63   CI                               |
+    36                     |              o 64-95   BVP                              |
+    37                     |              o 96-127  DSSI                             |
+    38                     |              o 128-247 Reserved for future use          |
+    39                     |              o 248-255 Host Software Servers            |
+    40                     |                                                         |
+    41                     | Note 2:   This controller serves SCSI devices. Refer to |
+    42                     |           Table H-4.                                    |
+    43                     |                                                         |
+    44                     +---------------------------------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           UNANNOUNCED PRODUCTS                                                               Page H-2
+                                                                                          11 June 1992
+
+
+     1                     Table H-2:  Unannounced Disk Drive/Media Identifier Values
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |    15    |    DU   | RD32  | 022544,040040 | 2564,4020 | 40   MB,  5.25",  fixed,  half |
+     8     |          |         |       |               |           | height                         |
+     9     +----------+---------+-------+---------------+-----------+--------------------------------+
+    10     |    16    | unassigned (was RX32)                                                        |
+    11     +----------+---------+-------+---------------+-----------+--------------------------------+
+    12     |    20    |    DU   | RX35  | 022545,100043 | 2565,8023 | TEAC   FD35F,   3.5",   floppy |
+    13     |          |         |       |               |           | (CSS, Reading Special)         |
+    14     +----------+---------+-------+---------------+-----------+--------------------------------+
+    15     |    23    |    DE   | SVS00 | 020547,064600 | 2167,6980 | Variable, depends on           |
+    16     |          |         |       |               |           | implementation.                |
+    17     +----------+---------+-------+---------------+-----------+--------------------------------+
+    18     |    24    |    DU   | RD33  | 022544,040041 | 2564,4021 | 80 MB, 5.25", fixed, half      |
+    19     |          |         |       |               |           | height                         |
+    20     +----------+---------+-------+---------------+-----------+--------------------------------+
+    21     |    31    |    DU   | ESE52 | 022513,031264 | 254B,32B4 | 120MB  Solid state disk with   |
+    22     |          |         |       |               |           | data retention.                |
+    23     +----------+---------+-------+---------------+-----------+--------------------------------+
+    24     |    47    |    DU   | RA73  | 022544,010111 | 2564,1049 | tbs, 5.25" fixed               |
+    25     |          |         |       |               |           |                                |
+    26     +----------+---------+-------+---------------+-----------+--------------------------------+
+    27     |    48    |    DU   | ESE56 | 022513,031270 | 254B,32B8 | 600 MB  Solid state disk with  |
+    28     |          |         |       |               |           | data retention.                |
+    29     +----------+---------+-------+---------------+-----------+--------------------------------+
+    30     |    49    |    DU   | ESE58 | 022513,031272 | 254B,32BA | 1 GB  Solid state disk without |
+    31     |          |         |       |               |           | data retention.                |
+    32     +----------+---------+-------+---------------+-----------+--------------------------------+
+    33     |    50    |    ST   | SST00 | 116447,035000 | 9D27,3A00 | Variable, depends on           |
+    34     |          |         |       |               |           | implementation.                |
+    35     +----------+---------+-------+---------------+-----------+--------------------------------+
+    36     |    51    |    DI   | EF51  | 021112,060063 | 224A,6033 | 100 MB Solid State Disk with   |
+    37     |          |         |       |               |           | data retention.                |
+    38     +----------+---------+-------+---------------+-----------+--------------------------------+
+    39     |    51    |    DI   | EF52  | 021112,060064 | 224A,6034 | 200 MB Solid State Disk with   |
+    40     |          |         |       |               |           | data retention.                |
+    41     +----------+---------+-------+---------------+-----------+--------------------------------+
+    42     |    51    |    DI   | EF53  | 021112,060065 | 224A,6035 | 250 MB Solid State Disk without|
+    43     |          |         |       |               |           | data retention.                |
+    44     +----------+---------+-------+---------------+-----------+--------------------------------+
+    45     |    51    |    DI   | EF54  | 021112,060066 | 224A,6036 | 400 MB Solid State Disk with   |
+    46     |          |         |       |               |           | data retention.                |
+    47     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           UNANNOUNCED PRODUCTS                                                               Page H-3
+                                                                                          11 June 1992
+
+
+
+     1                  Table H-2: Unannounced Disk Drive/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |    51    |    DI   | EF58  | 021112,060072 | 224A,603A | 1 GB Solid State Disk without  |
+     8     |          |         |       |               |           | data retention.                |
+     9     +----------+---------+-------+---------------+-----------+--------------------------------+
+    10     |    52    |    DI   | RF36  | 021144,060044 | 2264,6024 | TBS                            |
+    11     |          |         |       |               |           | (DSSI Disk Server)             |
+    12     +----------+---------+-------+---------------+-----------+--------------------------------+
+    13     |    53    |    DI   | RFH36 | 021144,062044 | 2264,6424 | TBS                            |
+    14     |          |         |       |               |           | (DSSI Disk Server)             |
+    15     +----------+---------+-------+---------------+-----------+--------------------------------+
+    16     |    54    |    DI   | RF74  | 021144,060112 | 2264,604A | 5 1/4" fixed, TBS GB.          |
+    17     |          |         |       |               |           | (DSSI Disk Server)             |
+    18     +----------+---------+-------+---------------+-----------+--------------------------------+
+    19     |    55    |    DI   | RFH74 | 021144,062112 | 2264,644A | 5 1/4" fixed, TBS GB.          |
+    20     |          |         |       |               |           | (DSSI Disk Server)             |
+    21     +----------+---------+-------+---------------+-----------+--------------------------------+
+    22     |    56    |    DI   | RF75  | 021144,060113 | 2264,604B | 5 1/4", fixed, TBS GB.         |
+    23     |          |         |       |               |           | (DSSI Disk Server)             |
+    24     +----------+---------+-------+---------------+-----------+--------------------------------+
+    25     |    57    |    DI   | RFH75 | 021144,062113 | 2264,644B | 5 1/4" fixed, TBS GB.          |
+    26     |          |         |       |               |           | (DSSI Disk Server)             |
+    27     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+    28                     Table H-3:  Unannounced Tape Drive/Media Identifier Values
+
+    29     +----------+---------+-------+---------------------------+--------------------------------+
+    30     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+    31     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+    32     |          |         |       |     octal     |    hex    |                                |
+    33     +----------+---------+-------+---------------+-----------+--------------------------------+
+    34     |     9    |    ME   | SVS00 | 064547,064600 | 6967,6980 | Variable, depends on           |
+    35     |          |         |       |               |           | implementation.                |
+    36     +----------+---------+-------+---------------+-----------+--------------------------------+
+    37     |    18    |    MU   | TD34  | 066550,040042 | 6D68,4022 | IBM 3480 product family using  |
+    38     |          |         |       |               |           | the FIPS-60 bus.               |
+    39     +----------+---------+-------+---------------+-----------+--------------------------------+
+    40     |    19    |    MU   | TD44  | 066550,040054 | 6D68,402C | STK version of IBM 3480 using  |
+    41     |          |         |       |               |           | the FIPS-60 bus.               |
+    42     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Mass Storage Control Protocol                                                 Version 2.4.0
+           UNANNOUNCED PRODUCTS                                                               Page H-4
+                                                                                          11 June 1992
+
+
+
+     1                  Table H-3: Unannounced Tape Drive/Media Identifier Values (cont.)
+
+     2     +----------+---------+-------+---------------------------+--------------------------------+
+     3     |Model Byte|  Device | Media |   Media Type Identifier   |             Drive              |
+     4     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+     5     |          |         |       |     octal     |    hex    |                                |
+     6     +----------+---------+-------+---------------+-----------+--------------------------------+
+     7     |    20    |    MU   | TAD85 | 066550,011125 | 6D68,1255 | TZ857 with integrated STI to   |
+     8     |          |         |       |               |           | SCSI adapter. Supports direct  |
+     9     |          |         |       |               |           | cartridge access via DISPLAY   |
+    10     |          |         |       |               |           | command.                       |
+    11     +----------+---------+-------+---------------+-----------+--------------------------------+
+
+
+    12                Table H-4:  Unannounced SCSI Storage Device/Media Identifier Values
+
+    13     +----------+---------+-------+---------------------------+--------------------------------+
+    14     |Model Byte|  Device | Media |   Media Type Identifier   |          Drive/Media           |
+    15     | (decimal)|Type Name| Name  +---------------+-----------+        Characteristics         |
+    16     |          |         |       |     octal     |    hex    |                                |
+    17     +----------+---------+-------+---------------+-----------+--------------------------------+
+    18     |    11    |    DK   | RZ35  | 021345,120043 | 22E5,A023 | 852 MB, 3.5", fixed            |
+    19     +----------+---------+-------+---------------+-----------+--------------------------------+
+    20     |    22    |    DK   | RZ73  | 021345,120111 | 22E5,A049 | 2 GB, 5.25", fixed             |
+    21     +----------+---------+-------+---------------+-----------+--------------------------------+
+    22     |    50    |    DK   |       |               |           | Reserved for New Dawn          |
+    23     +----------+---------+-------+---------------+-----------+--------------------------------+
+    24     |   139    |    MK   | TLZ06 | 065350,146406 | 6AE8,CC06 | RDAT-2                         |
+    25     +----------+---------+-------+---------------+-----------+--------------------------------+
+    26     |   141    |    MK   | TKZ10 | 065350,136412 | 6AE8,BD0A | QIC                            |
+    27     +----------+---------+-------+---------------+-----------+--------------------------------+
+    28     |   142    |    MK   | TKZ11 | 065350,136413 | 6AE8,BD0B | QIC-2                          |
+    29     +----------+---------+-------+---------------+-----------+--------------------------------+
+    30     | Notes: 1. The   preferred   method   for   assigning  a model  value  is according to   |
+    31     |           the device type (i.e. disk, tape, or optical). The  recommended model byte    |
+    32     |           ranges are as follows:                                                        |
+    33     |                                                                                         |
+    34     |              o 1-127   Disk devices                                                     |
+    35     |              o 128-191 Tape devices                                                     |
+    36     |              o 192-255 Optical disk devices                                             |
+    37     |                                                                                         |
+    38     +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+
+
+
+                                                        
+                                         *** RESTRICTED DISTRIBUTION ***
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@@ -0,0 +1,9676 @@
+
+
+
+
+                  
+
+
+
+
+
+
+
+
+
+
+                          Tape Mass Storage Control Protocol
+
+                             ECO Controlled Version 2.0.2
+
+                                    8 November 1987
+
+
+                    Send all inquiries and comments to COOKIE::SSAG
+
+
+
+
+
+               This document  defines  the  Tape  aspect  of  the  Mass
+               Storage Control Protocol (MSCP).
+
+
+
+
+
+
+
+
+
+
+
+
+              DIGITAL EQUIPMENT CORPORATION CONFIDENTIAL AND PROPRIETARY
+
+               This  document  is  an  unpublished  work  and  contains
+               valuable   trade  secrets  which  are  confidential  and
+               proprietary to Digital Equipment  Corporation,  and  may
+               only be disclosed to individuals who have entered into a
+               confidentiality agreement with Digital, and may  not  be
+               copied or reproduced in whole or in part.
+
+               Copyright (c) Digital Equipment Corporation 1982,  1983,
+               1984, 1985, 1986, 1987.  All Rights Reserved.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           CONTENTS                                                  Page ii
+                                                             8 November 1987
+
+
+                                           CONTENTS
+
+
+
+           CHAPTER 1       INTRODUCTION
+
+                   1.1     Scope  . . . . . . . . . . . . . . . . . . .  1-1
+                   1.2     References . . . . . . . . . . . . . . . . .  1-1
+                   1.2.1     MSCP Specification . . . . . . . . . . . .  1-1
+                   1.2.1.1     Table Of Contents Abstract . . . . . . .  1-1
+                   1.2.2     STI Specification  . . . . . . . . . . . .  1-3
+
+
+           CHAPTER 2       TERMINOLOGY
+
+
+           CHAPTER 3       TMSCP GENERAL OPERATIONAL CHARACTERISTICS
+
+                   3.1     Unit States  . . . . . . . . . . . . . . . .  3-1
+                   3.1.1     Unit Offline . . . . . . . . . . . . . . .  3-1
+                   3.1.2     Unit Available . . . . . . . . . . . . . .  3-1
+                   3.1.3     Unit Online  . . . . . . . . . . . . . . .  3-1
+                   3.1.4     Exclusive Access Of A Unit . . . . . . . .  3-1
+                   3.1.5     Unit Operating Beyond EOT  . . . . . . . .  3-1
+                   3.1.6     Unit Position Lost . . . . . . . . . . . .  3-1
+                   3.1.7     Unit Serious Exception Pending . . . . . .  3-2
+                   3.1.8     Unit Cached Data Lost  . . . . . . . . . .  3-2
+                   3.2     Command Categories And Execution Order . . .  3-2
+                   3.2.1     Lengthy Command Considerations . . . . . .  3-2
+                   3.2.1.1     Synchronous Vs Asynchronous  . . . . . .  3-2
+                   3.2.1.2     Abort  . . . . . . . . . . . . . . . . .  3-3
+                   3.3     Tape Motion Command Operation  . . . . . . .  3-6
+                   3.3.1     "read" Type  . . . . . . . . . . . . . . .  3-6
+                   3.3.1.1     Direction  . . . . . . . . . . . . . . .  3-6
+                   3.3.1.2     BOT Handling . . . . . . . . . . . . . .  3-6
+                   3.3.1.3     EOT Handling . . . . . . . . . . . . . .  3-7
+                   3.3.1.4     Tape Mark Handling . . . . . . . . . . .  3-7
+                   3.3.2     "write" Type . . . . . . . . . . . . . . .  3-7
+                   3.3.2.1     Direction  . . . . . . . . . . . . . . .  3-7
+                   3.3.2.2     BOT Handling . . . . . . . . . . . . . .  3-7
+                   3.3.2.3     EOT Handling . . . . . . . . . . . . . .  3-8
+                   3.3.2.4     Tape Mark Handling . . . . . . . . . . .  3-8
+                   3.3.3     "ancillary" Type . . . . . . . . . . . . .  3-8
+                   3.3.3.1     Direction  . . . . . . . . . . . . . . .  3-8
+                   3.3.3.2     BOT Handling . . . . . . . . . . . . . .  3-8
+                   3.3.3.3     EOT Handling . . . . . . . . . . . . . .  3-8
+                   3.3.3.4     Tape Mark Handling . . . . . . . . . . .  3-8
+                   3.4     Data Transfer Operations . . . . . . . . . .  3-8
+                   3.4.1     Record Length  . . . . . . . . . . . . . .  3-9
+                   3.4.2     Tape Format Employed . . . . . . . . . . .  3-9
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           CONTENTS                                                 Page iii
+                                                             8 November 1987
+
+
+                   3.4.3     Host Buffer Access . . . . . . . . . . . . 3-10
+                   3.4.3.1     WRITE Command  . . . . . . . . . . . . . 3-10
+                   3.4.3.2     READ Command . . . . . . . . . . . . . . 3-10
+                   3.4.3.2.1     Forward Direction  . . . . . . . . . . 3-11
+                   3.4.3.2.2     Reverse Direction  . . . . . . . . . . 3-11
+                   3.4.3.3     ACCESS Command . . . . . . . . . . . . . 3-11
+                   3.4.3.4     Compare Operations . . . . . . . . . . . 3-11
+                   3.4.4     End Message Byte Count Fields  . . . . . . 3-12
+                   3.5     General Error Processing . . . . . . . . . . 3-12
+                   3.5.1     Reporting Precedence . . . . . . . . . . . 3-12
+                   3.5.2     Serious Exception Handling . . . . . . . . 3-13
+                   3.5.3     Position Lost Handling . . . . . . . . . . 3-16
+                   3.5.4     Asynchronous Completion Handling Of 
+                             Non-Data Transfer Commands . . . . . . . . 3-16
+                   3.5.5     Data Error Recovery  . . . . . . . . . . . 3-17
+                   3.5.5.1     Suppression  . . . . . . . . . . . . . . 3-18
+                   3.5.5.2     Enhanced Write Error Recovery  . . . . . 3-18
+                   3.5.6     Error Log Requirements . . . . . . . . . . 3-19
+                   3.6     Tape Caching . . . . . . . . . . . . . . . . 3-19
+                   3.6.1     Write-Back Caching . . . . . . . . . . . . 3-20
+                   3.6.1.1     Unit Flag Conflicts  . . . . . . . . . . 3-23
+                   3.6.1.2     Command Modifier Conflicts . . . . . . . 3-23
+                   3.6.1.3     Error And Exception Handling . . . . . . 3-23
+                   3.6.1.4     Cached Data Lost Handling  . . . . . . . 3-25
+                   3.6.2     Read-Ahead Caching . . . . . . . . . . . . 3-28
+                   3.6.2.1     Unit Flag Conflicts  . . . . . . . . . . 3-29
+                   3.6.2.2     Command Modifier Conflicts . . . . . . . 3-29
+                   3.6.2.3     Error Handling . . . . . . . . . . . . . 3-30
+
+
+           CHAPTER 4       TMSCP MESSAGE OVERVIEW
+
+                   4.1     Tape MSCP Functions Having No Disk MSCP 
+                           Counterparts . . . . . . . . . . . . . . . .  4-1
+                   4.2     Identical Disk And Tape MSCP Functions . . .  4-1
+                   4.3     Tape Specific MSCP Commands And Responses  .  4-1
+                   4.4     Generic Command Message Format . . . . . . .  4-2
+                   4.4.1     Command Modifiers  . . . . . . . . . . . .  4-2
+                   4.5     End Message Format . . . . . . . . . . . . .  4-3
+                   4.5.1     Flags  . . . . . . . . . . . . . . . . . .  4-3
+                   4.5.2     Status . . . . . . . . . . . . . . . . . .  4-3
+                   4.5.3     Status Codes . . . . . . . . . . . . . . .  4-4
+                   4.5.4     Byte Count (Host Transfer) . . . . . . . .  4-7
+                   4.5.5     Byte Count (Tape Record) . . . . . . . . .  4-8
+                   4.5.6     Position (Object Count)  . . . . . . . . .  4-8
+
+
+           CHAPTER 5       TMSCP COMMAND DESCRIPTIONS
+
+                   5.1     ACCESS Command . . . . . . . . . . . . . . .  5-1
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           CONTENTS                                                  Page iv
+                                                             8 November 1987
+
+
+                   5.1.1     Command Message Format . . . . . . . . . .  5-1
+                   5.1.2     End Message Format . . . . . . . . . . . .  5-2
+                   5.1.3     Description  . . . . . . . . . . . . . . .  5-4
+                   5.2     AVAILABLE Command  . . . . . . . . . . . . .  5-5
+                   5.2.1     Command Message Format . . . . . . . . . .  5-5
+                   5.2.2     End Message Format . . . . . . . . . . . .  5-6
+                   5.2.3     Description  . . . . . . . . . . . . . . .  5-8
+                   5.3     COMPARE HOST DATA Command  . . . . . . . . .  5-9
+                   5.3.1     Command Message Format . . . . . . . . . .  5-9
+                   5.3.2     End Message Format . . . . . . . . . . . . 5-10
+                   5.3.3     Description  . . . . . . . . . . . . . . . 5-12
+                   5.4     ERASE Command  . . . . . . . . . . . . . . . 5-13
+                   5.4.1     Command Message Format . . . . . . . . . . 5-13
+                   5.4.2     End Message Format . . . . . . . . . . . . 5-14
+                   5.4.3     Description  . . . . . . . . . . . . . . . 5-15
+                   5.5     ERASE GAP Command  . . . . . . . . . . . . . 5-16
+                   5.5.1     Command Message Format . . . . . . . . . . 5-16
+                   5.5.2     End Message Format . . . . . . . . . . . . 5-17
+                   5.5.3     Description  . . . . . . . . . . . . . . . 5-18
+                   5.6     FLUSH Command  . . . . . . . . . . . . . . . 5-19
+                   5.6.1     Command Message Format . . . . . . . . . . 5-19
+                   5.6.2     End Message Format . . . . . . . . . . . . 5-20
+                   5.6.3     Description  . . . . . . . . . . . . . . . 5-22
+                   5.7     GET UNIT STATUS Command  . . . . . . . . . . 5-23
+                   5.7.1     Command Message Format . . . . . . . . . . 5-23
+                   5.7.2     End Message Format . . . . . . . . . . . . 5-24
+                   5.7.3     Description  . . . . . . . . . . . . . . . 5-33
+                   5.8     ONLINE Command . . . . . . . . . . . . . . . 5-34
+                   5.8.1     Command Message Format . . . . . . . . . . 5-34
+                   5.8.2     End Message Format . . . . . . . . . . . . 5-36
+                   5.8.3     Description  . . . . . . . . . . . . . . . 5-39
+                   5.9     READ Command . . . . . . . . . . . . . . . . 5-40
+                   5.9.1     Command Message Format . . . . . . . . . . 5-40
+                   5.9.2     End Message Format . . . . . . . . . . . . 5-41
+                   5.9.3     Description  . . . . . . . . . . . . . . . 5-43
+                   5.10    REPOSITION Command . . . . . . . . . . . . . 5-44
+                   5.10.1    Command Message Format . . . . . . . . . . 5-44
+                   5.10.2    End Message Format . . . . . . . . . . . . 5-46
+                   5.10.3    Description  . . . . . . . . . . . . . . . 5-48
+                   5.11    SET UNIT CHARACTERISTICS Command . . . . . . 5-56
+                   5.11.1    Command Message Format . . . . . . . . . . 5-56
+                   5.11.2    End Message Format . . . . . . . . . . . . 5-61
+                   5.11.3    Description  . . . . . . . . . . . . . . . 5-64
+                   5.12    WRITE Command  . . . . . . . . . . . . . . . 5-65
+                   5.12.1    Command Message Format . . . . . . . . . . 5-65
+                   5.12.2    End Message Format . . . . . . . . . . . . 5-67
+                   5.12.3    Description  . . . . . . . . . . . . . . . 5-69
+                   5.13    WRITE TAPE MARK Command  . . . . . . . . . . 5-70
+                   5.13.1    Command Message Format . . . . . . . . . . 5-70
+                   5.13.2    End Message Format . . . . . . . . . . . . 5-71
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           CONTENTS                                                   Page v
+                                                             8 November 1987
+
+
+                   5.13.3    Description  . . . . . . . . . . . . . . . 5-73
+
+
+           CHAPTER 6       TMSCP ERROR LOG MESSAGES
+
+                   6.1     TMSCP Error Log Message Format . . . . . . .  6-1
+                   6.1.1     Tape Errors  . . . . . . . . . . . . . . .  6-3
+                   6.1.2     STI Communication Or Command Failures  . .  6-5
+                   6.1.3     STI Formatter Error Log  . . . . . . . . .  6-6
+                   6.1.4     STI Drive Error Log  . . . . . . . . . . .  6-7
+
+
+           APPENDIX A      OPCODE, FLAG AND OFFSET DEFINITIONS
+
+
+           APPENDIX B      STATUS AND EVENT CODE DEFINITIONS
+
+
+           APPENDIX C      MISCELLANEOUS TABLES
+
+                           Table C-1: Tape Format Flag Values . . . . .  C-1
+                           Table C-2: Tape Format Bitflag Values  . . .  C-2
+                           Table C-3: Controller Specific Maximum Record 
+                           Size . . . . . . . . . . . . . . . . . . . .  C-3
+                           Table C-4: Format Specific Long Gap Values .  C-3
+
+
+           APPENDIX D      REPOSITION COMMAND VARIATIONS
+
+
+           APPENDIX E      WAIVERS AND EXCEPTIONS
+
+                   E.1     TU81 . . . . . . . . . . . . . . . . . . . .  E-1
+                   E.1.1     "EOT Encountered" Status Subcode . . . . .  E-1
+                   E.1.2     "Serious Exception" And "EOT Encountered"   E-1
+                   E.1.3     "Byte Count (Host Transfer)" On Read 
+                             Reverse  . . . . . . . . . . . . . . . . .  E-1
+                   E.2     TK50 . . . . . . . . . . . . . . . . . . . .  E-2
+                   E.2.1     Incorrect Command Processing While Position 
+                             Lost . . . . . . . . . . . . . . . . . . .  E-2
+                   E.2.2     COMPARE HOST DATA Error Reporting  . . . .  E-3
+                   E.2.3     Incorrect "Offline" Error Reporting  . . .  E-3
+                   E.2.4     Progress Indicator Processing  . . . . . .  E-3
+                   E.2.5     Multiple Error Logs Reported . . . . . . .  E-4
+                   E.2.6     Invalid "Records Skipped Count"  . . . . .  E-4
+                   E.2.7     Hardware Serial Number . . . . . . . . . .  E-5
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           CONTENTS                                                  Page vi
+                                                             8 November 1987
+                                                                            
+
+           APPENDIX F      REVISION HISTORY
+
+                   F.1     Changes Since Version 1.6  . . . . . . . . .  F-1
+                   F.2     Changes Since Version 1.6.4  . . . . . . . .  F-2
+	|          F.3     Changes Since Version 2.0.0  . . . . . . . .  F-3
+
+
+           TABLES
+
+                   A-1     Control Message Opcodes  . . . . . . . . . .  A-1
+                   A-2     Command Modifiers  . . . . . . . . . . . . .  A-3
+                   A-3     End Message Flags  . . . . . . . . . . . . .  A-4
+                   A-4     Controller Flags . . . . . . . . . . . . . .  A-4
+                   A-5     Unit Flags . . . . . . . . . . . . . . . . .  A-5
+                   A-6     Command Message Offsets  . . . . . . . . . .  A-6
+                   A-7     End and Attention Message Offsets  . . . . .  A-7
+                   A-8     Error Log Message Offsets  . . . . . . . . .  A-9
+                   A-9     Error Log Message Format Codes . . . . . . . A-11
+                   A-10    Error Log Message Flags  . . . . . . . . . . A-11
+                   A-11    Access Nonvolatile Memory Command Operation 
+                           Codes  . . . . . . . . . . . . . . . . . . . A-11
+                   B-1     Status and Event Codes . . . . . . . . . . .  B-2
+                   B-2     Status Only Subcode Values . . . . . . . . .  B-3
+                   B-3     "Compare Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . .  B-5
+                   B-4     "Data Error" Status or Event Subcode Values   B-5
+                   B-5     "Host Buffer Access Error" Status or Event 
+                           Subcode Values . . . . . . . . . . . . . . .  B-5
+                   B-6     "Controller Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . .  B-6
+                   B-7     "Drive Error" Status or Event Subcode Values  B-7
+                   B-8     "Formatter Error" Status or Event Subcode 
+                           Values . . . . . . . . . . . . . . . . . . .  B-7
+                   B-9     "Message From An Internal Diagnostic" Event 
+                           Only Subcode Values  . . . . . . . . . . . .  B-7
+                   C-1     Tape Format Flag Values  . . . . . . . . . .  C-1
+                   C-2     Tape Format Bitflag Values . . . . . . . . .  C-2
+                   C-3     Controller Specific Maximum Record Size  . .  C-3
+                   C-4     Format Specific Long Gap Values  . . . . . .  C-3
+                   D-1     REPOSITION Command Variations  . . . . . . .  D-1
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           INTRODUCTION                                             Page 1-1
+                                                             8 November 1987
+
+
+     1                                 CHAPTER 1
+
+     2                               INTRODUCTION
+
+
+     3     1.1  Scope
+
+     4     This  document  defines  the  Tape  Class  Mass  Storage  Control
+     5     Protocol  (TMSCP), a magnetic tape applications protocol intended
+     6     for use with  intelligent  mass  storage  controllers.   Although
+     7     Magnetic Tape Class Devices possess operation strategies inherent
+     8     to themselves, the architecture of TMSCP is not  autonomous,  but
+     9     rather  conforms  to  the overall philosophy defined in the "Mass
+    10     Storage Control Protocol" specification, hereinafter referred  to
+    11     as MSCP.
+
+    12     1.2  References
+
+    13     1.2.1  MSCP Specification
+
+    14     The  MSCP  specification  is  necessary  to   gain   a   complete
+    15     understanding  of  TMSCP  and its relationship to the overall I/O
+    16     architecture.   Numerous  areas  have  been  omitted  from   this
+    17     document  since they are fully defined in MSCP.  The next section
+    18     contains an abstract of the Table of Contents of  ECO  controlled
+    19     MSCP  Version  1.3  to aid in referencing particular subjects not
+    20     fully defined in this document.
+
+    21     1.2.1.1  Table Of Contents Abstract
+
+    22             CHAPTER 1       INTRODUCTION
+    23             1.1     Overview Of MSCP Subsystem
+    24             1.2     Purpose
+    25             1.3     Method Of Presentation
+    26             1.4     Scope
+
+    27             CHAPTER 2       TERMINOLOGY
+
+    28             CHAPTER 3       CLASS DRIVER / MSCP SERVER COMMUNICATIONS
+    29             3.1     Connection
+    30             3.2     Flow Control
+    31             3.3     Multihost Communication
+
+    32             CHAPTER 4       ALGORITHMS AND USAGE RULES
+    33             4.1     Controller States
+    34             4.2     Controls And Indicators
+    35             4.3     Unit States
+    36             4.4     Unit Numbers
+    37             4.5     Command Categories And Execution Order
+    38             4.6     Class Driver / MSCP Server Synchronization
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           INTRODUCTION                                             Page 1-2
+           References                                        8 November 1987
+
+
+     1             4.7     Class Driver Error Recovery
+     2             4.8     Serious Exceptions
+     3             4.9     Host Access Timeouts
+     4             4.10    Command Timeouts
+     5             4.11    Disk Geometry And Format
+     6             4.12    Bad Block Replacement
+     7             4.13    Write Protection
+     8             4.14    Compare Operations
+     9             4.15    Special Drive Topologies
+    10             4.16    Controller And Unit Identifiers
+    11             4.17    Media Type Identifiers
+
+    12             CHAPTER 5       MSCP CONTROL MESSAGES
+    13             5.1     Generic Control Message Format
+    14             5.2     Reserved And Undefined Fields
+    15             5.3     Command Messages
+    16             5.4     End Messages
+    17             5.5     Controller Flags
+    18             5.6     Unit Flags
+    19             5.7     Attention Messages
+    20             5.8     Error Log Messages
+
+    21             CHAPTER 6       MINIMAL MSCP COMMAND SET
+    22             6.1     Overview
+    23             6.2     ABORT Command
+    24             6.3     ACCESS Command
+    25             6.4     AVAILABLE Command
+    26             6.5     COMPARE HOST DATA Command
+    27             6.6     DETERMINE ACCESS PATHS Command
+    28             6.7     ERASE Command
+    29             6.8     GET COMMAND STATUS Command
+    30             6.9     GET UNIT STATUS Command
+    31             6.10    ONLINE Command
+    32             6.11    READ Command
+    33             6.12    REPLACE Command
+    34             6.13    SET CONTROLLER CHARACTERISTICS Command
+    35             6.14    SET UNIT CHARACTERISTICS Command
+    36             6.15    WRITE Command
+
+    37             CHAPTER 7       MULTIHOST SUPPORT FUNCTIONALITY
+    38             7.1     Overview
+    39             7.2     Multihost ABORT Command
+    40             7.3     Multihost ACCESS NONVOLATILE MEMORY Command
+    41             7.4     Multihost AVAILABLE Command
+    42             7.5     Multihost GET COMMAND STATUS Command
+    43             7.6     Multihost GET UNIT STATUS Command
+    44             7.7     Multihost ONLINE Command
+    45             7.8     Multihost SET CONTROLLER CHARACTERISTICS 
+    46                     Command
+    47             7.9     Multihost SET UNIT CHARACTERISTICS Command
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           INTRODUCTION                                             Page 1-3
+           References                                        8 November 1987
+
+
+     1             CHAPTER 8       CONTROLLER INITIATED BAD BLOCK
+     2             REPLACEMENT
+     3             8.1     Controller Initiated Bad Block Replacement 
+     4                     Overview
+     5             8.2     Data Safety Write Protection
+     6             8.3     Bad Block Replacement
+     7             8.4     Replacement Control Table Access
+     8             8.5     Atomic Bad Block Replacement
+     9             8.6     Error Log Messages
+    10             8.7     Unit Context Information
+    11             8.8     Actions During ONLINE
+    12             8.9     Actions During SET UNIT CHARACTERISTICS
+    13             8.10    Actions Before First Modification
+    14             8.11    MSCP Control Message Format Changes
+    15             8.12    Host Support For Controller Initiated Bad 
+    16                     Block Replacement
+
+    17             APPENDIX A      OPCODE, FLAG, AND OFFSET DEFINITIONS
+
+    18             APPENDIX B      STATUS AND EVENT CODE DEFINITIONS
+
+    19             APPENDIX C      CONTROLLER, UNIT, AND MEDIA TYPE
+    20             IDENTIFIERS
+
+    21             APPENDIX D      BUFFER DESCRIPTOR FORMATS
+
+    22             APPENDIX E      WAIVERS AND EXCEPTIONS
+
+    23             APPENDIX F      REVISION HISTORY
+
+    24     1.2.2  STI Specification
+
+    25     The Storage Tape Interface Specification, hereinafter referred to
+    26     as  STI,  defines a communications interface used between DIGITAL
+    27     tape formatters and controllers.  Chapter 6  makes  reference  to
+    28     various STI responses used in error log messages.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TERMINOLOGY                                              Page 2-1
+                                                             8 November 1987
+
+
+     1                                 CHAPTER 2
+
+     2                                TERMINOLOGY
+
+
+
+     3     Addressable Unit:
+
+     4         The host addressable unit in TMSCP is a tape  unit.   A  tape
+     5         formatter is only visible to a controller and not to a host.
+
+
+     6     Beginning-of-tape (BOT):
+
+     7         The beginning of the usable  recording  area  of  a  magnetic
+     8         tape.  There are two aspects used in defining BOT:
+
+
+     9         1.  Physical BOT, the position forward  of  the  tape  leader
+    10             signified by a Tape Format dependent delimiter.
+
+    11         2.  Logical BOT, the position just before  the  first  Object
+    12             recorded (i.e., object count 0) on the tape.
+
+
+    13         This  distinction  is  necessary  because  the  Tape   Format
+    14         Identification  Area  (TFIA)  used  in  certain  Tape Formats
+    15         extends beyond Physical BOT causing a  physical  displacement
+    16         of   Logical  BOT.   Depending  on  the  circumstance,  TMSCP
+    17         controllers will use either physical BOT or logical BOT  when
+    18         reporting BOT, as is convenient.
+
+
+    19     End-of-Tape (EOT):
+
+    20         That physical position on a magnetic tape where a Tape Format
+    21         dependent delimiter signals the beginning of the tape trailer
+    22         area.  Due to variations in delimiter sensing, it is possible
+    23         for the sensing of EOT to vary on a given piece of media from
+    24         forward to reverse direction, from drive to drive, from  read
+    25         to   write,  from  caching  to  non-caching,  and  for  other
+    26         device-specific reasons.  The  amount  of  variation  in  EOT
+    27         reporting  is device-specific.  Thus, applications should not
+    28         depend upon EOT deasserting in the reverse direction  at  the
+    29         same  point  it asserted in the forward direction, nor should
+    30         they depend upon EOT asserting at the same position each time
+    31         the media is accessed.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TERMINOLOGY                                              Page 2-2
+                                                             8 November 1987
+
+
+     1     Logical End-of-Tape (LEOT):
+
+     2         Two consecutive Tape  Marks  which  indicate  a  host-imposed
+     3         logical  end  of  recorded  data  (i.e.,  end of volume) on a
+     4         magnetic tape.  LEOT is detected and reported during  forward
+     5         REPOSITION operations at the option of the host.
+
+
+     6     Long Gap:
+
+     7         An extended Record gap (usually a group of contiguous  Record
+     8         gaps)  sufficient  enough  in  length or quantity to indicate
+     9         that the remainder of the magnetic tape is unrecorded.   Long
+    10         gap  values  are Tape Format dependent as defined in Appendix
+    11         C, Table C-4.
+
+
+    12     Object:
+
+    13         An "object" is either a Record or a  Tape  Mark.   The  first
+    14         tape   Object   is   recorded  forward  of  the  Tape  Format
+    15         Identification Area (TFIA).  Note that the TFIA, Record Gaps,
+    16         and    any    other    control   information   (e.g.,   error
+    17         detection/correction  frames)  are  not  considered   to   be
+    18         Objects.
+
+
+    19     Physical End-of-Tape (PEOT):
+
+    20         The end of usable recording area, located in the tape trailer
+    21         area,  beginning  at  EOT and extending through a Tape Format
+    22         dependent   length.    Refer   to   the   appropriate   media
+    23         specification for the PEOT size of a specific media.
+
+
+    24     Record:
+
+    25         The unit of data transfer, consisting of a collection of data
+    26         bytes  which  are written or read as a single entity, without
+    27         regard for Tape Format dependent  physical  or  host  defined
+    28         logical composition.
+
+
+    29     Record Gap:
+
+    30         A Tape Format dependent delimiter recorded on a magnetic tape
+    31         which  separates  Objects  and  control  information from one
+    32         another or is  used  to  by-pass  defects  on  the  recording
+    33         surface.   Normal Record Gaps have no special significance to
+    34         TMSCP or the host.  They are only relevant in describing tape
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TERMINOLOGY                                              Page 2-3
+                                                             8 November 1987
+
+
+     1         position.
+
+
+     2     Tape Format:
+
+     3         The  recording  method  used  to  store/retrieve  data  on  a
+     4         magnetic tape.  Refer to Appendix C, Tables C-1 and C-2 for a
+     5         list of recognized Tape Formats.
+
+
+     6     Tape Format Identification Area (TFIA):
+
+     7         The area (around physical BOT) where  Tape  Format  dependent
+     8         format determination information is recorded.
+
+
+     9     Tape Mark:
+
+    10         A Tape Format dependent delimiter recorded on a magnetic tape
+    11         which  usually  serves  as a separator for host defined files
+    12         (i.e., end of file mark).
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-1
+                                                             8 November 1987
+
+
+     1                                 CHAPTER 3
+
+     2                 TMSCP GENERAL OPERATIONAL CHARACTERISTICS
+
+
+     3     3.1  Unit States
+
+     4     The following subsections clarify Unit States as  they  apply  to
+     5     TMSCP.  For an in depth definition of Unit States, refer to MSCP.
+
+     6     3.1.1  Unit Offline
+
+     7     If a formatter is offline to a  controller,  the  addressed  tape
+     8     unit  connected  to  the  offline  formatter is also offline to a
+     9     host.  A tape unit connected to a formatter which is online to  a
+    10     controller is subsequently offline to any other controller.
+
+    11     3.1.2  Unit Available
+
+    12     To be Unit-Available, a tape unit must be powered up,  online  to
+    13     the  formatter,  have  a  magnetic  tape  loaded, and be ready to
+    14     perform I/O.
+
+    15     3.1.3  Unit Online
+
+    16     Same as MSCP.
+
+    17     3.1.4  Exclusive Access Of A Unit
+
+    18     Same as MSCP.
+
+    19     3.1.5  Unit Operating Beyond EOT
+
+    20     The "EOT Encountered" end flag, as described in section 4.5.1, is
+    21     considered part of the tape unit state.  The current state of the
+    22     "EOT encountered" end flag is returned in the end messages of all
+    23     commands  which  specify  the  Unit Number field (including those
+    24     defined only in MSCP).
+
+    25     3.1.6  Unit Position Lost
+
+    26     The "Position Lost" end flag, as described in section  4.5.1,  is
+    27     considered part of the tape unit state.  The current state of the
+    28     "Position Lost" end flag is returned in the end messages  of  all
+    29     commands  which  specify  the  Unit Number field (including those
+    30     defined only in MSCP).
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS              Page 3-1.1
+           Unit States                                       8 November 1987
+
+
+     1  |                                NOTE
+
+     2  |          It  is  important  to  distinguish  between   the
+     3  |          Position Lost state, as indicated by the Position
+     4  |          Lost end  flag,  and  the  Position  Lost  status
+     5  |          reported  in  end messages and error logs.  Refer
+     6  |          to sections 3.5.3, 4.5.1, and 4.5.3  for  further
+     7  |          information.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-2
+           Unit States                                       8 November 1987
+
+
+     1     3.1.7  Unit Serious Exception Pending
+
+     2     The "Serious Exception" end flag, as described in section  4.5.1,
+     3     is  considered part of the tape unit state.  The current state of
+     4     the "Serious Exception" end flag is returned in the end  messages
+     5     of  all  commands  which specify the Unit Number field (including
+     6     those defined only in MSCP).
+
+     7     3.1.8  Unit Cached Data Lost
+
+     8     The "Cached Data Lost" end flag, as described in  section  4.5.1,
+     9     is  considered part of the tape unit state.  The current state of
+    10     the "Cached Data Lost" end flag is returned in the  end  messages
+    11     of  all  commands  which specify the Unit Number field (including
+    12     those defined only in MSCP).
+
+    13     3.2  Command Categories And Execution Order
+
+    14     As noted in  MSCP,  TMSCP  only  uses  immediate  and  sequential
+    15     command  types.   TMSCP  implementation  of the immediate command
+    16     type and most sequential commands is identical to that  described
+    17     in  MSCP.   However,  certain  tape  operations  (e.g., ERASE and
+    18     REPOSITION) can take a considerable  amount  of  time  (up  to  5
+    19     minutes  or  more)  to  complete, depending on the amount of tape
+    20     traversed during the operation.  Special consideration of lengthy
+    21     commands  is  necessary in order to reduce the impact on the host
+    22     environment.   As  an  additional  performance   feature,   TMSCP
+    23     provides  support  for  the  caching of data transfer operations.
+    24     The remaining subsections provide a detailed description of those
+    25     special considerations.
+
+    26     3.2.1  Lengthy Command Considerations
+
+    27     3.2.1.1  Synchronous Vs Asynchronous
+
+    28     Treating lengthy  commands  as  purely  sequential  (synchronous)
+    29     operations  can  adversely  affect  host  operation  in that host
+    30     resources dedicated to the completion of the operation cannot  be
+    31     relinquished until completion.  Therefore, asynchronous operation
+    32     of lengthy, non-data transfer  commands  is  preferred.   On  the
+    33     other  hand,  confirmation  of  successful completion may be more
+    34     important to the host (e.g., data  erasure  in  a  high  security
+    35     installation).
+
+    36     TMSCP controllers accommodate both  situations  by  allowing  the
+    37     host to specify, for a limited set of non-data transfer commands,
+    38     whether a command is to complete in a synchronous or asynchronous
+    39     manner via the "Immediate Completion" command modifier.
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-3
+           Command Categories And Execution Order            8 November 1987
+
+
+     1     If the  "Immediate  Completion"  command  modifier  is  set,  the
+     2     controller  returns  the end message with the status field set to
+     3     Success (subcode "Normal") as soon as the specified operation  is
+     4     initiated  on  the tape unit (for non-data transfer commands), or
+     5     as soon as the data has been transferred  from  the  host  buffer
+     6     (for   data   transfer  commands).   The  command  completion  is
+     7     therefore  asynchronous  to  the  completion  of  the  tape  unit
+     8     operation.    Until   the   tape  unit  operation  completes  the
+     9     controller  must  preserve  the   sequentiality   of   subsequent
+    10     sequential   commands  issued  by  the  host.   Furthermore,  the
+    11     controller must also ensure that subsequent  sequential  commands
+    12     appear  to  be progressing towards completion in order to satisfy
+    13     command timeout requirements as defined in MSCP.
+
+    14     If the "Immediate Completion"  command  modifier  is  clear,  the
+    15     controller  will  not  return the end message until the tape unit
+    16     operation is completed.
+
+    17     The "Immediate Completion" command modifier can only be specified
+    18     in   the   following   non-data  transfer  commands:   ERASE  and
+    19     REPOSITION.  In the case of the REPOSITION command,  asynchronous
+    20     operation  will  only  occur  if  the  "Immediate Completion" and
+    21     "Rewind" command modifiers are set and  the  "record  count"  and
+    22     "tape  mark  count"  fields  are  both  zero (i.e., rewind to BOT
+    23     only).  The controller will  ignore  the  "Immediate  Completion"
+    24     command  modifier  in  all  other  variations  of  the REPOSITION
+    25     command.
+
+    26     The AVAILABLE  command  is  always  treated  as  an  asynchronous
+    27     completion command (without the need for a command modifier).
+
+    28     Refer to section 3.5.4 for information regarding  the  occurrence
+    29     of an error during asynchronous operations.
+
+    30     3.2.1.2  Abort
+
+    31     Host resource blockage comes into play again when the host  wants
+    32     to ABORT a lengthy synchronous command.  A second concern is that
+    33     the tape unit operation is ABORTED in a  controlled,  predictable
+    34     manner.
+
+    35     TMSCP controllers must ABORT  lengthy  commands  specifically  as
+    36     follows:
+
+    37           o  ERASE  command  with  "Immediate   Completion"   command
+    38              modifier clear:
+
+    39                  Abort process - end  message  returned  as  soon  as
+    40                  ABORT  command  is  received,  tape motion continues
+    41                  until erase is completed (ten feet  past  EOT),  and
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-4
+           Command Categories And Execution Order            8 November 1987
+
+
+     1                  tape has rewound to BOT.
+
+
+     2           o  REPOSITION command with "Rewind" command  modifier  set,
+     3              "Immediate  Completion"  command modifier clear, "Object
+     4              Count" and "Reverse" modifiers either set or clear, zero
+     5              "record  count  or  object  count" and "tape mark count"
+     6              fields:
+
+     7                  Abort process - end  message  returned  as  soon  as
+     8                  ABORT  command is received; tape motion continues to
+     9                  BOT.
+
+
+    10           o  REPOSITION command with "Rewind" command modifier clear,
+    11              "Object  Count"  and  "Reverse"  modifiers either set or
+    12              clear, non-zero "record count or object count" or  "tape
+    13              mark count" field(s):
+
+    14                  Abort process - after receipt of the ABORT  command,
+    15                  tape  motion  continues  to  the  first  record  gap
+    16                  encountered; end message returned as  soon  as  tape
+    17                  motion ceases (see exception described below).
+
+    18                                          NOTE
+
+    19                      Devices which support the Tape  Format  Flag
+    20                      value of TF.CTP ("TK50" compatible cartridge
+    21                      tapes) do not follow the exception condition
+    22                      described  below when the "Reverse" modifier
+    23                      is set.  Instead, they  have  the  following
+    24                      behavior:
+
+
+    25                  Abort Process - after receipt of the ABORT  command,
+    26                  tape  motion  occurs  to the beginning of the track,
+    27                  and proceeds forward  until  the  first  record  gap
+    28                  encountered.  This may be closer to logical BOT than
+    29                  the original target  object.   The  end  message  is
+    30                  returned as soon as tape motion ceases.
+
+
+    31           o  REPOSITION command with "Rewind" modifier  set,  "Object
+    32              Count"  and  "Reverse"  modifiers  either  set or clear,
+    33              non-zero "record count or object count"  or  "tape  mark
+    34              count" field(s):
+
+    35                  Abort process - if the rewind has not yet  completed
+    36                  (BOT  has  not  been  reached),  the  end message is
+    37                  returned as soon as the ABORT command  is  received,
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-5
+           Command Categories And Execution Order            8 November 1987
+
+
+     1                  and tape motion continues to BOT.  If the rewind has
+     2                  completed, tape motion continues to the first record
+     3                  gap  encountered  after receipt of the ABORT command
+     4                  (see exception described below).  The end message is
+     5                  returned as soon as tape motion ceases.
+
+
+
+     6     In the cases listed above where the end message  is  returned  as
+     7     soon as the ABORT command is received and the tape unit operation
+     8     in progress is allowed to continue (i.e.,  ERASE  and  Rewind  to
+     9     BOT), command completion is asynchronous to the completion of the
+    10     tape unit operation.  Until the tape unit operation completes the
+    11     controller   must   preserve   the  sequentiality  of  subsequent
+    12     sequential  commands  issued  by  the  host.   Furthermore,   the
+    13     controller  must  also ensure that subsequent sequential commands
+    14     appear to be progressing towards completion in order  to  satisfy
+    15     command  timeout  requirements  as  defined  in  MSCP.   Refer to
+    16     Section 3.5.4 for information  regarding  the  occurrence  of  an
+    17     error during asynchronous operations.
+
+    18                  FIRST RECORD GAP ENCOUNTERED EXCEPTION
+
+
+    19             As mentioned in Section 5.10.3 TMSCP  controllers
+    20             are  expressly  permitted  to optimize any of the
+    21             various positioning functions available  via  the
+    22             REPOSITION command.
+
+    23             If an ABORT command is received for a  REPOSITION
+    24             command  while  the  controller  is  performing a
+    25             positioning function optimization,  cessation  of
+    26             tape  motion  at  the  next  object  (as required
+    27             above) may result in a tape position outside  the
+    28             range   of  objects  spanned  by  the  REPOSITION
+    29             function.    If   this   condition   occurs   the
+    30             controller    must   continue   the   positioning
+    31             function.  During the function  continuation  the
+    32             action  the  controller  must  follow  depends on
+    33             whether the controller is capable of  determining
+    34             when  the tape is positioned within the objective
+    35             (i.e., that range of objects) of the  positioning
+    36             function or not:
+
+    37             a.  If so, the controller continues  tape  motion
+    38                 until   the   first  record  gap  within  the
+    39                 objective is encountered;  end  message  with
+    40                 Command  Aborted  status  returned as soon as
+    41                 tape motion ceases.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-6
+           Command Categories And Execution Order            8 November 1987
+
+
+     1             b.  If not, the controller continues tape  motion
+     2                 until  the final objective of the positioning
+     3                 function is found; end message  with  Success
+     4                 (subcode "Normal") status returned as soon as
+     5                 tape motion ceases.
+
+
+
+     6     3.3  Tape Motion Command Operation
+
+     7     Tape motion commands  are  classified  as  "read",  "write",  and
+     8     "ancillary"  types.   The  following  subsections  describe  tape
+     9     motion operations common  to  each  type.   A  separate  section,
+    10     section 3.4, describes common data transfer command operation.
+
+    11     3.3.1  "read" Type
+
+    12     The "read" type commands are as follows:
+
+    13           o  ACCESS
+    14           o  COMPARE HOST DATA
+    15           o  READ
+    16           o  REPOSITION
+
+
+    17     3.3.1.1  Direction
+
+    18     "Read" type commands can be performed in either  the  forward  or
+    19     reverse  direction.   Direction  is  selected  according  to  the
+    20     setting of the "Reverse"  command  modifier.   When  clear,  tape
+    21     motion  is away from BOT, or in the FORWARD direction.  When set,
+    22     tape motion is toward BOT, or in the REVERSE direction.
+
+    23     It is not mandatory for "non-industry standard" TMSCP devices  to
+    24     support  the  "Reverse"  command  modifier on the ACCESS, COMPARE
+    25     HOST DATA, and READ commands (see Appendix C, Table C-1 for those
+    26     devices considered "non-industry standard").  Devices that do not
+    27     implement the "Reverse" command modifier on  the  above  commands
+    28     must  treat  that  modifier  as reserved, and reject a command so
+    29     modified with a status of Invalid Command.
+
+    30     All TMSCP devices are required to support the "Reverse"  modifier
+    31     on the REPOSITION command.
+
+    32     3.3.1.2  BOT Handling
+
+    33     Any "read" type commands performed in the REVERSE  direction  can
+    34     unexpectedly  encounter  BOT  (usually  Logical  BOT)  before the
+    35     requested operation is completed.  The tape  cannot  be  made  to
+    36     cross  BOT as a result of a REVERSE operation.  Any attempt to do
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-7
+           Tape Motion Command Operation                     8 November 1987
+
+
+     1     so  will  result  in  termination  of  the  command  with  a  BOT
+     2     Encountered status when BOT is encountered.
+
+     3     3.3.1.3  EOT Handling
+
+     4     When EOT is  crossed  while  performing  a  FORWARD  "read"  type
+     5     command  the "EOT Encountered" end flag will be set.  When EOT is
+     6     encountered in this manner, tape motion will  not  be  suspended.
+     7     Command  operation will continue in the normal fashion.  The "EOT
+     8     Encountered" end flag remains set while the  tape  is  positioned
+     9     beyond   EOT   (i.e.,  in  the  tape  trailer  area).   The  "EOT
+    10     Encountered" end flag will be  cleared  only  when  the  tape  is
+    11     subsequently  positioned  ahead  of EOT (i.e., on the BOT side of
+    12     EOT) by any REVERSE "read" type command.
+
+    13     3.3.1.4  Tape Mark Handling
+
+    14     With one exception "read" type command operation is single record
+    15     oriented.   The  exception is the REPOSITION command which can be
+    16     Tape Mark, record, combined Tape Mark and record, or tape  object
+    17     oriented (See section 5.10 for complete details).  If a Tape Mark
+    18     is unexpectedly encountered before the requested record operation
+    19     of  any  "read"  type  command  is  completed the command will be
+    20     terminated with a Tape Mark Encountered status.   The  tape  will
+    21     remain  positioned  forward of the unexpected Tape Mark, relative
+    22     to the direction of travel specified in the command.
+
+    23     3.3.2  "write" Type
+
+    24     The "write" type commands are as follows:
+
+    25           o  ERASE GAP
+    26           o  WRITE
+    27           o  WRITE TAPE MARK
+
+
+    28     3.3.2.1  Direction
+
+    29     "write"  type  commands  are  always  performed  in  the  forward
+    30     direction.   The "Reverse" command modifier is not allowed in any
+    31     "write" type command and  is  therefore  treated  as  a  reserved
+    32     field.
+
+    33     3.3.2.2  BOT Handling
+
+    34     "write" type commands will never encounter BOT unexpectedly.
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-8
+           Tape Motion Command Operation                     8 November 1987
+
+
+     1     3.3.2.3  EOT Handling
+
+     2     When EOT is crossed while performing a "write" type  command  the
+     3     "EOT  Encountered" end flag will be set.  When EOT is encountered
+     4     in this manner, tape  motion  will  not  be  suspended.   Command
+     5     operation will continue in the normal fashion and complete with a
+     6     status  of  Success  (subcode  "EOT  encountered").    The   "EOT
+     7     Encountered"  end  flag  remains set while the tape is positioned
+     8     beyond  EOT  (i.e.,  in  the  tape  trailer  area).    The   "EOT
+     9     Encountered"  end  flag  will  be  cleared  only when the tape is
+    10     subsequently positioned ahead of EOT (i.e., on the  BOT  side  of
+    11     EOT)  by any REVERSE "read" type command.  Note that encountering
+    12     EOT while performing a "write"  type  command  causes  a  Serious
+    13     Exception condition.  Refer to section 3.5.2 for more detail.
+
+    14     3.3.2.4  Tape Mark Handling
+
+    15     "write"  type  commands  will  never  encounter   a   Tape   Mark
+    16     unexpectedly.
+
+    17     3.3.3  "ancillary" Type
+
+    18     The ERASE command is the only "ancillary" type command.
+
+    19     3.3.3.1  Direction
+
+    20     The  ERASE  command  always  begins  operation  in  the   forward
+    21     direction.   The "Reverse" command modifier is not allowed and is
+    22     therefore treated as a reserved field.
+
+    23     3.3.3.2  BOT Handling
+
+    24     The ERASE command will never encounter BOT unexpectedly.
+
+    25     3.3.3.3  EOT Handling
+
+    26     The ERASE command will never encounter EOT unexpectedly.
+
+    27     3.3.3.4  Tape Mark Handling
+
+    28     The ERASE command will never encounter a Tape Mark unexpectedly.
+
+    29     3.4  Data Transfer Operations
+
+    30     The following are considered data transfer operations:
+
+    31           o  ACCESS
+    32           o  Compare Operations (i.e., COMPARE HOST DATA command  and
+    33              Compare modified READ and WRITE)
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS                Page 3-9
+           Data Transfer Operations                          8 November 1987
+
+
+     1           o  READ
+     2           o  WRITE
+
+
+     3     The ACCESS and COMPARE HOST DATA commands  are  included  in  the
+     4     data    transfer    category    since    their    operation    is
+     5     characteristically the same, with the exception that no  data  is
+     6     transferred to or from the host.
+
+     7     The following subsections describe operations common to the  data
+     8     transfer  commands.   A  separate section, section 3.3, describes
+     9     common tape motion command operation.
+
+    10     3.4.1  Record Length
+
+    11     Records    and    transfer    specifications    are    inherently
+    12     variable-length and are specified as an integral number of bytes,
+    13     including odd byte counts.
+
+    14     There is no  formal  minimum  record  length.   However,  certain
+    15     recording   techniques   are   sufficiently  noise-prone  that  a
+    16     recommended minimum record size is required.
+
+    17     Maximum recommended record length is also required.   This  value
+    18     is  a  function  of  both  tape  format  and  error detection and
+    19     correction algorithms employed by a device.   Any  record  length
+    20     greater  than the recommended maximum may not be reliably written
+    21     and/or read.
+
+    22     The tape unit specific minimum and maximum record  length  values
+    23     are  available  to  the host in the end message of the ONLINE and
+    24     SET UNIT CHARACTERISTICS commands.
+
+    25     Although a record of greater length than the maximum  recommended
+    26     can  be  read  or  written,  there exists a true absolute maximum
+    27     size.  This absolute maximum  size  is  dependent  upon  specific
+    28     controller  characteristics.   Refer  to  Appendix  C, Table C-3.
+    29     Transfer  commands  specifying  a  byte  count  larger  than  the
+    30     absolute  maximum  are  rejected with an Invalid Command (subcode
+    31     "Invalid Byte Count") status.   In  addition,  transfer  commands
+    32     specifying  a  byte  count  of  zero  are rejected with a Invalid
+    33     Command (subcode "Invalid Byte Count") status.
+
+    34     3.4.2  Tape Format Employed
+
+    35     The tape format employed in data transfer operations is dependent
+    36     on  the  first tape motion command issued after the the tape unit
+    37     becomes "Unit Online" (the first time in the case  of  multi-host
+    38     access).
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-10
+           Data Transfer Operations                          8 November 1987
+
+
+     1     If the first tape motion command  is  a  "write"  type  operation
+     2     (i.e., ERASE GAP, WRITE, or WRITE TAPE MARK) the format specified
+     3     by the host via either the ONLINE  or  SET  UNIT  CHARACTERISTICS
+     4     command will be employed.
+
+     5     If the first tape motion command is a "read" type command  (i.e.,
+     6     ACCESS,  COMPARE  HOST  DATA,  READ or REPOSITION) the controller
+     7     will ignore host format  specifications  and  employ  the  format
+     8     implied by the information recorded in the TFIA when the tape was
+     9     created.  If the tape unit is unable to  determine  the  recorded
+    10     format  from the TFIA information, the command is terminated with
+    11     a Data Error (subcode "Unrecoverable  Read  Error")  end  message
+    12     status.   If  the  TFIA  information is nonexistent, implying the
+    13     tape is blank, the  command  is  terminated  with  a  Data  Error
+    14     (subcode  "Long  Gap  Encountered")  end message status.  In this
+    15     case the format employed in reading the TFIA information dictates
+    16     the  long  gap  value used in determining the long gap condition;
+    17     however, the host specified format will not be changed to reflect
+    18     that format.
+
+    19     3.4.3  Host Buffer Access
+
+    20     The COMPARE HOST DATA, READ, and WRITE command  messages  contain
+    21     byte  count  and  host  buffer  address  descriptors.  The ACCESS
+    22     command only contains a byte count field.
+
+    23     3.4.3.1  WRITE Command
+
+    24     In the case of a WRITE command, the specified number of bytes  is
+    25     written  to  tape from the host buffer, in the forward direction,
+    26     beginning at the host  buffer  base  address  proceeding  through
+    27     successively higher numbered addresses.
+
+    28     3.4.3.2  READ Command
+
+    29     For a READ command, the specified byte count is treated as advice
+    30     to  the  controller.   The amount of data actually transferred is
+    31     dictated by the number of bytes  contained  in  the  tape  record
+    32     being  read.   A  Success end message status is returned when the
+    33     tape record size is either equal to or less than  (short  record)
+    34     the  specified  byte  count.  However, if the tape record size is
+    35     greater  than  (long  record)  the  specified  byte   count   the
+    36     controller  will  terminate  the  transfer  at the specified byte
+    37     count and report a Record Data Truncated end message status.
+
+    38     READ commands can be performed in either the forward  or  reverse
+    39     direction.   "Non-industry  standard"  devices  may  reject  READ
+    40     commands with the "Reverse" command modifier set with a status of
+    41     Illegal  Command  (subcode  "Illegal Modifier").  See Appendix C,
+    42     Table C-1 for those devices considered "non-industry standard".
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-11
+           Data Transfer Operations                          8 November 1987
+
+
+     1     3.4.3.2.1  Forward Direction
+
+     2     If a READ in the forward direction is specified, the  data  bytes
+     3     of the record are stored in the host buffer beginning at the base
+     4     address,  as  specified  in  the  buffer  descriptor,  proceeding
+     5     through  successively  higher-numbered  addresses.   If  the tape
+     6     record is short, the host buffer contents from
+
+     7       [base address]+[tape record byte count]
+
+     8     through
+
+     9       [base address]+[specified byte count]-1
+
+    10     remain unchanged.  If the tape record is long,  the  end  of  the
+    11     tape record is truncated.
+
+    12     3.4.3.2.2  Reverse Direction
+
+    13     If a READ in the reverse direction is specified, the  data  bytes
+    14     of the record are stored in the host buffer beginning at
+
+    15       [base address]+[specified byte count]-1,
+
+    16     proceeding through successively lower-numbered addresses.  If the
+    17     tape  record  is long, the front of the tape record is truncated.
+    18     If the tape record is short the record image ends at host buffer
+
+    19       [base address]+[specified byte count]-[tape record byte count].
+
+    20     The host buffer contents from the base address to  the  beginning
+    21     of the short record image remain unchanged.
+
+    22     3.4.3.3  ACCESS Command
+
+    23     The ACCESS command is performed in the same manner  as  the  READ
+    24     command, described in section 3.4.3.2, with the exception that no
+    25     data is transferred to the host.
+
+    26     3.4.3.4  Compare Operations
+
+    27     Compare operations permit comparison of data contained in a  host
+    28     buffer to data contained in a tape record.
+
+    29     The compare process is performed in the same manner as  the  READ
+    30     command buffer storage process described in section 3.4.3.2.  The
+    31     only difference is that instead of storing the data read from the
+    32     tape record, a byte by byte comparison is performed.
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-12
+           Data Transfer Operations                          8 November 1987
+
+
+     1     The MSCP description of compare operations is identical to  TMSCP
+     2     implementation with the exceptions described below.
+
+     3     A compare operation will not be performed by  the  controller  in
+     4     the following cases:
+
+     5          1.  If a long tape record is encountered  during  a  compare
+     6              modified  READ  or  COMPARE  HOST DATA command.  In this
+     7              case the  command  is  terminated  with  a  Record  Data
+     8              Truncated status.
+
+     9          2.  If any error occurs during the original data transfer of
+    10              a compare modified READ or WRITE command which indicates
+    11              the data was not transferred correctly or  at  all.   In
+    12              the  case  of  the COMPARE HOST DATA command the compare
+    13              operation  is  performed  up  to  the  point  the  error
+    14              occurred.
+
+
+    15     Refer to sections 4.5.4 and 4.5.5 for details on end message byte
+    16     count fields.
+
+    17     "Compare after" operations (i.e., compare modified WRITE or READ)
+    18     require  positioning  in the direction opposite to that specified
+    19     in the original command.  Regardless of the success or failure of
+    20     the  compare operation, the tape will always be positioned in the
+    21     record gap forward of the tape record being operated on, relative
+    22  |  to  the  direction of the original command.  Failures not related
+    23  |  to the compare  operation  may  cause  the  tape  to  not  be  so
+    24  |  positioned  (see  section  3.5.3,  Position Lost Handling).  Such
+    25  |  failures will result in a "Position Lost" error condition.
+
+    26     3.4.4  End Message Byte Count Fields
+
+    27     Refer to sections 4.5.4 and 4.5.5 for details on end message byte
+    28     count fields.
+
+    29     3.5  General Error Processing
+
+    30     The following subsections describe  error  processing  common  to
+    31     TMSCP  commands.   Refer  to  section  4.5.2  for  definition  of
+    32     individual status codes.
+
+    33     3.5.1  Reporting Precedence
+
+    34     In  cases   where   multiple   errors   (or   conditions)   occur
+    35     simultaneously,  TMSCP controllers report errors according to the
+    36     following categorical precedence:
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-13
+           General Error Processing                          8 November 1987
+
+
+     1          1.  fatal condition (e.g., Drive Error, etc.)
+     2          2.  unrecoverable data error
+     3          3.  conditional success (e.g., EOT encountered)
+     4          4.  success
+
+
+     5     3.5.2  Serious Exception Handling
+
+     6     Tape Units are inherently sequential devices.   This  means  that
+     7     the  order  in  which  tape  motion commands are executed must be
+     8     identical to the order in which they are received.
+
+     9     In traditional tape systems, sequentiality is  preserved  because
+    10     the device is capable of having only one command outstanding to a
+    11     tape unit at any time.  If a tape motion command terminates  with
+    12     an  error, the host can take immediate action to prevent commands
+    13     waiting to be executed from being presented to the  device.   The
+    14     host, therefore, implicitly maintains control of sequentiality.
+
+    15     Since TMSCP  controllers  are  required  to  provide  queuing  of
+    16     multiple   commands,   control  of  sequentiality  following  the
+    17     occurrence  of  an  error  becomes  the  responsibility  of   the
+    18     controller.  When an error occurs and the tape unit remains "Unit
+    19     Online"  to  any  host,  a  TMSCP  controller  must  ensure  that
+    20     subsequent  commands  are  rejected until a host acknowledges the
+    21     occurrence of the error.  This condition is known  as  a  Serious
+    22     Exception condition.
+
+    23     Any of the conditions defined by the following Status Codes cause
+    24     a Serious Exception condition:
+
+    25           o  BOT Encountered
+    26           o  Compare Error
+    27           o  Controller Error
+    28           o  Data Error
+    29           o  Drive Error
+    30           o  Formatter Error
+    31           o  Host Buffer Access Error
+    32           o  Invalid Command, subcodes
+    33               o  Invalid Byte Count
+    34               o  Invalid Format
+    35               o  Invalid Unit Flags
+    36           o  LEOT Detected
+    37           o  Position Lost, only when  Position  Lost  state  is  not
+    38              already in effect (refer to section 3.5.3)
+    39           o  Record Data Truncated
+    40           o  Success, subcode "EOT Encountered", on  the  ERASE  GAP,
+    41              WRITE, and WRITE TAPE MARK commands
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-14
+           General Error Processing                          8 November 1987
+
+
+     1           o  Tape Mark Encountered
+     2           o  Write Protected
+
+
+     3     The following Status Codes will not  cause  a  Serious  Exception
+     4     condition:
+
+     5           o  Command Aborted
+     6           o  Invalid Command, all subcodes except
+     7               o  Invalid Byte Count
+     8               o  Invalid Format
+     9               o  Invalid Unit Flags
+
+    10           o  Position Lost, while Position Lost state  exists  (refer
+    11              to section 3.5.3)
+    12           o  Serious Exception
+    13           o  Success, all subcodes except EOT Encountered
+    14           o  Unit-Available
+    15           o  Unit-Offline
+
+
+    16     The occurrence of a Serious Exception  condition  causes  a  tape
+    17     unit  to  enter the Serious Exception state relative to all hosts
+    18     to which it is currently "Unit Online", regardless of which  host
+    19     issued the command encountering the error.
+
+    20     The Serious Exception state is reflected in the Serious Exception
+    21     end  flag (SEF).  Refer to section 4.5.1 for a description of End
+    22     Flags.
+
+    23     The end message of the command encountering the Serious Exception
+    24     condition  will  have  SEF  set  and  the status will reflect the
+    25     nature  of  the  error.   Subsequent  commands  (including  those
+    26     currently queued by the controller) are rejected with SEF set and
+    27     a status  of  Serious  Exception  in  their  end  messages.   The
+    28     controller  continues to reject commands in that manner until the
+    29     tape unit Serious Exception state is cleared by a host.
+
+    30     The following commands are subject to rejection while  a  Serious
+    31     Exception state exists:
+
+    32           o  ACCESS
+    33           o  AVAILABLE
+    34           o  COMPARE HOST DATA
+    35           o  ERASE
+    36           o  ERASE GAP
+    37           o  FLUSH
+    38           o  READ
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-15
+           General Error Processing                          8 November 1987
+
+
+     1           o  REPOSITION
+     2           o  WRITE
+     3           o  WRITE TAPE MARK
+
+
+     4     The Clear Serious Exception Command Modifier (CSE) is  the  means
+     5     by  which a host acknowledges the Serious Exception condition and
+     6     signals the controller that the tape unit Serious Exception State
+     7     can  be cleared and normal processing resumed.  Note that the CSE
+     8     modifier is acted upon before any other part of  a  command.   It
+     9     only  clears  the Serious Exception state that existed before the
+    10     command was received (i.e., a  command  cannot  clear  a  Serious
+    11     Exception  it  caused).  The CSE modifier is ignored if a Serious
+    12     Exception state does not exist.
+
+    13     The  CSE  modifier  can  only  be  specified  for  the  following
+    14     commands:
+
+    15           o  ACCESS
+    16           o  AVAILABLE
+    17           o  COMPARE HOST DATA
+    18           o  ERASE
+    19           o  ERASE GAP
+    20           o  FLUSH
+    21           o  ONLINE
+    22           o  READ
+    23           o  REPOSITION
+    24           o  SET UNIT CHARACTERISTICS
+    25           o  WRITE
+    26           o  WRITE TAPE MARK
+
+
+    27     The CSE modifier is reserved for all other commands.
+
+    28     The existence of a Serious Exception state does not prevent other
+    29     hosts  from bringing a tape unit into the "Unit Online" state via
+    30     the ONLINE command.   The  tape  unit  will  be  in  the  Serious
+    31     Exception  state relative to any host issuing a successful ONLINE
+    32     command while the Serious Exception state exists, unless the  CSE
+    33     modifier  was specified in that ONLINE command.  In that case the
+    34     Serious Exception State is cleared for all hosts which  have  the
+    35     tape unit in the "Unit Online" state.
+
+    36     If a tape unit leaves the "Unit Online"  state  relative  to  all
+    37     hosts,  for  any  reason,  the  tape unit will cease to be in the
+    38     Serious Exception state.
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-16
+           General Error Processing                          8 November 1987
+
+
+     1     3.5.3  Position Lost Handling
+
+     2     When a "Position Lost"  error  condition  occurs  the  tape  unit
+     3     enters  the Position Lost state relative to all hosts to which it
+     4     is currently "Unit Online", regardless of which host  issued  the
+     5     command encountering the error.
+
+     6     The Position Lost state is reflected in  the  Position  Lost  end
+     7     flag.  Refer to section 4.5.1 for a description of end flags.
+
+     8  |  A "Position Lost" error condition includes the following:
+
+     9  |       1.  The formatter is not certain of the unit's position.
+
+    10  |       2.  The change in unit position reported in the end messages
+    11  |           is  not  due  to  the  command corresponding to this end
+    12  |           message.  This includes the following case:
+
+    13  |            o  The unit enters the "Cached Data  Lost"  state  (see
+    14  |               section   3.6.1.4).    This   effectively  "revokes"
+    15  |               position that was previously reported (i.e., success
+    16  |               was  reported  on  previous cached operations, which
+    17  |               have subsequently failed).  The change  reported  in
+    18  |               the  position  field  cannot  be  attributed  to the
+    19  |               command   containing   the   "Cached   Data    Lost"
+    20  |               indication, and thus a Position Lost error condition
+    21  |               has occurred and the unit enters the  Position  Lost
+    22  |               state.
+
+
+
+    23     While the Position Lost state is in effect the host may issue any
+    24     tape motion command in an attempt to recover position on its own.
+    25     The controller must attempt to  execute  those  commands  in  the
+    26     normal  manner.   During  the  execution  of  those  commands the
+    27     controller will not generate a Serious  Exception  condition  for
+    28     "Position  Lost"  error occurrences.  The occurrence of all other
+    29     errors however, will be handled in the normal manner.
+
+    30     The tape unit will remain in the Position Lost  state  until  the
+    31     tape  is subsequently positioned to BOT, the tape unit enters the
+    32     "Unit Offline" state, or the tape is unloaded  via  an  AVAILABLE
+    33     command.
+
+    34     3.5.4  Asynchronous  Completion  Handling  Of  Non-Data  Transfer
+    35            Commands
+
+    36     TMSCP controllers cannot provide host end message notification of
+    37     errors  encountered  during  either  the  execution of "immediate
+    38     completion" non-data transfer type  commands  (refer  to  section
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-17
+           General Error Processing                          8 November 1987
+
+
+     1     3.2.1.1)  or  tape  motion  continuation  following  the abort of
+     2     certain lengthy I/O commands (refer to section  3.2.1.2)  because
+     3     the  end message has already been returned to the host.  Based on
+     4     the fact that all error conditions that  can  be  encountered  in
+     5     either  of  those  cases  imply that the tape unit is essentially
+     6     inoperable, the TMSCP controller will take the following actions:
+
+     7          1.  Issue an error log detailing the  error  that  occurred,
+     8              provided  the  error  normally requires logging and that
+     9              miscellaneous error logs are enabled (refer to  Appendix
+    10              B).
+
+    11          2.  Place  the  tape  unit  into   the   "Offline   -   Unit
+    12              Inoperative" state.
+
+    13     Subsequent  tape  motion  commands  will  therefore  be  rejected
+    14     because  of this "offline" state, precluding further operation of
+    15     the tape unit until the host has acknowledged the condition.
+
+    16     3.5.5  Data Error Recovery
+
+    17     TMSCP tape unit data error recovery procedures meet or exceed the
+    18     requirements  of  DEC  Standard  174  with certain qualifications
+    19     described below.  Note that error  recovery  techniques  used  by
+    20     block  oriented tape units differ from those used by 9-track tape
+    21     units, but will provide equivalent performance.
+
+    22     Only unrecoverable errors will be reported to the host in the end
+    23     message status field.
+
+    24     Recoverable errors are reported to the host in  error  log  form.
+    25     The  controller will normally issue only one error log during the
+    26     error recovery process  regardless  of  the  number  of  attempts
+    27     involved  in  recovering the data.  The error log is issued after
+    28     the final attempt whether recovery  was  successful  or  not.   A
+    29     separate  error log will be issued for non-transfer errors (e.g.,
+    30     an STI transmission error) which occur during any segment of  the
+    31     recovery process.  Refer to Chapter 6 for specific information on
+    32     error log content.
+
+    33     Error recovery algorithms require unsolicited positioning in  the
+    34     direction  opposite  to  that  specified in the original command.
+    35     For either a successful or unsuccessful recovery  operation,  the
+    36     tape  will  be  positioned  in the record gap forward of the data
+    37     record, relative  to  the  direction  of  the  original  command.
+    38  |  Failure  to  be  so  positioned  will result in a "Position Lost"
+    39  |  error condition.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-18
+           General Error Processing                          8 November 1987
+
+
+     1     Note that explicit recovery sequences may  be  specified  to  the
+     2     controller by a formatter.
+
+     3     3.5.5.1  Suppression
+
+     4     Data error recovery suppression is controlled  by  the  "Suppress
+     5     Error  Recovery"  command  modifier.  This modifier is allowed in
+     6     the ACCESS,  COMPARE  HOST  DATA,  READ,  REPOSITION,  and  WRITE
+     7     commands.   When  set,  the  controller will inhibit "Data Error"
+     8     recovery procedures.   If  a  "Data  Error"  is  encountered  the
+     9     controller  reports  it to the host immediately as unrecoverable.
+    10     The tape will remain positioned in the record gap forward of  the
+    11     tape  object in error, relative to the direction specified in the
+    12     command.
+
+    13                                   NOTE
+
+    14             Use of this modifier does not  prevent  the  tape
+    15             unit  from  entering  the Serious Exception state
+    16             when an error occurs.  Refer to section 3.5.2 for
+    17             more detail.
+
+
+    18     3.5.5.2  Enhanced Write Error Recovery
+
+    19     Enhanced  Write  Error  Recovery  is  a  non-host  settable  unit
+    20     characteristic  presented  only  if the TMSCP controller supports
+    21     the "Enable Re-write Error Recovery" command  modifier  on  WRITE
+    22     commands for the specified tape unit (see section 5.7.2).
+
+    23     When the "Enable Re-write Error  Recovery"  command  modifier  is
+    24     set,  the  unit  uses a special error recovery algorithm to write
+    25     the data on the media.  The algorithm  must  guarantee  that  the
+    26     probability  of  altering  or  destroying  previously written and
+    27     acknowledged  data  is  no  worse  than  the  probability  of  an
+    28     undetected  write  error.  It is permissible for the algorithm to
+    29     leave multiple copies of the record on the media.
+
+    30     An example algorithm that can be applied to 9-track tapes  is  as
+    31     follows:
+
+    32          1.  If the initial attempt to write a record fails, then the
+    33              record will be re-written sequentially down the tape for
+    34              up to three retries.
+
+    35          2.  If the  re-writing  of  the  record  fails  after  three
+    36              re-tries,  then  the TMSCP controller will institute its
+    37              normal over-write error recovery algorithm to over-write
+    38              the fourth copy of the record.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-19
+           General Error Processing                          8 November 1987
+
+
+     1     Units not utilizing the above algorithm must  document  in  their
+     2     device  specification  how  the  requirements  of "Enhanced Write
+     3     Error Recovery" are met.
+
+     4     The "Enable Re-write Error Recovery" modifier may not be  set  in
+     5     conjunction  with  the "Suppress Error Recovery" command modifier
+     6     and  is  therefore  treated  as  a  reserved  field.   The  TMSCP
+     7     controller  must  return  an  Invalid  Command  (subcode "Invalid
+     8     Modifier") status code for status if  both  the  "Suppress  Error
+     9     Recovery"   command  modifier  and  the  "Enable  Re-write  Error
+    10     Recovery" command modifiers are set in the same WRITE command.
+
+    11     3.5.6  Error Log Requirements
+
+    12     Controllers should not report error logs for events which  cannot
+    13     possibly be potential hardware problems.  Examples:
+
+    14          1.  On a tape device,  a  REPOSITION  command  returns  Data
+    15              Error  (subcode  "Long  Gap Encountered") as a result of
+    16              reading metadata previously recorded on the tape.   Such
+    17              metadata  may  be  a  result of the recording technology
+    18              (e.g., block mode cartridge tape).  This situation  does
+    19              not  indicate  a potential hardware problem, and thus an
+    20              error log should not be returned.
+
+    21          2.  On a tape device,  a  REPOSITION  command  returns  Data
+    22              Error  (subcode  "Long  Gap Encountered") as a result of
+    23              not reading a record within the required distance.  This
+    24              situation  has the potential of being a hardware failure
+    25              in the read electronics, and thus an error log should be
+    26              returned.
+
+
+    27     3.6  Tape Caching
+
+    28     There is a broad range of mass storage caches possible, including
+    29     both  one and two level caches (for large, slow, archival storage
+    30     devices), write-through and write-back operations,  and  volatile
+    31     and  non-volatile  caches.   Caching  may potentially be used for
+    32     both disk and tape class devices.  Caching for disk class devices
+    33     implies  what is normally meant by the term "cache".  Caching for
+    34     tape  class  devices  is  merely  a  mechanism  for  implementing
+    35     controller   based   read-ahead   and  write-behind.   Note  that
+    36     write-through  caching  applied  to  a  tape  class   device   is
+    37     meaningless; since the data has already been written to the tape,
+    38     it won't be retained in the cache.
+
+    39     A controller will typically only provide a subset of  this  cache
+    40     functionality,  if indeed it supports caching at all.  This broad
+    41     range  of  functionality  has  been  included  to  avoid  ad  hoc
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-20
+           Tape Caching                                      8 November 1987
+
+
+     1     extensions as future controllers are defined.
+
+     2     Performing write operations  as  write-behind,  sometimes  called
+     3     write-back,  operations offers the potential for greatly improved
+     4     performance with a decrease  in  reliability.   The  decrease  in
+     5     reliability  arises  from  the  fact that data may be lost if the
+     6     controller crashes or otherwise loses context before the data  is
+     7     written  back  to  the  unit.   The probability of losing data is
+     8     significantly  different  depending  on  whether  the  cache   is
+     9     volatile  or  non-volatile,  since  the  primary  cause of losing
+    10     write-back data is power failures.  Volatile caches  always  lose
+    11     data on power failures; non-volatile caches only lose data if the
+    12     unit, volume, or cache is removed from the controller during  the
+    13     power  failure.   For these reasons write-back operations must be
+    14     explicitly enabled by the host.  Write-back operations for  TMSCP
+    15     controllers with any type of cache may be enabled on either a per
+    16     unit basis, using a unit flag, or on a per command basis, using a
+    17     command  modifier  or both.  In addition, the FLUSH command, with
+    18     or without command modifiers, may be  used  to  ensure  that  all
+    19     write-back data has been written out to the specified tape unit.
+
+    20     Tape write-back caching can be viewed  as  having  the  following
+    21     three characteristics:
+
+    22          1.  The TMSCP controller's write-back cache is viewed  as  a
+    23              volatile cache.
+
+    24          2.  The TMSCP controller may have write-back caching enabled
+    25              on a per unit basis or on a per command basis or both.
+
+    26          3.  The TMSCP controller must provide support for  "Enhanced
+    27              Write Error Recovery".
+
+
+    28     3.6.1  Write-Back Caching
+
+    29     Because  normal  TMSCP  controller  data  transfer  commands  are
+    30     sequential  and  depend on the completion of transferring data to
+    31     the media,  write-back  caching  requires  specific  measures  to
+    32     handle  the  various  error  and  exception  conditions which can
+    33     occur.
+
+    34     If write-back caching is not enabled, either on a per unit  basis
+    35     or  on a per command basis, then the TMSCP controller will return
+    36     the end  message  when  the  write  data  transfer  operation  is
+    37     completed.
+
+    38     If write-back caching is enabled, either on a per unit  basis  or
+    39     on  a per command basis, then the TMSCP controller may return the
+    40     end message as soon as the data for the write data  operation  is
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-21
+           Tape Caching                                      8 November 1987
+
+
+     1     completely  buffered  by  the  TMSCP  controller.   If write-back
+     2     caching is enabled on a per command basis, the  TMSCP  controller
+     3     must  preserve  the sequentiality of subsequent sequential (i.e.,
+     4     non-write-back cached) commands issued by the  host  until  those
+     5     operations complete.  Furthermore, the TMSCP controller must also
+     6     ensure  that  subsequent  sequential  commands   appear   to   be
+     7     progressing  towards  completion  in order to satisfy the command
+     8     timeout requirements as defined in MSCP.  When write-back caching
+     9     is enabled, either on a per unit basis or on a per command basis,
+    10     then the TMSCP controller must, at all times, maintain the proper
+    11     sequentiality  of  all  previous non-write-back cached sequential
+    12     commands.
+
+    13     The "Immediate Completion" command modifier can only be specified
+    14     on the following data transfer "write" type commands (i.e., ERASE
+    15     GAP, WRITE, and WRITE TAPE MARK commands).
+
+    16     If write-back caching is enabled, either on a per  command  basis
+    17     or  on  a  per  unit  basis,  there must be some command sequence
+    18     available  to  guarantee  that  all  write-back  data  previously
+    19     buffered  for  "write"  type commands has been written out to the
+    20     specified tape unit.  The following  five  sections  outline  the
+    21     command sequences available.
+
+    22          1.  If write-back caching is enabled on a per command basis,
+    23              subsequent  non-write-back  cached "write" type commands
+    24              will  guarantee  that  all  write-back  data  previously
+    25              buffered  for "write" type commands has been written out
+    26              to  the  specified  tape  unit.   That  is,  the   TMSCP
+    27              controller   will   return  the  end  message  when  the
+    28              non-write-back  cached  "write"   type   data   transfer
+    29              operation  has  completed  to  the  specified tape unit.
+    30              Thus  implying  that  all  write-back  data   previously
+    31              buffered  for "write" type commands has been written out
+    32              to the specified tape unit.
+
+    33          2.  If write-back caching is enabled on a  per  unit  basis,
+    34              there  are  two  distinct command sequences available to
+    35              guarantee that write-back data buffered for all previous
+    36              "write"  type  commands  has  been  written  out  to the
+    37              specified tape unit.
+
+    38              i.  The first command sequence uses the  FLUSH  command.
+    39                  This   command  sequence  will  guarantee  that  all
+    40                  write-back data previously buffered for "write" type
+    41                  commands  has been written out to the specified tape
+    42                  unit before the TMSCP controller will return the end
+    43                  message indicating the FLUSH command has completed.
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-22
+           Tape Caching                                      8 November 1987
+
+
+     1             ii.  The second command sequence  depends  on  two  WRITE
+     2                  TAPE MARK commands with no other intervening "write"
+     3                  type commands.  This command sequence will guarantee
+     4                  that  all  write-back  data  previously buffered for
+     5                  "write" type commands has been written  out  to  the
+     6                  specified  tape  unit  prior to the TMSCP controller
+     7                  returning the end message indicating that the second
+     8                  WRITE TAPE MARK command has completed.
+
+
+     9          3.  Any command sequence that transitions from "write"  type
+    10              commands  to  either "read" type commands or "ancillary"
+    11              type commands will force the TMSCP controller to "flush"
+    12              all  of  the write-back cached data out to the specified
+    13              tape unit  prior  to  the  end  message  indicating  the
+    14              completion  of  either  the  "read"  type command or the
+    15              "ancillary" type command.
+
+    16          4.  At the time a response to a non-data transfer sequential
+    17              command   (i.e.,   AVAILABLE,   ONLINE,   or   SET  UNIT
+    18              CHARACTERISTICS) is generated, the cache must be  empty.
+    19              Thus,  any  of these commands may be used to "flush" all
+    20              of the write-back cached data out to the specified  tape
+    21              unit.   Note  that  although  the  Available  command is
+    22              considered "immediate completion", the response must not
+    23              be  generated  until  the cache on the specified unit is
+    24              empty.
+
+    25          5.  In addition to the above mechanisms available to  hosts,
+    26              the TMSCP controller may, at its discretion, "flush" all
+    27              of the TMSCP controller's write-back cached data out  to
+    28              the specified tape unit at any time.
+
+
+    29     Proper support for write-back caching requires  that  controlled,
+    30     predictable   results  be  specified  for  the  handling  of  the
+    31     following conditions:
+
+    32           o  Conflicting requirements imposed by unit flags.
+
+    33           o  Conflicting requirements imposed by command modifiers.
+
+    34           o  Data  transfer  errors  that  occur  prior  to   command
+    35              completion having been signaled to the host.
+
+    36           o  Data transfer errors that occur after command completion
+    37              has been signaled to the host.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-23
+           Tape Caching                                      8 November 1987
+
+
+     1     3.6.1.1  Unit Flag Conflicts
+
+     2     The  "Compare  Writes"  unit  flag  overrides  the  enabling   of
+     3     write-back  caching.   There  are no other unit flags in conflict
+     4     with the enabling of write-back caching.
+
+     5     3.6.1.2  Command Modifier Conflicts
+
+     6     The "Compare" modifier on WRITE commands overrides  the  enabling
+     7     of   write-back   caching   for  the  duration  of  the  command.
+     8     Write-back caching, if enabled, may be re-established  after  the
+     9     compare portion of the command if subsequent commands do not also
+    10     have the "Compare" modifier set.
+
+    11     The  "Suppress  Error  Recovery"  command  modifier  may   either
+    12     override  the  enabling of write-back caching for the duration of
+    13     the command, or be handled as specified in the following sections
+    14     (this  approach  assumes  that  the  first  data  transfer  error
+    15     terminates any further processing).
+
+    16     Write-back  caching,  if  enabled,  may  be   re-established   if
+    17     subsequent   commands  do  not  also  have  the  "Suppress  Error
+    18     Recovery" command modifier set.
+
+    19     There are  no  other  command  modifiers  in  conflict  with  the
+    20     enabling of write-back caching.
+
+    21     3.6.1.3  Error And Exception Handling
+
+    22     In general, TMSCP controllers cannot  provide  host  end  message
+    23     notification  of  errors  or  exceptions  encountered  during the
+    24     execution of write-back caching data  transfer  commands  because
+    25     the end message has already been returned to the host.  There are
+    26     three error conditions which must be accommodated.   These  cases
+    27     are:
+
+    28          1.  Failure to transfer the specified  data  from  the  host
+    29              because  of  some  type  of host bus access error (i.e.,
+    30              parity error, time out, bus interface failure, etc.).
+
+    31          2.  Failure to maintain the  specified  data  in  the  TMSCP
+    32              controller's  write-back cache (i.e., parity error, EDC,
+    33              or other indication of data corruption).
+
+    34          3.  Failure to successfully transfer the  buffered  data  to
+    35              the media.
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-24
+           Tape Caching                                      8 November 1987
+
+
+     1     The first error  condition  will  be  encountered  prior  to  the
+     2     signaling  of completion for the write-back caching data transfer
+     3     command.  This situation implies that all of the prior data  will
+     4     have been written on the media and is therefore accessible.  This
+     5     condition  requires  that  the  TMSCP  controller   perform   the
+     6     following steps:
+
+     7          i.  Issue an error log detailing the  error  that  occurred,
+     8              provided  the error normally requires logging, (refer to
+     9              Appendix B).
+
+    10         ii.  Position the specified tape unit immediately  after  the
+    11              last  successful record written, and just prior to where
+    12              the record in error would have occurred.
+
+    13        iii.  Place the tape unit into the "Serious Exception" state.
+
+    14         iv.  Report the specific nature of the error  as  the  status
+    15              for this specific command's end packet.
+
+
+    16     Subsequent tape data transfer commands will therefore be rejected
+    17     because  of  the  "Serious  Exception"  state, precluding further
+    18     operation  of  the  specified  tape  unit  until  the  host   has
+    19     acknowledged  the  condition.   Additionally,  the  host  will be
+    20     "synchronized" with the  tape  unit  and  positioned  immediately
+    21     after the last good record.
+
+    22     Based on the fact that the other error  conditions  that  can  be
+    23     encountered  after  a write-back caching data transfer command is
+    24     initiated  imply  that  the  data   on   media   is   essentially
+    25     inaccessible,  the second and third conditions require that TMSCP
+    26     controller perform the following steps:
+
+    27          i.  Issue an error log detailing the  error  that  occurred,
+    28              provided  the  error normally requires logging (refer to
+    29              Appendix B).
+
+    30         ii.  If the specified tape unit remains in the "Unit  Online"
+    31              state,  then  place  the  tape  unit  into  the "Serious
+    32              Exception", "Position  Lost",  and  "Cached  Data  Lost"
+    33              states.
+
+    34        iii.  Report the specific nature of the error as the status of
+    35              the  immediately  subsequent sequential end packet to be
+    36              returned.
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-25
+           Tape Caching                                      8 November 1987
+
+
+     1     Subsequent tape data transfer commands will therefore be rejected
+     2     because  of  the  "Serious  Exception"  state, precluding further
+     3     operation of the tape unit until the host  has  acknowledged  the
+     4     condition.   The  host  will  not be "synchronized" with the tape
+     5     unit and data written to the media subsequent to the  last  known
+     6     "synchronization" point will be effectively inaccessible.
+
+     7     There is one exception  condition  which  must  be  accommodated.
+     8     That  is  the case of encountering EOT.  This exception condition
+     9     will  be  handled  by  the  TMSCP  controller  anticipating   the
+    10     detection  of  EOT  through  some  "early  warning" mechanism and
+    11     "winding  down"  the  write-back  caching  such  that  the  TMSCP
+    12     controller  is  operating  in  a non-write-back caching mode when
+    13     "true" EOT is encountered.
+
+    14     3.6.1.4  Cached Data Lost Handling
+
+    15     Tape Units are inherently sequential devices.   This  means  that
+    16     the  order  in  which  "write"  type  data  transfer commands are
+    17     executed must be  identical  to  the  order  in  which  they  are
+    18     received.
+
+    19     In traditional tape  systems,  this  sequentiality  is  preserved
+    20     because  the  device  is  capable  of  having  only  one  command
+    21     outstanding to a tape unit at any time.  If a "write"  type  data
+    22     transfer  command  terminates  with  an  error, the host can take
+    23     immediate action to prevent commands waiting to be executed  from
+    24     being  presented  to the device.  The host, therefore, implicitly
+    25     maintains control of sequentiality.
+
+    26     A reliable detection mechanism for determining if write-back data
+    27     has  been  lost  needs  to  be defined.  This detection mechanism
+    28     needs to be hierarchical; partially implemented in the  host  and
+    29     partially  implemented  in  the  TMSCP  controller.   At the host
+    30     level, sufficient state needs to be maintained to  remember  that
+    31     write-back  operations  are  in  progress.   This  state  may  be
+    32     maintained either at the class driver level  or  the  application
+    33     level  or both.  At the TMSCP controller level, there needs to be
+    34     a reporting  mechanism  to  provide  information  concerning  the
+    35     efficacy   of   any  pending  write-back  caching  data  transfer
+    36     commands.  The effect of this detection mechanism  should  be  to
+    37     facilitate  either  the  restart  of the entire application or an
+    38     application dependent  restart  of  the  operations  within  some
+    39     context   (i.e.,   from   the  last  known  good  host/tape  unit
+    40     synchronization point).  The description of the TMSCP  controller
+    41     mechanism is contained below.
+
+    42     TMSCP controllers cannot provide host end message notification of
+    43     errors   or   exceptions  encountered  during  the  execution  of
+    44     write-back caching data transfer commands because the end message
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-26
+           Tape Caching                                      8 November 1987
+
+
+     1     has  already been returned to the host.  Therefore, when an error
+     2     occurs and the tape unit remains "Unit Online"  to  any  host,  a
+     3     TMSCP   controller  must  ensure  that  subsequent  commands  are
+     4     rejected until a host acknowledges the occurrence of  the  error.
+     5     This condition is known as a Cached Data Lost condition.
+     6     Any of the conditions  defined  by  the  following  Status  Codes
+     7     occurring  while  write-back  caching  is  enabled,  either  on a
+     8     command basis or on a unit basis, will cause a Cached  Data  Lost
+     9     condition:
+
+    10           o  Controller Error
+    11           o  Data Error
+    12           o  Drive Error
+    13           o  Formatter Error
+    14           o  Position Lost, only when  Position  Lost  state  is  not
+    15              already in effect (refer to section 3.5.3)
+    16           o  Write Protected
+
+    17     The following Status Codes will not  cause  a  Cached  Data  Lost
+    18     condition:
+
+    19           o  Invalid Command, all subcodes
+    20           o  Success, all subcodes
+    21           o  Unit-Available
+    22           o  Unit-Offline
+
+
+    23     Whenever one of the above error conditions occurs that causes the
+    24     TMSCP controller to lose cached data for the specified tape unit,
+    25     it enters the Cached Data Lost state relative  to  all  hosts  to
+    26     which  it  is  currently  "Unit Online", regardless of which host
+    27     issued the command encountering the error.
+
+    28     The Cached Data Lost state is reflected in the Cached  Data  Lost
+    29     end  flag (DLF).  Refer to section 4.5.1 for a description of end
+    30     flags.
+
+    31     The end message of the next sequential  command  processed  after
+    32     the  occurrence  of  the Cached Data Lost condition will have its
+    33     DLF set and its status will reflect  the  nature  of  the  error.
+    34     Subsequent  commands  (including  those  currently  queued by the
+    35     controller) are rejected with DLF set and  a  status  of  Serious
+    36     Exception  in  their  end  messages.  The controller continues to
+    37     reject commands in that manner until the tape  unit  Cached  Data
+    38     Lost  state  is  cleared by a host.  Immediate commands processed
+    39     prior to the  reporting  to  the  host  of  a  Cached  Data  Lost
+    40     condition will reflect the previous state of the Cached Data Lost
+    41     flag.
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-27
+           Tape Caching                                      8 November 1987
+
+
+     1     The following commands are subject to rejection  while  a  Cached
+     2     Data Lost state exists:
+
+     3           o  ACCESS
+     4           o  COMPARE HOST DATA
+     5           o  ERASE
+     6           o  ERASE GAP
+     7           o  FLUSH
+     8           o  READ
+     9           o  REPOSITION
+    10           o  WRITE
+    11           o  WRITE TAPE MARK
+
+
+    12     The Clear Cached Data Lost modifier (CDL) is the means by which a
+    13     host  acknowledges the Cached Data Lost condition and signals the
+    14     controller that the tape unit  Cached  Data  Lost  State  can  be
+    15     cleared  and  normal  processing  resumed.   Note  that  the  CDL
+    16     modifier is acted upon before any other part of  a  command.   It
+    17     only  clears  the  Cached Data Lost state that existed before the
+    18     command was received (i.e., a command cannot clear a Cached  Data
+    19     Lost  it  caused).   The  TMSCP controller must return an Invalid
+    20     Command (subcode "Invalid  Modifier")  status  code  if  the  CDL
+    21     modifier  is  specified  and  a  Cached  Data Lost state does not
+    22     currently exist.
+
+    23     The  CDL  modifier  can  only  be  specified  for  the  following
+    24     commands:
+
+    25           o  ACCESS
+    26           o  AVAILABLE
+    27           o  COMPARE HOST DATA
+    28           o  ERASE
+    29           o  ERASE GAP
+    30           o  FLUSH
+    31           o  ONLINE
+    32           o  READ
+    33           o  REPOSITION
+    34           o  SET UNIT CHARACTERISTICS
+    35           o  WRITE
+    36           o  WRITE TAPE MARK
+
+
+    37     The CDL modifier is reserved for all other commands.
+
+    38     The existence of a Cached Data Lost state does not prevent  other
+    39     hosts  from bringing a tape unit into the "Unit Online" state via
+    40     the ONLINE command.  The tape unit will be  in  the  Cached  Data
+    41     Lost  state  relative  to  any  host  issuing a successful ONLINE
+    42     command while the Cached Data Lost state exists, unless  the  CDL
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-28
+           Tape Caching                                      8 November 1987
+
+
+     1     modifier  was specified in that ONLINE command.  In that case the
+     2     Cached Data Lost State is cleared for all hosts  which  have  the
+     3     tape unit in the "Unit Online" state.
+
+     4     If a tape unit leaves the "Unit Online"  state  relative  to  all
+     5     hosts,  for  any  reason,  the  tape unit will cease to be in the
+     6     Cached Data Lost state.
+
+     7     3.6.2  Read-Ahead Caching
+
+     8  |  TMSCP controllers should normally read-ahead and cache data  from
+     9  |  the  media  in anticipation of host read requests.  However, each
+    10  |  read request must  be  satisfied  by  a  distinct  physical  read
+    11  |  operation.   If  the  host  issues multiple read requests for the
+    12  |  same data, then the TMSCP controller must use a separate physical
+    13  |  read  operation  to  satisfy  each request.  If the host issues a
+    14  |  read request for data  that  has  been  invalidated  by  a  write
+    15  |  operation  subsequent  to  the  read-ahead  operation,  the TMSCP
+    16  |  controller must physically read the tape again.
+
+    17  |  Hosts can request that data be read from tape in one of two ways:
+
+    18  |        o  Issuing a "read" type command (ACCESS, COMPARE, READ).
+
+    19  |        o  Issuing a command with an explicit or  implicit  compare
+    20  |           operation.   That  is,  either  a command with a compare
+    21  |           modifier or a command  to  which  a  compare  unit  flag
+    22  |           applies.
+
+
+    23  |  Note that a read-compare operation is two distinct read requests.
+
+    24  |  A "write" type operation invalidates  all  read-ahead  data  read
+    25  |  from tape objects at positions (object count) equal to or greater
+    26  |  than the position (object count) of the "write" type operation.
+
+    27  |  This effectively means that  a  TMSCP  controller  must  use  the
+    28  |  following policies to manage its read-ahead cache:
+
+    29  |       1.  Only  data  physically  read  from   tape   enters   the
+    30  |           read-ahead  cache.   In  particular,  write  data  never
+    31  |           enters the read-ahead cache.
+
+    32  |       2.  Any data in the read-ahead cache that is used to satisfy
+    33  |           a  host  read  request  is  immediately deleted from the
+    34  |           cache,  and  is  not  available  to  satisfy  subsequent
+    35  |           requests.   If  multiple  physical  read operations have
+    36  |           entered multiple copies of the same data in  the  cache,
+    37  |           then only one copy need be deleted.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-29
+           Tape Caching                                      8 November 1987
+
+
+     1  |       3.  A  "write"  type  operation  causes  any  data  in   the
+     2  |           read-ahead  cache  read  from  objects  with  a position
+     3  |           (object count) greater than or  equal  to  the  position
+     4  |           (object  count)  of  the  "write"  type  operation to be
+     5  |           immediately deleted from the cache.
+
+     6  |       4.  In addition to the above, the TMSCP controller  may,  at
+     7  |           its   discretion,   delete  any  or  all  of  the  TMSCP
+     8  |           controller's read-ahead cached data.
+
+
+     9     Proper support for read-ahead caching requires  that  controlled,
+    10     predictable   results  be  specified  for  the  handling  of  the
+    11     following conditions:
+
+
+    12           o  Conflicting requirements imposed by unit flags.
+
+    13           o  Conflicting requirements imposed by command modifiers.
+
+    14           o  Data transfer errors during read ahead operations.
+
+
+    15     3.6.2.1  Unit Flag Conflicts
+
+    16     The  "Compare  Reads"  and  the  "Suppress  Caching"  unit  flags
+    17     override  the enabling of read-ahead caching.  There are no other
+    18     unit flags in conflict with the enabling of read-ahead caching.
+
+    19     3.6.2.2  Command Modifier Conflicts
+
+    20     The "Compare" modifier on READ commands overrides the enabling of
+    21     read-ahead  caching  for the duration of the command.  Read-ahead
+    22     caching may be re-established after the compare  portion  of  the
+    23     command  if  subsequent  commands  do not also have the "Compare"
+    24     modifier set.
+
+    25     The "Suppress Caching" modifier on "read" type commands overrides
+    26     the  enabling  of  read-ahead  caching  for  the  duration of the
+    27     specified command.   Read-ahead  caching  may  be  re-established
+    28     after  the  completion  of  the  specified  command if subsequent
+    29     "read" type commands do not  also  have  the  "Suppress  Caching"
+    30     modifier set.
+
+    31     The  "Suppress  Error  Recovery"  command  modifier  may   either
+    32     override  the  enabling of read-ahead caching for the duration of
+    33     the command, or be handled as specified in the following sections
+    34     (this approach assumes that the first unrecoverable or suppressed
+    35     error  recovery  data  transfer  error  terminates  any   further
+    36     processing).
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP GENERAL OPERATIONAL CHARACTERISTICS               Page 3-30
+           Tape Caching                                      8 November 1987
+
+
+     1     Read-ahead caching may be re-established if  subsequent  commands
+     2     do  not  also have the "Suppress Error Recovery" command modifier
+     3     set.
+
+     4     There are  no  other  command  modifiers  in  conflict  with  the
+     5     enabling of read-ahead caching.
+
+     6     3.6.2.3  Error Handling
+
+     7     TMSCP controllers are responsible for handling  all  errors  that
+     8     occur  during  read-ahead  caching.   TMSCP  controllers issue an
+     9     error log detailing the error that occurred (provided that  error
+    10     normally  requires  logging  [refer  to  Appendix  B]), when data
+    11     associated with the specific data record  causing  the  error  is
+    12     requested  by  the  host.  In order to accomplish this objective,
+    13     the TMSCP controller must make a reasonable assumption about  the
+    14     format  of  data  records  and tape marks written on the magnetic
+    15     tape.  This assumption about tape format will  be  formalized  in
+    16     the  definition  of  a properly formatted tape segment which is a
+    17     portion of magnetic  tape  starting  at  BOT  which  contains  an
+    18     arbitrary  number  of  data  records  and tape marks in arbitrary
+    19     order,  terminating  in  two  consecutive  tape  marks  with   no
+    20     intervening data records.  A properly formatted tape segment must
+    21     be written from BOT or may result from the arbitrary appending of
+    22     additional  data  records  and tape marks to a properly formatted
+    23     tape segment which also results  in  a  properly  formatted  tape
+    24     segment.   The TMSCP controller must terminate read-ahead caching
+    25     upon detection of the end of a properly  formatted  tape  segment
+    26     while  reading  in the forward direction.  Subsequent Host issued
+    27     "read" type commands  in  the  forward  direction  may  re-enable
+    28     read-ahead  caching.   In  addition,  TMSCP controllers may apply
+    29     other recording technology or implementation dependent mechanisms
+    30     in  order  to  properly  control  read-ahead caching and minimize
+    31     their error handling requirements.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-1
+                                                             8 November 1987
+
+
+     1                                 CHAPTER 4
+
+     2                          TMSCP MESSAGE OVERVIEW
+
+
+     3     4.1  Tape MSCP Functions Having No Disk MSCP Counterparts
+
+     4           o  ERASE GAP
+     5           o  FLUSH
+     6           o  REPOSITION
+     7           o  WRITE TAPE MARK
+
+
+     8     4.2  Identical Disk And Tape MSCP Functions
+
+     9     The following functions are identical in both Disk MSCP and  Tape
+    10     MSCP.  Their descriptions have been omitted from this document.
+
+    11           o  ABORT  (Refer  to  section   3.2.1.2   for   operational
+    12              clarifications)
+    13           o  ACCESS NON-VOLATILE MEMORY
+    14           o  DETERMINE ACCESS PATHS
+    15           o  GET COMMAND STATUS
+    16           o  SET CONTROLLER CHARACTERISTICS
+    17           o  INVALID COMMAND End Message
+    18           o  ACCESS PATH Attention Message
+    19           o  AVAILABLE Attention Message
+    20           o  DUPLICATE UNIT NUMBER Attention Message
+
+
+    21     4.3  Tape Specific MSCP Commands And Responses
+
+    22     Following is a list of Tape specific  MSCP  commands,  which  are
+    23     described in Chapter 5.
+
+    24           o  ACCESS
+    25           o  AVAILABLE
+    26           o  COMPARE HOST DATA
+    27           o  ERASE
+    28           o  ERASE GAP
+    29           o  GET UNIT STATUS
+    30           o  ONLINE
+    31           o  READ
+    32           o  REPOSITION
+    33           o  SET UNIT CHARACTERISTICS
+    34           o  WRITE
+    35           o  WRITE TAPE MARK
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-2
+           Generic Command Message Format                    8 November 1987
+
+
+     1     4.4  Generic Command Message Format
+
+     2     Refer to MSCP  for  a  general  description  of  command  message
+     3     formats.
+
+     4     4.4.1  Command Modifiers
+
+     5     Refer to MSCP for  a  general  description  of  command  modifier
+     6     operations.
+
+     7     The following  command  modifiers  are  supported  by  TMSCP  but
+     8     described in MSCP:
+
+     9           o  All Class Drivers
+    10           o  Enable Set Write Protect
+    11           o  Exclusive Access
+    12           o  Next Unit
+    13           o  Suppress Error Correction
+
+
+    14     The following command modifiers apply to a single TMSCP  command.
+    15     Their  operation  is  described  within  the  identified  command
+    16     description:
+
+    17           o  Detect LEOT - REPOSITION
+    18           o  Enable Re-write Error Recovery - WRITE
+    19           o  Object Count - REPOSITION
+    20           o  Rewind - REPOSITION
+    21           o  Unload - AVAILABLE
+
+
+    22     The following command modifiers apply to various TMSCP  commands.
+    23     Their  operation  is  described  within  the  identified  general
+    24     operation sections:
+
+    25           o  Clear Cached Data Lost - 3.6.1.4
+    26           o  Clear Serious Exception - 3.5.2
+    27           o  Compare - 3.4.3.4
+    28           o  Immediate  Completion  -  3.2  for   non-data   transfer
+    29              commands
+    30           o  Immediate Completion - 3.6 for data transfer commands
+    31           o  Reverse - 3.3
+    32           o  Suppress Caching - 3.6.2.2
+    33           o  Suppress Error Recovery - 3.5.5.1
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-3
+           End Message Format                                8 November 1987
+
+
+     1     4.5  End Message Format
+
+     2     Refer to MSCP for  a  general  description  of  the  end  message
+     3     format.
+
+     4     4.5.1  Flags
+
+     5     The following end flag is supported by TMSCP but defined in MSCP:
+
+     6           o  Error Log Generated
+
+     7     End flags specific to TMSCP  and  common  to  many  commands  are
+     8     defined here:
+
+     9           o  Cached Data Lost
+
+    10                  Set when cached data is lost  because  a  tape  unit
+    11                  entered the Serious Exception state while write-back
+    12                  caching is enabled.  Refer to  section  3.6.1.4  for
+    13                  further details.
+
+
+    14           o  EOT Encountered
+
+    15                  Set whenever the magnetic tape loaded on a drive  is
+    16                  currently  positioned  at  or  beyond EOT.  Refer to
+    17                  section 3.3 for further details.
+
+
+    18           o  Position Lost
+
+    19                  Set when a tape unit is in the Position Lost  state.
+    20                  Refer to section 3.5.3 for further details.
+
+
+    21           o  Serious Exception
+
+    22                  Set when a tape unit is  in  the  Serious  Exception
+    23                  state.  Refer to section 3.5.2 for further details.
+
+
+
+    24     4.5.2  Status
+
+    25     Refer to MSCP for a complete  description  of  the  status  field
+    26     format.
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-4
+           End Message Format                                8 November 1987
+
+
+     1     4.5.3  Status Codes
+
+     2     The following status codes are common to  both  MSCP  and  TMSCP.
+     3     Refer to MSCP for a complete description:
+
+     4           o  Command Aborted
+     5           o  Controller Error
+     6           o  Drive Error
+     7           o  Host Buffer Access Error
+     8           o  Invalid Command (subcode "Invalid Unit Flags")
+     9           o  Unit-Available
+    10           o  Unit-Offline
+
+
+    11     Major status codes that are TMSCP specific  are  described  here.
+    12     Status  codes  specific  to  a single TMSCP command are described
+    13     with the associated command.
+
+    14           o  BOT Encountered
+
+    15                  This status code will be returned  whenever  BOT  is
+    16                  unexpectedly  encountered  during the execution of a
+    17                  reverse read type command.  See section 3.3.1.2  for
+    18                  more details.
+
+
+    19           o  Compare Error
+
+    20                  This  status  code  will  be  returned  whenever   a
+    21                  difference is found between the tape record data and
+    22                  the host buffer data during a compare operation.  It
+    23                  will  also  be  returned  if  a short tape record is
+    24                  encountered during a COMPARE HOST  DATA  or  compare
+    25                  modified READ command.
+
+
+    26           o  Data Error (subcode "Long Gap Encountered")
+
+    27                  This status code will be returned when a long gap is
+    28                  detected  while  attempting  to  execute a read type
+    29                  command (i.e., ACCESS, COMPARE HOST  DATA,  READ  or
+    30                  REPOSITION).    Tape  motion  ceases  and  the  tape
+    31                  remains positioned at  the  end  of  the  long  gap.
+    32                  Refer  to  Appendix  C,  Table  C-4  for tape format
+    33                  dependent long gap values.
+
+
+    34           o  Data Error (subcode "Unrecoverable Read Error")
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-5
+           End Message Format                                8 November 1987
+
+
+     1                  This status code will be returned:  1) if  the  tape
+     2                  format   cannot   be   determined   from   the  TFIA
+     3                  information during first read density selection; or,
+     4                  2) if invalid or uncorrectable data is obtained from
+     5                  the tape  unit,  as  determined  by  internal  error
+     6                  detecting or correcting codes.
+
+
+     7           o  Formatter Error
+
+     8                  This  status  will  be  returned  when  a  formatter
+     9                  reports   an   internally   detected  error  to  the
+    10                  controller.  The subcode  will  identify  the  exact
+    11                  cause of the error.
+
+
+    12           o  Invalid Command (subcode "Invalid Byte Count")
+
+    13                  The command specified a byte count greater than  the
+    14                  absolute  maximum  allowed by the controller.  Refer
+    15                  to Appendix C, Table C-3 for specifics.
+
+
+    16           o  Media Format Error
+
+    17                  This status code will be returned whenever the media
+    18                  is  formatted in a manner that cannot be written nor
+    19                  read by the controller (i.e., if  a  disk  formatted
+    20                  media   is   accessed  by  TMSCP  on  a  combination
+    21                  TMSCP/MSCP device).
+
+
+    22           o  Position Lost
+
+    23                  This status  code  will  be  returned  whenever  the
+    24                  controller, formatter, or tape unit loses context of
+    25                  the tape position (object count).
+
+    26  |                                       NOTE
+
+    27  |                   The intent of this status code is to  report
+    28  |                   the  loss of tape position when further tape
+    29  |                   motion  operations  are   believed   to   be
+    30  |                   possible  by  the  unit.  If an error occurs
+    31  |                   that is not recoverable and prevents further
+    32  |                   tape   motion   commands   (i.e.,  the  tape
+    33  |                   breaks), then the appropriate drive or  data
+    34  |                   error  should  be  reported as the status of
+    35  |                   the  command,  rather  than  a   status   of
+    36  |                   Position  Lost.   The Position Lost end flag
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-6
+           End Message Format                                8 November 1987
+
+
+     1  |                   should still be used as appropriate.
+
+
+
+     2           o  Record Data Truncated
+
+     3                  This error code will be returned whenever the length
+     4                  of  a  tape  record  read  is greater than the value
+     5                  supplied  in  the  "byte   count"   field   of   the
+     6                  corresponding command message.
+
+
+     7           o  Serious Exception
+
+     8                  The command is rejected because  the  tape  unit  is
+     9                  currently  in  the  Serious  Exception  state or the
+    10                  Cached Data Lost state.  Refer to section 3.5.2  and
+    11                  section 3.6.1.4 for further details.
+
+
+    12           o  Success (subcode "EOT Encountered")
+
+    13                  This status code will be returned  whenever  EOT  is
+    14                  encountered  during  the  execution  of a write type
+    15                  command (i.e.,  ERASE  GAP,  WRITE,  or  WRITE  TAPE
+    16                  MARK).   The  command  will  complete  in the normal
+    17                  fashion after EOT is encountered.
+
+
+    18           o  Tape Mark Encountered
+
+    19                  This status code will be returned  whenever  a  tape
+    20                  mark   is   unexpectedly   encountered   during  the
+    21                  execution of a  read  type  command.   See  sections
+    22                  3.3.1.4 and 5.10.3 for more details.
+
+
+    23           o  Write Protected
+    24                  (subcode "Unit is Hardware Write Protected")
+    25                  (subcode "Unit is Software Write Protected")
+    26                  (subcode "Unit is Data Safety Write Protected")
+
+    27                      This status will be returned if the tape unit is
+    28                      found  to  be hardware, software, or data safety
+    29                      Write Protected when a command requiring a write
+    30                      operation  is  issued.   The command is rejected
+    31                      immediately and no tape motion will occur.   The
+    32                      subcode   reflects   the   tape  unit  hardware,
+    33                      software and data safety write protect state(s),
+    34                      maintained by the controller.
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-7
+           End Message Format                                8 November 1987
+
+
+     1     The status codes that may be returned for each  command  and  any
+     2     special  meaning  that  they  have  are listed in the command end
+     3     message descriptions in Chapter 5.
+
+     4     4.5.4  Byte Count (Host Transfer)
+
+     5     This field, returned in the COMPARE HOST DATA,  READ,  and  WRITE
+     6     command  end  messages, reflects the number of bytes SUCCESSFULLY
+     7     compared or transferred.  The count begins with  the  first  byte
+     8     transferred to/from the tape record and ceases at either the byte
+     9     count specified in the command message or the byte count  of  the
+    10     first error encountered, if any.
+
+    11     If the controller cannot initiate the command for any reason  the
+    12     value returned in this field is zero.
+
+    13     If  an  error  occurs,  the  controller  must  have  SUCCESSFULLY
+    14     compared  or  transferred  all data up to the point identified by
+    15     "byte count".  Furthermore, the error identified by "status" must
+    16     have  actually  occurred at the byte position identified by "byte
+    17     count".
+
+    18     READ or WRITE commands  that  include  a  compare  operation  are
+    19     special  cases.   Such  a command makes two passes over the data:
+    20     one for  the  original  transfer  and  another  for  the  compare
+    21     operation.   For  most  errors,  there  is no way to determine in
+    22     which pass the error was detected.  Therefore, the only guarantee
+    23     is  that  error-free  transfer  was  performed  up  to  the point
+    24     identified by "byte count".  The compare operation may or may not
+    25     have  been  performed  up to that point.  If a "Compare Error" is
+    26     reported, both the original transfer and  the  compare  operation
+    27     have  been  successfully  performed up to the point identified by
+    28     "byte count".
+
+    29                                   NOTE
+
+    30             The state of a compare or transfer following  the
+    31             byte  position  identified  by  "byte  count"  is
+    32             undefined, since:
+
+    33             1.  None of the transfer following  "byte  count"
+    34                 may have been performed or attempted.
+
+    35             2.  All of  it  may  have  been  attempted  (with
+    36                 unknown success).
+
+    37             3.  Some parts may have been attempted and others
+    38                 not.
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP MESSAGE OVERVIEW                                   Page 4-8
+           End Message Format                                8 November 1987
+
+
+     1             This implies that the compare pass of a  transfer
+     2             command may be performed for the entire transfer,
+     3             or portions thereof.
+
+
+     4     4.5.5  Byte Count (Tape Record)
+
+     5     This field, returned in the ACCESS, COMPARE HOST DATA, READ,  and
+     6     WRITE   command  end  messages,  reflects  the  number  of  bytes
+     7     contained in the tape record.  This field is only  valid  if  the
+     8     controller  can determine the actual size of the tape record.  If
+     9     the controller is unable to determine the tape record  size,  the
+    10     value returned in this field is zero.
+
+    11     4.5.6  Position (Object Count)
+
+    12     This field, returned in the ACCESS, COMPARE HOST DATA, ERASE GAP,
+    13     FLUSH,  READ,  REPOSITION,  WRITE and WRITE TAPE MARK command end
+    14     messages, reflects the current position of the tape in  terms  of
+    15     object  counts  forward  of  BOT.   Because  of the potential for
+    16     command  pipelining  within  the  controller  and/or   the   tape
+    17     formatter,  the  most  currently  reported "object count" may lag
+    18     behind the actual physical drive  position.   Nevertheless,  this
+    19     information is sufficient for host recovery from power failure or
+    20     position lost situations.
+
+    21     All caching units must report Position in a manner that makes the
+    22     caching  transparent to higher levels.  Thus, the contents of the
+    23     Position field is the position  on  tape,  in  terms  of  objects
+    24     forward of BOT, of the object whose end message has most recently
+    25     been generated for the indicated unit.  I.e.,  a  unit  caches  a
+    26     WRITE  command,  and  sends  an end message indicating successful
+    27     completion.  In  that  and  subsequent  end  messages,  the  unit
+    28     reports position as if the WRITE had already taken place, when in
+    29     reality the WRITE may still be waiting in the cache.
+
+    30     When a "position lost" condition is encountered or the tape  unit
+    31     is  in  the  Position  Lost  state  (refer to section 3.5.3), the
+    32     controller will return its  "best  guess"  object  count  in  the
+    33     Position  field.  The "best guess" can be obtained by setting the
+    34     object count to be the last known (good) position when  the  unit
+    35     first  enters the Position Lost state.  Subsequent positioning of
+    36     the drive causes corresponding changes to the "best guess" object
+    37     count.
+
+    38     This field must be valid when the tape unit is "Unit Online"  and
+    39     not  in  the  Position  Lost  state.  This field must contain the
+    40     unit's "best guess" object count when  the  tape  unit  is  "Unit
+    41     Online" and in the Position Lost state.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-1
+                                                             8 November 1987
+
+
+     1                                 CHAPTER 5
+
+     2                        TMSCP COMMAND DESCRIPTIONS
+
+
+     3     5.1  ACCESS Command
+
+     4     Command Category:
+
+     5              Sequential
+
+
+     6     5.1.1  Command Message Format
+
+     7                     31                             0 
+     8                     +-------------------------------+
+     9                     |    command reference number   |
+    10                     +---------------+---------------+
+    11                     |    reserved   |  unit number  |
+    12                     +---------------+-------+-------+
+    13                     |   modifiers   |  rsvd | opcode|
+    14                     +---------------+-------+-------+
+    15                     |           byte count          |
+    16                     +-------------------------------+
+
+
+    17     Allowable Modifiers:
+
+    18              Clear Cached Data Lost
+    19              Clear Serious Exception
+    20              Reverse (see section 3.3.1.1)
+    21              Suppress Caching
+    22              Suppress Error Correction
+    23              Suppress Error Recovery
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-2
+           ACCESS Command                                    8 November 1987
+
+
+     1     5.1.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |                               |
+    11                     +----                       ----+
+    12                     |                               |
+    13                     +----       undefined       ----+
+    14                     |                               |
+    15                     +----                       ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |    position  (object count)   |
+    19                     +-------------------------------+
+    20                     |    byte count (tape record)   |
+    21                     +-------------------------------+
+
+
+    22     Status Codes:
+
+    23              Success
+    24                  (subcode "Normal")
+
+    25              BOT Encountered
+    26              Command Aborted
+    27              Controller Error
+    28              Data Error
+    29              Drive Error
+    30              Formatter Error
+
+    31              Invalid Command
+    32                  (subcode "Invalid Byte Count")
+    33                  (subcode "Illegal Modifier") (see section 3.3.1.1)
+
+    34              Position Lost
+    35              Record Data Truncated
+    36              Serious Exception
+    37              Tape Mark Encountered
+    38              Unit-Available
+    39              Unit-Offline
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-3
+           ACCESS Command                                    8 November 1987
+
+
+     1     Flags:
+
+     2              Cached Data Lost
+     3              EOT Encountered
+     4              Error Log Generated
+     5              Position Lost
+     6              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-4
+           ACCESS Command                                    8 November 1987
+
+
+     1     5.1.3  Description
+
+     2     Beginning at the current position, data is read from the magnetic
+     3     tape  loaded  on the specified unit, checked for errors, and then
+     4     discarded.
+
+     5     Refer to sections 3.3, 3.4, and 3.5 for additional detail.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-5
+           AVAILABLE Command                                 8 November 1987
+
+
+     1     5.2  AVAILABLE Command
+
+     2     Command Category:
+
+     3              Sequential - Immediate Completion
+
+
+     4     5.2.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable Modifiers:
+
+    14              All Class Drivers (ignored in single host case)
+    15              Clear Cached Data Lost
+    16              Clear Serious Exception
+    17              Exclusive Access
+    18              Unload
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-6
+           AVAILABLE Command                                 8 November 1987
+
+
+     1     5.2.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status Codes:
+
+    11              Success
+
+    12                  (subcode "Normal")
+
+    13                  (subcode "Still Connected")
+
+    14                      The "still connected" subcode is returned if the
+    15                      tape   unit  may  potentially  be  accessed  via
+    16                      another controller  but  some  other  tape  unit
+    17                      which  shares  the  same  access  path  is still
+    18                      "Unit-Online" to another host.
+
+                           
+    19                  (subcode "Still Online")
+
+    20                      The "still online" subcode  indicates  that  the
+    21                      tape   unit   is  now  "Unit-Available"  to  the
+    22                      requesting Host, but  the  tape  unit  is  still
+    23                      "Unit-Online" to another Host.
+
+
+    24                  (subcode "Still Online/Unload Ignored")
+
+    25                      The "Still  Online/Unload  Ignored"  subcode  is
+    26                      returned if the "Unload" modifier was set in the
+    27                      command message and  the  unload  was  inhibited
+    28                      because  the tape unit is still "Unit Online" to
+    29                      another host.
+
+
+    30              Command Aborted
+    31              Controller Error
+    32              Drive Error
+    33              Exclusive Access
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-7
+           AVAILABLE Command                                 8 November 1987
+
+
+     1              Formatter Error
+     2              Serious Exception
+     3              Unit-Offline
+
+     4     Flags:
+
+     5              Cached Data Lost
+     6              EOT Encountered
+     7              Error Log Generated
+     8              Position Lost
+     9              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-8
+           AVAILABLE Command                                 8 November 1987
+
+
+     1     5.2.3  Description
+
+     2     All outstanding host commands are completed after which the  tape
+     3     unit  is  placed  in  the  "Unit Available" state relative to the
+     4     host.
+
+     5     If the tape unit is not "Unit Online" to any other host, the tape
+     6     is rewound to BOT.
+
+     7     If the "Unload" command modifier is set and the tape unit is  not
+     8     "Unit  Online" to any other host (i.e., "Still Online"), the tape
+     9     will be unloaded, rendering the tape unit  "Unit-Offline".   Note
+    10     that   a  "Still  Connected"  condition  will  not  inhibit  tape
+    11     unloading, if unload was requested.
+
+    12     If the "Unload" command modifier was not set, the  tape  unit  is
+    13     not  already "Unit-Available" and no other tape units sharing the
+    14     same access path are "Unit-Online" (i.e., the  "Still  Connected"
+    15     status  subcode  flag  is  clear),  then  an  AVAILABLE attention
+    16     message is sent by any other controller to which the tape unit is
+    17     connected.   The  controller  to which this command was sent need
+    18     not send an AVAILABLE attention message.
+
+    19     This command will be accepted if the tape unit is  "Unit  Online"
+    20     or  "Unit  Available".  There is no point in issuing this command
+    21     to a "Unit Available" tape unit without specifying  the  "Unload"
+    22     modifier.
+
+    23     Refer to section 3.2 for additional information.
+
+    24     The actions of this command with the  Exclusive  Access  modifier
+    25     are described in the MSCP specification.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                               Page 5-9
+           COMPARE HOST DATA Command                         8 November 1987
+
+
+     1     5.3  COMPARE HOST DATA Command
+
+     2     Command Category:
+
+     3              Sequential
+
+
+     4     5.3.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+    13                     |           byte count          |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---          buffer         ---+
+    17                     |                               |
+    18                     +---        descriptor       ---+
+    19                     |                               |
+    20                     +-------------------------------+
+
+
+    21     Allowable Modifiers:
+
+    22              Clear Cached Data Lost
+    23              Clear Serious Exception
+    24              Reverse (see section 3.3.1.1)
+    25              Suppress Caching
+    26              Suppress Error Correction
+    27              Suppress Error Recovery
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-10
+           COMPARE HOST DATA Command                         8 November 1987
+
+
+     1     5.3.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |   byte count (host transfer)  |
+    11                     +-------------------------------+
+    12                     |                               |
+    13                     +----                       ----+
+    14                     |           undefined           |
+    15                     +----                       ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |    position  (object count)   |
+    19                     +-------------------------------+
+    20                     |    byte count (tape record)   |
+    21                     +-------------------------------+
+
+
+    22     Status Codes:
+
+    23              Success
+    24                  (subcode "Normal")
+
+    25              BOT Encountered
+    26              Command Aborted
+    27              Compare Error
+    28              Controller Error
+    29              Data Error
+    30              Drive Error
+    31              Formatter Error
+    32              Host Buffer Access Error
+
+    33              Invalid Command
+    34                  (subcode "Invalid Byte Count")
+    35                  (subcode "Illegal Modifier") (see section 3.3.1.1)
+
+    36              Position Lost
+    37              Record Data Truncated
+    38              Serious Exception
+    39              Tape Mark Encountered
+    40              Unit-Available
+    41              Unit-Offline
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-11
+           COMPARE HOST DATA Command                         8 November 1987
+
+
+     1     Flags:
+
+     2              Cached Data Lost
+     3              EOT Encountered
+     4              Error Log Generated
+     5              Position Lost
+     6              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-12
+           COMPARE HOST DATA Command                         8 November 1987
+
+
+     1     5.3.3  Description
+
+     2     Beginning at the current position, data is read from a record  on
+     3     the  magnetic tape and compared to the data in the specified host
+     4     buffer.
+
+     5     Refer to sections 3.3, 3.4, and 3.5 for additional detail.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-13
+           ERASE Command                                     8 November 1987
+
+
+     1     5.4  ERASE Command
+
+     2     Command Category:
+
+     3              Sequential (optionally Immediate Completion)
+
+
+     4     5.4.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   | rsvd  | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable Modifiers:
+
+    14              Clear Cached Data Lost
+    15              Clear Serious Exception
+
+    16              Immediate Completion
+
+    17                  If this  modifier  is  set,  this  command  will  be
+    18                  executed   in  an  asynchronous  manner.   Refer  to
+    19                  section 3.2 for complete details.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-14
+           ERASE Command                                     8 November 1987
+
+
+     1     5.4.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status Codes:
+
+    11              Success
+    12                  (subcode "Normal")
+
+    13              Command Aborted
+    14              Controller Error
+    15              Drive Error
+    16              Formatter Error
+    17              Serious Exception
+    18              Unit-Available
+    19              Unit-Offline
+
+    20              Write Protected
+    21                  (subcode "Unit is Hardware Write Protected")
+    22                  (subcode "Unit is Software Write Protected")
+    23                  (subcode "Unit is Data Safety Write Protected")
+
+
+
+    24     Flags:
+
+    25              Cached Data Lost
+    26              EOT Encountered
+    27              Error Log Generated
+    28              Position Lost
+    29              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-15
+           ERASE Command                                     8 November 1987
+
+
+     1     5.4.3  Description
+
+     2     Tape objects are erased starting at  the  current  tape  position
+     3     through  PEOT  (see  chapter  2).  The tape is then positioned to
+     4     BOT.
+
+     5     Refer to sections 3.3 and 3.5 for additional detail.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-16
+           ERASE GAP Command                                 8 November 1987
+
+
+     1     5.5  ERASE GAP Command
+
+     2     Command Category:
+
+     3              Sequential (optionally Immediate Completion)
+
+
+     4     5.5.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable Modifiers:
+
+    14              Clear Cached Data Lost
+    15              Clear Serious Exception
+
+    16              Immediate Completion
+
+    17                  If  this  modifier  is  set,  this  command  may  be
+    18                  executed  as  a "write-back" caching enabled "write"
+    19                  type command.  Refer to section 3.6.1  for  complete
+    20                  details.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-17
+           ERASE GAP Command                                 8 November 1987
+
+
+     1     5.5.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+
+
+    10     Status Codes:
+
+    11              Success
+    12                  (subcode "Normal")
+    13                  (subcode "EOT Encountered")
+
+    14              Command Aborted
+    15              Controller Error
+    16              Drive Error
+    17              Formatter Error
+    18              Position Lost
+    19              Serious Exception
+    20              Unit-Available
+    21              Unit-Offline
+                           
+    22              Write Protected
+    23                  (subcode "Unit is Hardware Write Protected")
+    24                  (subcode "Unit is Software Write Protected")
+    25                  (subcode "Unit is Data Safety Write Protected")
+
+
+
+    26     Flags:
+
+    27              Cached Data Lost
+    28              EOT Encountered
+    29              Error Log Generated
+    30              Position Lost
+    31              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-18
+           ERASE GAP Command                                 8 November 1987
+
+
+     1     5.5.3  Description
+
+     2     Beginning at the current tape position, a format dependent record
+     3     gap is recorded on the magnetic tape loaded on the specified tape
+     4     unit.  This command is useful for  bypassing  tape  defects  when
+     5     write error recovery is host controlled.
+
+     6     Refer to sections 3.3 and 3.5 for additional detail.
+
+     7                                  WARNING
+
+     8             The use of this  command  may  cause  undesirable
+     9             interaction   with   formatter   directed   error
+    10             recovery  sequences  resulting  in   non-standard
+    11             record  gaps.  Affected tapes may not be readable
+    12             on all systems.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-19
+           FLUSH Command                                     8 November 1987
+
+
+     1     5.6  FLUSH Command
+
+     2     Command Category:
+
+     3              Sequential
+
+
+     4     5.6.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable Modifiers:
+
+    14              Clear Cached Data Lost
+    15              Clear Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-20
+           FLUSH Command                                     8 November 1987
+
+
+     1     5.6.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |                               |
+    11                     +----                       ----+
+    12                     |                               |
+    13                     +----       undefined       ----+
+    14                     |                               |
+    15                     +----                       ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |    position  (object count)   |
+    19                     +-------------------------------+
+
+
+    20     Status Codes:
+
+    21              Success
+    22                  (subcode "Normal")
+    23                  (subcode "EOT Encountered")
+
+    24              Command Aborted
+    25              Controller Error
+    26              Data Error
+    27              Formatter Error
+    28              Position Lost
+    29              Serious Exception
+    30              Unit-Available
+    31              Unit-Offline
+
+    32              Write Protected
+    33                  (subcode "Unit is Hardware Write Protected")
+    34                  (subcode "Unit is Software Write Protected")
+    35                  (subcode "Unit is Data Safety Write Protected")
+
+
+
+    36     Flags:
+
+    37              Cached Data Lost
+    38              EOT Encountered
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-21
+           FLUSH Command                                     8 November 1987
+
+
+     1              Error Log Generated
+     2              Position Lost
+     3              Serious Exception
+
+
+     4     Position (Object Count):
+
+     5              This field, returned in the FLUSH command  end  message,
+     6              reflects  the  current  position of the tape in terms of
+     7              object counts forward of BOT.  Because of the  potential
+     8              for  command pipelining within the controller and/or the
+     9              tape formatter,  the  most  currently  reported  "object
+    10              count"   may   lag  behind  the  actual  physical  drive
+    11              position.  Nevertheless, this information is  sufficient
+    12              for  host  recovery  from power failure or position lost
+    13              situations when write-back caching is enabled.
+
+    14              When a "position lost" condition is encountered  or  the
+    15              tape  unit  is  in  the  Position  Lost  state (refer to
+    16              3.5.3), the controller will return the last known (good)
+    17              "object count".
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-22
+           FLUSH Command                                     8 November 1987
+
+
+     1     5.6.3  Description
+
+     2  |  The FLUSH command is used to ensure that  all  previously  issued
+     3  |  cached  "write"  type commands have fully completed.  Hosts would
+     4     normally   use   this   command   to   establish   or    maintain
+     5     synchronization  with  "write"  type commands being issued to the
+     6     specified tape unit.
+
+     7  |  The end  message  to  FLUSH  command  is  not  issued  until  all
+     8  |  previously issued sequential commands have fully completed.  This
+     9  |  means that the data for all previous "write"  type  commands  has
+    10  |  been  successfully  written  to  media,  or  a  Cached  Data Lost
+    11  |  condition has caused the write-back cache  to  be  cleared.   The
+    12     order  in  which the write-back cached data is transferred to the
+    13     specified tape unit is the exactly the same as the order in which
+    14  |  the  write-back  cached  "write"  type  commands occurred.  TMSCP
+    15  |  controllers should continue to fetch and  buffer  host  data  for
+    16  |  subsequent  "write"  type  commands, provided that such action is
+    17  |  required in order to ensure high performance (i.e.,  required  to
+    18  |  keep  the unit streaming).  The TMSCP controller must be prepared
+    19  |  to take the appropriate action with such pre-fetched data in  the
+    20  |  event  of  a  Cached  Data  Lost  condition or similar unexpected
+    21  |  event.  The TMSCP controller must also preserve sequentiality  of
+    22  |  such  subsequent  "write" type commands with the FLUSH command as
+    23  |  well as all other sequential commands.
+
+    24     Refer to sections 3.3, 3.4, 3.5, and 3.6 for additional detail.
+
+
+
+
+
+
+
+
+
+
+
+
+        
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-23
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1     5.7  GET UNIT STATUS Command
+
+     2     Command Category:
+
+     3              Immediate
+
+
+     4     5.7.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable Modifiers:
+
+    14              Next Unit
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                           
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-24
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1     5.7.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |   unit flags  |multi-unit code|
+    11                     +-------------------------------+
+    12                     |            reserved           |
+    13                     +-------------------------------+
+    14                     |                               |
+    15                     +---     unit identifier     ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |     media type identifier     |
+    19                     +---------------+---------------+
+    20                     |     speed     |     format    |
+    21                     +-------+-------+---------------+
+    22                     |  rsvd |freecap|  format menu  |
+    23                     +-------+-----------------------+
+    24                     | uhvrsn| usvrsn| fhvrsn| fsvrsn|
+    25                     +-------+-------+-------+-------+
+
+
+    26     Status Codes:
+
+    27              Success
+
+    28                  (subcode "Normal") = Unit is "UNIT ON-LINE"
+
+    29                  (subcode "Duplicate Unit Number")
+
+    30                      Implies that the tape unit is "Unit-Online", but
+    31                      that  its  unit  number  is identical to that of
+    32                      some other unit known to  the  controller.   The
+    33                      tape unit will become "Unit-Offline", due to the
+    34                      duplicate Unit Number, as soon as it  ceases  to
+    35                      be "Unit-Online".
+
+
+    36                  (subcode "Read-only Volume Format")
+
+    37                      The unit only implements  read  access  for  the
+    38                      format  of  the volume mounted in the unit.  For
+    39                      example, a single-density volume is mounted in a
+    40                      double-density drive that can read but not write
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-25
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1                      it.  Note that the  unit  will  be  Data  Safety
+     2                      Write   Protected   whenever   this  subcode  is
+     3                      returned.
+
+
+     4              Unit-Available
+     5              Unit-Offline
+
+
+     6     Flags:
+
+     7              Cached Data Lost
+     8              Exclusive Access
+     9              EOT Encountered
+    10              Position Lost
+    11              Serious Exception
+
+
+    12     Multi-Unit code:
+
+    13              The low byte of this field identifies  the  access  path
+    14              between the controller and the tape unit.  The high byte
+    15              identifies the particular tape unit on the access path.
+
+    16              This  field  must  be  valid  when  the  returned   Unit
+    17              Identifier is non-zero.
+
+    18     Unit Flags:
+
+    19              Caching Support
+
+    20                  A non-host settable characteristic, set only if  the
+    21                  tape unit supports tape write-back caching.
+
+    22                  This flag must  be  valid  when  the  returned  Unit
+    23                  Identifier is non-zero.
+
+
+    24              Compare Reads
+
+    25                  A host settable characteristic, enabled either  with
+    26                  ONLINE  or  SET  UNIT  CHARACTERISTICS commands.  If
+    27                  set, all READ  commands  will  be  verified  with  a
+    28                  compare  operation  (equivalent  to  specifying  the
+    29                  compare modifier on each command).
+
+    30                  This flag must  be  valid  when  the  tape  unit  is
+    31                  "Unit-Online".
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-26
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1              Compare Writes
+
+     2                  A host settable characteristic, enabled either  with
+     3                  ONLINE  or  SET  UNIT  CHARACTERISTICS Commands.  If
+     4                  set, all WRITE operations will be compare verified.
+
+     5                  This flag must  be  valid  when  the  tape  unit  is
+     6                  "Unit-Online".
+
+
+     7              Data Safety Write Protect
+
+     8                  A non-host  settable  characteristic,  set  whenever
+     9                  some  condition  in  the  unit  or  volume  prevents
+    10                  reliable  modification  of  data  on   the   volume.
+    11                  Possible causes include:
+
+    12                   o  An invalid volume format.
+
+    13                   o  The unit is only capable of reading the volume's
+    14                      format.   I.e.,  a  single-density  volume  in a
+    15                      double-density drive.
+
+
+    16                  Note that there can be multiple causes  for  a  unit
+    17                  being  Data  Safety  Write  Protected.  Furthermore,
+    18                  some causes represent errors while others  represent
+    19                  normal occurrences.
+
+    20                  Each individual cause can only be established when a
+    21                  unit   is  brought  "Unit-Online".   Causes  can  be
+    22                  eliminated - possibly allowing  the  unit  to  cease
+    23                  being  Data  Safety Write Protected - while the unit
+    24                  remains "Unit-Online".  However, the server can only
+    25                  establish a new cause for the unit to be Data Safety
+    26                  Write  Protected  by  forcing   the   unit   to   be
+    27                  "Unit-Available"  to  all class drivers.  Thus class
+    28                  drivers  are  guaranteed  that  a  unit   will   not
+    29                  spontaneously  become  Data  Safety  Write Protected
+    30                  while the unit is "Unit-Online".
+
+    31                  This flag must  be  valid  when  the  tape  unit  is
+    32                  "Unit-Online".
+
+
+    33              Enhanced Write Error Recovery
+
+    34                  A non-host settable characteristic, set only if  the
+    35                  tape   unit  supports  the  "Enable  Re-write  Error
+    36                  Recovery" command modifier on WRITE commands.
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-27
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1                  This flag must  be  valid  when  the  returned  Unit
+     2                  Identifier is non-zero.
+
+
+     3              Hardware Write Protect
+
+     4                  A non-host settable characteristic, set if the  tape
+     5                  unit's write protect mechanism is activated, causing
+     6                  the tape unit to be hardware write protected.
+
+     7                  This flag must be valid when the tape unit is either
+     8                  "Unit-Available" or "Unit-Online".
+
+
+     9              Software Write Protect
+
+    10                  A host settable characteristic, enabled or  disabled
+    11                  either  with  the ONLINE or SET UNIT CHARACTERISTICS
+    12                  commands.  If set, the tape unit will act as  though
+    13                  it  were  hardware  write  protected.  If clear, the
+    14                  tape unit is virtually  write  enabled.   This  flag
+    15                  cannot override hardware write protection.
+
+    16                  This flag must  be  valid  when  the  tape  unit  is
+    17                  "Unit-Online".
+
+
+    18              Suppress Caching
+
+    19                  A host settable characteristic, enabled or disabled,
+    20                  either  with  an  ONLINE or SET UNIT CHARACTERISTICS
+    21                  command.   If   set,   caching   using   the   TMSCP
+    22                  controller's  volatile  cache  is  disabled for this
+    23                  unit.  If clear, read-ahead caching using the  TMSCP
+    24                  controller's  volatile  cache  is  enabled  for this
+    25                  unit.   Equivalent  to  specifying   the   "Suppress
+    26                  Caching"  modifier  on  all "read" type commands for
+    27                  which it is legal.
+
+    28                  This flag must  be  valid  when  the  tape  unit  is
+    29                  "Unit-Online".
+
+
+    30              Variable Speed Mode Suppression
+
+    31                  A host settable characteristic, enabled or  disabled
+    32                  either  with  an  ONLINE or SET UNIT CHARACTERISTICS
+    33                  command.  If set, a variable speed tape unit's speed
+    34                  change  algorithm  is  disabled,  the tape unit will
+    35                  operate at a fixed  speed.   If  clear,  a  variable
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-28
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1                  speed tape unit's speed change algorithm is enabled,
+     2                  the tape unit's operating speed may vary.  This flag
+     3                  is always clear for a fixed speed tape unit.
+
+     4                  This flag must  be  valid  when  the  tape  unit  is
+     5                  "Unit-Online".
+
+
+     6              Variable Speed Unit
+
+     7                  A non-host settable characteristic, set only if  the
+     8                  tape   unit   has   variable  speed  capability.   A
+     9                  controller or  formatter  directed  automatic  speed
+    10                  change  algorithm (based on host data throughput) is
+    11                  employed for tape units with this capability.
+
+    12                  This flag must  be  valid  when  the  returned  Unit
+    13                  Identifier is non-zero.
+
+
+    14              Write Back
+
+    15                  A host settable characteristic, enabled or  disabled
+    16                  either  with  an  ONLINE or SET UNIT CHARACTERISTICS
+    17                  command.   If  set,  write-back  caching  using  the
+    18                  controller's  volatile  cache  is  enabled  for this
+    19                  unit.   Equivalent  to  specifying  the   "Immediate
+    20                  Completion"  modifier  on  all "write" type commands
+    21                  for which it is legal.
+
+    22                  This flag must  be  valid  when  the  tape  unit  is
+    23                  "Unit-Online".
+
+
+    24     Unit Identifier:
+
+    25              This field identifies a unique  device  number  for  the
+    26              tape  unit  as  defined in the MSCP specification.  This
+    27              field must be valid and non-zero when the unit is in any
+    28              of the following states:
+
+    29                  "Unit-Available", all sub-states.
+    30                  "Unit-Online", all sub-states.
+    31                  "Unit-Offline", the following sub-states:
+    32                      "Duplicate unit number"
+    33                      "Exclusive use"
+    34                      "No  media  mounted  or  disabled   via   switch
+    35                      setting"
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-29
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1     Media Type Identifier:
+
+     2              This field identifies the type of  media  used  by  this
+     3              tape  unit,  for  use  by Host Generic device allocation
+     4              mechanisms.
+
+     5              This  field  must  be  valid  when  the  returned   Unit
+     6              Identifier is non-zero.
+
+     7     Format:
+
+     8              The tape format currently employed  on  the  tape  unit.
+     9              The  value in the low byte of this field is a binary bit
+    10              representing the nominal or formatted density  of  tape.
+    11              The  high byte is a code value associated with the media
+    12              type.  The high byte always has the  same  value  for  a
+    13              given unit.  (Refer to Appendix C, Tables C-1 and C-2.)
+
+    14              This  field  must  be  valid  when  the  tape  unit   is
+    15              "Unit-Online".
+
+    16     Speed:
+
+    17              The  value  in  this   field   is   expressed   as   the
+    18              instantaneous data rate in kilo-bytes per second.
+
+    19              For 9-track tape units it is computed, without rounding,
+    20              as follows:
+
+    21                  Speed = ( ips * bpi) / 1000
+
+    22                  where:
+
+    23                  ips = Tape Transport speed in Inches per Second
+
+    24                  bpi = nominal  recording  format  frame  density  in
+    25                  linear bits per inch
+
+
+    26              For a fixed speed tape unit,  the  value  returned  will
+    27              indicate the speed at which the tape unit operates.
+
+    28              The value returned for a variable  speed  tape  unit  is
+    29              dependent  upon  the  state  of the "variable speed mode
+    30              suppression" unit flag.  If that flag is set, the  value
+    31              returned  will  indicate the tape unit's fixed speed set
+    32              via an ONLINE or SET UNIT CHARACTERISTICS  command.   If
+    33              clear,  the  value  returned  will  indicate the highest
+    34              speed for the current density, unless the tape  unit  is
+    35              actively  performing I/O.  Then the value is the current
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-30
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1              operating speed, set by the speed change algorithm.
+
+     2              This  field  must  be  valid  when  the  tape  unit   is
+     3              "Unit-Online".
+
+     4     Format Menu:
+
+     5              The high byte of this field is a code  value  associated
+     6              with the media type.  This value is constant for a given
+     7              device.  The low byte of this field contains a  menu  of
+     8              available  tape  unit  operating  formats  expressed  as
+     9              binary bit values.  Each bit in the low byte  represents
+    10              unique  nominal  or formatted density in bits per linear
+    11              inch of tape.  (Refer to  Appendix  C,  Tables  C-1  and
+    12              C-2.)
+
+    13              This  field  must  be  valid  when  the  returned   Unit
+    14              Identifier is non-zero.
+
+    15     Freecap:
+
+    16              This field  indicates  the  amount  of  media  currently
+    17              available  for  writing as a function of the total media
+    18              size.
+
+    19              The following values of this field have special meaning.
+    20              If  the  value is reported, it must be for that meaning.
+    21              If one of the meanings is to be  reported,  it  must  be
+    22              with  the  corresponding  value.  Devices are allowed to
+    23              always report zero in this field.
+
+    24              0       The amount of media available is unknown.   This
+    25                      is  for  backwards  compatibility  with existing
+    26                      TMSCP devices.
+
+    27              1       There is no space available before PEOT.
+
+    28              251     The entire media is available (i.e., new tape).
+
+    29              252-255 Reserved.
+
+    30              Values from  2  to  251  have  an  approximately  linear
+    31              relationship to the fraction of free capacity available.
+    32              A larger value indicates a larger free capacity.
+
+    33                                        NOTE
+
+    34                  Due  to  bad  spots,  extended   gaps   due   to
+    35                  streaming,  or other format and device dependent
+    36                  anomalies, the value of  this  field  is  almost
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-31
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1                  certainly    optimistic.    Because   of   these
+     2                  anomalies, the controller may,  at  its  option,
+     3                  choose  not to use the full 8 bits of resolution
+     4                  in reporting free capacity.  The controller  may
+     5                  also,  at  its  option,  attempt  to account for
+     6                  these anomalies.
+
+
+     7              Devices reporting values other than zero in  this  field
+     8              must   document   in   their  device  specification  the
+     9              algorithm that is used to generate the value.
+
+    10              It is strongly recommended that all new devices  support
+    11              this feature.
+
+    12              This  field  must  be  valid  when  the  tape  unit   is
+    13              "Unit-Online".
+
+    14     Fsvrsn:
+
+    15              The  formatter's  software,   firmware,   or   microcode
+    16              revision   number.    Always   zero   if  the  product's
+    17              development and maintainability  groups  determine  that
+    18              there  is  no  benefit  from supporting machine readable
+    19              revision numbers.
+
+    20              This  field  must  be  valid  when  the  returned   Unit
+    21              Identifier is non-zero.
+
+    22     Fhvrsn:
+
+    23              The formatter's hardware revision number.   Always  zero
+    24              if  the product's development and maintainability groups
+    25              determine that  there  is  no  benefit  from  supporting
+    26              machine readable revision numbers.
+
+    27              This  field  must  be  valid  when  the  returned   Unit
+    28              Identifier is non-zero.
+
+    29     Usvrsn:
+
+    30              The unit's software,  firmware,  or  microcode  revision
+    31              number.   Always  zero  if the product's development and
+    32              maintainability  groups  determine  that  there  is   no
+    33              benefit   from   supporting  machine  readable  revision
+    34              numbers.
+
+    35              This  field  must  be  valid  when  the  returned   Unit
+    36              Identifier is non-zero.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-32
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1     Uhvrsn:
+
+     2              The unit's hardware revision number.  Always zero if the
+     3              product's   development   and   maintainability   groups
+     4              determine that  there  is  no  benefit  from  supporting
+     5              machine readable revision numbers.
+
+     6              This  field  must  be  valid  when  the  returned   Unit
+     7              Identifier is non-zero.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-33
+           GET UNIT STATUS Command                           8 November 1987
+
+
+     1     5.7.3  Description
+
+     2     The GET UNIT STATUS command returns the current connection  state
+     3     of  a  tape  unit as well as unit characteristics.  Note that the
+     4     validity of the unit characteristics is dependent on the  current
+     5     connection state.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-34
+           ONLINE Command                                    8 November 1987
+
+
+     1     5.8  ONLINE Command
+
+     2     Command Category:
+
+     3              Sequential
+
+
+     4     5.8.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+    13                     |   unit flags  |    reserved   |
+    14                     +---------------+---------------+
+    15                     |                               |
+    16                     +---                         ---+
+    17                     |            reserved           |
+    18                     +---                         ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |  device dependent parameters  |
+    22                     +---------------+---------------+
+    23                     |     speed     |     format    |
+    24                     +---------------+---------------+
+
+
+    25     Allowable Modifiers:
+
+    26              Clear Cached Data Lost
+    27              Clear Serious Exception
+
+    28              Enable Set Write Protect
+
+    29                  Causes the "Write Protect"  unit  flag  to  be  host
+    30                  settable.   This  modifier  causes  the state of the
+    31                  "Software Write Protect" unit flag to be  copied  to
+    32                  the  controller's  software  write  protect flag for
+    33                  this tape unit.
+
+    34              Exclusive Access
+
+
+
+                           
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-35
+           ONLINE Command                                    8 November 1987
+
+
+     1     Unit Flags:
+
+     2              If the tape unit is already "Unit-Online" to  any  class
+     3              driver,  then  the  class  driver  must specify the same
+     4              values for controller supported host settable unit flags
+     5              as  are  currently  in  effect  on  the  tape unit.  The
+     6              controller may or may not, at its option, check that the
+     7              flag  values  are  the  same.  If it does check, then it
+     8              must return an Invalid Command  (subcode  "Invalid  Unit
+     9              Flags")  status  code  if  they  are  different.  If the
+    10              controller does not check that they are the  same,  then
+    11              it  must  ignore  the  unit flags specified by the class
+    12              driver and  preserve  the  flag  settings  currently  in
+    13              effect  on  the  tape  unit.   Note  that  this checking
+    14              becomes  mandatory,  rather  than   optional,   if   the
+    15              controller provides Multi-Host Support.
+
+    16              Refer to section 5.11.1 for a description of  individual
+    17              flag usage.
+
+
+    18     Device Dependent Parameters:
+    19     Format:
+    20     Speed:
+
+    21              Refer to section 5.11.1 for a description of  the  usage
+    22              of these fields.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-36
+           ONLINE Command                                    8 November 1987
+
+
+     1     5.8.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |   unit flags  |multi-unit code|
+    11                     +---------------+---------------+
+    12                     |            reserved           |
+    13                     +-------------------------------+
+    14                     |                               |
+    15                     +---     unit identifier     ---+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |     media type identifier     |
+    19                     +---------------+---------------+
+    20                     |     speed     |     format    |
+    21                     +---------------+---------------+
+    22                     |    maximum write byte count   |
+    23                     +---------------+---------------+
+    24                     |    reserved   |  noise record |
+    25                     +---------------+---------------+
+
+
+    26     Status Codes:
+
+    27              Success
+
+    28                  (subcode "Normal") - tape unit brought online.
+
+    29                  (subcode "Already Online")
+
+    30                      tape  unit  is   already   online,   state   and
+    31                      characteristics  are not altered.  When the tape
+    32                      unit is already  "Unit-Online",  the  controller
+    33                      merely  returns  the Unit Characteristics in the
+    34                      End message  with  this  status  flag  bit  set,
+    35                      without performing any other actions.
+
+
+    36                  (subcode "Read-only Volume Format")
+
+    37                      The unit only implements  read  access  for  the
+    38                      format  of  the volume mounted in the unit.  For
+    39                      example, a single-density volume is mounted in a
+    40                      double-density drive that can read but not write
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-37
+           ONLINE Command                                    8 November 1987
+
+
+     1                      it.  Note that the  unit  will  be  Data  Safety
+     2                      Write   Protected   whenever   this  subcode  is
+     3                      returned.
+
+
+     4              Command Aborted
+
+     5                  The tape unit's state is unchanged.  The  host  must
+     6                  assume  that the return characteristics are invalid,
+     7                  since the tape unit may be "Unit-Offline".
+
+     8              Controller Error
+     9              Drive Error
+    10              Exclusive Access
+    11              Formatter Error
+
+    12              Invalid Command
+
+    13                  (subcode "Invalid Unit Flags")
+
+    14                      The tape unit is already "Unit-Online" and those
+    15                      host settable characteristics that are supported
+    16                      by  the  controller  are  different  from  those
+    17                      currently  in effect on the tape unit.  The tape
+    18                      unit remains "Unit-Online" and the host settable
+    19                      Unit  Flags  are  NOT  changed.   Note  that the
+    20                      controller  returns  an  Invalid   Command   End
+    21                      message if a reserved Unit Flag is set.
+
+
+    22                  (subcode "Invalid Format")
+
+    23                      More than  one  Format  was  specified,  or  the
+    24                      Format  specified  is  not supported by the tape
+    25                      unit, or the tape unit is already  "Unit-Online"
+    26                      and  the  value supplied in the Format field was
+    27                      neither a zero nor the Format specified  by  the
+    28                      host  that  initially brought the tape unit into
+    29                      the "Unit-Online" state.   The  original  format
+    30                      selected remains unchanged.
+
+
+    31              Media Format Error
+    32              Unit-Offline
+
+    33     Flags:
+
+    34              Cached Data Lost
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-38
+           ONLINE Command                                    8 November 1987
+
+
+     1              EOT Encountered
+     2              Error Log Generated
+     3              Position Lost
+     4              Serious Exception
+
+     5     Multi-Unit code:
+     6     Unit Flags:
+     7     Unit Identifier:
+     8     Media Type Identifier:
+     9     Format:
+    10     Speed:
+
+    11              Refer to section 5.7.2 for descriptions of these fields.
+
+    12     Recommended Maximum WRITE Byte Count:
+
+    13              The largest size record likely to have  acceptable  data
+    14              reliability   statistics,   considering   the   selected
+    15              recording mode.  This  limit  is  not  enforced  by  the
+    16              controller   although  the  controller  does  enforce  a
+    17              maximum controller dependent limit.
+
+    18              This  field  must  be  valid  when  the  tape  unit   is
+    19              "Unit-Online".
+
+    20     Noise Record:
+
+    21              A non-host-settable characteristic advising the host  of
+    22              the  record  length  at  or  below which legitimate data
+    23              records are subject to confusion with noise records.  If
+    24              zero,  the tape unit's recording technology is immune to
+    25              noise records.
+
+    26              This  field  must  be  valid  when  the  tape  unit   is
+    27              "Unit-Online".
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-39
+           ONLINE Command                                    8 November 1987
+
+
+     1     5.8.3  Description
+
+     2     The ONLINE Command  is  used  to  bring  a  tape  unit  into  the
+     3     "Unit-Online"  state, set host settable unit characteristics, and
+     4     obtain those unit characteristics that are essential  for  proper
+     5     class driver operation.
+
+     6     Host settable characteristics are set exactly as if  a  SET  UNIT
+     7     CHARACTERISTICS command was issued (refer to section 5.11).  Host
+     8     settable characteristics are not set  until  tape  unit  specific
+     9     validity checks have succeeded, if applicable.  The host settable
+    10     characteristics are not altered  if  the  tape  unit  is  already
+    11     "Unit-Online".
+
+    12     If the tape unit is not already "Unit-Online"  to  another  host,
+    13     then  a  successful ONLINE command will position the tape to BOT,
+    14     if it is  not  already  there.   If  the  tape  unit  is  already
+    15     "Unit-Online"  to  another host, then a successful ONLINE command
+    16     will not alter the position of the tape.
+
+    17     The actions of this command with the  Exclusive  Access  modifier
+    18     are described in the MSCP specification.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-40
+           READ Command                                      8 November 1987
+
+
+     1     5.9  READ Command
+
+     2     Command Category:
+
+     3              Sequential
+
+
+     4     5.9.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+    13                     |           byte count          |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---          buffer         ---+
+    17                     |                               |
+    18                     +---        descriptor       ---+
+    19                     |                               |
+    20                     +-------------------------------+
+
+
+    21     Allowable Modifiers:
+
+    22              Clear Cached Data Lost
+    23              Clear Serious Exception
+    24              Compare
+    25              Reverse (see section 3.3.1.1)
+    26              Suppress Caching
+    27              Suppress Error Correction
+    28              Suppress Error Recovery
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-41
+           READ Command                                      8 November 1987
+
+
+     1     5.9.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |   byte count (host transfer)  |
+    11                     +-------------------------------+
+    12                     |                               |
+    13                     +----                       ----+
+    14                     |           undefined           |
+    15                     +----                       ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |    position  (object count)   |
+    19                     +-------------------------------+
+    20                     |    byte count (tape record)   |
+    21                     +-------------------------------+
+
+
+    22     Status Codes:
+
+    23              Success
+    24                  (subcode "Normal")
+
+    25              BOT Encountered
+    26              Command Aborted
+    27              Compare Error
+    28              Controller Error
+    29              Data Error
+    30              Drive Error
+    31              Formatter Error
+    32              Host Buffer Access Error
+
+    33              Invalid Command
+    34                  (subcode "Invalid Byte Count")
+    35                  (subcode "Illegal Modifier") (see section 3.3.1.1)
+
+    36              Position Lost
+    37              Record Data Truncated
+    38              Serious Exception
+    39              Tape Mark Encountered
+    40              Unit-Available
+    41              Unit-Offline
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-42
+           READ Command                                      8 November 1987
+
+
+     1     Flags:
+
+     2              Cached Data Lost
+     3              EOT Encountered
+     4              Error Log Generated
+     5              Position Lost
+     6              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-43
+           READ Command                                      8 November 1987
+
+
+     1     5.9.3  Description
+
+     2     Beginning at the current position, data is read from a record  on
+     3     the magnetic tape loaded on the specified unit and transferred to
+     4     the specified host buffer.
+
+     5     Refer to sections 3.3, 3.4, and 3.5 for additional detail.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-44
+           REPOSITION Command                                8 November 1987
+
+
+     1     5.10  REPOSITION Command
+
+     2     Command Category:
+
+     3              Sequential  (REWIND  to  BOT  variation  is   optionally
+     4              Immediate Completion)
+
+
+     5     5.10.1  Command Message Format
+
+     6                     31                             0 
+     7                     +-------------------------------+
+     8                     |    command reference number   |
+     9                     +---------------+---------------+
+    10                     |    reserved   |  unit number  |
+    11                     +---------------+-------+-------+
+    12                     |   modifiers   |  rsvd | opcode|
+    13                     +---------------+-------+-------+
+    14                     |  record count or object count |
+    15                     +-------------------------------+
+    16                     |        tape mark count        |
+    17                     +-------------------------------+
+
+
+    18     Allowable Modifiers:
+
+    19              Clear Cached Data Lost
+    20              Clear Serious Exception
+
+    21              Detect LEOT
+
+    22                  If this modifier is set, and the "Reverse"  modifier
+    23                  is  clear,  and either or both a Skip Tape Mark or a
+    24                  Skip Record positioning function  is  also  enabled,
+    25                  the  automatic detection of LEOT (i.e., two adjacent
+    26                  tape  marks  with  no   intervening   records),   as
+    27                  described in section 5.10.3, is enabled.
+
+    28              Immediate Completion
+
+    29                  If this modifier is set, and only the Rewind to  BOT
+    30                  positioning  function  is enabled, the Rewind to BOT
+    31                  will be executed in an asynchronous  manner.   Refer
+    32                  to section 3.2.1.1 for additional detail.
+
+    33              Object Count
+
+    34                  The setting of this modifier and the value specified
+    35                  in  the "record count or object count" and the "tape
+    36                  mark  count"  command  message  fields  selects  the
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-45
+           REPOSITION Command                                8 November 1987
+
+
+     1                  skipping operation to be performed:
+
+     2                   o  If set, and the "record count or  object  count"
+     3                      field  is  non-zero,  the Skip Tape Object (both
+     4                      records and tape marks) positioning function  is
+     5                      enabled.
+
+     6                   o  If clear, and the "record count or object count"
+     7                      field  is  non-zero, the Skip Record positioning
+     8                      function is enabled.
+
+     9                   o  If clear, and the "tape  mark  count"  field  is
+    10                      non-zero,   the   Skip   Tape  Mark  positioning
+    11                      function is enabled.
+
+
+    12                  All of the positioning functions mentioned above are
+    13                  described in section 5.10.3.
+
+    14              Reverse
+
+    15              Rewind
+
+    16                  If  this  modifier  is  set,  the  rewind   to   BOT
+    17                  positioning  function,  described in section 5.10.3,
+    18                  is enabled.
+
+    19              Suppress Caching
+    20              Suppress Error Correction
+    21              Suppress Error Recovery
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-46
+           REPOSITION Command                                8 November 1987
+
+
+     1     5.10.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |   records skipped/undefined   |
+    11                     +-------------------------------+
+    12                     |  tape marks skipped/undefined |
+    13                     +-------------------------------+
+    14                     |                               |
+    15                     +----       undefined       ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |    position (object count)    |
+    19                     +-------------------------------+
+
+
+    20     Status Codes:
+
+    21              Success
+    22                  (subcode "Normal")
+
+    23              BOT Encountered
+    24              Command Aborted
+    25              Controller Error
+
+    26              Data Error
+    27                  (subcode "Long Gap Encountered")
+
+    28              Drive Error
+    29              Formatter Error
+    30              LEOT Encountered
+    31              Position Lost
+    32              Serious Exception
+    33              Tape Mark Encountered
+    34              Unit-Available
+    35              Unit-Offline
+
+    36     Flags:
+
+    37              Cached Data Lost
+    38              EOT Encountered
+    39              Error Log Generated
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-47
+           REPOSITION Command                                8 November 1987
+
+
+     1              Position Lost
+     2              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-48
+           REPOSITION Command                                8 November 1987
+
+
+     1     5.10.3  Description
+
+     2     This command is used to position the tape from  one  location  to
+     3     another.
+
+     4     For additional  information  related  to  the  operation  of  the
+     5     REPOSITION command refer to the following:
+
+     6           o  Chapter 2 Terminology, especially the definitions of:
+
+     7               >  Record
+     8               >  Tape Mark
+     9               >  Object
+    10               >  Record Gap
+    11               >  Beginning-of-Tape (BOT)
+    12               >  End-of-Tape (EOT)
+    13               >  Logical End-of-Tape (LEOT)
+
+
+    14           o  Sections 3.2 Command  Categories  And  Execution  Order,
+    15              3.2.1    Lengthy    Command    Considerations,   3.2.1.1
+    16              Synchronous Vs Asynchronous, and 3.2.1.2 Abort.
+
+    17           o  Sections 3.3.1 (Tape Motion  Command  Operation)  "read"
+    18              Type,  3.3.1.1  Direction, 3.3.1.2 BOT Handling, 3.3.1.3
+    19              EOT Handling, and 3.3.1.4 Tape Mark Handling.
+
+    20           o  Section 3.4.2 Tape Format Employed.
+
+    21           o  Section 3.5 General Error Processing and all  associated
+    22              sub-sections.
+
+    23           o  Sections 4.5.1 (End Message Format) Flags, 4.5.2 Status,
+    24              4.5.3  Status  Codes; especially those listed in Section
+    25              5.10.2, and 4.5.6 Position (Object Count).
+
+
+    26     The manner in which the positioning operation is to be  performed
+    27     is dictated by the state of the following command modifiers:
+
+    28           o  "Object Count"
+    29           o  "Rewind"
+    30           o  "Reverse"
+
+    31     and the values supplied in the  "tape  mark  count"  and  "record
+    32     count or object count" command message fields.
+
+    33     In addition, the following two special case modifiers affect  the
+    34     manner in which the positioning operation will complete:
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-49
+           REPOSITION Command                                8 November 1987
+
+
+     1           o  "Immediate Completion"
+     2           o  "Detect LEOT"
+
+
+     3     Four positioning functions are available for use:
+
+     4           o  Rewind to BOT
+     5           o  Skip Tape Mark
+     6           o  Skip Record
+     7           o  Skip Tape Object (i.e., both records and tape marks).
+
+
+     8     Either of the  following  mutually  exclusive  sets  of  combined
+     9     positioning  functions  can  be  specified  in  a  single command
+    10     message:
+
+    11           o  Rewind to BOT
+    12           o  Skip Tape Mark
+    13           o  Skip Record
+
+    14              or
+
+    15           o  Rewind to BOT
+    16           o  Skip Tape Object
+
+
+    17     When combined, the controller must execute the set of positioning
+    18     functions synchronously as a single atomic command.  Furthermore,
+    19     the functions within a set must be  executed  in  the  sequential
+    20     order shown above, with the following conditional exceptions:
+
+    21          1.  If the function which is next in line for  execution  is
+    22              not  enabled the controller must continue execution with
+    23              the next function which is enabled (if any).
+
+    24          2.  If a function is terminated due to the detection  of  an
+    25              error   or   a   special   condition  (e.g.,  tape  mark
+    26              unexpectedly encountered, LEOT condition satisfied,  BOT
+    27              unexpectedly  encountered, etc.) the controller must not
+    28              attempt to execute any remaining enabled functions.
+
+    29          3.  If none of the functions are enabled the controller,  as
+    30              implied, must take no positioning action.
+
+    31              The  last  case  listed  is   actually   considered   an
+    32              additional function, a sequential no-operation.
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-50
+           REPOSITION Command                                8 November 1987
+
+
+     1     A model of operation for  each  of  the  available  functions  is
+     2     described below.  Controller dependent algorithms formulated with
+     3     the intent of optimizing the positioning functions are  expressly
+     4     permitted,  provided the models of operation are not perturbed in
+     5     any manner.
+
+     6                                   NOTE
+
+     7             Occurrence of an  error  which  prevents  further
+     8             continuation  of  tape  unit  operation while the
+     9             controller is performing a  positioning  function
+    10             optimization   may  result  in  a  tape  position
+    11             outside the  range  of  objects  spanned  by  the
+    12             REPOSITION function.  In this case the controller
+    13             must first report the error in the normal  manner
+    14             and  then  place  the  tape unit in the "Position
+    15             Lost" state, if appropriate.
+
+
+    16     Table D-1 shows all the possible  variations  of  the  REPOSITION
+    17     command  which  can  be  specified.   Table  D-1 also depicts the
+    18     conditions under which the modifiers and count field  values  are
+    19     ignored.   Those  conditions  are  not  addressed in the model of
+    20     operation descriptions.
+
+    21           o  Rewind to BOT
+
+    22                  The Rewind to BOT positioning  function  is  enabled
+    23                  when the "Rewind" modifier is set.
+
+    24                  When this function is enabled and the  tape  is  not
+    25                  already  resting  at BOT, the controller starts tape
+    26                  motion in the reverse direction  and  continues  the
+    27                  tape  motion until either BOT is reached or an error
+    28                  occurs.
+
+    29                  If the "Immediate Completion" modifier is set and no
+    30                  other  positioning function is enabled the rewind to
+    31                  BOT  is  completed  in  an  asynchronous  manner  as
+    32                  described in Section 3.2.1.1.
+
+
+    33           o  Skip Tape Mark
+
+    34                  The Skip Tape Mark positioning function  is  enabled
+    35                  when  the  "Object  Count" modifier is clear and the
+    36                  "tape mark count" command message field is non-zero.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-51
+           REPOSITION Command                                8 November 1987
+
+
+     1                  When this function is enabled the controller  starts
+     2                  tape  motion,  in  the  direction  indicated  by the
+     3                  "Reverse" modifier, and  begins  searching  for  the
+     4                  occurrence of tape marks.
+
+     5                  The controller continues the search until one of the
+     6                  following occurs:
+
+     7                  1.  An   error   is    detected    (including    BOT
+     8                      Encountered).
+
+     9                  2.  A tape mark which satisfies the LEOT  condition,
+    10                      if  LEOT  detection  was enabled (see discussion
+    11                      below) is detected.
+
+    12                  3.  The number of tape marks specified in the  "tape
+    13                      mark  count"  command  message  field  have been
+    14                      found.
+
+
+    15                  During the tape mark search the controller will keep
+    16                  a  count  of  the  number of tape marks found (Note:
+    17                  records found are not counted during the  tape  mark
+    18                  search).
+
+    19                  LEOT detection is  enabled  only  if  the  "Reverse"
+    20                  modifier is clear.  If LEOT detection is enabled the
+    21                  controller will determine that the  tape  mark  just
+    22                  found  satisfies the LEOT condition if either of the
+    23                  following is true:
+
+    24                  1.  The previous object  found  during  the  current
+    25                      search was also a tape mark.
+
+    26                  2.  Based  on   context   (the   controller)   saved
+    27                      following  execution  of  the last command which
+    28                      caused tape motion, the tape was left positioned
+    29                      forward (i.e., on the EOT side of BOT) of a tape
+    30                      mark.
+
+    31                      In  the  latter  case  the  saved  context  must
+    32                      specifically  indicate that the last tape motion
+    33                      command was one of the following:
+
+    34                      a.  An  ACCESS,  COMPARE  HOST  DATA,  READ,  or
+    35                          REPOSITION  (with  Skip  Record  positioning
+    36                          function enabled) command performed  in  the
+    37                          forward  direction,  which terminated with a
+    38                          status of Tape Mark Encountered.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-52
+           REPOSITION Command                                8 November 1987
+
+
+     1                      b.  A REPOSITION (with Skip Tape  Mark  function
+     2                          enabled   and   the   Skip  Record  function
+     3                          disabled) command performed in  the  forward
+     4                          direction  which completed successfully when
+     5                          that tape mark was found.
+
+
+
+     6                  The controller performs the following  actions  upon
+     7                  detecting LEOT:
+
+     8                  1.  Stops tape motion.
+
+     9                  2.  Resumes tape motion in the reverse direction and
+    10                      positions  the tape to the record gap separating
+    11                      the two adjacent tape marks.
+
+    12                  3.  Stops tape motion once again.
+
+
+    13                  The controller must not  add  the  tape  mark  which
+    14                  satisfies  the  LEOT condition to the number of tape
+    15                  marks found count.
+
+
+    16           o  Skip Record
+
+    17                  The Skip Record positioning function is enabled when
+    18                  the "Object Count" modifier is clear and the "record
+    19                  count or object  count"  command  message  field  is
+    20                  non-zero.
+
+    21                  When this function is enabled the controller  starts
+    22                  tape  motion  (or  continues tape motion if the Skip
+    23                  Tape Mark  function  was  just  completed),  in  the
+    24                  direction  indicated  by the "Reverse" modifier, and
+    25                  begins searching for the occurrence of records.
+
+    26                  The controller continues the search until one of the
+    27                  following occurs:
+
+    28                  1.  An   error   is    detected    (including    BOT
+    29                      Encountered).
+
+    30                  2.  A tape mark is detected.
+
+    31                  3.  The number of records specified in  the  "record
+    32                      count  or  object  count"  command message field
+    33                      have been found.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-53
+           REPOSITION Command                                8 November 1987
+
+
+     1                  During the record search the controller will keep  a
+     2                  count of the number of records found.
+
+     3                  The action the  controller  follows  if  the  record
+     4                  search  is  terminated  by  detecting  a  tape  mark
+     5                  depends on whether  LEOT  detection  is  enabled  or
+     6                  disabled:
+
+     7                   o  If LEOT Detection  is  disabled  (i.e.,  "Detect
+     8                      LEOT"  modifier  clear  or "Reverse" modifier is
+     9                      set, regardless of the setting  of  the  "Detect
+    10                      LEOT"  modifier)  the tape mark is considered as
+    11                      having been unexpectedly encountered.   In  this
+    12                      case the controller must add the unexpected tape
+    13                      mark to the number of tape marks counted  during
+    14                      the   immediately   preceding   Skip  Tape  Mark
+    15                      function, or, set the tape mark count to one  if
+    16                      the Skip Tape Mark function was not enabled.
+
+    17                   o  If LEOT detection is enabled (i.e., the  "Detect
+    18                      LEOT" modifier is set and the "Reverse" modifier
+    19                      is clear) the controller must determine  whether
+    20                      or   not   the  tape  mark  satisfies  the  LEOT
+    21                      condition as described in  the  Skip  Tape  Mark
+    22                      function  above.   If  LEOT  was  satisfied  the
+    23                      controller performs the LEOT  detected  actions,
+    24                      again,  as  described  in  the  Skip  Tape  Mark
+    25                      function above.  Otherwise,  the  tape  mark  is
+    26                      unexpected   and  the  controller  performs  the
+    27                      unexpected   tape   mark    actions    described
+    28                      immediately above.
+
+
+
+    29           o  Skip Tape Object (i.e., both records and tape marks)
+
+    30                  The Skip Tape Object positioning function is enabled
+    31                  when  the  "Object  Count"  modifier  is set and the
+    32                  "record count or object count" command message field
+    33                  is non-zero.
+
+    34                  When this function is enabled the controller  starts
+    35                  tape  motion,  in  the  direction  indicated  by the
+    36                  "Reverse" modifier, and  begins  searching  for  the
+    37                  occurrence of tape objects.
+
+    38                  The controller continues the search until one of the
+    39                  following occurs:
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-54
+           REPOSITION Command                                8 November 1987
+
+
+     1                  1.  An   error   is    detected    (including    BOT
+     2                      Encountered).
+
+     3                  2.  The number of  tape  objects  specified  in  the
+     4                      "record  count  or object count" command message
+     5                      field have been found.
+
+
+     6                  During the tape object search  the  controller  need
+     7                  not  keep  a count of the number of records and tape
+     8                  marks found.
+
+
+     9           o  Sequential No-operation
+
+    10                  The Sequential No-operation is enabled when none  of
+    11                  the positioning functions are enabled.
+
+    12                  The controller must not initiate any tape motion.
+
+
+
+    13     After executing the  enabled  functions  to  the  fullest  extent
+    14     possible  (i.e.,  terminate  execution due to the detection of an
+    15     error or special condition, or complete execution successfully by
+    16     satisfying  the  count  value(s) etc.), the controller prepares a
+    17     REPOSITION command end message in  standard  MSCP/TMSCP  fashion,
+    18     with exception of the following specific actions:
+
+    19           o  Where a Rewind to BOT function with no other positioning
+    20              function enabled and the "immediate completion" modifier
+    21              clear was executed, the controller  will  set  both  the
+    22              "tape  marks  skipped"  and  "records skipped" fields to
+    23              zero.
+
+    24           o  Where a Rewind to BOT function with no other positioning
+    25              function enabled and the "immediate completion" modifier
+    26              set was executed, the controller will set both the "tape
+    27              marks  skipped" and "records skipped" fields to zero and
+    28              set the "status" field to Success (subcode "Normal").
+
+    29           o  Where  a  Skip  Tape  Mark  positioning   function   was
+    30              executed,  the  controller  will  set  the  "tape  marks
+    31              skipped" field equal to the number of tape  marks  found
+    32              during  execution of the Skip Tape Mark function or as a
+    33              result  of   the   subsequent   Skip   Record   function
+    34              unexpectedly encountering a tape mark.  If the Skip Tape
+    35              Mark function was not executed and a tape mark  was  not
+    36              unexpectedly  encountered  in the subsequent Skip Record
+    37              function the controller will set the "tape marks"  field
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-55
+           REPOSITION Command                                8 November 1987
+
+
+     1              to zero.
+
+     2           o  Where a Skip Record positioning  function  was  executed
+     3              the  controller  will  set  the  "records skipped" field
+     4              equal to the number of records found  value  accumulated
+     5              during  execution  of  the function.  If the Skip Record
+     6              function was not executed the controller  will  set  the
+     7              "records skipped" field to zero.
+
+     8           o  If either a Skip Tape Mark or  Skip  Record  positioning
+     9              function  terminated due to detecting LEOT (as described
+    10              above) the controller will set the "status" field  equal
+    11              to LEOT Detected.
+
+    12           o  Where a Skip Object positioning  function  was  executed
+    13              the  controller  may set the "records skipped" and "tape
+    14              marks skipped" fields to any value it  chooses.   (Those
+    15              fields are undefined in this case.)
+
+    16           o  For a Sequential No-operation the  controller  will  set
+    17              the  "status"  field  to  Success (subcode "Normal") and
+    18              both the "tape  marks  skipped"  and  "records  skipped"
+    19              fields to zero.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-56
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1     5.11  SET UNIT CHARACTERISTICS Command
+
+     2     Command Category:
+
+     3              Sequential
+
+
+     4     5.11.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+    13                     |   unit flags  |    reserved   |
+    14                     +---------------+---------------+
+    15                     |                               |
+    16                     +---                         ---+
+    17                     |            reserved           |
+    18                     +---                         ---+
+    19                     |                               |
+    20                     +-------------------------------+
+    21                     |  device dependent parameters  |
+    22                     +---------------+---------------+
+    23                     |     speed     |     format    |
+    24                     +---------------+---------------+
+
+
+    25     Allowable Modifiers:
+
+    26              Clear Cached Data Lost
+    27              Clear Serious Exception
+    28              Enable Set Write Protect
+    29              Exclusive Access
+
+    30     Unit Flags:
+
+    31              Caching Support
+
+    32                  A non-host settable characteristic, set only if  the
+    33                  tape unit supports tape write-back caching.
+
+    34                  This  flag  is  invalid  when  the   returned   Unit
+    35                  Identifier is zero.
+
+                           
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-57
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1              Compare Reads
+
+     2                  If set, all READ commands will be  verified  with  a
+     3                  compare  operation.   Equivalent  to  specifying the
+     4                  compare modifier on each such command.
+
+
+     5              Compare Writes
+
+     6                  If set, then all WRITE  commands  will  be  verified
+     7                  with  a compare operation.  Equivalent to specifying
+     8                  the Compare modifier on each such command.
+
+
+     9              Enhanced Write Error Recovery
+
+    10                  A non-host settable characteristic, set only if  the
+    11                  tape   unit  supports  the  "Enable  Re-write  Error
+    12                  Recovery" command modifier on WRITE commands.
+
+    13                  This  flag  is  invalid  when  the   returned   Unit
+    14                  Identifier is zero.
+
+
+    15              Software Write Protect (SWP)
+
+    16                  If the SWP unit flag is  set  and  the  "Enable  Set
+    17                  Write  Protect"  command  modifier  is also set, the
+    18                  tape unit will act as though it were Hardware  Write
+    19                  Protected.
+
+    20                  If the SWP unit flag is clear and  the  "Enable  Set
+    21                  Write  Protect"  command  modifier  is set, the tape
+    22                  unit is virtually write enabled.
+
+    23                  This flag cannot override hardware write protection.
+
+
+    24              Suppress Caching
+
+    25                  If  set,  caching  using  the   TMSCP   controller's
+    26                  volatile cache is disabled for this unit.  If clear,
+    27                  read-ahead  caching  using  the  TMSCP  controller's
+    28                  volatile cache is enabled for this unit.  Equivalent
+    29                  to  specifying  the   "Suppress   Caching"   command
+    30                  modifier on all "read" type commands for which it is
+    31                  legal.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-58
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1              Variable Speed Mode Suppression
+
+     2                  If set, a variable speed tape  unit's  speed  change
+     3                  algorithm is disabled.  The tape unit speed is fixed
+     4                  at  the  speed  specified  in  the   "speed"   field
+     5                  (described  below).  If clear, a variable speed tape
+     6                  unit's speed change algorithm is enabled,  the  tape
+     7                  unit's  operating  speed  may  vary.   This  flag is
+     8                  ignored for a fixed speed tape unit.
+
+
+     9              Write Back
+
+    10                  If set, write-back caching  using  the  controller's
+    11                  volatile  cache is enabled for this unit.  If clear,
+    12                  write-back caching using the  controller's  volatile
+    13                  cache  is  disabled  for  this  unit.  Equivalent to
+    14                  specifying the "Immediate  Completion"  modifier  on
+    15                  all "write" type commands for which it is legal.
+
+
+
+    16     Device Dependent Parameters:
+
+    17              Device  and/or  controller   dependent   device   tuning
+    18              parameters.   The  value  zero  in this field means that
+    19              default or normal  tuning  parameters  should  be  used.
+    20              Non-zero  values  for  this  field  should  normally  be
+    21              established through system-dependent appropriate  means.
+    22              Examples  of  the  use  of  this field include selecting
+    23              alternative  optimization  algorithms  or  enabling  and
+    24              disabling automatic (online) diagnosis of the tape unit.
+
+
+    25     Format:
+
+    26              Assuming that the tape is positioned at BOT,  the  value
+    27              supplied  in  this field is the host suggested format to
+    28              be employed for tape unit operations (Refer  to  section
+    29              3.4.2  for  related information).  It can be a zero or a
+    30              binary bit  value  in  the  low  byte  representing  the
+    31              density  of the tape.  If the value is zero, the highest
+    32              density format  supported  by  the  tape  unit  will  be
+    33              employed.   If  the  low byte is non-zero, then the high
+    34              byte must have the value corresponding to the media used
+    35              by  the  unit.   See  Appendix C, Tables C-1 and C-2 for
+    36              details.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-59
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1              If the value is non-zero, but  the  high  byte  has  the
+     2              wrong  value,  the  command  will  be  rejected  with an
+     3              Invalid Command (subcode "Invalid Format") status code.
+
+     4              If the tape is not positioned at BOT, a zero value  will
+     5              not  cause  the  format  to  change  from  the currently
+     6              selected density.  Furthermore, if the value is non-zero
+     7              the  command  will  be  rejected with an Invalid Command
+     8              (subcode  "Invalid  Format")  status  code.   Refer   to
+     9              sections 5.8.2 and 5.11.2 for additional Invalid Command
+    10              (subcode "Invalid Format") information.
+
+
+    11     Speed:
+
+    12              The speed field value  is  used  to  select  the  "fixed
+    13              speed"  at which a variable speed tape unit will operate
+    14              when the "Variable Speed  Mode  Suppression"  unit  flag
+    15              (described  above)  is  set  in the command packet.  The
+    16              speed field is ignored if that flag is clear or the tape
+    17              unit is not capable of variable speed operation.
+
+    18              The  value  supplied  in  this  field  is   the   host's
+    19              prediction  of  the  maximum  data  transfer  rate it is
+    20              capable of sustaining during data transfers to  or  from
+    21              the  tape  unit.   It  is  expressed as the maximum data
+    22              transfer rate, in kilo-bytes per  second,  that  can  be
+    23              achieved using a particular tape speed and format.  (For
+    24              9-track  tape  units,  the  actual  data  transfer  rate
+    25              approaches  this  value  as  the  record  length becomes
+    26              arbitrarily large.)
+
+    27              For 9-track tape units it is computed, without rounding,
+    28              as follows:
+
+    29                  Speed = ( ips * bpi ) / 1000
+
+    30                  where:
+
+    31                  ips = nominal tape transport  speed  in  Inches  per
+    32                  Second
+
+    33                  bpi = nominal  recording  format  frame  density  in
+    34                  linear Bits Per Inch (for nine track, actually Bytes
+    35                  Per Inch)
+
+
+    36              The controller will automatically select the  tape  unit
+    37              speed, from those available on the tape unit, which best
+    38              accommodates the host's data transfer  rate  prediction.
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-60
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1              The following rules govern speed selection:
+
+     2              1.  The tape unit speed will be set equal to  the  speed
+     3                  which   is  closest  to  providing  a  maximum  data
+     4                  transfer rate  at  the  currently  employed  format,
+     5                  which is less than or equal to the value supplied.
+
+     6              2.  The lowest possible tape unit speed will be selected
+     7                  if  the  maximum data transfer rate at the currently
+     8                  employed format of all available tape unit speeds is
+     9                  greater than the value supplied.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-61
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1     5.11.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |   unit flags  |multi-unit code|
+    11                     +---------------+---------------+
+    12                     |            reserved           |
+    13                     +-------------------------------+
+    14                     |                               |
+    15                     +----    unit identifier    ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |     media type identifier     |
+    19                     +---------------+---------------+
+    20                     |     speed     |     format    |
+    21                     +---------------+---------------+
+    22                     |    maximum write byte count   |
+    23                     +---------------+---------------+
+    24                     |    reserved   |  noise record |
+    25                     +---------------+---------------+
+
+
+    26     Status Codes:
+
+    27              Success
+
+    28                  (subcode "Normal")
+
+    29                      The tape unit is "Unit-Online".
+
+
+    30                  (subcode "Duplicate Unit Number")
+
+    31                      Implies that the tape unit is "Unit-Online"  but
+    32                      that  its  unit  number  is identical to that of
+    33                      some other tape unit known  to  the  controller.
+    34                      The tape unit will become "Unit-Offline", due to
+    35                      the duplicate Unit Number, as soon as it  ceases
+    36                      to be "Unit-Online".
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-62
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1                  (subcode "Read-only Volume Format")
+
+     2                      The unit only implements  read  access  for  the
+     3                      format  of  the volume mounted in the unit.  For
+     4                      example, a single-density volume is mounted in a
+     5                      double-density drive that can read but not write
+     6                      it.  Note that the  unit  will  be  Data  Safety
+     7                      Write   Protected   whenever   this  subcode  is
+     8                      returned.
+
+
+     9              Command Aborted
+
+    10                  The tape unit state is  unchanged.   The  host  must
+    11                  assume  that  the  returned unit characteristics are
+    12                  invalid, since the unit may be "Unit-Offline".
+
+    13              Controller Error
+    14              Drive Error
+    15              Formatter Error
+
+    16              Invalid Command
+    17                  (subcode "Invalid Format")
+
+    18                      More than  one  Format  was  specified,  or  the
+    19                      Format   specified   is  not  supported  by  the
+    20                      specified unit, or the magnetic tape  loaded  on
+    21                      the tape unit was not positioned at BOT.
+
+
+    22              Unit-Available
+    23              Unit-Offline
+
+
+    24     Flags:
+
+    25              Cached Data Lost
+    26              EOT Encountered
+    27              Error Log Generated
+    28              Exclusive Access
+    29              Position Lost
+    30              Serious Exception
+
+
+    31     Multi-Unit code:
+    32     Unit Flags:
+    33     Unit Identifier:
+    34     Media Type Identifier:
+    35     Format:
+    36     Speed:
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-63
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1              Refer to  section  5.7.2  for  a  description  of  these
+     2              fields.
+
+     3     Recommended Maximum WRITE Byte Count:
+     4     Noise Record:
+
+     5              Refer to  section  5.8.2  for  a  description  of  these
+     6              fields.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-64
+           SET UNIT CHARACTERISTICS Command                  8 November 1987
+
+
+     1     5.11.3  Description
+
+     2     The SET UNIT CHARACTERISTICS Command is used to set host-settable
+     3     unit  characteristics  and  to  obtain those unit characteristics
+     4     essential for proper class driver operation.
+
+     5     Successful execution of this command never alters the tape unit's
+     6     host-relative    connection    state    ("Unit-Online",    etc.).
+     7     Unsuccessful  execution  leaves  the  tape  unit  in  either  the
+     8     "Unit-Available" or "Unit-Offline" states.
+
+     9     Setting host settable characteristics for a  tape  unit  that  is
+    10     "Unit-Offline" or "Unit Available" has no effect.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-65
+           WRITE Command                                     8 November 1987
+
+
+     1     5.12  WRITE Command
+
+     2     Command Category:
+
+     3              Sequential (optionally Immediate Completion)
+
+
+     4     5.12.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+    13                     |           byte count          |
+    14                     +-------------------------------+
+    15                     |                               |
+    16                     +---          buffer         ---+
+    17                     |                               |
+    18                     +---        descriptor       ---+
+    19                     |                               |
+    20                     +-------------------------------+
+
+
+    21     Allowable Modifiers:
+
+    22              Clear Cached Data Lost
+    23              Clear Serious Exception
+    24              Compare
+
+    25              Enable Re-write Error Recovery
+
+    26                  This modifier may not be set in conjunction with the
+    27                  "Suppress  Error  Recovery"  command modifier and is
+    28                  therefore treated as a reserved  field.   The  TMSCP
+    29                  controller  must  return an Invalid Command (subcode
+    30                  "Invalid  Modifiers")  status  code  if   both   the
+    31                  "Suppress  Error  Recovery" command modifier and the
+    32                  "Enable Re-write Error  Recovery"  command  modifier
+    33                  are set in the same command (see section 3.5.5.2).
+
+    34              Immediate Completion
+
+    35                  If  this  modifier  is  set,  this  command  may  be
+    36                  executed  as  a "write-back" caching enabled "write"
+    37                  type command.  Refer to section 3.6.1  for  complete
+    38                  details.
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-66
+           WRITE Command                                     8 November 1987
+
+
+     1              Suppress Error Correction
+
+     2              Suppress Error Recovery
+
+     3                  This modifier may not be set in conjunction with the
+     4                  "Enable  Re-write  Error  Recovery" command modifier
+     5                  and is therefore treated as a reserved  field.   The
+     6                  TMSCP  controller  must  return  an  Invalid Command
+     7                  (subcode "Invalid Modifiers") status  code  if  both
+     8                  the  "Suppress  Error Recovery" command modifier and
+     9                  the  "Enable  Re-write   Error   Recovery"   command
+    10                  modifier  are  set  in the same command (see section
+    11                  3.5.5.2).
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-67
+           WRITE Command                                     8 November 1987
+
+
+     1     5.12.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |   byte count (host transfer)  |
+    11                     +-------------------------------+
+    12                     |                               |
+    13                     +----                       ----+
+    14                     |           undefined           |
+    15                     +----                       ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |    position  (object count)   |
+    19                     +-------------------------------+
+    20                     |    byte count (tape record)   |
+    21                     +-------------------------------+
+
+
+    22     Status Codes:
+
+    23              Success
+    24                  (subcode "Normal")
+    25                  (subcode "EOT Encountered")
+
+    26              Command Aborted
+    27              Compare Error
+    28              Controller Error
+    29              Data Error
+    30              Drive Error
+    31              Formatter Error
+    32              Host Buffer Access Error
+
+    33              Invalid Command
+    34                  (subcode "Invalid Byte Count")
+
+    35              Position Lost
+    36              Serious Exception
+    37              Unit-Available
+    38              Unit-Offline
+
+    39              Write-Protected
+    40                  (subcode "Unit is Hardware Write Protected")
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-68
+           WRITE Command                                     8 November 1987
+
+
+     1                  (subcode "Unit is Software Write Protected")
+     2                  (subcode "Unit is Data Safety Write Protected")
+
+
+
+     3     Flags:
+
+     4              Cached Data Lost
+     5              EOT Encountered
+     6              Error Log Generated
+     7              Position Lost
+     8              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-69
+           WRITE Command                                     8 November 1987
+
+
+     1     5.12.3  Description
+
+     2     Beginning at the current position, data is written from the  host
+     3     buffer to the magnetic tape in record form.
+
+     4     Refer to sections 3.3, 3.4, and 3.5 for additional detail.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-70
+           WRITE TAPE MARK Command                           8 November 1987
+
+
+     1     5.13  WRITE TAPE MARK Command
+
+     2     Command Category:
+
+     3              Sequential (optionally Immediate Completion)
+
+
+     4     5.13.1  Command Message Format
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |    reserved   |  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   modifiers   |  rsvd | opcode|
+    12                     +---------------+-------+-------+
+
+
+    13     Allowable Modifiers:
+
+    14              Clear Cached Data Lost
+    15              Clear Serious Exception
+
+    16              Immediate Completion
+
+    17                  If  this  modifier  is  set,  this  command  may  be
+    18                  executed  as  a "write-back" caching enabled "write"
+    19                  type command.  Refer to section 3.6.1  for  complete
+    20                  details.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-71
+           WRITE TAPE MARK Command                           8 November 1987
+
+
+     1     5.13.2  End Message Format
+
+     2                     31                             0 
+     3                     +-------------------------------+
+     4                     |    command reference number   |
+     5                     +---------------+---------------+
+     6                     |sequence number|  unit number  |
+     7                     +---------------+-------+-------+
+     8                     |     status    | flags |endcode|
+     9                     +---------------+-------+-------+
+    10                     |                               |
+    11                     +----                       ----+
+    12                     |                               |
+    13                     +---        undefined        ---+
+    14                     |                               |
+    15                     +----                       ----+
+    16                     |                               |
+    17                     +-------------------------------+
+    18                     |    position  (object count)   |
+    19                     +-------------------------------+
+
+
+    20     Status Codes:
+
+    21              Success
+    22                  (subcode "Normal")
+    23                  (subcode "EOT Encountered")
+
+    24              Command Aborted
+    25              Controller Error
+    26              Data Error
+    27              Drive Error
+    28  |           Formatter Error  
+    29              Position Lost
+    30              Serious Exception
+    31              Unit-Available
+    32              Unit-Offline
+
+    33              Write-Protected
+    34                  (subcode "Unit is Hardware Write Protected")
+    35                  (subcode "Unit is Software Write Protected")
+    36                  (subcode "Unit is Data Safety Write Protected")
+
+
+    37     Flags:
+
+    38              Cached Data Lost
+    39              EOT Encountered
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-72
+           WRITE TAPE MARK Command                           8 November 1987
+
+
+     1              Error Log Generated
+     2              Position Lost
+     3              Serious Exception
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
+
+
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+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP COMMAND DESCRIPTIONS                              Page 5-73
+           WRITE TAPE MARK Command                           8 November 1987
+
+
+     1     5.13.3  Description
+
+     2     A tape mark is written to the specified  unit  beginning  at  the
+     3     current position.
+
+     4     Refer to sections 3.3 and 3.5 for additional detail.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP ERROR LOG MESSAGES                                 Page 6-1
+                                                             8 November 1987
+
+
+     1                                 CHAPTER 6
+
+     2                         TMSCP ERROR LOG MESSAGES
+
+
+     3     6.1  TMSCP Error Log Message Format
+
+     4     For a detailed description of error log message  handling,  refer
+     5     to MSCP.
+
+     6     The "volume serial number" field of the MSCP  generic  error  log
+     7     message  format is used as the "Position (object count)" field in
+     8     TMSCP.
+
+     9     The following error log message formats are supported  by  TMSCP,
+    10     but described in MSCP:
+
+    11           o  Controller Errors
+    12           o  Memory Errors
+
+
+    13     The subsections which follow describe TMSCP  specific  error  log
+    14     formats.   Note  that  where  an  STI response is included in the
+    15     error log format the response code  and  checksum  bytes  of  the
+    16     response  are  not  returned by the controller.  Refer to the STI
+    17     specification for a description of the individual STI responses.
+
+    18     Fields common to the TMSCP error log messages are described here:
+
+    19           o  position (object count)
+
+    20                  "position  (object  count)"  at  which   the   error
+    21                  occurred.   This  value  may not necessarily reflect
+    22                  the actual object count  since  certain  errors  can
+    23                  result  in a position lost condition.  In that case,
+    24                  the last known good object count will be specified.
+
+
+    25           o  fsvrsn
+
+    26                  Formatter software, firmware, or microcode  revision
+    27                  number.    Always   zero   if  the  development  and
+    28                  maintainability groups for a product determine  that
+    29                  there is no benefit from supporting machine readable
+    30                  revision numbers.
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP ERROR LOG MESSAGES                                 Page 6-2
+           TMSCP Error Log Message Format                    8 November 1987
+
+
+     1           o  fhvrsn
+
+     2                  Formatter hardware revision number.  Always zero  if
+     3                  the  development  and  maintainability  groups for a
+     4                  product determine that  there  is  no  benefit  from
+     5                  supporting machine readable revision numbers.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP ERROR LOG MESSAGES                                 Page 6-3
+           TMSCP Error Log Message Format                    8 November 1987
+
+
+     1     6.1.1  Tape Errors
+
+     2     The following format is  used  to  report  generic  tape  errors,
+     3     including   errors   that  occur  during  a  tape  data  transfer
+     4     operation.
+
+
+     5                     31                             0 
+     6                     +-------------------------------+
+     7                     |    command reference number   |
+     8                     +---------------+---------------+
+     9                     |sequence number|  unit number  |
+    10                     +---------------+-------+-------+
+    11                     |   event code  | flags | format|
+    12                     +---------------+-------+-------+
+    13                     |                               |
+    14                     +---  controller identifier  ---+
+    15                     |                               |
+    16                     +---------------+-------+-------+
+    17                     |multi-unit code| chvrsn| csvrsn|
+    18                     +---------------+-------+-------+
+    19                     |                               |
+    20                     +---     unit identifier     ---+
+    21                     |                               |
+    22                     +-------+-------+-------+-------+
+    23                     | retry | level | uhvrsn| usvrsn|
+    24                     +-------+-------+-------+-------+
+    25                     |    position (object count)    |
+    26                     +---------------+-------+-------+
+    27                     |    reserved   | fhvrsn| fsvrsn|
+    28                     +---------------+-------+-------+
+    29                     |                               |
+    30                     /             device            /
+    31                     /     dependent information     /
+    32                     /           (optional)          /
+    33                     |                               |
+    34                     +-------------------------------+
+
+    35     Level/Retry:
+
+    36              The error recovery level and retry  count  of  the  most
+    37              recent  attempt at the transfer.  Note that these values
+    38              always identify the last attempt --  i.e.,  the  attempt
+    39              that  succeeded  or  the  last attempt before giving up.
+    40              Refer to MSCP for more detail on  the  Level  and  Retry
+    41              fields.
+                           
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP ERROR LOG MESSAGES                                 Page 6-4
+           TMSCP Error Log Message Format                    8 November 1987
+
+
+     1     Device Dependent Information:
+
+     2              An optional, variable  length  field  containing  device
+     3              dependent  information.   The  length  of  this field is
+     4              implied by the total length of the  error  log  message,
+     5              passed  to  the  class  driver  by the port driver.  The
+     6              contents  of  the  field  must  be  described   in   the
+     7              functional  specification  of  any device providing this
+     8              field.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP ERROR LOG MESSAGES                                 Page 6-5
+           TMSCP Error Log Message Format                    8 November 1987
+
+
+     1     6.1.2  STI Communication Or Command Failures
+
+     2     The following format is used by STI Tape  Controllers  to  report
+     3     STI  communication or command failures.  Note that the controller
+     4     may attempt several retries.  A separate error log  message  will
+     5     be generated for each attempt that fails.
+
+
+     6                     31                             0 
+     7                     +-------------------------------+
+     8                     |    command reference number   |
+     9                     +---------------+---------------+
+    10                     |sequence number|  unit number  |
+    11                     +---------------+-------+-------+
+    12                     |   event code  | flags | format|
+    13                     +---------------+-------+-------+
+    14                     |                               |
+    15                     +---  controller identifier  ---+
+    16                     |                               |
+    17                     +---------------+-------+-------+
+    18                     |multi-unit code| chvrsn| csvrsn|
+    19                     +---------------+-------+-------+
+    20                     |                               |
+    21                     +---     unit identifier     ---+
+    22                     |                               |
+    23                     +---------------+-------+-------+
+    24                     |    reserved   | uhvrsn| usvrsn|
+    25                     +---------------+-------+-------+
+    26                     |    position (object count)    |
+    27                     +---------------+-------+-------+
+    28                     |    reserved   | fhvrsn| fsvrsn|
+    29                     +---------------+-------+-------+
+    30                     |                               |
+    31                     +---     STI UNSUCCESSFUL    ---+
+    32                     |            Response           |
+    33                     +---         12 bytes        ---+
+    34                     |                               |
+    35                     +-------------------------------+
+
+    36     STI UNSUCCESSFUL Response:
+
+    37              Twelve bytes of information received from the  formatter
+    38              as  an  UNSUCCESSFUL  response, or in response to an STI
+    39              GET SUMMARY STATUS command (the format is identical).
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP ERROR LOG MESSAGES                                 Page 6-6
+           TMSCP Error Log Message Format                    8 November 1987
+
+
+     1     6.1.3  STI Formatter Error Log
+
+     2     The following format is used by STI Tape  Controllers  to  return
+     3     formatter requested error logs.
+
+
+     4                     31                             0 
+     5                     +-------------------------------+
+     6                     |    command reference number   |
+     7                     +---------------+---------------+
+     8                     |sequence number|  unit number  |
+     9                     +---------------+-------+-------+
+    10                     |   event code  | flags | format|
+    11                     +---------------+-------+-------+
+    12                     |                               |
+    13                     +---  controller identifier  ---+
+    14                     |                               |
+    15                     +---------------+-------+-------+
+    16                     |multi-unit code| chvrsn| csvrsn|
+    17                     +---------------+-------+-------+
+    18                     |                               |
+    19                     +---     unit identifier     ---+
+    20                     |                               |
+    21                     +---------------+-------+-------+
+    22                     |    reserved   | uhvrsn| usvrsn|
+    23                     +---------------+-------+-------+
+    24                     |    position (object count)    |
+    25                     +---------------+-------+-------+
+    26                     |    reserved   | fhvrsn| fsvrsn|
+    27                     +---------------+-------+-------+
+    28                     |                               |
+    29                     +---    STI GET FORMATTER    ---+
+    30                     /       ERROR LOG Response      /
+    31                     +---         20 bytes        ---+
+    32                     |                               |
+    33                     +-------------------------------+
+
+
+
+    34     STI GET FORMATTER ERROR LOG Response:
+
+    35              Twenty bytes of information received from the  formatter
+    36              in  response  to an STI GET FORMATTER ERROR LOG command.
+    37              Note that the format of this  information  is  formatter
+    38              and  error  type  dependent,  and will therefore vary in
+    39              length and content.  If the  information  received  from
+    40              the  formatter is less than twenty bytes, the controller
+    41              will zero fill the remaining unused bytes of  the  error
+    42              log message.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           TMSCP ERROR LOG MESSAGES                                 Page 6-7
+           TMSCP Error Log Message Format                    8 November 1987
+
+
+     1     6.1.4  STI Drive Error Log
+
+     2     The following format is used by STI Tape  Controllers  to  return
+     3     drive requested error logs.
+
+
+     4                     31                             0 
+     5                     +-------------------------------+
+     6                     |    command reference number   |
+     7                     +---------------+---------------+
+     8                     |sequence number|  unit number  |
+     9                     +---------------+-------+-------+
+    10                     |   event code  | flags | format|
+    11                     +---------------+-------+-------+
+    12                     |                               |
+    13                     +---  controller identifier  ---+
+    14                     |                               |
+    15                     +---------------+-------+-------+
+    16                     |multi-unit code| chvrsn| csvrsn|
+    17                     +---------------+-------+-------+
+    18                     |                               |
+    19                     +---     unit identifier     ---+
+    20                     |                               |
+    21                     +---------------+-------+-------+
+    22                     |    reserved   | uhvrsn| usvrsn|
+    23                     +---------------+-------+-------+
+    24                     |    position (object count)    |
+    25                     +---------------+-------+-------+
+    26                     |    reserved   | fhvrsn| fsvrsn|
+    27                     +---------------+-------+-------+
+    28                     |                               |
+    29                     +---     STI GET EXTENDED    ---+
+    30                     /     DRIVE STATUS Response     /
+    31                     +---         62 bytes        ---+
+    32                     |                               |
+    33                     +-------------------------------+
+
+
+
+    34     STI GET EXTENDED DRIVE STATUS Response:
+
+    35              Sixty  two  bytes  of  information  received  from   the
+    36              formatter  in  response  to  an  STI  GET EXTENDED DRIVE
+    37              STATUS  command.   Note  that   the   format   of   this
+    38              information  is drive and error type dependent, and will
+    39              therefore  vary  in  length   and   content.    If   the
+    40              information  received  from the drive is less than sixty
+    41              two bytes, the controller will zero fill  the  remaining
+    42              unused bytes of the error log message.
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-1
+                                                                                       8 November 1987
+
+
+     1                                             APPENDIX A
+
+     2                                 OPCODE, FLAG AND OFFSET DEFINITIONS
+
+
+     3                                Table A-1:  Control Message Opcodes
+
+     4     +--------------------+--------------------------+-----------------------------------------+
+     5     |    Opcode Value    |                          |                                         |
+     6     |    (See note 1)    |   Preferred Mnemonics    |                                         |
+     7     |------+------+------+--------+-----------------+     Control Message Type                |
+     8     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+     9     +------+------+------+--------+-----------------+-----------------------------------------+
+    10     |   0  |  00  |  00  |    -   |        -        | reserved - must not be assigned         |
+    11     +------+------+------+--------+-----------------+-----------------------------------------+
+    12     | Standard Command Messages (See note 2):       |                                         |
+    13     |   1  |  01  |  01  | OP.ABO | MSCP$K_OP_ABORT | ABORT                                   |
+    14     |  16  |  20  |  10  | OP.ACC | MSCP$K_OP_ACCES | ACCESS                                  |
+    15     |   8  |  10  |  08  | OP.AVL | MSCP$K_OP_AVAIL | AVAILABLE                               |
+    16     |  32  |  40  |  20  | OP.CMP | MSCP$K_OP_COMP  | COMPARE HOST DATA                       |
+    17     |  11  |  13  |  0B  | OP.DAP | MSCP$K_OP_DTACP | DETERMINE ACCESS PATHS                  |
+    18     |  18  |  22  |  12  | OP.ERS | MSCP$K_OP_ERASE | ERASE                                   |
+    19     |   2  |  02  |  02  | OP.GCS | MSCP$K_OP_GTCMD | GET COMMAND STATUS                      |
+    20     |   3  |  03  |  03  | OP.GUS | MSCP$K_OP_GTUNT | GET UNIT STATUS                         |
+    21     |   9  |  11  |  09  | OP.ONL | MSCP$K_OP_ONLIN | ONLINE                                  |
+    22     |  33  |  41  |  21  | OP.RD  | MSCP$K_OP_READ  | READ                                    |
+    23     |   4  |  04  |  04  | OP.SCC | MSCP$K_OP_STCON | SET CONTROLLER CHARACTERISTICS          |
+    24     |  10  |  12  |  0A  | OP.SUC | MSCP$K_OP_STUNT | SET UNIT CHARACTERISTICS                |
+    25     |  34  |  42  |  22  | OP.WR  | MSCP$K_OP_WRITE | WRITE                                   |
+    26     +------+------+------+--------+-----------------+-----------------------------------------+
+    27     | Optional Command Messages:  |                 |                                         |
+    28     |   5  |  05  |  05  | OP.ANM | MSCP$K_OP_ACCNM | ACCESS NON-VOLATILE MEMORY              |
+    29     +------+------+------+--------+-----------------+-----------------------------------------+
+    30     | Implementation Specific Command Messages (See note 4):                                  |
+    31     |  40  |  50  |  28  | OP.SP1 | MSCP$K_OP_SPEC1 | # 1                                     |
+    32     |  41  |  51  |  29  | OP.SP2 | MSCP$K_OP_SPEC2 | # 2                                     |
+    33     |  42  |  52  |  2A  | OP.SP3 | MSCP$K_OP_SPEC3 | # 3                                     |
+    34     |  43  |  53  |  2B  | OP.SP4 | MSCP$K_OP_SPEC4 | # 4                                     |
+    35     |  44  |  54  |  2C  | OP.SP5 | MSCP$K_OP_SPEC5 | # 5                                     |
+    36     |  45  |  55  |  2D  | OP.SP6 | MSCP$K_OP_SPEC6 | # 6                                     |
+    37     |  46  |  56  |  2E  | OP.SP7 | MSCP$K_OP_SPEC7 | # 7                                     |
+    38     |  47  |  57  |  2F  | OP.SP8 | MSCP$K_OP_SPEC8 | # 8                                     |
+    39     +------+------+------+--------+-----------------+-----------------------------------------+
+    40     | Tape MSCP Specific Command Messages:          |                                         |
+    41     |  22  |  26  |  16  | OP.ERG | MSCP$K_OP_ERGAP | ERASE GAP                               |
+    42     |  19  |  23  |  13  | OP.FLU | MSCP$K_OP_FLUSH | FLUSH                                   |
+    43     |  37  |  45  |  25  | OP.REP | MSCP$K_OP_REPOS | REPOSITION                              |
+    44     |  36  |  44  |  24  | OP.WTM | MSCP$K_OP_WRITM | WRITE TAPE MARK                         |
+    45     +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-2
+                                                                                       8 November 1987
+
+
+
+     1                             Table A-1: Control Message Opcodes (cont.)
+
+     2     +--------------------+--------------------------+-----------------------------------------+
+     3     |    Opcode Value    |                          |                                         |
+     4     |    (See note 1)    |   Preferred Mnemonics    |                                         |
+     5     |------+------+------+--------+-----------------+     Control Message Type                |
+     6     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+     7     +------+------+------+--------+-----------------+-----------------------------------------+
+     8     | End Messages (See note 3):                    |                                         |
+     9     | 128  | 200  |  80  | OP.END | MSCP$K_OP_END   | END message flag                        |
+    10     +------+------+------+--------+-----------------+-----------------------------------------+
+    11     | Attention Messages:|        |                 |                                         |
+    12     |  64  | 100  |  40  | OP.AVA | MSCP$K_OP_AVATN | AVAILABLE                               |
+    13     |  65  | 101  |  41  | OP.DUP | MSCP$K_OP_DUPUN | DUPLICATE UNIT NUMBER                   |
+    14     |  66  | 102  |  42  | OP.ACP | MSCP$K_OP_APATH | ACCESS PATH                             |
+    15     +------+------+------+--------+-----------------+-----------------------------------------+
+    16     | Notes: 1. Message  opcode  bits 6 and 7 indicate the type of message (command, end,  or |
+    17     |           attention).                                                                   |
+    18     |                                                                                         |
+    19     |        2. Standard  command  opcode  bits  3 through 5 indicate  the  command  category |
+    20     |           (Immediate,  Sequential,  or Non-Sequential) and whether or not  the  command |
+    21     |           includes a buffer descriptor.                                                 |
+    22     |                                                                                         |
+    23     |        3. End message endcodes are formed by adding the end message flag to the command |
+    24     |           opcode.   For  example, a READ command's end message contains (using  16  bit |
+    25     |           mnemonics)  the value OP.RD+OP.END in its endcode field.  As described in the |
+    26     |           MSCP  specification,  an MSCP protocol violation produces an Invalid  Command |
+    27     |           end  message which contains just the end message flag (OP.END) in its endcode |
+    28     |           field.                                                                        |
+    29     |                                                                                         |
+    30     |        4. The implementation specific command opcodes may be used by manufacturing test |
+    31     |           equipment to verify controller operation. They must never be issued by normal |
+    32     |           host  software,  or  when a host is running anything other  than  stand-alone |
+    33     |           diagnostics.  The operation of these commands when issued under anything  but |
+    34     |           the expected manufacturing and/or diagnostic environment is UNDEFINED.        |
+    35     +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-3
+                                                                                       8 November 1987
+
+
+     1                                   Table A-2:  Command Modifiers
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+     Command Modifier                   |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (See note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     | ACCESS, AVAILABLE, COMPARE HOST DATA, ERASE, ERASE GAP, FLUSH, ONLINE, READ,           |
+     9     | REPOSITION, SET UNIT CHARACTERISTICS, WRITE and WRITE TAPE MARK Command Modifiers:      |
+    10     |  13  | 20000 | 2000 | MD.CSE | MSCP$x_MD_CLSEX | Clear Serious Exception                |
+    11     |  12  | 10000 | 1000 | MD.CDL | MSCP$x_MD_CDATL | Clear Cached Data Lost                 |
+    12     +------+-------+------+--------+-----------------+----------------------------------------+
+    13     | ACCESS, COMPARE HOST DATA, READ, and REPOSITION Command Modifiers:                      |
+    14     |  11  |  4000 |  800 | MD.SCH | MSCP$x_MD_SCCHH | Suppress Caching                       |
+    15     |   3  |    10 |    8 | MD.REV | MSCP$x_MD_REVRS | Reverse                                |
+    16     +------+-------+------+--------+-----------------+----------------------------------------+
+    17     | ACCESS, COMPARE HOST DATA, READ, REPOSITION, and WRITE Command Modifiers:               |
+    18     |   9  |  1000 |  200 | MD.SEC | MSCP$x_MD_SECOR | Suppress Error Correction              |
+    19     |   8  |   400 |  100 | MD.SER | MSCP$x_MD_SEREC | Suppress Error Recovery                |
+    20     +------+-------+------+--------+-----------------+----------------------------------------+
+    21     | AVAILABLE Command Modifiers: |                 |                                        |
+    22     |   4  |    20 |   10 | MD.UNL | MSCP$x_MD_UNLOD | Unload                                 |
+    23     |   1  |     2 |    2 | MD.ALL | MSCP$x_MD_ALLCD | All Class Drivers                      |
+    24     +------+-------+------+--------+-----------------+----------------------------------------+
+    25     | AVAILABLE, ONLINE, and SET UNIT CHARACTERISTICS Command Modifiers:                      |
+    26     |   5  |    40 |   20 | MD.EXC | MSCP$x_MD_EXCAC | Exclusive Access                       |
+    27     +------+-------+------+--------+-----------------+----------------------------------------+
+    28     | ERASE, ERASE GAP, REPOSITION, WRITE and WRITE TAPE MARK Command Modifier:               |
+    29     |   6  |   100 |   40 | MD.IMM | MSCP$x_MD_IMMED | Immediate Completion                   |
+    30     +------+-------+------+--------+-----------------+----------------------------------------+
+    31     | GET UNIT STATUS Command Modifier:              |                                        |
+    32     |   0  |     1 |    1 | MD.NXU | MSCP$x_MD_NXUNT | Next Unit                              |
+    33     +------+-------+------+--------+-----------------+----------------------------------------+
+    34     | ONLINE and SET UNIT CHARACTERISTICS Command Modifier:                                   |
+    35     |   2  |     4 |    4 | MD.SWP | MSCP$x_MD_STWRP | Enable Set Write Protect               |
+    36     +------+-------+------+--------+-----------------+----------------------------------------+
+    37     | READ and WRITE Command Modifier:               |                                        |
+    38     |  14  | 40000 | 4000 | MD.CMP | MSCP$x_MD_COMP  | Compare                                |
+    39     +------+-------+------+--------+-----------------+----------------------------------------+
+    40     | REPOSITION Command Modifiers:|                 |                                        |
+    41     |   7  |   200 |   80 | MD.DLE | MSCP$x_MD_DLEOT | Detect LEOT                            |
+    42     |   2  |     4 |    4 | MD.OBC | MSCP$x_MD_OBJCT | Object Count                           |
+    43     |   1  |     2 |    2 | MD.RWD | MSCP$x_MD_REWND | Rewind                                 |
+    44     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-4
+                                                                                       8 November 1987
+
+
+
+     1                                Table A-2: Command Modifiers (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------+-----------------+     Command Modifier                   |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (See note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     | WRITE Command Modifier:      |                 |                                        |
+     9     |   4  |    20 |   10 | MD.ERW | MSCP$x_MD_ENRWR | Enable Re-Write Error Recovery         |
+    10     +------+-------+------+--------+-----------------+----------------------------------------+
+    11     | Notes: 1. The  "x" in  the 32 bit mnemonic for a bit flag will be "V" if the symbol  is |
+    12     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    13     +-----------------------------------------------------------------------------------------+
+
+
+    14                                   Table A-3:  End Message Flags
+
+    15     +------+--------------+--------------------------+----------------------------------------+
+    16     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    17     |Number+-------+------+--------+-----------------+     End Message Flag                   |
+    18     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+    19     |      |       |      |        |   (See note 1)  |                                        |
+    20     +------+-------+------+--------+-----------------+----------------------------------------+
+    21     |   5  |    40 |   20 | EF.LOG | MSCP$x_EF_ERLOG | Error Log Generated                    |
+    22     |   4  |    20 |   10 | EF.SEX | MSCP$x_EF_SEREX | Serious Exception                      |
+    23     |   3  |    10 |    8 | EF.EOT | MSCP$x_EF_EOT   | End of Tape encountered                |
+    24     |   2  |     4 |    4 | EF.PLS | MSCP$x_EF_PLS   | Position Lost                          |
+    25     |   1  |     2 |    2 | EF.DLS | MSCP$x_EF_DLS   | Cached Data Lost                       |
+    26     +------+-------+------+--------+-----------------+----------------------------------------+
+    27     | Notes: 1. The  "x" in  the 32 bit mnemonic for a bit flag will be "V" if the symbol  is |
+    28     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    29     +-----------------------------------------------------------------------------------------+
+
+
+    30                                    Table A-4:  Controller Flags
+
+    31     +------+--------------+--------------------------+----------------------------------------+
+    32     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    33     |Number+-------+------+--------------------------+     Controller Flag                    |
+    34     |      | Octal | Hex. | 16 bit |     32 bit      |     (See note 2)                       |
+    35     |      |       |      |        |   (See note 1)  |                                        |
+    36     +------+-------+------+--------+-----------------+----------------------------------------+
+    37     |   7  |   200 |   80 | CF.ATN | MSCP$x_CF_ATTN  | Enable Attention Messages              |
+    38     |   6  |   100 |   40 | CF.MSC | MSCP$x_CF_MISC  | Enable Miscellaneous Error Log Messages|
+    39     |   5  |    40 |   20 | CF.OTH | MSCP$x_CF_OTHER | Enable Other Host's Error Log Messages |
+    40     |   4  |    20 |   10 | CF.THS | MSCP$x_CF_THIS  | Enable This Host's Error Log Messages  |
+    41     |   2  |     4 |    4 | CF.MLH | MSCP$x_CF_MLHT  | Multi-Host Support                     |
+    42     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-5
+                                                                                       8 November 1987
+
+
+
+     1                                 Table A-4: Controller Flags (cont.)
+
+     2     +------+--------------+--------------------------+----------------------------------------+
+     3     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+     4     |Number+-------+------+--------------------------+     Controller Flag                    |
+     5     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+     6     |      |       |      |        |   (See note 1)  |                                        |
+     7     +------+-------+------+--------+-----------------+----------------------------------------+
+     8     | Notes: 1. The  "x" in  the 32 bit mnemonic for a bit flag will be "V" if the symbol  is |
+     9     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    10     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+    11                                       Table A-5:  Unit Flags
+
+    12     +------+--------------+--------------------------+----------------------------------------+
+    13     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    14     |Number+-------+------+--------+-----------------+     Unit Flag                          |
+    15     |      | Octal | Hex. | 16 bit |     32 bit      |                                        |
+    16     |      |       |      |        |   (See note 1)  |                                        |
+    17     +------+-------+------+--------+-----------------+----------------------------------------+
+    18     |  15  |100000 | 8000 | UF.CAC | MSCP$x_UF_CACH  | Tape Write-back caching                |
+    19     |  13  | 20000 | 2000 | UF.WPH | MSCP$x_UF_WRTPH | Hardware Write protect                 |
+    20     |  12  | 10000 | 1000 | UF.WPS | MSCP$x_UF_WRTPS | Software Write protect                 |
+    21     |  11  |  4000 |  800 | UF.SCH | MSCP$x_UF_SCCHH | Suppress Caching                       |
+    22     |  10  |  2000 |  400 | UF.EXA | MSCP$x_UF_EXACC | Exclusive Access                       |
+    23     |   8  |   400 |  100 | UF.WPD | MSCP$x_UF_WRTPD | Data Safety Write Protect              |
+    24     |   6  |   100 |   40 | UF.WBN | MSCP$x_UF_WBKNV | Write-back                             |
+    25     |   5  |    40 |   20 | UF.VSS | MSCP$x_UF_VSMSU | Variable Speed Mode Suppression        |
+    26     |   4  |    20 |   10 | UF.VSU | MSCP$x_UF_VARSP | Variable Speed Unit                    |
+    27     |   3  |    10 |    8 | UF.EWR | MSCP$x_UF_EWRER | Enhanced Write Error Recovery          |
+    28     |   1  |     2 |    2 | UF.CMW | MSCP$x_UF_CMPWR | Compare Writes                         |
+    29     |   0  |     1 |    1 | UF.CMR | MSCP$x_UF_CMPRD |                                        |
+    30     +------+-------+------+--------+-----------------+----------------------------------------+
+    31     | Notes: 1. The  "x" in  the 32 bit mnemonic for a bit flag will be "V" if the symbol  is |
+    32     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    33     +------+-------+------+--------+-----------------+----------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-6
+                                                                                       8 November 1987
+
+
+     1                                Table A-6:  Command Message Offsets
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | Generic Command Message Offsets:              |       |                                 |
+     8     |   0  |   0  |   0  | P.CRF  | MSCP$L_CMD_REF  |   4   | Command reference number        |
+     9     |   4  |   4  |   4  | P.UNIT | MSCP$W_UNIT     |   2   | Unit number                     |
+    10     |   6  |   6  |   6  |        |                 |   2   | Reserved                        |
+    11     |   8  |  10  |   8  | P.OPCD | MSCP$B_OPCODE   |   1   | Opcode                          |
+    12     |   9  |  11  |   9  |        |                 |   1   | Reserved                        |
+    13     |  10  |  12  |   A  | P.MOD  | MSCP$W_MODIFIER |   2   | Modifiers                       |
+    14     |  12  |  14  |   C  | P.BCNT | MSCP$L_BYTE_CNT |   4   | Byte count                      |
+    15     |  16  |  20  |  10  | P.BUFF | MSCP$B_BUFFER   |  12   | Buffer descriptor               |
+    16     +------+------+------+--------+-----------------+-------+---------------------------------+
+    17     | ABORT and GET COMMAND STATUS Command Message Offsets: |                                 |
+    18     |  12  |  14  |   C  | P.OTRF | MSCP$L_OUT_REF  |   4   | Outstanding reference number    |
+    19     +------+------+------+--------+-----------------+-------+---------------------------------+
+    20     | ACCESS NON-VOLATILE MEMORY Command Message Offsets:   |                                 |
+    21     |   4  |   4  |   4  |        |                 |   2   | Reserved                        |
+    22     |  12  |  14  |   C  |        |                 |   4   | Reserved                        |
+    23     |  16  |  20  |  10  | P.ANOF | MSCP$L_ANM_OFFS |   4   | Offset                          |
+    24     |  20  |  24  |  14  | P.ANOP | MSCP$W_ANM_OPER |   2   | Operation (See Table A-11)      |
+    25     |  22  |  26  |  16  | P.ANDL | MSCP$W_ANM_DLGH |   2   | Data length                     |
+    26     |  24  |  30  |  18  | P.ANMD | MSCP$T_ANM_MEMD |   *   | Memory data                     |
+    27     +------+------+------+--------+-----------------+-------+---------------------------------+
+    28     | ONLINE and SET UNIT CHARACTERISTICS Command Message Offsets:                            |
+    29     |  12  |  14  |   C  |        |                 |   2   | Reserved                        |
+    30     |  14  |  16  |   E  | P.UNFL | MSCP$W_UNT_FLGS |   2   | Unit flags                      |
+    31     |  16  |  20  |  10  |        |                 |  12   | Reserved                        |
+    32     |  28  |  34  |  1C  | P.DVPM | MSCP$L_DEV_PARM |   4   | Device dependent parameters     |
+    33     |  32  |  40  |  20  | P.FORM | MSCP$W_FORMAT   |   2   | Format                          |
+    34     |  34  |  42  |  22  | P.SPED | MSCP$L_SPEED    |   2   | Speed                           |
+    35     +------+------+------+--------+-----------------+-------+---------------------------------+
+    36     | REPOSITION Command Message Offsets:           |       |                                 |
+    37     |  12  |  14  |   C  | P.RECC | MSCP$L_REC_CNT  |   4   | Record/Object Count             |
+    38     |  16  |  20  |  10  | P.TMGC | MSCP$L_TMGP_CNT |   4   | Tape Mark Count or N/A          |
+    39     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-7
+                                                                                       8 November 1987
+
+
+
+     1                             Table A-6: Command Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | SET CONTROLLER CHARACTERISTICS Command Message Offsets:                                 |
+     8     |   4  |   4  |   4  |        |                 |   2   | Reserved                        |
+     9     |  12  |  14  |   C  | P.VRSN | MSCP$W_VERSION  |   2   | MSCP version                    |
+    10     |  14  |  16  |   E  | P.CNTF | MSCP$W_CNT_FLGS |   2   | Controller flags                |
+    11     |  16  |  20  |  10  | P.HTMO | MSCP$W_HST_TMO  |   2   | Host timeout                    |
+    12     |  18  |  22  |  12  |        |                 |   2   | Reserved                        |
+    13     |  20  |  24  |  14  | P.TIME | MSCP$Q_TIME     |   8   | Quad-word time and date         |
+    14     |  28  |  34  |  1C  | P.CNPM | MSCP$L_CNT_PARM |   4   | Controller dependent parameters |
+    15     +------+------+------+--------+-----------------+-------+---------------------------------+
+    16     | Notes: 1. An asterisk (*) in the field size indicates that the size varies.             |
+    17     +-----------------------------------------------------------------------------------------+
+
+
+    18                           Table A-7:  End and Attention Message Offsets
+
+    19     +--------------------+--------------------------+-------+---------------------------------+
+    20     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+    21     +------+------+------+--------+-----------------+ Size  | Field Description               |
+    22     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+    23     +------+------+------+--------+-----------------+-------+---------------------------------+
+    24     | Generic End Message Offsets:|                 |       |                                 |
+    25     |   0  |   0  |   0  | P.CRF  | MSCP$L_CMD_REF  |   4   | Command reference number        |
+    26     |   4  |   4  |   4  | P.UNIT | MSCP$W_UNIT     |   2   | Unit number                     |
+    27     |   6  |   6  |   6  | P.SEQ  | MSCP$W_SEQ_NUM  |   2   | Sequence Number (LAST error log)|
+    28     |   8  |  10  |   8  | P.OPCD | MSCP$B_OPCODE   |   1   | Opcode (also called endcode)    |
+    29     |   9  |  11  |   9  | P.FLGS | MSCP$B_FLAGS    |   1   | End message flags               |
+    30     |  10  |  12  |   A  | P.STS  | MSCP$W_STATUS   |   2   | Status                          |
+    31     |  12  |  14  |   C  | P.BCNT | MSCP$L_BYTE_CNT |   4   | Byte count (host transfer)      |
+    32     +------+------+------+--------+-----------------+-------+---------------------------------+
+    33     | ABORT and GET COMMAND STATUS End Message Offsets:     |                                 |
+    34     |  12  |  14  |   C  | P.OTRF | MSCP$L_OUT_REF  |   4   | Outstanding reference number    |
+    35     +------+------+------+--------+-----------------+-------+---------------------------------+
+    36     | ACCESS NON-VOLATILE MEMORY End Message Offsets:       |                                 |
+    37     |   4  |   4  |   4  |        |                 |   2   | Reserved                        |
+    38     |  12  |  14  |   C  | P.ANSZ | MSCP$L_ANM_SIZE |   4   | Memory Size                     |
+    39     |  16  |  20  |  10  | P.ANOF | MSCP$L_ANM_OFFS |   4   | Offset                          |
+    40     |  20  |  24  |  14  | P.ANOP | MSCP$W_ANM_OPER |   2   | Operation (See Table A-11)      |
+    41     |  22  |  26  |  16  | P.ANDL | MSCP$W_ANM_DLGH |   2   | Data length                     |
+    42     |  24  |  30  |  18  | P.ANMD | MSCP$T_ANM_MEMD |   *   | Memory data                     |
+    43     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-8
+                                                                                       8 November 1987
+
+
+
+     1                        Table A-7: End and Attention Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | GET COMMAND STATUS End Message Offsets:       |       |                                 |
+     8     |  16  |  20  |  10  | P.CMST | MSCP$L_CMD_STS  |   4   | Command status                  |
+     9     +------+------+------+--------+-----------------+-------+---------------------------------+
+    10     | GET UNIT STATUS End Message Offsets:          |       |                                 |
+    11     |  12  |  14  |   C  | P.MLUN | MSCP$W_MULT_UNT |   2   | Multiunit code                  |
+    12     |  14  |  16  |   E  | P.UNFL | MSCP$W_UNT_FLGS |   2   | Unit flags                      |
+    13     |  16  |  20  |  10  |        |                 |   4   | Reserved                        |
+    14     |  20  |  24  |  14  | P.UNTI | MSCP$Q_UNIT_ID  |   8   | Unit identifier                 |
+    15     |  28  |  34  |  1C  | P.MEDI | MSCP$L_MEDIA_ID |   4   | Media type identifier           |
+    16     |  32  |  40  |  20  | P.FORM | MSCP$W_FORMAT   |   2   | Tape Format                     |
+    17     |  34  |  42  |  22  | P.SPED | MSCP$W_SPEED    |   2   | Speed                           |
+    18     |  36  |  44  |  24  | P.FMEN | MSCP$W_FORMENU  |   2   | Format Menu                     |
+    19     |  38  |  46  |  26  | P.FCAP | MSCP$B_FREECAP  |   1   | Free capacity                   |
+    20     |  39  |  47  |  27  |        |                 |   1   | Reserved                        |
+    21     |  40  |  50  |  28  | P.FSVR | MSCP$B_FMTR_SVR |   1   | Formatter software version      |
+    22     |  41  |  51  |  29  | P.FHVR | MSCP$B_FMTR_HVR |   1   | Formatter hardware version      |
+    23     |  42  |  52  |  2A  | P.USVR | MSCP$B_UNIT_SVR |   1   | Unit software version           |
+    24     |  43  |  53  |  2B  | P.UHVR | MSCP$B_UNIT_HVR |   1   | Unit hardware version           |
+    25     +------+------+------+--------+-----------------+-------+---------------------------------+
+    26     | ONLINE and SET UNIT CHARACTERISTICS End Message and AVAILABLE Attention Message offsets:|
+    27     |  12  |  14  |   C  | P.MLUN | MSCP$W_MULT_UNT |   2   | Multiunit code                  |
+    28     |  14  |  16  |   E  | P.UNFL | MSCP$W_UNT_FLGS |   2   | Unit flags                      |
+    29     |  16  |  20  |  10  |        |                 |   4   | Reserved                        |
+    30     |  20  |  24  |  14  | P.UNTI | MSCP$Q_UNIT_ID  |   8   | Unit identifier                 |
+    31     |  28  |  34  |  1C  | P.MEDI | MSCP$L_MEDIA_ID |   4   | Media type identifier           |
+    32     |  32  |  40  |  20  | P.FORM | MSCP$W_FORMAT   |   2   | Tape Format                     |
+    33     |  34  |  42  |  22  | P.SPED | MSCP$W_SPEED    |   2   | Speed                           |
+    34     |  36  |  44  |  24  | P.MXWR | MSCP$L_MAXWRREC |   4   | Maximum WRITE record size       |
+    35     |  40  |  50  |  28  | P.NREC | MSCP$W_NOISEREC |   2   | Noise Record                    |
+    36     |  42  |  52  |  2A  |        |                 |   2   | Reserved                        |
+    37     +------+------+------+--------+-----------------+-------+---------------------------------+
+    38     | SET CONTROLLER CHARACTERISTICS End Message Offsets:   |                                 |
+    39     |   4  |   4  |   4  |        |                 |   2   | Reserved                        |
+    40     |  12  |  14  |   C  | P.VRSN | MSCP$W_VERSION  |   2   | MSCP version                    |
+    41     |  14  |  16  |   E  | P.CNTF | MSCP$W_CNT_FLGS |   2   | Controller flags                |
+    42     |  16  |  20  |  10  | P.CTMO | MSCP$W_CNT_TMO  |   2   | Controller timeout              |
+    43     |  18  |  22  |  12  | P.CSVR | MSCP$B_CNT_SVR  |   1   | Controller software version     |
+    44     |  19  |  23  |  13  | P.CHVR | MSCP$B_CNT_HVR  |   1   | Controller hardware version     |
+    45     |  20  |  24  |  14  | P.CNTI | MSCP$Q_CNT_ID   |   8   | Controller ID                   |
+    46     |  28  |  34  |  1C  | P.CMBC | MSCP$L_MAXBCNT  |   4   | Controller  maximum  byte count |
+    47     |      |      |      |        |                 |       | (optional)                      |
+    48     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                                Page A-9
+                                                                                       8 November 1987
+
+
+
+     1                        Table A-7: End and Attention Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | ACCESS, COMPARE HOST DATA, FLUSH, READ, REPOSITION, WRITE, and WRITE TAPE MARK end      |
+     8     | message offsets:   |        |                 |       |                                 |
+     9     |  28  |  34  |  1C  | P.POS  | MSCP$L_POSITION |   4   | Position (Object Count)         |
+    10     +------+------+------+--------+-----------------+-------+---------------------------------+
+    11     | ACCESS, COMPARE HOST DATA, READ and WRITE end message offsets:                          |
+    12     |  32  |  40  |  20  | P.TRBC | MSCP$L_TAPEREC  |   4   | Tape Record Byte Count          |
+    13     +------+------+------+--------+-----------------+-------+---------------------------------+
+    14     | REPOSITION end message offsets:               |       |                                 |
+    15     |  12  |  14  |   C  | P.RCSK | MSCP$L_RCSKIPED |   4   | Records Skipped / Undefined     |
+    16     |  16  |  20  |  10  | P.TMSK | MSCP$L_TMSKIPED |   4   | Tape Marks Skipped / Undefined  |
+    17     +------+------+------+--------+-----------------+-------+---------------------------------+
+    18     | Notes: 1. An asterisk (*) in the field size indicates that the size varies.             |
+    19     +-----------------------------------------------------------------------------------------+
+
+
+    20                               Table A-8:  Error Log Message Offsets
+
+    21     +--------------------+--------------------------+-------+---------------------------------+
+    22     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+    23     +------+------+------+--------+-----------------+ Size  | Field Description               |
+    24     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+    25     +------+------+------+--------+-----------------+-------+---------------------------------+
+    26     | Generic Error Log Message Offsets:            |       |                                 |
+    27     |   0  |   0  |   0  | L.CRF  | MSLG$L_CMD_REF  |   4   | Command reference number        |
+    28     |   4  |   4  |   4  | L.UNIT | MSLG$W_UNIT     |   2   | Unit number                     |
+    29     |   6  |   6  |   6  | L.SEQ  | MSLG$W_SEQ_NUM  |   2   | Sequence number                 |
+    30     |   8  |  10  |   8  | L.FMT  | MSLG$B_FORMAT   |   1   | Format                          |
+    31     |   9  |  11  |   9  | L.FLGS | MSLG$B_FLAGS    |   1   | Error log message flags         |
+    32     |  10  |  12  |   A  | L.EVNT | MSLG$W_EVENT    |   2   | Event code                      |
+    33     |  12  |  14  |   C  | L.CNTI | MSLG$Q_CNT_ID   |   8   | Controller ID                   |
+    34     |  20  |  24  |  14  | L.CSVR | MSLG$B_CNT_SVR  |   1   | Controller software version     |
+    35     |  21  |  25  |  15  | L.CHVR | MSLG$B_CNT_HVR  |   1   | Controller hardware version     |
+    36     |  22  |  26  |  16  | L.MLUN | MSLG$W_MULT_UNT |   2   | Multiunit code                  |
+    37     |  24  |  30  |  18  | L.UNTI | MSLG$Q_UNIT_ID  |   8   | Unit ID                         |
+    38     |  32  |  40  |  20  | L.USVR | MSLG$B_UNIT_SVR |   1   | Unit software version           |
+    39     |  33  |  41  |  21  | L.UHVR | MSLG$B_UNIT_HVR |   1   | Unit hardware version           |
+    40     |  34  |  42  |  22  |        |                 |   2   | Reserved                        |
+    41     |  36  |  44  |  24  | L.GPCT | MSLG$L_GAP_CNT  |   4   | Position (object count)         |
+    42     |  40  |  50  |  28  | L.FSVR | MSLG$B_FMTR_SVR |   1   | Formatter software version      |
+    43     |  41  |  51  |  29  | L.FHVR | MSLG$B_FMTR_HVR |   1   | Formatter hardware version      |
+    44     +------+------+------+--------+-----------------+-------+---------------------------------+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                               Page A-10
+                                                                                       8 November 1987
+
+
+
+     1                            Table A-8: Error Log Message Offsets (cont.)
+
+     2     +--------------------+--------------------------+-------+---------------------------------+
+     3     |    Offset Value    |   Preferred Mnemonics    | Field |                                 |
+     4     +------+------+------+--------+-----------------+ Size  | Field Description               |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |(bytes)|                                 |
+     6     +------+------+------+--------+-----------------+-------+---------------------------------+
+     7     | Controller Error with Controller Dependent Information Offsets:                         |
+     8     |  22  |  26  |  16  |        |                 |   *   | Controller dependent information|
+     9     |      |      |      |        |                 |       | (optional)                      |
+    10     +------+------+------+--------+-----------------+-------+---------------------------------+
+    11     | Memory Errors with Host Bus Address Error Log Message Offsets:                          |
+    12     |  22  |  26  |  16  |        |                 |   2   | Reserved                        |
+    13     |  24  |  30  |  18  | L.BADR | MSLG$L_BUS_ADDR |   4   | Host bus memory address         |
+    14     |  28  |  34  |  1C  |        |                 |   *   | Controller dependent information|
+    15     |      |      |      |        |                 |       | (optional)                      |
+    16     +------+------+------+--------+-----------------+-------+---------------------------------+
+    17     | Memory Errors with Controller Memory Address Error Log Message Offsets:                 |
+    18     |  22  |  26  |  16  |        |                 |   2   | Reserved                        |
+    19     |  24  |  30  |  18  | L.BADR | MSLG$L_BUS_ADDR |   4   | Host bus memory address         |
+    20     |  28  |  34  |  1C  |        |                 |   *   | Controller dependent information|
+    21     |      |      |      |        |                 |       | (optional)                      |
+    22     +------+------+------+--------+-----------------+-------+---------------------------------+
+    23     | Tape Errors Error Log Message Offsets:        |       |                                 |
+    24     |  34  |  42  |  22  | L.LVL  | MSLG$B_LEVEL    |   1   | Level                           |
+    25     |  35  |  43  |  23  | L.RTRY | MSLG$B_RETRY    |   1   | Retry                           |
+    26     |  42  |  52  |  2A  |        |                 |   2   | Reserved                        |
+    27     |  44  |  54  |  2C  |        |                 |   *   | Device Dependent Information    |
+    28     +------+------+------+--------+-----------------+-------+---------------------------------+
+    29     | STI Communication Error Error Log Message Offsets:    |                                 |
+    30     |  42  |  52  |  2A  |        |                 |   2   | Reserved                        |
+    31     |  44  |  54  |  2C  | L.STI  | MSLG$Z_STI      |  12   | STI UNSUCCESSFUL Response       |
+    32     +------+------+------+--------+-----------------+-------+---------------------------------+
+    33     | STI Formatter Error Error Log Message Offsets:|       |                                 |
+    34     |  42  |  52  |  2A  |        |                 |   2   | Reserved                        |
+    35     |  44  |  54  |  2C  | L.STI  | MSLG$Z_STI      |  20   | STI GET EXTENDED FORMATTER      |
+    36     |      |      |      |        |                 |       | STATUS Response                 |
+    37     +------+------+------+--------+-----------------+-------+---------------------------------+
+    38     | STI Drive Error Error Log Message Offsets:    |       |                                 |
+    39     |  42  |  52  |  2A  |        |                 |   2   | Reserved                        |
+    40     |  44  |  54  |  2C  | L.STI  | MSLG$Z_STI      |  62   | STI GET EXTENDED DRIVE STATUS   |
+    41     |      |      |      |        |                 |       | Response                        |
+    42     +------+------+------+--------+-----------------+-------+---------------------------------+
+    43     | Note: An asterisk (*) in the field size indicates that the size varies.                 |
+    44     +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           OPCODE, FLAG AND OFFSET DEFINITIONS                                               Page A-11
+                                                                                       8 November 1987
+
+
+     1                             Table A-9:  Error Log Message Format Codes
+
+     2     +--------------------+--------------------------+-----------------------------------------+
+     3     |    Format Code     |   Preferred Mnemonics    |                                         |
+     4     +------+------+------+--------+-----------------+   Format Description                    |
+     5     | Dec. | Oct. | Hex. | 16 bit |      32 bit     |                                         |
+     6     +------+------+------+--------+-----------------+-----------------------------------------+
+     7     |   0  |   0  |   0  | FM.CNT | MSLG$K_CNT_ERR  | Controller Errors                       |
+     8     |   1  |   1  |   1  | FM.BAD | MSLG$K_BUS_ADDR | Memory Errors with (host/controller)    |
+     9     |      |      |      |        |                 | Memory Address                          |
+    10     |   5  |   5  |   5  | FM.TPE | MSLG$K_TAPE_ERR | Tape Errors                             |
+    11     |   6  |   6  |   6  | FM.STI | MSLG$K_STI_ERR  | STI Communication or Command Failure    |
+    12     |   7  |   7  |   7  | FM.DEL | MSLG$K_STI_DEL  | STI Drive Error Log                     |
+    13     |   8  |  10  |   8  | FM.FEL | MSLG$K_STI_FEL  | STI Formatter Error Log                 |
+    14     +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+    15                                Table A-10:  Error Log Message Flags
+
+    16     +------+--------------+--------------------------+----------------------------------------+
+    17     | Bit  |   Bit Mask   |   Preferred Mnemonics    |                                        |
+    18     |Number+-------+------+--------+-----------------+   Format Description                   |
+    19     |      | Octal | Hex. | 16 bit |32 bit (See note)|                                        |
+    20     +------+-------+------+--------+-----------------+----------------------------------------+
+    21     |   7  |   200 |   80 | LF.SUC | MSLG$x_LF_SUCC  | Operation Successful                   |
+    22     |   6  |   100 |   40 | LF.CON | MSLG$x_LF_CONT  | Operation Continuing                   |
+    23     |   0  |     1 |    1 | LF.SNR | MSLG$x_LF_SQNRS | Sequence Number Reset                  |
+    24     +------+-------+------+--------+-----------------+----------------------------------------+
+    25     | Notes: 1. The  "x" in  the 32 bit mnemonic for a bit flag will be "V" if the symbol  is |
+    26     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    27     +-----------------------------------------------------------------------------------------+
+
+
+    28                   Table A-11:  Access Nonvolatile Memory Command Operation Codes
+
+    29     +--------------------+--------------------------+-----------------------------------------+
+    30     |    Opcode Value    |   Preferred Mnemonics    |                                         |
+    31     +------+------+------+--------+-----------------+   Operation                             |
+    32     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+    33     +------+------+------+--------+-----------------+-----------------------------------------+
+    34     |   0  |   0  |   0  | ANM.RD | MSCP$K_ANM_READ | Read                                    |
+    35     |   1  |   1  |   1  | ANM.EX | MSCP$K_ANM_EXCG | Exchange                                |
+    36     |   2  |   2  |   2  | ANM.TS | MSCP$K_ANM_TSST | Test and Set                            |
+    37     +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-1
+                                                                                       8 November 1987
+                           
+
+     1                                             APPENDIX B
+
+     2                                  STATUS AND EVENT CODE DEFINITIONS
+
+
+
+     3                                                NOTES
+
+
+     4             1.  The combination of a status or event code  with  a  subcode  should  be
+     5                 expressed (assuming 16 bit symbols) as:
+
+     6                                        (subcode * ST.SUB) + code
+
+     7             2.  The subcode values shown in Table B-2 are only returned as status codes
+     8                 in  end  messages.  They are never returned as event codes in error log
+     9                 messages.  In addition, the references to other tables within Table B-2
+    10                 indicates that those tables contain subcodes which are only returned as
+    11                 status codes in end messages.
+
+    12             3.  The subcode values shown in Tables B-3 through B-7 may be returned  (as
+    13                 indicated in the "EV" and "ST" columns) as status codes, event codes or
+    14                 both.  In those tables, an asterisk in the "EV" column  indicates  that
+    15                 the  code and subcode may be returned as an event code.  An asterisk in
+    16                 the "ST" column indicates that the code and subcode may be returned  as
+    17                 a status code.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-2
+                                                                                       8 November 1987
+
+
+     1                                 Table B-1:  Status and Event Codes
+
+     2     +--------------------+--------------------------+-----------------------------------------+
+     3     |       Value        |   Preferred Mnemonics    |                                         |
+     4     +------+------+------+--------+-----------------+     Status or Event Code                |
+     5     | Dec. | Oct. | Hex. | 16 bit |     32 bit      |                                         |
+     6     +------+------+------+--------+-----------------+-----------------------------------------+
+     7     |   31 |   37 |   1F | ST.MSK | MSCP$M_ST_MASK  | Status / event code mask                |
+     8     |   32 |   40 |   20 | ST.SUB | MSCP$M_ST_SUBCD | Subcode multiplier                      |
+     9     |      |      |      |        |                 |                                         |
+    10     |    0 |    0 |    0 | ST.SUC | MSCP$K_ST_SUCC  | Success                                 |
+    11     |    1 |    1 |    1 | ST.CMD | MSCP$K_ST_ICMD  | Invalid command                         |
+    12     |    2 |    2 |    2 | ST.ABO | MSCP$K_ST_ABRTD | Command Aborted                         |
+    13     |    3 |    3 |    3 | ST.OFL | MSCP$K_ST_OFFLN | Unit-Offline                            |
+    14     |    4 |    4 |    4 | ST.AVL | MSCP$K_ST_AVLBL | Unit-Available                          |
+    15     |    5 |    5 |    5 | ST.MFE | MSCP$K_ST_MFMTE | Media Format Error                      |
+    16     |    6 |    6 |    6 | ST.WPR | MSCP$K_ST_WRTPR | Write Protected                         |
+    17     |    7 |    7 |    7 | ST.CMP | MSCP$K_ST_COMP  | Compare Error                           |
+    18     |    8 |   10 |    8 | ST.DAT | MSCP$K_ST_DATA  | Data Error                              |
+    19     |    9 |   11 |    9 | ST.HST | MSCP$K_ST_HSTBF | Host Buffer Access Error                |
+    20     |   10 |   12 |    A | ST.CNT | MSCP$K_ST_CNTLR | Controller Error                        |
+    21     |   11 |   13 |    B | ST.DRV | MSCP$K_ST_DRIVE | Drive Error                             |
+    22     |   12 |   14 |    C | ST.FMT | MSCP$K_ST_FMTER | Formatter Error                         |
+    23     |   13 |   15 |    D | ST.BOT | MSCP$K_ST_BOT   | BOT Encountered                         |
+    24     |   14 |   16 |    E | ST.TM  | MSCP$K_ST_TAPEM | Tape Mark Encountered                   |
+    25     |   15 |   17 |    F |   -    |        -        | unassigned                              |
+    26     |   16 |   20 |   10 | ST.RDT | MSCP$K_ST_RDTRN | Record Data Truncated                   |
+    27     |   17 |   21 |   11 | ST.POL | MSCP$K_ST_PLOST | Position Lost                           |
+    28     |   18 |   22 |   12 | ST.SEX | MSCP$K_ST_SEX   | Serious Exception                       |
+    29     |   19 |   23 |   13 | ST.LED | MSCP$K_ST_LED   | LEOT Detection                          |
+    30     | 20-30| 24-36| 14-1E|    -   |        -        | unassigned                              |
+    31     |   31 |   37 |   1F | ST.DIA | MSCP$K_ST_DIAG  | Message from an internal diagnostic     |
+    32     +------+------+------+--------+-----------------+-----------------------------------------+
+
+
+
+
+
+
+
+           
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-3
+                                                                                       8 November 1987
+
+
+     1                               Table B-2:  Status Only Subcode Values
+
+     2      +------+---------------------+---------------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |                                                         |
+     4      | code +------+-------+------+                 Status Subcode                          |
+     5      |      | Dec. |  Oct. | Hex. |                                                         |
+     6      +------+------+-------+------+---------------------------------------------------------+
+     7      | "Success" Subcode values:  |                                                         |
+     8      |    0 |    0 |     0 |    0 | Normal                                                  |
+     9      |    1 |   32 |    40 |   20 | Unload Ignored                                          |
+    10      |    2 |   64 |   100 |   40 | Still Connected                                         |
+    11      |    4 |  128 |   200 |   80 | Duplicate Unit Number                                   |
+    12      |    8 |  256 |   400 |  100 | Already Online                                          |
+    13      |   16 |  512 |  1000 |  200 | Still Online                                            |
+    14      |   32 | 1024 |  2000 |  400 | EOT Encountered                                         |
+    15      |  128 | 4096 | 10000 | 1000 | Read Only Volume Format                                 |
+    16      +------+------+-------+------+---------------------------------------------------------+
+    17      | "Invalid Command" Subcode values:                                                    |
+    18      |    0 |    1 |     1 |    1 | Invalid Message Length                                  |
+    19      | many |      |       |      | Other  "Invalid  Command"  subcodes  values  should  be |
+    20      |      |      |       |      | referenced  as  follows  (note  that  this  is combined |
+    21      |      |      |       |      | with the status code):                                  |
+    22      |      |      |       |      |                                                         |
+    23      |      |      |       |      |      offset*256.+code                                   |
+    24      |      |      |       |      |                                                         |
+    25      |      |      |       |      | where "offset" is the command message offset symbol for |
+    26      |      |      |       |      | the  field  in  error  and "code" is the symbol for the |
+    27      |      |      |       |      | "Invalid Command" status code.                          |
+    28      +------+------+-------+------+---------------------------------------------------------+
+    29      | "Command Aborted" Subcode values:                                                    |
+    30      |    0 |    2 |     2 |    2 | Subcodes are not used.                                  |
+    31      +------+------+-------+------+---------------------------------------------------------+
+    32      | "Unit-Offline" Subcode values:                                                       |
+    33      |    0 |    3 |     3 |    3 | Unit unknown or online to another controller.           |
+    34      |    1 |   35 |    43 |   23 | No media mounted or drive disabled via switch setting   |
+    35      |    2 |   67 |   103 |   43 | Unit is inoperative                                     |
+    36      |    4 |  131 |   203 |   83 | Duplicate unit number                                   |
+    37      |    8 |  259 |   403 |  103 | Unit disabled by field service or diagnostic.           |
+    38      |   16 |  515 |  1003 |  203 | Exclusive Use                                           |
+    39      +------+------+-------+------+---------------------------------------------------------+
+    40      | "Unit-Available" Subcode values:                                                     |
+    41      |    0 |    4 |     4 |    4 | Available                                               |
+    42      |   32 | 1028 |  2004 |  404 | Already In use                                          |
+    43      +------+------+-------+------+---------------------------------------------------------+
+    44      | "Media Format Error" Subcode values:                                                 |
+    45      |    5 |  165 |   245 |   A5 | Block mode device not formatted for tape operations.    |
+    46      +------+------+-------+------+---------------------------------------------------------+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-4
+                                                                                       8 November 1987
+
+
+
+     1                            Table B-2: Status Only Subcode Values (cont.)
+
+     2      +------+---------------------+---------------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |                                                         |
+     4      | code +------+-------+------+                 Status Subcode                          |
+     5      |      | Dec. |  Oct. | Hex. |                                                         |
+     6      +------+------+-------+------+---------------------------------------------------------+
+     7      | "Write Protected" Subcode values:                                                    |
+     8      |    8 |  262 |   406 |  106 | Unit is Data Safety Write Protected                     |
+     9      |  128 | 4102 | 10006 | 1006 | Unit is Software Write Protected                        |
+    10      |  256 | 8198 | 20006 | 2006 | Unit is Hardware Write Protected                        |
+    11      +------+------+-------+------+---------------------------------------------------------+
+    12      | "Compare Error" Subcode values - see Table B-3                                       |
+    13      +------+------+-------+------+---------------------------------------------------------+
+    14      | "Data Error" Subcode values - see Table B-4                                          |
+    15      +------+------+-------+------+---------------------------------------------------------+
+    16      | "Host Buffer Access Error" Subcode values - see Table B-5                            |
+    17      +------+------+-------+------+---------------------------------------------------------+
+    18      | "Controller Error" Subcode values - see Table B-6                                    |
+    19      +------+------+-------+------+---------------------------------------------------------+
+    20      | "Drive Error" Subcode values - see Table B-7                                         |
+    21      +------+------+-------+------+---------------------------------------------------------+
+    22      | "Formatter Error" Subcode values - see Table B-8                                     |
+    23      +------+------+-------+------+---------------------------------------------------------+
+    24      | "BOT Encountered" Subcode values:                                                    |
+    25      |    0 |   13 |    15 |    D | Subcodes are not used                                   |
+    26      +------+------+-------+------+---------------------------------------------------------+
+    27      | "Tape Mark Encountered" Subcode values:                                              |
+    28      |    0 |   14 |    16 |    E | Subcodes are not used                                   |
+    29      +------+------+-------+------+---------------------------------------------------------+
+    30      | "Record Data Truncated" Subcode values:                                              |
+    31      |    0 |   16 |    20 |   10 | Subcodes are not used.                                  |
+    32      +------+------+-------+------+---------------------------------------------------------+
+    33      | "Position Lost" Subcode values:                                                      |
+    34      |    0 |   17 |    21 |   11 | Subcodes are not used.                                  |
+    35      +------+------+-------+------+---------------------------------------------------------+
+    36      | "Serious Exception" Subcode values:                                                  |
+    37      |    0 |   18 |    22 |   12 | Subcodes are not used.                                  |
+    38      +------+------+-------+------+---------------------------------------------------------+
+    39      | "LEOT Encountered" Subcode values:                                                   |
+    40      |    0 |   19 |    23 |   13 | Subcodes are not used.                                  |
+    41      +------+------+-------+------+---------------------------------------------------------+
+    42      | "Message from an internal diagnostic" Subcode values - see Table B-9                 |
+    43      +------+------+-------+------+---------------------------------------------------------+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-5
+                                                                                       8 November 1987
+
+
+     1                     Table B-3:  "Compare Error" Status or Event Subcode Values
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    0 |    7 |    7  |    7 |*|*| Subcodes  are  not  used.  Note: the  compare error |
+     8      |      |      |       |      | | | code  is only used  as an event code when the error |
+     9      |      |      |       |      | | | occurs   during   a read-compare  or  write-compare |
+    10      |      |      |       |      | | | operation.                                          |
+    11      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+    12                      Table B-4:  "Data Error" Status or Event Subcode Values
+
+    13      +------+---------------------+-+-+-----------------------------------------------------+
+    14      | Sub- |   Code + Subcode    |E|S|                                                     |
+    15      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    16      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    17      +------+------+-------+------+-+-+-----------------------------------------------------+
+    18      |    0 |    8 |    10 |    8 |*|*| Long Gap Encountered                                |
+    19      |    7 |  232 |   350 |   E8 |*|*| Unrecoverable Read Error                            |
+    20      | many |   -  |   -   |   -  |*|*| controller and/or driver/formatter type dependent   |
+    21      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+    22               Table B-5:  "Host Buffer Access Error" Status or Event Subcode Values
+
+    23      +------+---------------------+-+-+-----------------------------------------------------+
+    24      | Sub- |   Code + Subcode    |E|S|                                                     |
+    25      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    26      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    27      +------+------+-------+------+-+-+-----------------------------------------------------+
+    28      |    0 |    9 |    11 |    9 | |*| Host Buffer Access error, cause not available.      |
+    29      |      |      |       |      | | |   The controller was unable to access a host buffer |
+    30      |      |      |       |      | | |   to perform a transfer, but has no visibility into |
+    31      |      |      |       |      | | |   the cause of the error.                           |
+    32      +------+------+-------+------+-+-+-----------------------------------------------------+
+    33      |    3 |  105 |   151 |   69 |*|*| Nonexistent memory error                            |
+    34      +------+------+-------+------+-+-+-----------------------------------------------------+
+    35      |    4 |  137 |   211 |   89 |*|*| Host memory parity error                            |
+    36      +------+------+-------+------+-+-+-----------------------------------------------------+
+    37      |    5 |  169 |   251 |   A9 |*|*| Invalid page  table entry                           |
+    38      |      |      |       |      | | |   (See    UNIBUS/QBUS    Storage    Systems    Port |
+    39      |      |      |       |      | | |   Specification for  additional detail.)            |
+    40      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-6
+                                                                                       8 November 1987
+
+
+
+     1            Table B-5: "Host Buffer Access Error" Status or Event Subcode Values (cont.)
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      |    6 |  201 |   311 |   C9 |*|*| Invalid buffer name                                 |
+     8      |      |      |       |      | | |   The key in the buffer name does not match the key |
+     9      |      |      |       |      | | |   in the buffer descriptor, the V bit in the buffer |
+    10      |      |      |       |      | | |   descriptor is clear, or the index into the Buffer |
+    11      |      |      |       |      | | |   Descriptor  Table  is  too large. (See the BI Vax |
+    12      |      |      |       |      | | |   Port Specification for additional detail.)        |
+    13      +------+------+-------+------+-+-+-----------------------------------------------------+
+    14      |    7 |  233 |   351 |   E9 |*|*| Buffer length violation                             |
+    15      |      |      |       |      | | |   The  number  of  bytes  requested  in  the  TMSCP |
+    16      |      |      |       |      | | |   command   exceeds  the buffer length as specified |
+    17      |      |      |       |      | | |   in   the  buffer  descriptor.  (See  the  BI  Vax |
+    18      |      |      |       |      | | |   Port Specification for additional detail.)        |
+    19      +------+------+-------+------+-+-+-----------------------------------------------------+
+    20      |    8 |  265 |   411 |  109 |*|*| Access control violation                            |
+    21      |      |      |       |      | | |   The   access   mode  specified  in   the   buffer |
+    22      |      |      |       |      | | |   descriptor is protected against by the PROT field |
+    23      |      |      |       |      | | |   in  the PTE.  (See the BI Vax Port  Specification |
+    24      |      |      |       |      | | |   for additional detail.)                           |
+    25      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+    26                   Table B-6:  "Controller Error" Status or Event Subcode Values
+
+    27      +------+---------------------+-+-+-----------------------------------------------------+
+    28      | Sub- |   Code + Subcode    |E|S|                                                     |
+    29      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    30      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    31      +------+------+-------+------+-+-+-----------------------------------------------------+
+    32      |    0 |   10 |    12 |    A |-|-| Reserved  for host  detected command  timeout error |
+    33      |      |      |       |      | | | logging.  This  subcode  is  never  reported  by  a |
+    34      |      |      |       |      | | | controller.                                         |
+    35      +------+------+-------+------+-+-+-----------------------------------------------------+
+    36      | many |   -  |   -   |   -  |*|*| controller and/or drive/formatter type dependent    |
+    37      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           STATUS AND EVENT CODE DEFINITIONS                                                  Page B-7
+                                                                                       8 November 1987
+
+
+     1                      Table B-7:  "Drive Error" Status or Event Subcode Values
+
+     2      +------+---------------------+-+-+-----------------------------------------------------+
+     3      | Sub- |   Code + Subcode    |E|S|                                                     |
+     4      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+     5      |      | Dec. |  Oct. | Hex. | | |                                                     |
+     6      +------+------+-------+------+-+-+-----------------------------------------------------+
+     7      | many |   -  |   -   |   -  |*|*| controller and/or drive/formatter type dependent    |
+     8      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+     9                    Table B-8:  "Formatter Error" Status or Event Subcode Values
+
+    10      +------+---------------------+-+-+-----------------------------------------------------+
+    11      | Sub- |   Code + Subcode    |E|S|                                                     |
+    12      | code +------+-------+------+V|T|     Status or Event Subcode                         |
+    13      |      | Dec. |  Oct. | Hex. | | |                                                     |
+    14      +------+------+-------+------+-+-+-----------------------------------------------------+
+    15      | many |   -  |   -   |   -  |*|*| controller and/or drive/formatter type dependent    |
+    16      +------+------+-------+------+-+-+-----------------------------------------------------+
+
+
+    17            Table B-9:  "Message From An Internal Diagnostic" Event Only Subcode Values
+
+    18      +------+---------------------+---------------------------------------------------------+
+    19      | Sub- |   Code + Subcode    |                                                         |
+    20      | code +------+-------+------+         Event Subcode                                   |
+    21      |      | Dec. |  Oct. | Hex. |                                                         |
+    22      +------+------+-------+------+---------------------------------------------------------+
+    23      | many |   -  |   -   |   -  | controller and/or drive/formatter type dependent        |
+    24      +------+------+-------+------+---------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           MISCELLANEOUS TABLES                                                               Page C-1
+                                                                                       8 November 1987
+
+
+     1                                             APPENDIX C
+
+     2                                        MISCELLANEOUS TABLES
+
+
+     3                                Table C-1:  Tape Format Flag Values
+
+     4     +---------------------+--------------------------+-+--------------------------------------+
+     5     |        Value        |  Preferred Code Mnemonic |N|                                      |
+     6     |-------+------+------+--------+-----------------+I|    Tape Format Flag                  |
+     7     |  Dec. | Oct. | Hex. | 16 bit |      32 bit     |S|                                      |
+     8     +-------+------+------+--------+-----------------+-+--------------------------------------+
+     9     |  255  |  377 |   FF | TF.MSK | MSCP$M_TF_MASK  | | Bitflag mask                         |
+    10     |  256  |  400 |  100 | TF.COD | MSCP$K_TF_CODE  | | Format code multiplier               |
+    11     +-------+------+------+--------+-----------------+-+--------------------------------------+
+    12     |    0  |    0 |    0 | TC.OLD | MSCP$K_TC_OLD   | | Old value (See notes 1, 4)           |
+    13     |  256  |  400 |  100 | TC.9TR | MSCP$K_TC_9TRACK| | New "9 track" devices                |
+    14     |  512  | 1000 |  200 | TC.CTP | MSCP$K_TC_CTP   |*| "TK50" compatible cart tapes (note 3)|
+    15     |  768  | 1400 |  300 | TC.348 | MSCP$K_TC_3480  | | "IBM 3480" compatible cartridge tapes|
+    16     | 1024  | 2000 |  400 | TC.W1  | MSCP$K_TC_W1    |*| "RV80" compatible write once cart.   |
+    17     +-------+------+------+--------+-----------------+----------------------------------------+
+    18     | Note:  1. The  TC.OLD value is for backwards compatibility with existing products.  New |
+    19     |           devices  are  prohibited from using the TC.OLD value.  Products having  media |
+    20     |           different  from those associated with the above TC.xxx values must  ECO  this |
+    21     |           specification to include a new TC.xxx value for their media.  Products having |
+    22     |           media  identical to one of those associated with the above TC.xxx value  must |
+    23     |           use that TC.xxx value.                                                        |
+    24     |                                                                                         |
+    25     |        2. Formats  with  an asterisk in the "NIS" column are  considered  "non-industry |
+    26     |           standard".  Devices  which  utilize these formats thus need not  support  the |
+    27     |           "Reverse"  command modifier on ACCESS, COMPARE HOST DATA, and READ  commands. |
+    28     |           Whether  or  not  a  device  supporting one of  these  formats  supports  the |
+    29     |           "Reverse"  command modifier on the above commands must be documented in  that |
+    30     |           device's  functional  specification  (See section 3.3.1.1).                   |
+    31     |                                                                                         |
+    32     |        3. Devices  that support this format may not follow the normal ABORT process for |
+    33     |           REPOSITION commands with the "Reverse" modifier set.  See section 3.2.1.2.    |
+    34     |                                                                                         |
+    35     |        4. Some  devices  that  report  a  tape code of  TC.OLD  are  considered  to  be |
+    36     |           equivalent to devices that report a tape code of TC.CTP with respect to notes |
+    37     |           2 and 3 (see Table C-2).                                                      |
+    38     +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           MISCELLANEOUS TABLES                                                               Page C-2
+           Table C-1: Tape Format Flag Values                                          8 November 1987
+
+
+     1                               Table C-2:  Tape Format Bitflag Values
+
+     2     +------+---------------------+--------------------------+---------------------------------+
+     3     |      |    Code + Bitflag   |Preferred Bitflag Mnemonic|                                 |
+     4     | Bit- +-------+------+------+--------+-----------------+    Tape Format Bitflag          |
+     5     | flag |  Dec. | Oct. | Hex. | 16 bit |32 bit (see note)|                                 |
+     6     +------+-------+------+------+--------+-----------------+---------------------------------+
+     7     | "OLD" bitflag values:      |        |                 |                                 |
+     8     |   0  |     1 |    1 |    1 | TF.800 | MSCP$x_TF_800   | NRZI 800 bpi                    |
+     9     |   1  |     2 |    2 |    2 | TF.PE  | MSCP$x_TF_PE    | Phase Encoded 1600 bpi          |
+    10     |   2  |     4 |    4 |    4 | TF.GCR | MSCP$x_TF_GCR   | Group Code Recording 6250 bpi   |
+    11     |   3  |     8 |   10 |    8 | TF.BLK | MSCP$x_TF_BLOCK | Cartridge Block Mode (note 3)   |
+    12     +------+-------+------+------+--------+-----------------+---------------------------------+
+    13     | "9TR" bitflag values:      |        |                 |                                 |
+    14     |   0  |   257 |  401 |  101 | TF.800 | MSCP$x_TF_800   | NRZI 800 bpi                    |
+    15     |   1  |   258 |  402 |  102 | TF.PE  | MSCP$x_TF_PE    | Phase Encoded 1600 bpi          |
+    16     |   2  |   260 |  404 |  104 | TF.GCR | MSCP$x_TF_GCR   | Group Code Recording 6250 bpi   |
+    17     +------+-------+------+------+--------+-----------------+---------------------------------+
+    18     | "CTP" bitflag values:      |        |                 |                                 |
+    19     |   0  |   513 | 1001 |  201 | TF.NOR | MSCP$x_TF_NORML | Normal (low) density            |
+    20     |   1  |   514 | 1002 |  202 | TF.BHD | MSCP$x_TF_BLKHD | High density                    |
+    21     +------+-------+------+------+--------+-----------------+---------------------------------+
+    22     | "IBM 3480" bitflag values: |        |                 |                                 |
+    23     |   0  |   769 | 1401 |  301 | TF.NOR | MSCP$x_TF_NORML | Normal density                  |
+    24     +------+-------+------+------+--------+-----------------+---------------------------------+
+    25     | "W1" bitflag values:       |        |                 |                                 |
+    26     |   0  |  1025 | 2001 |  401 | TF.NOR | MSCP$x_TF_NORML | Normal density                  |
+    27     +------+-------+------+------+--------+-----------------+---------------------------------+
+    28     | Notes: 1. The  "x" in  the 32 bit mnemonic for a bit flag will be "V" if the symbol  is |
+    29     |           defined as a bit number (offset) or an "M" if defined as a mask.              |
+    30     |                                                                                         |
+    31     |        2. The  "Preferred Bitflag Mnemonic" is the mnemonic for the bitflag  only.  The |
+    32     |           "Code + Bitflag" values are the sum of the tape format flag from the previous |
+    33     |           table  and  the bitflag from this table.  Thus, bitflags that have  the  same |
+    34     |           meaning  and  bit  value for different tape formats have  the  same  mnemonic |
+    35     |           (i.e., TF.800 for the TC.OLD and TC.9TR tape formats).                        |
+    36     |                                                                                         |
+    37     |        3. A  code + bitflag value of TC.OLD + TF.BLK is equivalent to a  code + bitflag |
+    38     |           value of TC.CTP + TF.NOR with respect to functional behavior of devices.      |
+    39     +-----------------------------------------------------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           MISCELLANEOUS TABLES                                                               Page C-3
+           Table C-2: Tape Format Bitflag Values                                       8 November 1987
+
+
+     1                        Table C-3:  Controller Specific Maximum Record Size
+
+     2     +----------------------------------------------------------+------------------------------+
+     3     | Controller Type                                          | Maximum Record Size in Bytes |
+     4     |                                                          |                              |
+     5     +----------------------------------------------------------+------------------------------+
+     6     | U/Q Bus Direct Access Controllers                        | 64K (preferred)              |
+     7     | HSC50                                                    | 64K                          |
+     8     | TU81                                                     | 64K-1                        |
+     9     | TK50                                                     | 64K-1                        |
+    10     +----------------------------------------------------------+------------------------------+
+
+
+    11                            Table C-4:  Format Specific Long Gap Values
+
+    12     +-----------------------------------------+-----------------------------------------------+
+    13     | Encoding Format Descriptor              | Log Gap Value                                 |
+    14     |                                         |                                               |
+    15     +-----------------------------------------+-----------------------------------------------+
+    16     | Phase Encoding (PE) 1600 bpi            | 25 linear feet                                |
+    17     | Group Code Recording (GCR) 6250 bpi     | 15 linear feet                                |
+    18     | Cartridge Block Format                  | device dependent, see device spec for details |
+    19     +-----------------------------------------+-----------------------------------------------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           REPOSITION COMMAND VARIATIONS                                                      Page D-1
+                                                                                       8 November 1987
+
+
+     1                                             APPENDIX D
+
+     2                                    REPOSITION COMMAND VARIATIONS
+
+
+
+     3                                                NOTES
+
+
+     4             1.  Square brackets ([]) indicate optional setting.
+
+     5             2.  An asterisk (*) indicates that the  modifier  setting  or  count  field
+     6                 value  is  ignored  by  the controller due to an interdependence on the
+     7                 setting of another modifier or the value of the count fields.
+
+     8             3.  See Section 5.10.3 specific information related to controller execution
+     9                 of combined basic functions.
+
+    10             4.  Certain  combinations  of   the   repositioning   functions,   although
+    11                 permissible,  are  illogical  since the result is always termination of
+    12                 the command with a status of BOT Encountered.
+
+
+
+    13                             Table D-1:  REPOSITION Command Variations
+
+    14     +------------------------------+----------------------------------------+--------+-------+
+    15     |                              |                                        |        |       |
+    16     |          REPOSITION          |                Modifiers               | Record | Tape  |
+    17     |          Command             +----------+-------+------+------+-------+   or   | Mark  |
+    18     |          Variation           |Immediate |Detect |Object|Rewind|Reverse| Object | Count |
+    19     |                              |Completion|LEOT   |Count |      |       | Count  |       |
+    20     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    21     |   a. sequential no-operation |     *    |   *   |  0   |  0   |   *   |    0   |   0   |
+    22     |                              |     *    |   *   |  1   |  0   |   *   |    0   |   *   |
+    23     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    24     |   a. rewind to BOT           |   0 [1]  |   *   |  0   |  1   |   *   |    0   |   0   |
+    25     |                              |   0 [1]  |   *   |  1   |  1   |   *   |    0   |   *   |
+    26     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    27     |   a. rewind to BOT           |     *    |   *   |  1   |  1   |   0   |   >0   |   *   |
+    28     |   b. skip   "object   count" |          |       |      |      |       |        |       |
+    29     |      tape    objects   (both |          |       |      |      |       |        |       |
+    30     |      records and tape marks) |          |       |      |      |       |        |       |
+    31     |      in      the     forward |          |       |      |      |       |        |       |
+    32     |      direction               |          |       |      |      |       |        |       |
+    33     +------------------------------+----------+-------+------+------+-------+--------+-------+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           REPOSITION COMMAND VARIATIONS                                                      Page D-2
+                                                                                       8 November 1987
+
+
+
+     1                          Table D-1: REPOSITION Command Variations (cont.)
+
+     2     +------------------------------+----------------------------------------+--------+-------+
+     3     |                              |                                        |        |       |
+     4     |          REPOSITION          |                Modifiers               | Record | Tape  |
+     5     |          Command             +----------+-------+------+------+-------+   or   | Mark  |
+     6     |          Variation           |Immediate |Detect |Object|Rewind|Reverse| Object | Count |
+     7     |                              |Completion|LEOT   |Count |      |       | Count  |       |
+     8     +------------------------------+----------+-------+------+------+-------+--------+-------+
+     9     |   a. rewind to BOT           |     *    |   *   |  1   |  1   |   1   |   >0   |   *   |
+    10     |   b. attempt to skip "object |          |       |      |      |       |        |       |
+    11     |      count"   tape   objects |          |       |      |      |       |        |       |
+    12     |      (both records and  tape |          |       |      |      |       |        |       |
+    13     |      marks) in  the  reverse |          |       |      |      |       |        |       |
+    14     |      direction               |          |       |      |      |       |        |       |
+    15     |   (See Note 4 above)         |          |       |      |      |       |        |       |
+    16     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    17     |   a. rewind to BOT           |     *    | 0 [1] |  0   |  1   |   0   |    0   |  >0   |
+    18     |   b. skip "tape mark  count" |          |       |      |      |       |        |       |
+    19     |      tape   marks   in   the |          |       |      |      |       |        |       |
+    20     |      forward direction       |          |       |      |      |       |        |       |
+    21     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    22     |   a. rewind to BOT           |     *    |   *   |  0   |  1   |   1   |    0   |  >0   |
+    23     |   b. attempt to  skip  "tape |          |       |      |      |       |        |       |
+    24     |      mark count" tape  marks |          |       |      |      |       |        |       |
+    25     |      in      the     reverse |          |       |      |      |       |        |       |
+    26     |      direction               |          |       |      |      |       |        |       |
+    27     |   (See Note 4 above)         |          |       |      |      |       |        |       |
+    28     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    29     |   a. rewind to BOT           |     *    | 0 [1] |  0   |  1   |   0   |   >0   |   0   |
+    30     |   b. skip   "record   count" |          |       |      |      |       |        |       |
+    31     |      records in the  forward |          |       |      |      |       |        |       |
+    32     |      direction               |          |       |      |      |       |        |       |
+    33     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    34     |   a. rewind to BOT           |     *    |   *   |  0   |  1   |   1   |   >0   |   0   |
+    35     |   b. attempt to skip "record |          |       |      |      |       |        |       |
+    36     |      count" records  in  the |          |       |      |      |       |        |       |
+    37     |      reverse direction       |          |       |      |      |       |        |       |
+    38     |   (See Note 4 above)         |          |       |      |      |       |        |       |
+    39     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    40     |   a. rewind to BOT           |     *    | 0 [1] |  0   |  1   |   0   |   >0   |  >0   |
+    41     |   b. skip "tape mark  count" |          |       |      |      |       |        |       |
+    42     |      tape   marks   in   the |          |       |      |      |       |        |       |
+    43     |      forward direction       |          |       |      |      |       |        |       |
+    44     |   c. skip   "record   count" |          |       |      |      |       |        |       |
+    45     |      records  in  that  same |          |       |      |      |       |        |       |
+    46     |      direction               |          |       |      |      |       |        |       |
+    47     +------------------------------+----------+-------+------+------+-------+--------+-------+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           REPOSITION COMMAND VARIATIONS                                                      Page D-3
+                                                                                       8 November 1987
+
+
+
+     1                          Table D-1: REPOSITION Command Variations (cont.)
+
+     2     +------------------------------+----------------------------------------+--------+-------+
+     3     |                              |                                        |        |       |
+     4     |          REPOSITION          |                Modifiers               | Record | Tape  |
+     5     |          Command             +----------+-------+------+------+-------+   or   | Mark  |
+     6     |          Variation           |Immediate |Detect |Object|Rewind|Reverse| Object | Count |
+     7     |                              |Completion|LEOT   |Count |      |       | Count  |       |
+     8     +------------------------------+----------+-------+------+------+-------+--------+-------+
+     9     |   a. rewind to BOT           |     *    |   *   |  0   |  1   |   1   |   >0   |  >0   |
+    10     |   b. attempt to  skip  "tape |          |       |      |      |       |        |       |
+    11     |      mark count" tape  marks |          |       |      |      |       |        |       |
+    12     |      in      the     reverse |          |       |      |      |       |        |       |
+    13     |      direction               |          |       |      |      |       |        |       |
+    14     |   c. attempt to skip "record |          |       |      |      |       |        |       |
+    15     |      count" records in  that |          |       |      |      |       |        |       |
+    16     |      same direction          |          |       |      |      |       |        |       |
+    17     |   (See Note 4 above)         |          |       |      |      |       |        |       |
+    18     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    19     |   a. skip   "object   count" |     *    |   *   |  1   |  0   |   0   |   >0   |   *   |
+    20     |      tape  objects  in   the |          |       |      |      |       |        |       |
+    21     |      forward direction       |          |       |      |      |       |        |       |
+    22     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    23     |   a. skip   "object   count" |     *    |   *   |  1   |  0   |   1   |   >0   |   *   |
+    24     |      tape  objects  in   the |          |       |      |      |       |        |       |
+    25     |      reverse direction       |          |       |      |      |       |        |       |
+    26     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    27     |   a. skip "tape mark  count" |     *    | 0 [1] |  0   |  0   |   0   |    0   |  >0   |
+    28     |      tape   marks   in   the |          |       |      |      |       |        |       |
+    29     |      forward direction       |          |       |      |      |       |        |       |
+    30     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    31     |   a. skip "tape mark  count" |     *    |   *   |  0   |  0   |   1   |    0   |  >0   |
+    32     |      tape   marks   in   the |          |       |      |      |       |        |       |
+    33     |      reverse direction       |          |       |      |      |       |        |       |
+    34     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    35     |   a. skip   "record   count" |     *    | 0 [1] |  0   |  0   |   0   |   >0   |   0   |
+    36     |      records in the  forward |          |       |      |      |       |        |       |
+    37     |      direction               |          |       |      |      |       |        |       |
+    38     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    39     |   a. skip   "record   count" |     *    |   *   |  0   |  0   |   1   |   >0   |   0   |
+    40     |      records in the  reverse |          |       |      |      |       |        |       |
+    41     |      direction               |          |       |      |      |       |        |       |
+    42     +------------------------------+----------+-------+------+------+-------+--------+-------+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation                                  Confidential And Proprietary
+           Tape Mass Storage Control Protocol                                            Version 2.0.0
+           REPOSITION COMMAND VARIATIONS                                                      Page D-4
+                                                                                       8 November 1987
+
+
+
+     1                          Table D-1: REPOSITION Command Variations (cont.)
+
+     2     +------------------------------+----------------------------------------+--------+-------+
+     3     |                              |                                        |        |       |
+     4     |          REPOSITION          |                Modifiers               | Record | Tape  |
+     5     |          Command             +----------+-------+------+------+-------+   or   | Mark  |
+     6     |          Variation           |Immediate |Detect |Object|Rewind|Reverse| Object | Count |
+     7     |                              |Completion|LEOT   |Count |      |       | Count  |       |
+     8     +------------------------------+----------+-------+------+------+-------+--------+-------+
+     9     |   a. skip "tape mark  count" |     *    | 0 [1] |  0   |  0   |   0   |   >0   |  >0   |
+    10     |      tape   marks   in   the |          |       |      |      |       |        |       |
+    11     |      forward direction       |          |       |      |      |       |        |       |
+    12     |   b. skip   "record   count" |          |       |      |      |       |        |       |
+    13     |      records  in  that  same |          |       |      |      |       |        |       |
+    14     |      direction               |          |       |      |      |       |        |       |
+    15     +------------------------------+----------+-------+------+------+-------+--------+-------+
+    16     |   a. skip "tape mark  count" |     *    |   *   |  0   |  0   |   1   |   >0   |  >0   |
+    17     |      tape   marks   in   the |          |       |      |      |       |        |       |
+    18     |      reverse direction       |          |       |      |      |       |        |       |
+    19     |   b. skip   "record   count" |          |       |      |      |       |        |       |
+    20     |      records  in  that  same |          |       |      |      |       |        |       |
+    21     |      direction               |          |       |      |      |       |        |       |
+    22     +------------------------------+----------+-------+------+------+-------+--------+-------+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                              *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           WAIVERS AND EXCEPTIONS                                   Page E-1
+                                                             8 November 1987
+
+
+     1                                APPENDIX E
+
+     2                          WAIVERS AND EXCEPTIONS
+
+
+     3     E.1  TU81
+
+     4     E.1.1  "EOT Encountered" Status Subcode
+
+     5     Waiver:
+
+     6         Early  serial  number  TU81  integrated  controllers   return
+     7         "Success (subcode EOT Encountered)" status in the end message
+     8         of all WRITE commands executed while the tape  is  positioned
+     9         on  the  trailer side of the EOT marker.  That status is only
+    10         supposed to be returned when the EOT  marker  is  encountered
+    11         during  the  execution of a "write" type command.  WRITE TAPE
+    12         MARK and ERASE GAP commands operate properly, only the  WRITE
+    13         command is affected.
+
+
+    14     Justification:
+
+    15         This waiver is required to allow the shipment  of  the  first
+    16         200  units  to  customers.   The  microcode  fixes  for these
+    17         problems are extensive and would delay the shipments of TU81s
+    18         for a quarter.
+
+    19     E.1.2  "Serious Exception" And "EOT Encountered"
+
+    20     Waiver:
+
+    21         "Serious exception" end message flag  and  "EOT  encountered"
+    22         are  not  reported  in  the  end messages of ABORT, DETERMINE
+    23         ACCESS PATHS, and GET COMMAND STATUS commands.
+
+
+    24     Justification:
+
+    25         This waiver is required to allow the shipment  of  the  first
+    26         200  units  to  customers.   The  microcode  fixes  for these
+    27         problems are extensive and would delay the shipments of TU81s
+    28         for a quarter.
+
+    29     E.1.3  "Byte Count (Host Transfer)" On Read Reverse
+
+    30     Wavier:
+
+    31         The "byte count (host transfer)" end message field of reverse
+    32         (i.e., "reverse" modifier set) COMPARE HOST DATA commands and
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           WAIVERS AND EXCEPTIONS                                   Page E-2
+           TU81                                              8 November 1987
+
+
+     1         reverse READ with compare  after  (i.e.,  "compare"  modifier
+     2         set) commands are erroneous under certain conditions.
+
+     3         The general problem is that the TU81 can not do a  real  read
+     4         reverse.   The  TU81  emulates  the  read  reverse by doing a
+     5         series of position and read forwards.  When attempting to  do
+     6         the  host  data compare operation the compare is taking place
+     7         in the wrong direction.  This causes the  TU81  to  flag  the
+     8         wrong byte in error if there are two or more errors.
+
+
+     9     Justification:
+
+    10         This problem has little system impact because the command  is
+    11         not  used  by  any  DEC operating system.  The command can be
+    12         issued by a user program.  In most cases the program  is  not
+    13         interested  in  the byte in error, just the fact there was an
+    14         error or not.
+
+    15         The change  to  conform  to  the  specification  is  a  major
+    16         hardware  and  micro  code  change  (the  compare  is done in
+    17         hardware).  The redesign  effort  would  take  at  least  one
+    18         quarter.
+
+    19         The point is best summed up by the following quote  from  Jim
+    20         Zahrobsky:   "What  is  described in the specification is the
+    21         ideal.  Traditional usage shows that there is no  clear  need
+    22         for  devices  to  conform to that ideal (TE16 and TU77 do not
+    23         meet the ideal.  The TU78 on the other hand does)." The  TU81
+    24         doesn't meet the ideal, but it does come very close.
+
+    25         Zahrobsky also went on to say that we will strive to meet the
+    26         ideal on all future devices.
+
+    27     E.2  TK50
+
+    28     E.2.1  Incorrect Command Processing While Position Lost
+
+    29     Waiver:
+
+    30         Whenever the TK50 is in the "Position Lost" state, it rejects
+    31         further   read/write/position   commands  until  one  of  the
+    32         following takes place:
+
+    33          o  The tape is returned to BOT;
+    34          o  The tape unit enters the "Unit Offline" state;
+    35          o  The tape is unloaded via the AVAILABLE command.
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           WAIVERS AND EXCEPTIONS                                   Page E-3
+           TK50                                              8 November 1987
+
+
+     1         The intention of the TMSCP specification in the case  of  the
+     2         "Position  Lost" condition is to allow the host to attempt to
+     3         recover positioning of the tape behind and/or  ahead  of  the
+     4         object  where  the  "Position Lost" condition occurred.  This
+     5         implies that the  host  can  issue  any  I/O  or  positioning
+     6         command  while  the  unit is in the "Position Lost" state and
+     7         expect reasonable operation of those  commands  provided  the
+     8         unit  is  still  able  to  read  the  tape  (i.e.,  its  read
+     9         electronics are still in good shape).
+
+
+    10     Justification:
+
+    11         Corrective action would have prevented FRS with MicroVAX  II.
+    12         This waiver only applies to early serial number TK50s.
+
+    13     E.2.2  COMPARE HOST DATA Error Reporting
+
+    14     Waiver:
+
+    15         While performing a COMPARE HOST DATA command, the  TK50  will
+    16         report  a  "Record  Data  Truncated"  error  subordinate to a
+    17         "Compare" error.  The operating systems indicated  that  this
+    18         was not a major error but that it must be fixed.
+
+
+    19     Justification:
+
+    20         Corrective action would have prevented FRS with MicroVAX  II.
+    21         This waiver only applies to early serial number TK50s.
+
+    22     E.2.3  Incorrect "Offline" Error Reporting
+
+    23     Wavier:
+
+    24         When the TK50 drive is powered off after the  controller  has
+    25         been  initialized the status reported is Offline (subcode "No
+    26         Media Loaded").
+
+
+    27     Justification:
+
+    28         Corrective action would have prevented FRS with MicroVAX  II.
+    29         This waiver only applies to early serial number TK50s.
+
+    30     E.2.4  Progress Indicator Processing
+
+    31     Waiver:
+
+    32         The Progress Indicator stops  indicating  progress  after  11
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           WAIVERS AND EXCEPTIONS                                   Page E-4
+           TK50                                              8 November 1987
+
+
+     1         minutes  and  20 seconds following the execution of the ERASE
+     2         command with the  "Immediate"  qualifier  set.   One  of  the
+     3         consequences  of  this is that 2/3 of the data may be left on
+     4         the tape.  Another consequence is that the host may flag  the
+     5         device  as  broken.  All of the operating systems agreed that
+     6         this is a problem that must be fixed.
+
+
+     7     Justification:
+
+     8         Corrective action would have prevented FRS with MicroVAX  II.
+     9         This waiver only applies to early serial number TK50s.
+
+    10     E.2.5  Multiple Error Logs Reported
+
+    11     Waiver:
+
+    12         The TK50 returns individual error logs for each of the  retry
+    13         attempts  when  a  "Long Gap Encountered" situation occurs at
+    14         BOT.  Also, the Event Code contained in the error log differs
+    15         from the value reported in the command's end message.
+
+    16         These situations only occur at BOT since other tests  perform
+    17         correctly when positioned away from BOT.
+
+
+    18     Justification:
+
+    19         Corrective action would have prevented FRS with MicroVAX  II.
+    20         This waiver only applies to early serial number TK50s.
+
+    21     E.2.6  Invalid "Records Skipped Count"
+
+    22     Waiver:
+
+    23         The "Records Skipped Count" field (P.RCSK) is set to all ones
+    24         in  the end message of a REPOSITION command that encounters a
+    25         "Long Gap" condition.  The value should be  zero.   The  spec
+    26         was ambiguous on this point.  The operating systems indicated
+    27         that this was not a major problem.
+
+
+    28     Justification:
+
+    29         Corrective action would have prevented FRS with MicroVAX  II.
+    30         This waiver only applies to early serial number TK50s.
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           WAIVERS AND EXCEPTIONS                                   Page E-5
+           TK50                                              8 November 1987
+
+
+     1     E.2.7  Hardware Serial Number
+
+     2     Waiver:
+
+     3         Early TK50 controllers and drives do not implement a Hardware
+     4         Serial Number that is unique to each component.
+
+
+     5     Justification:
+
+     6         Corrective action would have prevented FRS with MicroVAX  II.
+     7         This waiver only applies to early serial number TK50s.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           REVISION HISTORY                                         Page F-1
+                                                             8 November 1987
+
+
+     1                                APPENDIX F
+
+     2                             REVISION HISTORY
+
+
+     3     F.1  Changes Since Version 1.6
+
+     4     The following changes were made from TMSCP Version 1.6  to  TMSCP
+     5     Version 1.6.4:
+
+     6              The following approved ECOs to V1.6 were incorporated.
+
+     7               o  TMSCP16-1   BOT   and    Tape    Mark    Encountered
+     8                  Clarification
+     9               o  TMSCP16-5 STI Formatter Error Log Changed
+    10               o  TMSCP16-6 Eliminate Command Aborted as SE Cause
+    11               o  TMSCP16-8 64kb Transfer ECO
+    12               o  TMSCP16-9 ONL and SUC Speed Field Clarification
+    13               o  TMSCP16-10 Expanded Transfer Error Log
+    14               o  TMSCP16-12 Byte Count Equals Zero is Invalid
+    15               o  TMSCP16-13  TMSCP  Extension  to  Get  Unit   Status
+    16                  Command
+    17               o  TMSCP16-17 TU81 "EOT Encountered" Waiver
+    18               o  TMSCP16-19 TU81 Reverse Compare Byte Count Waiver
+    19               o  TMSCP16-21 Tape Caching
+    20               o  TMSCP16-22 "Non-industry standard" Read Reverse
+    21               o  TMSCP16-25 TK50 Maximum Record Length
+    22               o  TMSCP16-26 TK50 Various Temporary Waivers
+    23               o  TMSCP16-28 TK50 Hardware Serial Number
+    24               o  TMSCP16-29 CTP ABORT during REPOSITION
+    25               o  TMSCP16-31    REPOSITION     Command     Description
+    26                  Clarification
+    27               o  TMSCP16-32 Lengthy I/O Command Abort Clarification
+    28               o  TMSCP16-33    Asynchronous    Command     Completion
+    29                  Clarification
+    30               o  TMSCP16-34    Position    Lost    Error     Handling
+    31                  Clarification
+    32               o  TMSCP16-36 Error Log Generation Clarification
+    33               o  TMSCP16-37 Media Format Error Status Code
+    34               o  TMSCP16-42 GET UNIT STATUS Modification
+    35               o  TMSCP16-43 Tape Format Flag Redefinition
+    36               o  TMSCP16-44    End    Message    Field     Validation
+    37                  Clarification
+    38               o  TMSCP16-45 "Position" Field Clarification
+    39               o  TMSCP16-46 PEOT Redefinition
+    40               o  TMSCP16-47 "Position" Field Clarification
+    41               o  TMSCP16-49 TA90 Density Flag
+    42               o  TMSCP16-52 Sequential  Command  Clarification  while
+    43                  Caching
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           REVISION HISTORY                                         Page F-2
+           Changes Since Version 1.6                         8 November 1987
+
+
+     1               o  TMSCP16-56 "Position" Field Changes
+     2               o  TMSCP16-57 EOT Can Move
+     3               o  TMSCP16-58 TAPE TRANSFER ERRORS Errorlog Renamed
+     4               o  TMSCP16-60 Enhanced Write Error Recovery Changes
+
+
+     5              The  following  approved  ECOs   to   MSCP   V1.2   were
+     6              incorporated.
+
+     7               o  MSCP12-2 Error Log Sequence Numbers
+     8               o  MSCP12-9 Reserve OPCODE Zero
+     9               o  MSCP12-16 Error Log Format Generalization
+    10               o  MSCP12-27 Read-Only Device Write Protect
+    11               o  MSCP12-34 Exclusive Access [Multi-Host]
+    12               o  MSCP12-35 ACCESS NON-VOLATILE MEMORY Command
+    13               o  MSCP12-39 TU81 Waiver Request
+    14               o  MSCP12-53 Diagnostic Opcodes
+    15               o  MSCP12-56 Multi-Host Flag Definition
+    16               o  MSCP12-57 Minor Errors in ECOs
+
+
+    17              The following editorial changes were performed  (on  the
+    18              original and ECO text):
+
+    19               o  Table and  "picture"  formats  changed  to  be  more
+    20                  readable.
+
+    21               o  Spelling and grammatical errors corrected.
+
+    22               o  List and item formats changed to be consistent.
+
+    23               o  List items were placed in alphabetical order.
+
+    24               o  Wording changes were made to increase readability.
+
+
+
+    25  |  F.2  Changes Since Version 1.6.4
+
+    26  |  The following changes were made from TMSCP Version 1.6.4 to TMSCP
+    27  |  Version 2.0.0:
+
+    28  |           The following approved ECOs to V1.6 were incorporated.
+
+    29  |            o  TMSCP16-65 Position Lost Clarification
+    30  |            o  TMSCP16-68 No Host Buffer Access Error for WTM
+
+
+
+
+
+
+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***
+           Digital Equipment Corporation        Confidential And Proprietary
+           Tape Mass Storage Control Protocol                  Version 2.0.0
+           REVISION HISTORY                                         Page F-3
+           Changes Since Version 2.0.0                       8 November 1987
+
+
+     1  |  F.3  Changes Since Version 2.0.0
+
+     2  |  The following changes were made from TMSCP Version 2.0.0 to TMSCP
+     3  |  Version 2.0.2:
+
+     4  |           The following approved ECOs to V2.0.0 were incorporated.
+
+
+     5  |            o  TMSCP20-1 Discarding Read-Ahead Cache Contents
+
+     6  |            o  TMSCP20-4 Position Lost Terminology
+
+     7  |            o  TMSCP20-5 FLUSH Should Not Block Further Caching
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
+
+
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+
+
+
+
+
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+                 *** R E S T R I C T E D   D I S T R I B U T I O N ***