diff --git a/src/main/kotlin/com/thequux/mcpdp/core/CPU.kt b/src/main/kotlin/com/thequux/mcpdp/core/CPU.kt
index ce4a76e..3a65fb6 100644
--- a/src/main/kotlin/com/thequux/mcpdp/core/CPU.kt
+++ b/src/main/kotlin/com/thequux/mcpdp/core/CPU.kt
@@ -42,7 +42,12 @@ class CPU(val mbus: MemBus, var tracer: ITracer = NullTracer()) {
                     } // RTI
                     0x0003 -> trap(0x0Cu) // BPT
                     0x0004 -> trap(0x10u) // IOT
-                    0x0005 -> mbus.unibus.reset() // bus reset TODO: bus init
+                    0x0005 -> {
+                        stack_limit = 0u
+                        pirq = 0u
+                        core.reset()
+                        mbus.unibus.reset()
+                    } // bus reset TODO: bus init
                     0x0006 -> {
                         // TODO: handle suspending the trace trap
                         pc = stack_pop()
@@ -225,7 +230,7 @@ class CPU(val mbus: MemBus, var tracer: ITracer = NullTracer()) {
                 when (opcode shr 6 and 3) {
                     0 -> { // MARK
                         val count = opcode and 0x3F
-                        sp = (pc + (2 * count).toUInt()).toUShort()
+                        sp = check_sp((pc + (2 * count).toUInt()).toUShort())
                         pc = registers[5]
                         registers[5] = stack_pop()
                     } // MARK
@@ -712,6 +717,7 @@ class CPU(val mbus: MemBus, var tracer: ITracer = NullTracer()) {
     private val registers = UShortArray(8)
     private val general_registers = Array<UShortArray>(2) { UShortArray(6) }
     private val shadow_r6 = UShortArray(4) //
+    private var stack_limit: UShort = 0u
     val core = PagingUnit(mbus)
 
     var runState: RunState = RunState.HALTED
@@ -1046,7 +1052,7 @@ class CPU(val mbus: MemBus, var tracer: ITracer = NullTracer()) {
             0x3FFF4u -> 0x1170u // system ID
             0x3FFF6u -> cpu_err
             0x3FFFAu -> pirq // PIRQ
-            0x3FFFCu -> 0x0u // stack limit
+            0x3FFFCu -> stack_limit // stack limit
             0x3FFFEu -> psw
             else -> throw BusTimeoutError(addr)
         }
@@ -1059,7 +1065,7 @@ class CPU(val mbus: MemBus, var tracer: ITracer = NullTracer()) {
             0x3_FFF0u , 0x3_FFF2u , 0x3_FFF4u, 0x3_FFF8u -> {} // read-only registers
             0x3_FFF6u -> cpu_err = value
             0x3_FFFAu -> pirq = value
-            0x3_FFFCu -> {} // stack limit
+            0x3_FFFCu -> stack_limit = value and 0xFF00u // stack limit
             0x3_FFFEu -> psw = value or (cur_mode.toUShort() shl 14) // writing to PSW can only increase current mode
             else -> {
                 println("Bus error at ${addr.toString(16)}: ${value.toString(16)}")
@@ -1068,6 +1074,22 @@ class CPU(val mbus: MemBus, var tracer: ITracer = NullTracer()) {
         }
     }
 
+    fun check_sp(addr: UShort): UShort {
+        if (cur_mode != 0) return addr
+        if (sp < stack_limit + 224u) {
+            // setCPUERR(CPUE_RED)
+            // we can always assume that this is running in kernel mode, as no stack check occurs in exec or user mode
+            sp = 4u
+            throw Trap(0x04)
+            // stack limit red
+        } else if (sp < stack_limit + 256u) {
+            // stack limit yellow
+            // setTRAP(TRAP_YELLOW)
+            // setCPUERR(CPUE_YELLOW)
+        }
+        return addr
+    }
+
     enum class RunState(val handleInterrupt: Boolean) {
         RUNNING(handleInterrupt = true),
         HALTED(handleInterrupt = false),
diff --git a/src/main/kotlin/com/thequux/mcpdp/core/PagingUnit.kt b/src/main/kotlin/com/thequux/mcpdp/core/PagingUnit.kt
index f188214..45fe6b6 100644
--- a/src/main/kotlin/com/thequux/mcpdp/core/PagingUnit.kt
+++ b/src/main/kotlin/com/thequux/mcpdp/core/PagingUnit.kt
@@ -267,6 +267,17 @@ class PagingUnit(val pspace: PAddressSpace): VAddressSpace {
         }
     }
 
+    fun reset() {
+        enable22bit = false
+        enableUnibusMap = false
+        modeVTabs[0].useDSpace = false
+        modeVTabs[1].useDSpace = false
+        modeVTabs[2].useDSpace = false
+        mmr[3] = 0u
+        updateMode()
+
+    }
+
 
     private inner class UnibusMap: PAddressSpace {
         // This implements an *18-bit* address space, for the view of unibus peripherals