diff --git a/PDP10/kx10_cpu.c b/PDP10/kx10_cpu.c
index 4f13244..3bfe4d1 100644
--- a/PDP10/kx10_cpu.c
+++ b/PDP10/kx10_cpu.c
@@ -230,7 +230,10 @@ uint32  fault_addr;                           /* Fault address */
 uint64  opc;                                  /* Saved PC and Flags */
 uint64  mar;                                  /* Memory address compare */
 uint32  qua_time;                             /* Quantum clock value */
-#endif
+#if MAGIC_SWITCH
+int     MAGIC = 1;                            /* Magic switch. */
+#endif /* MAGIC_SWITCH */
+#endif /* ITS */
 #if KL_ITS
 #define dbr1    FM[(6<<4)|1]
 #define dbr2    FM[(6<<4)|2]
@@ -464,7 +467,10 @@ REG cpu_reg[] = {
     { ORDATAD (OPC, opc, 36, "Saved PC and flags")},
     { ORDATAD (MAR, mar, 18, "Memory address register")},
     { ORDATAD (QUA_TIME, qua_time, 36, "Quantum timer"), REG_RO},
-#endif
+#if MAGIC_SWITCH
+    { ORDATAD (MAGIC, MAGIC, 1, "Magic switch"), REG_FIT},
+#endif /* MAGIC_SWITCH */
+#endif /* ITS */
 #if KL
     { ORDATAD (EXT_AC, ext_ac, 4, "Extended Instruction AC"), REG_HRO},
     { ORDATAD (PREV_CTX, prev_ctx, 5, "Previous context"), REG_HRO},
@@ -4037,6 +4043,13 @@ if ((reason = build_dev_tab ()) != SCPE_OK)            /* build, chk dib_tab */
          break;
     }
 
+#if MAGIC_SWITCH
+    if (!MAGIC) {
+         reason = STOP_MAGIC;
+         break;
+    }
+#endif /* MAGIC_SWITCH */
+
     check_apr_irq();
     /* Normal instruction */
     if (f_load_pc) {
diff --git a/PDP10/kx10_defs.h b/PDP10/kx10_defs.h
index 90db1cf..f7f9434 100644
--- a/PDP10/kx10_defs.h
+++ b/PDP10/kx10_defs.h
@@ -86,6 +86,10 @@
 #define PDP6_DEV PDP6|WAITS
 #endif
 
+#ifndef MAGIC_SWITCH   /* Infamous MIT magic switch. */
+#define MAGIC_SWITCH 0
+#endif
+
 
 /* MPX interrupt multiplexer for ITS systems */
 #define MPX_DEV ITS
@@ -132,6 +136,10 @@ typedef t_uint64     uint64;
 
 #define STOP_HALT       1                               /* halted */
 #define STOP_IBKPT      2                               /* breakpoint */
+#define STOP_ACCESS     3                               /* invalid access */
+#if MAGIC_SWITCH
+#define STOP_MAGIC      4                               /* low on magic */
+#endif
 
 /* Debuging controls */
 #define DEBUG_CMD       0x0000001       /* Show device commands */
@@ -628,6 +636,10 @@ int auxcpu_write (t_addr addr, t_uint64);
 #define NUM_DEVS_DPK    ITS
 #define NUM_DEVS_AI     ITS
 #endif
+#if MAGIC_SWITCH && !KA && !ITS
+#error "Magic switch only valid on KA10 with ITS mods"
+#endif
+
 /* Global data */
 
 
diff --git a/PDP10/kx10_sys.c b/PDP10/kx10_sys.c
index e0db37b..f5eb1d0 100644
--- a/PDP10/kx10_sys.c
+++ b/PDP10/kx10_sys.c
@@ -213,7 +213,10 @@ const char *sim_stop_messages[] = {
     "Unknown error",
     "HALT instruction",
     "Breakpoint",
-    "Invalid access"
+    "Invalid access",
+#if MAGIC_SWITCH
+    "No magic"
+#endif
      };
 
 /* Simulator debug controls */