diff --git a/VAX/vax610_defs.h b/VAX/vax610_defs.h
index b0930001..e5a7da17 100644
--- a/VAX/vax610_defs.h
+++ b/VAX/vax610_defs.h
@@ -132,7 +132,7 @@ extern t_stat cpu_show_memory (FILE* st, UNIT* uptr, int32 val, CONST void* desc
 #define ADDR_IS_ROM(x)  (0)
 #define ADDR_IS_NVR(x)  (0)
 
-/* Machine specific reserved operand tests (all NOPs) */
+/* Machine specific reserved operand tests (mostly NOPs) */
 
 #define ML_PA_TEST(r)
 #define ML_LR_TEST(r)
@@ -142,6 +142,8 @@ extern t_stat cpu_show_memory (FILE* st, UNIT* uptr, int32 val, CONST void* desc
 #define LP_MBZ84_TEST(r)
 #define LP_MBZ92_TEST(r)
 
+#define MT_AST_TEST(r)  if ((r) > AST_MAX) RSVD_OPND_FAULT
+
 /* Qbus I/O modes */
 
 #define READ            0                               /* PDP-11 compatibility */
diff --git a/VAX/vax630_defs.h b/VAX/vax630_defs.h
index 39ea5770..59d5f5f3 100644
--- a/VAX/vax630_defs.h
+++ b/VAX/vax630_defs.h
@@ -182,7 +182,7 @@ extern t_stat cpu_show_memory (FILE* st, UNIT* uptr, int32 val, CONST void* desc
 
 #define ADDR_IS_CDG(x)  (0)
 
-/* Machine specific reserved operand tests (all NOPs) */
+/* Machine specific reserved operand tests (mostly NOPs) */
 
 #define ML_PA_TEST(r)
 #define ML_LR_TEST(r)
@@ -192,6 +192,8 @@ extern t_stat cpu_show_memory (FILE* st, UNIT* uptr, int32 val, CONST void* desc
 #define LP_MBZ84_TEST(r)
 #define LP_MBZ92_TEST(r)
 
+#define MT_AST_TEST(r)  if ((r) > AST_MAX) RSVD_OPND_FAULT
+
 /* Qbus I/O modes */
 
 #define READ            0                               /* PDP-11 compatibility */
diff --git a/VAX/vax730_defs.h b/VAX/vax730_defs.h
index 3946f4c3..c863ede4 100644
--- a/VAX/vax730_defs.h
+++ b/VAX/vax730_defs.h
@@ -116,6 +116,9 @@
 #define LP_MBZ84_TEST(r) if ((((uint32)(r)) & 0xF8C00000) != 0) RSVD_OPND_FAULT
 #define LP_MBZ92_TEST(r) if ((((uint32)(r)) & 0x7FC00000) != 0) RSVD_OPND_FAULT
 
+#define MT_AST_TEST(r)  r = (r) & 07; \
+                        if ((r) > AST_MAX) RSVD_OPND_FAULT
+
 /* Memory */
 
 #define MAXMEMWIDTH     21                              /* max mem, 16k chips */
diff --git a/VAX/vax750_defs.h b/VAX/vax750_defs.h
index 6bce6e23..a534410e 100644
--- a/VAX/vax750_defs.h
+++ b/VAX/vax750_defs.h
@@ -137,6 +137,9 @@
 #define LP_MBZ84_TEST(r) if ((((uint32)(r)) & 0xF8C00000) != 0) RSVD_OPND_FAULT
 #define LP_MBZ92_TEST(r) if ((((uint32)(r)) & 0x7FC00000) != 0) RSVD_OPND_FAULT
 
+#define MT_AST_TEST(r)  r = (r) & 07; \
+                        if ((r) > AST_MAX) RSVD_OPND_FAULT
+
 /* Memory */
 
 #define MAXMEMWIDTH     21                              /* max mem, 16k chips */
diff --git a/VAX/vax780_defs.h b/VAX/vax780_defs.h
index 42b0aaef..bbbbf452 100644
--- a/VAX/vax780_defs.h
+++ b/VAX/vax780_defs.h
@@ -23,6 +23,7 @@
    used in advertising or otherwise to promote the sale, use or other dealings
    in this Software without prior written authorization from Robert M Supnik.
 
+   18-May-17    RMS     Added model-specific AST validation test
    19-Jan-17    RMS     Moved CR to BR6 (Mark Pizzolato)
    29-Mar-15    RMS     Added model specific IPR max
    16-Dec-14    RMS     Removed TQ boot code (780 VMB doesn't support tape boot)
@@ -154,6 +155,9 @@
 #define LP_MBZ84_TEST(r) if ((((uint32)(r)) & 0xF8C00000) != 0) RSVD_OPND_FAULT
 #define LP_MBZ92_TEST(r) if ((((uint32)(r)) & 0x7FC00000) != 0) RSVD_OPND_FAULT
 
+#define MT_AST_TEST(r)  r = (r) & 07; \
+                        if ((r) > AST_MAX) RSVD_OPND_FAULT
+
 /* Memory */
 
 #define MAXMEMWIDTH     23                              /* max mem, MS780C */
diff --git a/VAX/vax860_defs.h b/VAX/vax860_defs.h
index 5a7d312b..c281f38b 100644
--- a/VAX/vax860_defs.h
+++ b/VAX/vax860_defs.h
@@ -174,6 +174,9 @@
 #define LP_MBZ84_TEST(r) if ((((uint32)(r)) & 0xF8C00000) != 0) RSVD_OPND_FAULT
 #define LP_MBZ92_TEST(r) if ((((uint32)(r)) & 0x7FC00000) != 0) RSVD_OPND_FAULT
 
+#define MT_AST_TEST(r)  r = (r) & 07; \
+                        if ((r) > AST_MAX) RSVD_OPND_FAULT
+
 /* Memory */
 
 #define MAXMEMWIDTH     25                              /* max mem, 4MB boards */
diff --git a/VAX/vax_cpu1.c b/VAX/vax_cpu1.c
index c7c0372b..144ee826 100644
--- a/VAX/vax_cpu1.c
+++ b/VAX/vax_cpu1.c
@@ -1506,8 +1506,7 @@ switch (prn) {                                          /* case on reg # */
         break;
 
     case MT_ASTLVL:                                     /* ASTLVL */
-        if (val > AST_MAX)                              /* > 4? fault */
-            RSVD_OPND_FAULT;
+        MT_AST_TEST (val);                              /* trim, test val */
         ASTLVL = val;
         break;
 
diff --git a/VAX/vaxmod_defs.h b/VAX/vaxmod_defs.h
index 46544a46..5bd7ec9c 100644
--- a/VAX/vaxmod_defs.h
+++ b/VAX/vaxmod_defs.h
@@ -1,6 +1,6 @@
 /* vaxmod_defs.h: VAX model-specific definitions file
 
-   Copyright (c) 1998-2013, Robert M Supnik
+   Copyright (c) 1998-2017, Robert M Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -23,6 +23,7 @@
    used in advertising or otherwise to promote the sale, use or other dealings
    in this Software without prior written authorization from Robert M Supnik.
 
+   18-May-17    RMS     Added model-specific AST validation test
    20-Dec-13    RMS     Added prototypes for unaligned IO and register handling
    11-Dec-11    RMS     Moved all Qbus devices to BR4; deleted RP definitions
    25-Nov-11    RMS     Added VEC_QBUS definition
@@ -232,7 +233,7 @@ extern t_stat cpu_show_memory (FILE* st, UNIT* uptr, int32 val, CONST void* desc
 #define ADDR_IS_QVM(x)  ((((uint32) (x)) >= QVMBASE) && \
                         (((uint32) (x)) < (QVMBASE + QVMSIZE)))
 
-/* Machine specific reserved operand tests (all NOPs) */
+/* Machine specific reserved operand tests (mostly NOPs) */
 
 #define ML_PA_TEST(r)
 #define ML_LR_TEST(r)
@@ -242,6 +243,9 @@ extern t_stat cpu_show_memory (FILE* st, UNIT* uptr, int32 val, CONST void* desc
 #define LP_MBZ84_TEST(r)
 #define LP_MBZ92_TEST(r)
 
+#define MT_AST_TEST(r)  if ((r) > AST_MAX) RSVD_OPND_FAULT
+
+
 /* Qbus I/O modes */
 
 #define READ            0                               /* PDP-11 compatibility */