diff --git a/VAX/vax780_defs.h b/VAX/vax780_defs.h
index 19a7ee72..eb162318 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)
@@ -148,6 +149,9 @@
 #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/vax_cpu1.c b/VAX/vax_cpu1.c
index 6b6f0251..865c8fd0 100644
--- a/VAX/vax_cpu1.c
+++ b/VAX/vax_cpu1.c
@@ -1526,8 +1526,7 @@ 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 499f03bf..67ecf3f5 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) 1998-2015, 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
    29-Mar-15    RMS     Added model-specific IPR max
    20-Dec-13    RMS     Added prototypes for unaligned IO and register handling
    12-Dec-12    RMS     Fixed IO base address for RQB, RQC, RQD
@@ -207,7 +208,7 @@
 #define CQMAMASK        (CQMSIZE - 1)                   /* Qmem addr mask */
 #define CQMBASE         0x30000000                      /* Qmem base */
 
-/* Machine specific reserved operand tests (all NOPs) */
+/* Machine specific reserved operand tests (mostly NOPs) */
 
 #define ML_PA_TEST(r)
 #define ML_LR_TEST(r)
@@ -217,6 +218,9 @@
 #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 */