diff --git a/VAX/vax_cpu.c b/VAX/vax_cpu.c
index ee349b72..504015da 100644
--- a/VAX/vax_cpu.c
+++ b/VAX/vax_cpu.c
@@ -1,6 +1,6 @@
 /* vax_cpu.c: VAX CPU
 
-   Copyright (c) 1998-2019, Robert M Supnik
+   Copyright (c) 1998-2020, 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"),
@@ -25,6 +25,7 @@
 
    cpu          VAX central processor
 
+   20-May-20    RMS     Added idle test for VMS 5.0/5.1 (Mark Pizzolato)
    23-Apr-19    RMS     Added hook for unpredictable indexed immediate .aw
    14-Apr-19    RMS     Added hook for non-standard MxPR CC's
    31-Mar-17    RMS     Fixed uninitialized variable on FPD path (COVERITY)
@@ -1580,13 +1581,16 @@ for ( ;; ) {
     case TSTL:
         CC_IIZZ_L (op0);                                /* set cc's */
         if ((cc == CC_Z) &&                             /* zero result and */
-            ((((cpu_idle_mask & VAX_IDLE_ULTOLD) &&     /* running Old Ultrix or friends? */
+            ((PC - fault_PC) == 6) &&                   /* 6 byte instruction? */
+            (fault_PC & 0x80000000) &&                  /* in system space? */
+            (((cpu_idle_mask & VAX_IDLE_VMS) &&         /* VMS 5.0 and 5.1 */
+              (PSL_GETIPL (PSL) == 0x3) &&              /*  at IPL 3 */
+              (PSL_IS & PSL)) ||                        /*  on the interrupt stack */
+             ((((cpu_idle_mask & VAX_IDLE_ULTOLD) &&    /* running Old Ultrix or friends? */
                (PSL_GETIPL (PSL) == 0x1)) ||            /*  at IPL 1? */
               ((cpu_idle_mask & VAX_IDLE_QUAD) &&       /* running Quasijarus or friends? */
                (PSL_GETIPL (PSL) == 0x0))) &&           /*  at IPL 0? */
-             (fault_PC & 0x80000000) &&                 /* in system space? */
-             ((PC - fault_PC) == 6) &&                  /* 6 byte instruction? */
-             ((fault_PC & 0x7fffffff) < 0x4000)))       /* in low system space? */
+              ((fault_PC & 0x7fffffff) < 0x4000))))     /* in low system space? */
             cpu_idle();                                 /* idle loop */
         break;