diff --git a/doc/simh.doc b/doc/simh.doc
index 94f038ef..6fda17d0 100644
Binary files a/doc/simh.doc and b/doc/simh.doc differ
diff --git a/scp.c b/scp.c
index 38489f3b..d2f2e5f6 100644
--- a/scp.c
+++ b/scp.c
@@ -451,6 +451,7 @@ void (*sim_vm_fprint_addr) (FILE *st, DEVICE *dptr, t_addr addr) = NULL;
 t_addr (*sim_vm_parse_addr) (DEVICE *dptr, CONST char *cptr, CONST char **tptr) = NULL;
 t_value (*sim_vm_pc_value) (void) = NULL;
 t_bool (*sim_vm_is_subroutine_call) (t_addr **ret_addrs) = NULL;
+void (*sim_vm_reg_update) (REG *rptr, uint32 idx, t_value prev_val, t_value new_val) = NULL;
 t_bool (*sim_vm_fprint_stopped) (FILE *st, t_stat reason) = NULL;
 const char *sim_vm_release = NULL;
 const char *sim_vm_release_message = NULL;
@@ -3148,11 +3149,13 @@ DEVICE *tdptr;
 CONST char *tptr;
 char *namebuf;
 char rangebuf[32];
+int side_effects = 0;
 
 if (dptr->registers)
     for (rptr = dptr->registers; rptr->name != NULL; rptr++) {
         if (rptr->flags & REG_HIDDEN)
             continue;
+        side_effects += ((rptr->flags & REG_DEPOSIT) != 0);
         if (rptr->depth > 1)
             sprintf (rangebuf, "[%d:%d]", 0, rptr->depth-1);
         else
@@ -3172,8 +3175,11 @@ else {
     namebuf = (char *)calloc (max_namelen + 1, sizeof (*namebuf));
     fprintf (st, "\nThe %s device implements these registers:\n\n", dptr->name);
     for (rptr = dptr->registers; rptr->name != NULL; rptr++) {
+        char note[2];
+
         if (rptr->flags & REG_HIDDEN)
             continue;
+        strlcpy (note, (side_effects != 0) ? ((rptr->flags & REG_DEPOSIT) ? "+" : " ") : "", sizeof (note));
         if (rptr->depth <= 1)
             sprintf (namebuf, "%*s", -((int)max_namelen), rptr->name);
         else {
@@ -3181,16 +3187,22 @@ else {
             sprintf (namebuf, "%s%*s", rptr->name, (int)(strlen(rptr->name))-((int)max_namelen), rangebuf);
             }
         if (all_unique) {
-            fprintf (st, "  %s %4d  %s\n", namebuf, rptr->width, rptr->desc ? rptr->desc : "");
+            fprintf (st, "  %s %4d %s %s\n", namebuf, rptr->width, note, rptr->desc ? rptr->desc : "");
             continue;
             }
         trptr = find_reg_glob (rptr->name, &tptr, &tdptr);
         if ((trptr == NULL) || (tdptr != dptr))
-            fprintf (st, "  %s %s %4d  %s\n", dptr->name, namebuf, rptr->width, rptr->desc ? rptr->desc : "");
+            fprintf (st, "  %s %s %4d %s %s\n", dptr->name, namebuf, rptr->width, note, rptr->desc ? rptr->desc : "");
         else
-            fprintf (st, "  %*s %s %4d  %s\n", (int)strlen(dptr->name), "", namebuf, rptr->width, rptr->desc ? rptr->desc : "");
+            fprintf (st, "  %*s %s %4d %s %s\n", (int)strlen(dptr->name), "", namebuf, rptr->width, note, rptr->desc ? rptr->desc : "");
         }
     free (namebuf);
+    if (side_effects)
+        fprintf (st, "\n  +  Deposits to %s register%s will have some additional\n"
+                       "     side effects which can be suppressed if the deposit is\n"
+                       "     done with the -Z switch specified\n", 
+                                 (side_effects == 1) ? "this" : "these", 
+                                 (side_effects == 1) ? "" : "s");
     }
 }
 
@@ -9441,6 +9453,11 @@ void put_rval_pcchk (REG *rptr, uint32 idx, t_value val, t_bool pc_chk)
 size_t sz;
 t_value mask;
 uint32 *ptr;
+t_value prev_val;
+
+if ((!(sim_switches & SWMASK ('Z'))) && 
+    (rptr->flags & REG_DEPOSIT) && sim_vm_reg_update)
+    prev_val = get_rval (rptr, idx);
 
 #define PUT_RVAL(sz,rp,id,v,m) \
     *(((sz *) rp->loc) + id) = \
@@ -9499,6 +9516,9 @@ else PUT_RVAL (t_uint64, rptr, idx, val, mask);
 #else
 else PUT_RVAL (uint32, rptr, idx, val, mask);
 #endif
+if ((!(sim_switches & SWMASK ('Z'))) && 
+    (rptr->flags & REG_DEPOSIT) && sim_vm_reg_update)
+    sim_vm_reg_update (rptr, idx, prev_val, val);
 }
 
 void put_rval (REG *rptr, uint32 idx, t_value val)
diff --git a/scp.h b/scp.h
index 5f423afe..711a8ca7 100644
--- a/scp.h
+++ b/scp.h
@@ -371,6 +371,7 @@ extern t_addr (*sim_vm_parse_addr) (DEVICE *dptr, CONST char *cptr, CONST char *
 extern t_bool (*sim_vm_fprint_stopped) (FILE *st, t_stat reason);
 extern t_value (*sim_vm_pc_value) (void);
 extern t_bool (*sim_vm_is_subroutine_call) (t_addr **ret_addrs);
+extern void (*sim_vm_reg_update) (REG *rptr, uint32 idx, t_value prev_val, t_value new_val);
 extern const char **sim_clock_precalibrate_commands;
 extern int32 sim_vm_initial_ips;                        /* base estimate of simulated instructions per second */
 extern const char *sim_vm_interval_units;               /* Simulator can change this - default "instructions" */
diff --git a/sim_defs.h b/sim_defs.h
index 6beb2146..dbc8c96b 100644
--- a/sim_defs.h
+++ b/sim_defs.h
@@ -710,6 +710,7 @@ struct REG {
 #define REG_VMIO        00400                           /* use VM data print/parse */
 #define REG_VMAD        01000                           /* use VM addr print/parse */
 #define REG_FIT         02000                           /* fit access to size */
+#define REG_DEPOSIT     04000                           /* call VM routine after update */
 #define REG_HRO         (REG_RO | REG_HIDDEN)           /* hidden, read only */
 
 #define REG_V_UF        16                              /* device specific */