diff --git a/doc/simh.doc b/doc/simh.doc
index 97014662..87369297 100644
Binary files a/doc/simh.doc and b/doc/simh.doc differ
diff --git a/scp.c b/scp.c
index 0effafdc..ba0f0aab 100644
--- a/scp.c
+++ b/scp.c
@@ -7637,6 +7637,7 @@ void put_rval (REG *rptr, uint32 idx, t_value val)
 {
 size_t sz;
 t_value mask;
+t_value old_val;
 uint32 *ptr;
 
 #define PUT_RVAL(sz,rp,id,v,m) \
@@ -7644,6 +7645,8 @@ uint32 *ptr;
             (sz)((*(((sz *) rp->loc) + id) & \
             ~((m) << (rp)->offset)) | ((v) << (rp)->offset))
 
+if (rptr->write_callback)
+    old_val = get_rval (rptr, idx);
 if (rptr == sim_PC)
     sim_brk_npc (0);
 sz = SZ_R (rptr);
@@ -7696,6 +7699,8 @@ else PUT_RVAL (t_uint64, rptr, idx, val, mask);
 #else
 else PUT_RVAL (uint32, rptr, idx, val, mask);
 #endif
+if ((rptr->write_callback) && (!(sim_switches & SIM_SW_REST)))
+    rptr->write_callback (old_val, rptr, idx);
 return;
 }
 
diff --git a/sim_defs.h b/sim_defs.h
index 88dafd90..c76d2125 100644
--- a/sim_defs.h
+++ b/sim_defs.h
@@ -648,9 +648,12 @@ struct REG {
     uint32              depth;                          /* save depth */
     const char          *desc;                          /* description */
     BITFIELD            *fields;                        /* bit fields */
-    uint32              flags;                          /* flags */
     uint32              qptr;                           /* circ q ptr */
     size_t              str_size;                       /* structure size */
+    void                (*write_callback)(t_value old_value, REG *rptr, int idx);
+                                                        /* called during DEPOSIT */
+    /* NOTE: Flags MUST always be last since it is initialized outside of macro definitions */
+    uint32              flags;                          /* flags */
     };
 
 /* Register flags */
@@ -855,111 +858,113 @@ struct MEMFILE {
 
 #define UDATA(act,fl,cap) NULL,act,NULL,NULL,NULL,0,0,(fl),0,(cap),0,NULL,0,0
 
+/* Internal use ONLY (see below) Generic Register declaration for all fields */
+#define _REGDATANF(nm,loc,rdx,wd,off,dep,desc,flds,qptr,siz,cbak) \
+    nm, (loc), (rdx), (wd), (off), (dep), (desc), (flds), (qptr), (siz), (cbak)
+
+/* Right Justified Octal Register Data */
+#define ORDATA(nm,loc,wd) ORDATAD(nm,loc,wd,NULL)
+/* Right Justified Decimal Register Data */
+#define DRDATA(nm,loc,wd) DRDATAD(nm,loc,wd,NULL) 
+/* Right Justified Hexadecimal Register Data */
+#define HRDATA(nm,loc,wd) HRDATAD(nm,loc,wd,NULL)
+/* Right Justified Binary Register Data */
+#define BINRDATA(nm,loc,wd) BINRDATAD(nm,loc,wd,NULL)
+/* One-bit binary flag at an arbitrary offset in a 32-bit word Register */
+#define FLDATA(nm,loc,pos) FLDATAD(nm,loc,pos,NULL)
+/* Arbitrary location and Radix Register */
+#define GRDATA(nm,loc,rdx,wd,pos) GRDATAD(nm,loc,rdx,wd,pos,NULL)
+/* Arrayed register whose data is kept in a standard C array Register */
+#define BRDATA(nm,loc,rdx,wd,dep) BRDATAD(nm,loc,rdx,wd,dep,NULL)
+/* Arrayed register whose data is part of the UNIT structure */
+#define URDATA(nm,loc,rdx,wd,off,dep,fl) URDATAD(nm,loc,rdx,wd,off,dep,fl,NULL)
+/* Arrayed register whose data is part of an arbitrary structure */
+#define STRDATA(nm,loc,rdx,wd,off,dep,siz,fl) STRDATAD(nm,loc,rdx,wd,off,dep,siz,fl,NULL)
+/* Same as above, but with additional description initializer */
+#define ORDATAD(nm,loc,wd,desc) ORDATADF(nm,loc,wd,desc,NULL)
+#define DRDATAD(nm,loc,wd,desc) DRDATADF(nm,loc,wd,desc,NULL)
+#define HRDATAD(nm,loc,wd,desc) HRDATADF(nm,loc,wd,desc,NULL) 
+#define BINRDATAD(nm,loc,wd,desc) BINRDATADF(nm,loc,wd,desc,NULL)
+#define FLDATAD(nm,loc,pos,desc) FLDATADF(nm,loc,pos,desc,NULL)
+#define GRDATAD(nm,loc,rdx,wd,pos,desc) GRDATADF(nm,loc,rdx,wd,pos,desc,NULL)
+#define BRDATAD(nm,loc,rdx,wd,dep,desc) BRDATADF(nm,loc,rdx,wd,dep,desc,NULL)
+#define URDATAD(nm,loc,rdx,wd,off,dep,fl,desc) URDATADF(nm,loc,rdx,wd,off,dep,fl,desc,NULL)
+#define STRDATAD(nm,loc,rdx,wd,off,dep,siz,fl,desc) STRDATADF(nm,loc,rdx,wd,off,dep,siz,fl,desc,NULL)
+/* Same as above, but with additional description initializer, and bitfields */
+#define ORDATADF(nm,loc,wd,desc,flds) ORDATADFC(nm,loc,wd,desc,flds,NULL)
+#define DRDATADF(nm,loc,wd,desc,flds) DRDATADFC(nm,loc,wd,desc,flds,NULL)
+#define HRDATADF(nm,loc,wd,desc,flds) HRDATADFC(nm,loc,wd,desc,flds,NULL)
+#define BINRDATADF(nm,loc,wd,desc,flds) BINRDATADFC(nm,loc,wd,desc,flds,NULL)
+#define FLDATADF(nm,loc,pos,desc,flds) FLDATADFC(nm,loc,pos,desc,flds,NULL)
+#define GRDATADF(nm,loc,rdx,wd,pos,desc,flds) GRDATADFC(nm,loc,rdx,wd,pos,desc,flds,NULL)
+#define BRDATADF(nm,loc,rdx,wd,dep,desc,flds) BRDATADFC(nm,loc,rdx,wd,dep,desc,flds,NULL)
+#define URDATADF(nm,loc,rdx,wd,off,dep,fl,desc,flds) URDATADFC(nm,loc,rdx,wd,off,dep,fl,desc,flds,NULL)
+#define STRDATADF(nm,loc,rdx,wd,off,dep,siz,fl,desc,flds) STRDATADFC(nm,loc,rdx,wd,off,dep,siz,fl,desc,flds,NULL)
+
 #if defined (__STDC__) || defined (_WIN32) /* Variants which depend on how macro arguments are convered to strings */
 /* Generic Register declaration for all fields.  
    If the register structure is extended, this macro will be retained and a 
    new macro will be provided that populates the new register structure */
 #define REGDATA(nm,loc,rdx,wd,off,dep,desc,flds,fl,qptr,siz) \
-    #nm, &(loc), (rdx), (wd), (off), (dep), (desc), (flds), (fl), (qptr), (siz)
-/* Internal use ONLY (see below) Generic Register declaration for all fields */
-#define _REGDATA(nm,loc,rdx,wd,off,dep,desc,flds,fl,qptr,siz) \
-    nm, &(loc), (rdx), (wd), (off), (dep), (desc), (flds), (fl), (qptr), (siz)
-/* Right Justified Octal Register Data */
-#define ORDATA(nm,loc,wd) #nm, &(loc), 8, (wd), 0, 1, NULL, NULL
-/* Right Justified Decimal Register Data */
-#define DRDATA(nm,loc,wd) #nm, &(loc), 10, (wd), 0, 1, NULL, NULL
-/* Right Justified Hexadecimal Register Data */
-#define HRDATA(nm,loc,wd) #nm, &(loc), 16, (wd), 0, 1, NULL, NULL
-/* Right Justified Binary Register Data */
-#define BINRDATA(nm,loc,wd) #nm, &(loc), 2, (wd), 0, 1, NULL, NULL
-/* One-bit binary flag at an arbitrary offset in a 32-bit word Register */
-#define FLDATA(nm,loc,pos) #nm, &(loc), 2, 1, (pos), 1, NULL, NULL
-/* Arbitrary location and Radix Register */
-#define GRDATA(nm,loc,rdx,wd,pos) #nm, &(loc), (rdx), (wd), (pos), 1, NULL, NULL
-/* Arrayed register whose data is kept in a standard C array Register */
-#define BRDATA(nm,loc,rdx,wd,dep) #nm, (loc), (rdx), (wd), 0, (dep), NULL, NULL
-/* Same as above, but with additional description initializer */
-#define ORDATAD(nm,loc,wd,desc) #nm, &(loc), 8, (wd), 0, 1, (desc), NULL
-#define DRDATAD(nm,loc,wd,desc) #nm, &(loc), 10, (wd), 0, 1, (desc), NULL
-#define HRDATAD(nm,loc,wd,desc) #nm, &(loc), 16, (wd), 0, 1, (desc), NULL
-#define BINRDATAD(nm,loc,wd,desc) #nm, &(loc), 2, (wd), 0, 1, (desc), NULL
-#define FLDATAD(nm,loc,pos,desc) #nm, &(loc), 2, 1, (pos), 1, (desc), NULL
-#define GRDATAD(nm,loc,rdx,wd,pos,desc) #nm, &(loc), (rdx), (wd), (pos), 1, (desc), NULL
-#define BRDATAD(nm,loc,rdx,wd,dep,desc) #nm, (loc), (rdx), (wd), 0, (dep), (desc), NULL
-/* Same as above, but with additional description initializer, and bitfields */
-#define ORDATADF(nm,loc,wd,desc,flds) #nm, &(loc), 8, (wd), 0, 1, (desc), (flds)
-#define DRDATADF(nm,loc,wd,desc,flds) #nm, &(loc), 10, (wd), 0, 1, (desc), (flds)
-#define HRDATADF(nm,loc,wd,desc,flds) #nm, &(loc), 16, (wd), 0, 1, (desc), (flds)
-#define BINRDATADF(nm,loc,wd) #nm, &(loc), 2, (wd), 0, 1, NULL, NULL
-#define FLDATADF(nm,loc,pos,desc,flds) #nm, &(loc), 2, 1, (pos), 1, (desc), (flds)
-#define GRDATADF(nm,loc,rdx,wd,pos,desc,flds) #nm, &(loc), (rdx), (wd), (pos), 1, (desc), (flds)
-#define BRDATADF(nm,loc,rdx,wd,dep,desc,flds) #nm, (loc), (rdx), (wd), 0, (dep), (desc), (flds)
+    _REGDATANF(#nm,&(loc),rdx,wd,off,dep,desc,flds,qptr,siz,NULL),(fl)
+/* Same as above, but with callback initializer */
+#define ORDATADFC(nm,loc,wd,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),8,wd,0,1,desc,flds,0,0,cbk)
+#define DRDATADFC(nm,loc,wd,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),10,wd,0,1,desc,flds,0,0,cbk)
+#define HRDATADFC(nm,loc,wd,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),16,wd,0,1,desc,flds,0,0,cbk)
+#define BINRDATADFC(nm,loc,wd,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),2,wd,0,1,desc,flds,0,0,cbk)
+#define FLDATADFC(nm,loc,pos,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),2,1,pos,1,desc,flds,0,0,cbk)
+#define GRDATADFC(nm,loc,rdx,wd,pos,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),rdx,wd,pos,1,desc,flds,0,0,cbk)
+#define BRDATADFC(nm,loc,rdx,wd,dep,desc,flds,cbk) \
+    _REGDATANF(#nm,loc,rdx,wd,0,dep,desc,flds,0,0,cbk)
+#define URDATADFC(nm,loc,rdx,wd,off,dep,fl,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),rdx,wd,off,dep,desc,flds,0,0,cbk),((fl) | REG_UNIT)
+#define STRDATADFC(nm,loc,rdx,wd,off,dep,siz,fl,desc,flds,cbk) \
+    _REGDATANF(#nm,&(loc),rdx,wd,off,dep,desc,flds,0,siz,cbk),((fl) | REG_STRUCT)
 #define BIT(nm)              {#nm, 0xffffffff, 1}             /* Single Bit definition */
 #define BITNC                {"",  0xffffffff, 1}             /* Don't care Bit definition */
 #define BITF(nm,sz)          {#nm, 0xffffffff, sz}            /* Bit Field definition */
 #define BITNCF(sz)           {"",  0xffffffff, sz}            /* Don't care Bit Field definition */
 #define BITFFMT(nm,sz,fmt)   {#nm, 0xffffffff, sz, NULL, #fmt}/* Bit Field definition with Output format */
 #define BITFNAM(nm,sz,names) {#nm, 0xffffffff, sz, names}     /* Bit Field definition with value->name map */
-/* Arrayed register whose data is part of the UNIT structure */
-#define URDATA(nm,loc,rdx,wd,off,dep,fl) \
-    _REGDATA(#nm,(loc),(rdx),(wd),(off),(dep),NULL,NULL,((fl) | REG_UNIT),0,0)
-/* Arrayed register whose data is part of an arbitrary structure */
-#define STRDATA(nm,loc,rdx,wd,off,dep,siz,fl) \
-    _REGDATA(#nm,(loc),(rdx),(wd),(off),(dep),NULL,NULL,((fl) | REG_STRUCT),0,(siz))
-/* Same as above, but with additional description initializer */
-#define URDATAD(nm,loc,rdx,wd,off,dep,fl,desc) \
-    _REGDATA(#nm,(loc),(rdx),(wd),(off),(dep),(desc),NULL,((fl) | REG_UNIT),0,0)
-#define STRDATAD(nm,loc,rdx,wd,off,dep,siz,fl,desc) \
-    _REGDATA(#nm,(loc),(rdx),(wd),(off),(dep),(desc),NULL,((fl) | REG_STRUCT),0,(siz))
-/* Same as above, but with additional description initializer, and bitfields */
-#define URDATADF(nm,loc,rdx,wd,off,dep,fl,desc,flds) \
-    _REGDATA(#nm,(loc),(rdx),(wd),(off),(dep),(desc),(flds),((fl) | REG_UNIT),0,0)
-#define STRDATADF(nm,loc,rdx,wd,off,dep,siz,fl,desc,flds) \
-    _REGDATA(#nm,(loc),(rdx),(wd),(off),(dep),(desc),(flds),((fl) | REG_STRUCT),0,(siz))
 #else /* For non-STD-C compiler which can't stringify macro arguments with # */
+/* Generic Register declaration for all fields.  
+   If the register structure is extended, this macro will be retained and a 
+   new macro will be provided that populates the new register structure */
 #define REGDATA(nm,loc,rdx,wd,off,dep,desc,flds,fl,qptr,siz) \
-    "nm", &(loc), (rdx), (wd), (off), (dep), (desc), (flds), (fl), (qptr), (siz)
-#define _REGDATA(nm,loc,rdx,wd,off,dep,desc,flds,fl,qptr,siz) \
-    nm, &(loc), (rdx), (wd), (off), (dep), (desc), (flds), (fl), (qptr), (siz)
-#define ORDATA(nm,loc,wd) "nm", &(loc), 8, (wd), 0, 1, NULL, NULL
-#define DRDATA(nm,loc,wd) "nm", &(loc), 10, (wd), 0, 1, NULL, NULL
-#define HRDATA(nm,loc,wd) "nm", &(loc), 16, (wd), 0, 1, NULL, NULL
-#define BINRDATA(nm,loc,wd) "nm", &(loc), 2, (wd), 0, 1, NULL, NULL
-#define FLDATA(nm,loc,pos) "nm", &(loc), 2, 1, (pos), 1, NULL, NULL
-#define GRDATA(nm,loc,rdx,wd,pos) "nm", &(loc), (rdx), (wd), (pos), 1, NULL, NULL
-#define BRDATA(nm,loc,rdx,wd,dep) "nm", (loc), (rdx), (wd), 0, (dep), NULL, NULL
-#define ORDATAD(nm,loc,wd,desc) "nm", &(loc), 8, (wd), 0, 1, (desc), NULL
-#define DRDATAD(nm,loc,wd,desc) "nm", &(loc), 10, (wd), 0, 1, (desc), NULL
-#define HRDATAD(nm,loc,wd,desc) "nm", &(loc), 16, (wd), 0, 1, (desc), NULL
-#define BINRDATAD(nm,loc,wd,desc) "nm", &(loc), 2, (wd), 0, 1, (desc), NULL
-#define FLDATAD(nm,loc,pos,desc) "nm", &(loc), 2, 1, (pos), 1, (desc), NULL
-#define GRDATAD(nm,loc,rdx,wd,pos,desc) "nm", &(loc), (rdx), (wd), (pos), 1, (desc), NULL
-#define BRDATAD(nm,loc,rdx,wd,dep,desc) "nm", (loc), (rdx), (wd), 0, (dep), (desc), NULL
-#define ORDATADF(nm,loc,wd,desc,flds) "nm", &(loc), 8, (wd), 0, 1, (desc), (flds)
-#define DRDATADF(nm,loc,wd,desc,flds) "nm", &(loc), 10, (wd), 0, 1, (desc), (flds)
-#define HRDATADF(nm,loc,wd,desc,flds) "nm", &(loc), 16, (wd), 0, 1, (desc), (flds)
-#define BINRDATADF(nm,loc,wd,desc,flds) "nm", &(loc), 2, (wd), 0, 1, (desc), (flds)
-#define FLDATADF(nm,loc,pos,desc,flds) "nm", &(loc), 2, 1, (pos), 1, (desc), (flds)
-#define GRDATADF(nm,loc,rdx,wd,pos,desc,flds) "nm", &(loc), (rdx), (wd), (pos), 1, (desc), (flds)
-#define BRDATADF(nm,loc,rdx,wd,dep,desc,flds) "nm", (loc), (rdx), (wd), 0, (dep), (desc), (flds)
+    _REGDATANF("nm",&(loc),rdx,wd,off,dep,desc,flds,qptr,siz,NULL),(fl)
+/* Same as above, but with callback initializer */
+#define ORDATADFC(nm,loc,wd,desc,flds,cbk) \
+    _REGDATANF("nm",&(loc),8,wd,0,1,desc,flds,0,0,cbk)
+#define DRDATADFC(nm,loc,wd,desc,flds,cbk) \
+    _REGDATANF("nm",&(loc),10,wd,0,1,desc,flds,0,0,cbk)
+#define HRDATADFC(nm,loc,wd,desc,flds,cbk) \
+    _REGDATANF("nm",&(loc),16,wd,0,1,desc,flds,0,0,cbk)
+#define BINRDATADFC(nm,loc,wd,cbk) \
+    _REGDATANF("nm",&(loc),2,wd,0,1,desc,flds,0,0,cbk)
+#define FLDATADFC(nm,loc,pos,desc,flds,cbk) \
+    _REGDATANF("nm",&(loc),2,1,pos,1,desc,flds,0,0,cbk)
+#define GRDATADFC(nm,loc,rdx,wd,pos,desc,flds,cbk) \
+    _REGDATANF("nm",&(loc),rdx,wd,pos,1,desc,flds,0,0,cbk)
+#define BRDATADFC(nm,loc,rdx,wd,dep,desc,flds,cbk) \
+    _REGDATANF("nm",loc,rdx,wd,0,dep,desc,flds,0,0,cbk)
+#define URDATADFC(nm,loc,rdx,wd,off,dep,fl,desc,flds,cbk) \
+    _REGDATANF("nm",&(loc),rdx,wd,off,dep,desc,flds,0,0,cbk),((fl) | REG_UNIT)
+#define STRDATADFC(nm,loc,rdx,wd,off,dep,siz,fl,desc,flds,cbk) \
+    _REGDATANF("nm",&(loc),rdx,wd,off,dep,desc,flds,0,siz,cbk),((fl) | REG_STRUCT)
+#define REGDATA(nm,loc,rdx,wd,off,dep,desc,flds,fl,qptr,siz) \
+    "nm", &(loc), (rdx), (wd), (off), (dep), (desc), (flds), (qptr), (siz), NULL, (fl)
 #define BIT(nm)              {"nm", 0xffffffff, 1}              /* Single Bit definition */
 #define BITNC                {"",   0xffffffff, 1}              /* Don't care Bit definition */
 #define BITF(nm,sz)          {"nm", 0xffffffff, sz}             /* Bit Field definition */
 #define BITNCF(sz)           {"",   0xffffffff, sz}             /* Don't care Bit Field definition */
 #define BITFFMT(nm,sz,fmt)   {"nm", 0xffffffff, sz, NULL, "fmt"}/* Bit Field definition with Output format */
 #define BITFNAM(nm,sz,names) {"nm", 0xffffffff, sz, names}      /* Bit Field definition with value->name map */
-#define URDATA(nm,loc,rdx,wd,off,dep,fl) \
-    _REGDATA("nm",(loc),(rdx),(wd),(off),(dep),NULL,NULL,((fl) | REG_UNIT),0,0)
-#define STRDATA(nm,loc,rdx,wd,off,dep,siz,fl) \
-    _REGDATA("nm",(loc),(rdx),(wd),(off),(dep),NULL,NULL,((fl) | REG_STRUCT),0,(siz))
-#define URDATAD(nm,loc,rdx,wd,off,dep,fl,desc) \
-    _REGDATA("nm",(loc),(rdx),(wd),(off),(dep),(desc),NULL,((fl) | REG_UNIT),0,0)
-#define STRDATAD(nm,loc,rdx,wd,off,dep,siz,fl,desc) \
-    _REGDATA("nm",(loc),(rdx),(wd),(off),(dep),(desc),NULL,((fl) | REG_STRUCT),0,(siz))
-#define URDATADF(nm,loc,rdx,wd,off,dep,fl,desc,flds) \
-    _REGDATA("nm",(loc),(rdx),(wd),(off),(dep),(desc),(flds),((fl) | REG_UNIT),0,0)
-#define STRDATADF(nm,loc,rdx,wd,off,dep,siz,fl,desc,flds) \
-    _REGDATA("nm",(loc),(rdx),(wd),(off),(dep),(desc),(flds),((fl) | REG_STRUCT),0,(siz))
 #endif
 #define ENDBITS {NULL}  /* end of bitfield list */