From 6d376b2fd97d46db92d9227e5f853bda31a80ced Mon Sep 17 00:00:00 2001
From: Mark Pizzolato <mark@infocomm.com>
Date: Mon, 4 Dec 2023 10:57:31 -1000
Subject: [PATCH] ALL simulators with instruction history support: Minor
 history enhancements

- Issue reasonable error message explaining invalid SET CPU HISTORY=n input
- Let SIGINT (^C) abort SHOW HISTORY output
---
 3B2/3b2_cpu.c                |  6 +++++-
 AltairZ80/altairz80_cpu.c    |  4 ++++
 B5500/b5500_cpu.c            | 10 ++++++++--
 H316/h316_cpu.c              | 12 +++++++++---
 I1401/i1401_cpu.c            | 12 +++++++++---
 I1620/i1620_cpu.c            | 12 +++++++++---
 I7000/i7010_cpu.c            | 10 ++++++++--
 I7000/i701_cpu.c             | 10 ++++++++--
 I7000/i7070_cpu.c            | 10 ++++++++--
 I7000/i7080_cpu.c            | 10 ++++++++--
 I7000/i7090_cpu.c            | 10 ++++++++--
 I7094/i7094_cpu.c            | 10 ++++++++--
 Intel-Systems/common/i8080.c | 12 +++++++++---
 Interdata/id16_cpu.c         | 12 +++++++++---
 Interdata/id32_cpu.c         | 12 +++++++++---
 PDP1/pdp1_cpu.c              | 12 +++++++++---
 PDP10/kx10_cpu.c             | 10 ++++++++--
 PDP10/pdp10_cpu.c            | 14 ++++++++++----
 PDP11/pdp11_cpu.c            | 12 +++++++++---
 PDP18B/pdp18b_cpu.c          | 12 +++++++++---
 PDP8/pdp8_cpu.c              | 12 +++++++++---
 SDS/sds_cpu.c                | 12 +++++++++---
 SEL32/sel32_cpu.c            | 10 ++++++++--
 TX-0/tx0_cpu.c               | 12 ++++++++++--
 VAX/vax_cpu.c                | 20 ++++++++++++--------
 alpha/alpha_cpu.c            | 12 ++++++++++--
 imlac/imlac_cpu.c            |  4 ++++
 sigma/sigma_cpu.c            | 13 ++++++++++---
 swtp6800/common/m6800.c      | 12 +++++++++---
 29 files changed, 245 insertions(+), 74 deletions(-)

diff --git a/3B2/3b2_cpu.c b/3B2/3b2_cpu.c
index b86d2df4..d16e0218 100644
--- a/3B2/3b2_cpu.c
+++ b/3B2/3b2_cpu.c
@@ -1468,7 +1468,7 @@ t_stat cpu_set_hist(UNIT *uptr, int32 val, CONST char *cptr, void *desc)
 
     /* If no length was provided, give up */
     if (result != SCPE_OK) {
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, MAX_HIST_SIZE);
     }
 
     /* Legnth 0 is a special flag that means disable the feature. */
@@ -1535,6 +1535,10 @@ t_stat cpu_show_hist(FILE *st, UNIT *uptr, int32 val, CONST void *desc)
     fprintf(st, "PSW      SP       PC        IR\n");
 
     for (i = 0; i < count; i++) {
+        if (stop_cpu) {                                     /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                                          /* abandon remaining output */
+        }
         ip = &INST[(di++) % (int32) cpu_hist_size];
         if (ip->valid) {
             /* Show the opcode mnemonic */
diff --git a/AltairZ80/altairz80_cpu.c b/AltairZ80/altairz80_cpu.c
index 9afd4ce9..2b7e8132 100644
--- a/AltairZ80/altairz80_cpu.c
+++ b/AltairZ80/altairz80_cpu.c
@@ -7186,6 +7186,10 @@ t_stat cpu_show_hist (FILE *st, UNIT *uptr, int32 val, CONST void *desc)
     if (di < 0) di = di + hst_lnt;
 
     for (k = 0; k < lnt; k++) {
+        if (stop_cpu) {                              /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                                   /* abandon remaining output */
+        }
         h = &hst[(di++) % hst_lnt];
 
         if (h->valid) {                              /* valid entry? */
diff --git a/B5500/b5500_cpu.c b/B5500/b5500_cpu.c
index 70ce9b50..291ee419 100644
--- a/B5500/b5500_cpu.c
+++ b/B5500/b5500_cpu.c
@@ -3929,8 +3929,10 @@ cpu_set_hist(UNIT * uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
     }
     lnt = (int32) get_uint(cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free(hst);
@@ -3974,6 +3976,10 @@ cpu_show_hist(FILE * st, UNIT * uptr, int32 val, CONST void *desc)
                 "                       X     S     F     R      M  GH KV Flags"
                 "  Q Intruction     IAR\n\n");
     for (k = 0; k < lnt; k++) { /* print specified */
+        if (stop_cpu) {                 /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                      /* abandon remaining output */
+        }
         h = &hst[(++di) % hst_lnt];     /* entry pointer */
         if (h->c & HIST_PC) {   /* instruction? */
             int i;
diff --git a/H316/h316_cpu.c b/H316/h316_cpu.c
index 2c5d8c02..6dbc73b2 100644
--- a/H316/h316_cpu.c
+++ b/H316/h316_cpu.c
@@ -1735,8 +1735,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -1769,7 +1771,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -1778,6 +1780,10 @@ if (di < 0)
 fprintf (st, "  PC   C    A       B       X       ea     IR\n");
 fprintf (st, "-----  - ------  ------  ------  -----  -----------\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         cr = (h->pc & HIST_C)? 1: 0;                    /* carry */
diff --git a/I1401/i1401_cpu.c b/I1401/i1401_cpu.c
index 3d6c7cf4..4f623eae 100644
--- a/I1401/i1401_cpu.c
+++ b/I1401/i1401_cpu.c
@@ -1893,8 +1893,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -1925,7 +1927,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -1933,6 +1935,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "IS     IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->ilnt) {                                      /* instruction? */
         fprintf (st, "%05d  ", h->is);
diff --git a/I1620/i1620_cpu.c b/I1620/i1620_cpu.c
index b7a6d89e..633fb44e 100644
--- a/I1620/i1620_cpu.c
+++ b/I1620/i1620_cpu.c
@@ -2408,8 +2408,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -2442,7 +2444,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -2450,6 +2452,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC     IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->vld) {                                       /* instruction? */
         fprintf (st, "%05d  ", h->pc);
diff --git a/I7000/i7010_cpu.c b/I7000/i7010_cpu.c
index e4aaf027..593e0293 100644
--- a/I7000/i7010_cpu.c
+++ b/I7000/i7010_cpu.c
@@ -3850,8 +3850,10 @@ cpu_set_hist(UNIT * uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
     }
     lnt = (int32) get_uint(cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free(hst);
@@ -3892,6 +3894,10 @@ cpu_show_hist(FILE * st, UNIT * uptr, int32 val, CONST void *desc)
         di = di + hst_lnt;
     fprintf(st, "IC     A     B    Aend  Bend   \n");
     for (k = 0; k < lnt; k++) { /* print specified */
+        if (stop_cpu) {                 /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                      /* abandon remaining output */
+        }
         h = &hst[(++di) % hst_lnt];     /* entry pointer */
         if (h->ic & HIST_PC) {  /* instruction? */
             pc = h->ic & HIST_MSK;
diff --git a/I7000/i701_cpu.c b/I7000/i701_cpu.c
index cc76ef8a..0b6229da 100644
--- a/I7000/i701_cpu.c
+++ b/I7000/i701_cpu.c
@@ -838,8 +838,10 @@ cpu_set_hist(UNIT * uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
     }
     lnt = (int32) get_uint(cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free(hst);
@@ -880,6 +882,10 @@ cpu_show_hist(FILE * st, UNIT * uptr, int32 val, CONST void *desc)
         di = di + hst_lnt;
     fprintf(st, "IC      AC            MQ            EA      SR\n\n");
     for (k = 0; k < lnt; k++) { /* print specified */
+        if (stop_cpu) {                 /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                      /* abandon remaining output */
+        }
         h = &hst[(++di) % hst_lnt];     /* entry pointer */
         if (h->ic & HIST_PC) {  /* instruction? */
             fprintf(st, "%06lo ", h->ic & AMASK);
diff --git a/I7000/i7070_cpu.c b/I7000/i7070_cpu.c
index da09ef8b..e207a2ea 100644
--- a/I7000/i7070_cpu.c
+++ b/I7000/i7070_cpu.c
@@ -2887,8 +2887,10 @@ cpu_set_hist(UNIT * uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
     }
     lnt = (int32) get_uint(cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free(hst);
@@ -2929,6 +2931,10 @@ cpu_show_hist(FILE * st, UNIT * uptr, int32 val, CONST void *desc)
         di = di + hst_lnt;
     fprintf(st, "IC    EA    BEFORE      AFTER       INST\n\n");
     for (k = 0; k < lnt; k++) { /* print specified */
+        if (stop_cpu) {                 /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                      /* abandon remaining output */
+        }
         h = &hst[(++di) % hst_lnt];     /* entry pointer */
         if (h->ic & HIST_PC) {  /* instruction? */
             fprintf(st, "%05d ", h->ic & 0xffff);
diff --git a/I7000/i7080_cpu.c b/I7000/i7080_cpu.c
index 69fa0cc5..ef7d7eb8 100644
--- a/I7000/i7080_cpu.c
+++ b/I7000/i7080_cpu.c
@@ -3320,8 +3320,10 @@ cpu_set_hist(UNIT * uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
     }
     lnt = (int32) get_uint(cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free(hst);
@@ -3363,6 +3365,10 @@ cpu_show_hist(FILE * st, UNIT * uptr, int32 val, CONST void *desc)
         di = di + hst_lnt;
     fprintf(st, "IC      OP   MA      REG\n\n");
     for (k = 0; k < lnt; k++) { /* print specified */
+        if (stop_cpu) {                 /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                      /* abandon remaining output */
+        }
         h = &hst[(++di) % hst_lnt];     /* entry pointer */
         if (h->ic & HIST_PC) {  /* instruction? */
             fprintf(st, "%06d %c %06d %02d ", h->ic & 0x3ffff,
diff --git a/I7000/i7090_cpu.c b/I7000/i7090_cpu.c
index ca71b5f7..e2080860 100644
--- a/I7000/i7090_cpu.c
+++ b/I7000/i7090_cpu.c
@@ -4272,8 +4272,10 @@ cpu_set_hist(UNIT * uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
     }
     lnt = (int32) get_uint(cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free(hst);
@@ -4315,6 +4317,10 @@ cpu_show_hist(FILE * st, UNIT * uptr, int32 val, CONST void *desc)
     fprintf(st,
 "IC      AC            MQ            EA      SR             XR1    XR2   XR4\n\n");
     for (k = 0; k < lnt; k++) { /* print specified */
+        if (stop_cpu) {                 /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                      /* abandon remaining output */
+        }
         h = &hst[(++di) % hst_lnt];     /* entry pointer */
         if (h->ic & HIST_PC) {  /* instruction? */
             fprintf(st, "%06o%c", h->ic & 077777, ((h->ic>>19)&1)?'b':' ');
diff --git a/I7094/i7094_cpu.c b/I7094/i7094_cpu.c
index a8ad9450..0a4174e1 100644
--- a/I7094/i7094_cpu.c
+++ b/I7094/i7094_cpu.c
@@ -2373,8 +2373,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -2468,6 +2470,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "    PC    repeat AC            MQ           SI           EA     IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     cpu_fprint_one_inst (st, h->pc, h->rpt, h->ea, h->ir, h->ac, h->mq, h->si, h->opnd);
     }                                                   /* end for */
diff --git a/Intel-Systems/common/i8080.c b/Intel-Systems/common/i8080.c
index ca5f02f5..90209a7e 100644
--- a/Intel-Systems/common/i8080.c
+++ b/Intel-Systems/common/i8080.c
@@ -1555,8 +1555,10 @@ t_stat cpu_set_hist(UNIT *uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
         }
     lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free (hst);
@@ -1586,7 +1588,7 @@ t_stat cpu_show_hist(FILE *st, UNIT *uptr, int32 val, CONST void *desc)
     if (cptr) {
         lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
         if ((r != SCPE_OK) || (lnt == 0))
-            return SCPE_ARG;
+            return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
         }
     else lnt = hst_lnt;
     di = hst_p - lnt;                       /* work forward */
@@ -1594,6 +1596,10 @@ t_stat cpu_show_hist(FILE *st, UNIT *uptr, int32 val, CONST void *desc)
         di = di + hst_lnt;
     fprintf (st, "PC   SP   CC A  B  C  D  E  F  H  L  Instruction\n\n");
     for (k = 0; k < lnt; k++) {             /* print specified */
+        if (stop_cpu) {                     /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                          /* abandon remaining output */
+            }
         h = &hst[(di++) % hst_lnt];         /* entry pointer */
         ir = h->inst[0];
         fprintf (st, "%04X %04X %02X ", h->pc , h->sp, h->psw);
diff --git a/Interdata/id16_cpu.c b/Interdata/id16_cpu.c
index d2929d13..9d074c79 100644
--- a/Interdata/id16_cpu.c
+++ b/Interdata/id16_cpu.c
@@ -1979,8 +1979,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (uint32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -2011,7 +2013,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -2019,6 +2021,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC    r1    opnd  ea    IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(di++) % hst_lnt];                         /* entry pointer */
     if (h->vld) {                                       /* instruction? */
         fprintf (st, "%04X  %04X  %04X  ", h->pc, h->r1, h->opnd);
diff --git a/Interdata/id32_cpu.c b/Interdata/id32_cpu.c
index 9dc132b6..bd46b5a4 100644
--- a/Interdata/id32_cpu.c
+++ b/Interdata/id32_cpu.c
@@ -2366,8 +2366,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (uint32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -2398,7 +2400,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -2406,6 +2408,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC     r1       operand  ea     IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(di++) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         fprintf (st, "%06X %08X %08X ", h->pc & VAMASK32, h->r1, h->opnd);
diff --git a/PDP1/pdp1_cpu.c b/PDP1/pdp1_cpu.c
index e4f789d7..122534ee 100644
--- a/PDP1/pdp1_cpu.c
+++ b/PDP1/pdp1_cpu.c
@@ -1642,8 +1642,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -1673,7 +1675,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -1681,6 +1683,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC      OV AC     IO      PF EA      IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         ov = (h->ovac >> HIST_V_SHF) & 1;               /* overflow */
diff --git a/PDP10/kx10_cpu.c b/PDP10/kx10_cpu.c
index db86c6e6..0eec1c67 100644
--- a/PDP10/kx10_cpu.c
+++ b/PDP10/kx10_cpu.c
@@ -13956,8 +13956,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -13995,6 +13997,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC       AC             EA        AR            RES           FLAGS IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+    }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
 #if KL
diff --git a/PDP10/pdp10_cpu.c b/PDP10/pdp10_cpu.c
index 3bcbd522..34dd92d1 100644
--- a/PDP10/pdp10_cpu.c
+++ b/PDP10/pdp10_cpu.c
@@ -2512,15 +2512,17 @@ t_stat cpu_set_hist (UNIT *uptr, int32 val, CONST char *cptr, void *desc)
 int32 i, lnt;
 t_stat r;
 
-if (cptr == NULL) {
+if (cptr == NULL) { /* Clear History */
     for (i = 0; i < hst_lnt; i++)
         hst[i].pc = 0;
     hst_p = 0;
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -2550,7 +2552,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -2558,6 +2560,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC      AC            EA      IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         fprintf (st, "%06o  ", h->pc & AMASK);
diff --git a/PDP11/pdp11_cpu.c b/PDP11/pdp11_cpu.c
index 1fbe97e7..89fa7073 100644
--- a/PDP11/pdp11_cpu.c
+++ b/PDP11/pdp11_cpu.c
@@ -3630,8 +3630,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -3662,7 +3664,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -3670,6 +3672,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC     SP     PSW     src    dst     IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(di++) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_VLD) {                             /* instruction? */
         ir = h->inst[0];
diff --git a/PDP18B/pdp18b_cpu.c b/PDP18B/pdp18b_cpu.c
index 7ae54a04..cdabc085 100644
--- a/PDP18B/pdp18b_cpu.c
+++ b/PDP18B/pdp18b_cpu.c
@@ -2329,8 +2329,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -2362,7 +2364,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -2370,6 +2372,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC      L AC      MQ      IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(di++) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         l = (h->lac >> 18) & 1;                         /* link */
diff --git a/PDP8/pdp8_cpu.c b/PDP8/pdp8_cpu.c
index 1fce64d9..e1d9f215 100644
--- a/PDP8/pdp8_cpu.c
+++ b/PDP8/pdp8_cpu.c
@@ -1572,8 +1572,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -1603,7 +1605,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -1611,6 +1613,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "PC     L AC    MQ    ea     IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         l = (h->lac >> 12) & 1;                         /* link */
diff --git a/SDS/sds_cpu.c b/SDS/sds_cpu.c
index d4130c04..103691cd 100644
--- a/SDS/sds_cpu.c
+++ b/SDS/sds_cpu.c
@@ -1871,8 +1871,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (sim_switches & SWMASK('M'))
     hst_exclude = MON_MODE;
@@ -1912,7 +1914,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -1920,6 +1922,10 @@ if (di < 0)
     di = di + hst_lnt;
 fprintf (st, "CYC PC    MD OV A        B        X        EA      C\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->typ) {                                       /* instruction? */
         ov = (h->typ >> 4) & 1;                         /* overflow */
diff --git a/SEL32/sel32_cpu.c b/SEL32/sel32_cpu.c
index 67a09238..7bfbef71 100644
--- a/SEL32/sel32_cpu.c
+++ b/SEL32/sel32_cpu.c
@@ -7548,8 +7548,10 @@ cpu_set_hist(UNIT *uptr, int32 val, CONST char *cptr, void *desc)
     }
     /* the user has specified options, process them */
     lnt = (int32)get_uint(cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;                            /* arg error for bad input or too small a value */
+    if (r != SCPE_OK)
+        return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+    if (lnt && (lnt < HIST_MIN))
+        return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;                                      /* start at beginning */
     if (hst_lnt) {                                  /* if a new length was input, resize history buffer */
         free(hst);                                  /* out with the old */
@@ -7586,6 +7588,10 @@ t_stat cpu_show_hist(FILE *st, UNIT *uptr, int32 val, CONST void *desc)
     if (di < 0)
         di = di + hst_lnt;                          /* wrap */
     for (k = 0; k < lnt; k++) {                     /* print specified entries */
+        if (stop_cpu) {                             /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                                  /* abandon remaining output */
+        }
         h = &hst[(++di) % hst_lnt];                 /* entry pointer */
         /* display the instruction and results */
         if (MODES & BASEBIT)
diff --git a/TX-0/tx0_cpu.c b/TX-0/tx0_cpu.c
index 993c2125..0b6be86c 100644
--- a/TX-0/tx0_cpu.c
+++ b/TX-0/tx0_cpu.c
@@ -1140,7 +1140,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN))) return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -1167,13 +1170,18 @@ InstHistory *h;
 if (hst_lnt == 0) return SCPE_NOFNC;                    /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
-    if ((r != SCPE_OK) || (lnt == 0)) return SCPE_ARG;
+    if ((r != SCPE_OK) || (lnt == 0))
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
 if (di < 0) di = di + hst_lnt;
 fprintf (st, "PC      OV AC     IO      PF EA      IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         ov = (h->ovac >> HIST_V_SHF) & 1;               /* overflow */
diff --git a/VAX/vax_cpu.c b/VAX/vax_cpu.c
index 45befe4b..3be7f0cf 100644
--- a/VAX/vax_cpu.c
+++ b/VAX/vax_cpu.c
@@ -1586,7 +1586,7 @@ for ( ;; ) {
         hst_p = hst_p + 1;
         if (hst_p >= hst_lnt)
             hst_p = 0;
-        if (hst_log && (hst_p == hst_log_p))
+        if (hst_log && (hst_p == hst_log_p))    /* File Logging and Full, then write all records*/
             cpu_show_hist_records (hst_log, FALSE, hst_log_p, hst_lnt);
         }
 
@@ -3533,21 +3533,21 @@ int32 i, lnt;
 char gbuf[CBUFSIZE];
 t_stat r;
 
-if (cptr == NULL) {
-    for (i = 0; i < hst_lnt; i++)
-        hst[i].iPC = 0;
-    hst_p = 0;
+if (cptr == NULL) { /* Clear History */
     if (hst_log) {
         sim_set_fsize (hst_log, (t_addr)0);
         hst_log_p = 0;
         cpu_show_hist_records (hst_log, TRUE, 0, 0);
         }
+    for (i = 0; i < hst_lnt; i++)
+        hst[i].iPC = 0;
+    hst_p = 0;
     return SCPE_OK;
     }
 cptr = get_glyph (cptr, gbuf, ':');
 lnt = (int32) get_uint (gbuf, 10, HIST_MAX, &r);
 if (r != SCPE_OK)
-    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s\n", gbuf);
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", gbuf, HIST_MAX);
 if (lnt && (lnt < HIST_MIN))
     return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
@@ -3594,7 +3594,7 @@ if (hst_lnt == 0)                                       /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
     if ((r != SCPE_OK) || (lnt == 0))
-        return SCPE_ARG;
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
@@ -3616,6 +3616,10 @@ if (do_header) {
     fprintf (st, "PC       PSL       IR\n\n");
     }
 for (k = 0; k < count; k++) {                           /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(start++) % hst_lnt];                      /* entry pointer */
     if (h->iPC == 0)                                    /* filled in? */
         continue;
@@ -4010,7 +4014,7 @@ fprintf (st, "   sim> SET CPU HISTORY                 clear history buffer\n");
 fprintf (st, "   sim> SET CPU HISTORY=0               disable history\n");
 fprintf (st, "   sim> SET CPU {-T} HISTORY=n{:file}   enable history, length = n\n");
 fprintf (st, "   sim> SHOW CPU HISTORY                print CPU history\n");
-fprintf (st, "   sim> SHOW CPU HISTORY=n              print first n entries of CPU history\n\n");
+fprintf (st, "   sim> SHOW CPU HISTORY=n              print most recent n entries of history\n\n");
 fprintf (st, "The -T switch causes simulator time to be recorded (and displayed)\n");
 fprintf (st, "with each history entry.\n");
 fprintf (st, "When writing history to a file (SET CPU HISTORY=n:file), 'n' specifies\n");
diff --git a/alpha/alpha_cpu.c b/alpha/alpha_cpu.c
index a8dcd063..dee7b4fd 100644
--- a/alpha/alpha_cpu.c
+++ b/alpha/alpha_cpu.c
@@ -1768,7 +1768,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (uint32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN))) return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -1847,13 +1850,18 @@ InstHistory *h;
 if (hst_lnt == 0) return SCPE_NOFNC;                    /* enabled? */
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
-    if ((r != SCPE_OK) || (lnt == 0)) return SCPE_ARG;
+    if ((r != SCPE_OK) || (lnt == 0))
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
 if (di < 0) di = di + hst_lnt;
 fprintf (st, "PC               Ra               Rb               IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->pc & HIST_PC) {                              /* instruction? */
         cpu_fprint_one_inst (st, h->ir, h->pc, h->ra, h->rb);
diff --git a/imlac/imlac_cpu.c b/imlac/imlac_cpu.c
index 2c05ef4d..610b1925 100644
--- a/imlac/imlac_cpu.c
+++ b/imlac/imlac_cpu.c
@@ -525,6 +525,10 @@ cpu_show_hist (FILE *st, UNIT *uptr, int32 val, CONST void *desc)
     j = history_m + history_i - history_n;
 
   for (i = 0; i < history_n; i++) {
+    if (stop_cpu) {                 /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                      /* abandon remaining output */
+    }
     fprintf (st, "%06o %06o %06o %06o %06o %d  ",
              history[j].PC,
              history[j].IR,
diff --git a/sigma/sigma_cpu.c b/sigma/sigma_cpu.c
index e7469bbb..edd760d2 100755
--- a/sigma/sigma_cpu.c
+++ b/sigma/sigma_cpu.c
@@ -2777,8 +2777,10 @@ if (cptr == NULL) {
     return SCPE_OK;
     }
 lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-    return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
 hst_p = 0;
 if (hst_lnt) {
     free (hst);
@@ -2839,13 +2841,18 @@ if (hst_lnt == 0)                                   /* enabled? */
     return SCPE_NOFNC;
 if (cptr) {
     lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
-    if ((r != SCPE_OK) || (lnt == 0)) return SCPE_ARG;
+    if ((r != SCPE_OK) || (lnt == 0))
+        return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
     }
 else lnt = hst_lnt;
 di = hst_p - lnt;                                       /* work forward */
 if (di < 0) di = di + hst_lnt;
 fprintf (st, "  PC   CC Rn       Rn|1     EA    operand  operand1 IR\n\n");
 for (k = 0; k < lnt; k++) {                             /* print specified */
+    if (stop_cpu) {                                     /* Control-C (SIGINT) */
+        stop_cpu = FALSE;
+        break;                                          /* abandon remaining output */
+        }
     h = &hst[(++di) % hst_lnt];                         /* entry pointer */
     if (h->typ_cc_pc)                                   /* instruction? */
         cpu_fprint_one_inst (st, h->typ_cc_pc, h->ir, h->rn, h->rn1, h->ea, h->op, h->op1);
diff --git a/swtp6800/common/m6800.c b/swtp6800/common/m6800.c
index 915efe50..e662077e 100644
--- a/swtp6800/common/m6800.c
+++ b/swtp6800/common/m6800.c
@@ -2173,8 +2173,10 @@ t_stat cpu_set_hist (UNIT *uptr, int32 val, CONST char *cptr, void *desc)
         return SCPE_OK;
         }
     lnt = (int32) get_uint (cptr, 10, HIST_MAX, &r);
-    if ((r != SCPE_OK) || (lnt && (lnt < HIST_MIN)))
-        return SCPE_ARG;
+if (r != SCPE_OK)
+    return sim_messagef (SCPE_ARG, "Invalid Numeric Value: %s.  Maximum is %d\n", cptr, HIST_MAX);
+if (lnt && (lnt < HIST_MIN))
+    return sim_messagef (SCPE_ARG, "%d is less than the minumum history value of %d\n", lnt, HIST_MIN);
     hst_p = 0;
     if (hst_lnt) {
         free (hst);
@@ -2204,7 +2206,7 @@ t_stat cpu_show_hist (FILE *st, UNIT *uptr, int32 val, CONST void *desc)
     if (cptr) {
         lnt = (int32) get_uint (cptr, 10, hst_lnt, &r);
         if ((r != SCPE_OK) || (lnt == 0))
-            return SCPE_ARG;
+            return sim_messagef (SCPE_ARG, "Invalid count specifier: %s, max is %d\n", cptr, hst_lnt);
         }
     else lnt = hst_lnt;
     di = hst_p - lnt;                       /* work forward */
@@ -2212,6 +2214,10 @@ t_stat cpu_show_hist (FILE *st, UNIT *uptr, int32 val, CONST void *desc)
         di = di + hst_lnt;
     fprintf (st, "PC   SP   CC A  B  IX   Instruction\n\n");
     for (k = 0; k < lnt; k++) {             /* print specified */
+        if (stop_cpu) {                     /* Control-C (SIGINT) */
+            stop_cpu = FALSE;
+            break;                          /* abandon remaining output */
+            }
         h = &hst[(di++) % hst_lnt];         /* entry pointer */
         ir = h->inst[0];
         fprintf (st, "%04X %04X %02X ", h->pc , h->sp, h->cc);