diff --git a/AltairZ80/altairz80_sio.c b/AltairZ80/altairz80_sio.c
index cb184c65..0e51bca8 100644
--- a/AltairZ80/altairz80_sio.c
+++ b/AltairZ80/altairz80_sio.c
@@ -170,7 +170,6 @@ extern void setClockFrequency(const uint32 Value);
 extern uint32 PCX;
 extern int32 SR;
 extern UNIT cpu_unit;
-extern volatile int32 stop_cpu;
 
 /* Debug Flags */
 static DEBTAB generic_dt[] = {
diff --git a/BESM6/besm6_tty.c b/BESM6/besm6_tty.c
index 9f934d8d..779c00a1 100644
--- a/BESM6/besm6_tty.c
+++ b/BESM6/besm6_tty.c
@@ -257,8 +257,7 @@ t_stat vt_clk (UNIT * this)
         static int divider;
         if (++divider == CLK_TPS/10) {
             divider = 0;
-            if (SCPE_STOP == sim_poll_kbd())
-                stop_cpu = 1;
+            sim_poll_kbd();
         }
     }
 
@@ -1093,9 +1092,6 @@ int vt_getc (int num)
     } else {
         /* Console (keyboard) input. */
         c = sim_poll_kbd();
-        if (c == SCPE_STOP) {
-            stop_cpu = 1;   /* just in case */ 
-        }
         if (! (c & SCPE_KFLAG))
             return -1;
     }
diff --git a/I1401/i1401_iq.c b/I1401/i1401_iq.c
index f07d2508..7832a15b 100644
--- a/I1401/i1401_iq.c
+++ b/I1401/i1401_iq.c
@@ -38,7 +38,6 @@
 #define UNIT_V_PCH      (UNIT_V_UF + 0)                 /* output conv */
 #define UNIT_PCH        (1 << UNIT_V_PCH)
 
-extern volatile int32 stop_cpu;
 extern uint8 M[];
 extern int32 BS, iochk, ind[64];
 extern UNIT cpu_unit;
diff --git a/scp.c b/scp.c
index 43c27572..576c9226 100644
--- a/scp.c
+++ b/scp.c
@@ -541,7 +541,7 @@ UNIT *sim_dfunit = NULL;
 DEVICE **sim_internal_devices = NULL;
 uint32 sim_internal_device_count = 0;
 int32 sim_opt_out = 0;
-int32 sim_is_running = 0;
+volatile t_bool sim_is_running = FALSE;
 t_bool sim_processing_event = FALSE;
 uint32 sim_brk_summ = 0;
 uint32 sim_brk_types = 0;
@@ -564,7 +564,7 @@ size_t *sim_sub_instr_off = NULL;
 static double sim_time;
 static uint32 sim_rtime;
 static int32 noqueue_time;
-volatile int32 stop_cpu = 0;
+volatile t_bool stop_cpu = FALSE;
 static unsigned int sim_stop_sleep_ms = 250;
 static char **sim_argv;
 t_value *sim_eval = NULL;
@@ -2264,12 +2264,12 @@ if (sim_vm_init != NULL)                                /* call once only */
     (*sim_vm_init)();
 sim_finit ();                                           /* init fio package */
 setenv ("SIM_NAME", sim_name, 1);                       /* Publish simulator name */
-stop_cpu = 0;
+stop_cpu = FALSE;
 sim_interval = 0;
 sim_time = sim_rtime = 0;
 noqueue_time = 0;
 sim_clock_queue = QUEUE_LIST_END;
-sim_is_running = 0;
+sim_is_running = FALSE;
 sim_log = NULL;
 if (sim_emax <= 0)
     sim_emax = 1;
@@ -4189,7 +4189,7 @@ while (*cptr) {
     if ((wait > 0.0) && (!stop_cpu))
         sim_os_ms_sleep ((unsigned)wait);
     }
-stop_cpu = 0;                   /* Clear in case sleep was interrupted */
+stop_cpu = FALSE;                   /* Clear in case sleep was interrupted */
 return SCPE_OK;
 }
 
@@ -7206,30 +7206,26 @@ for (i = 1; (dptr = sim_devices[i]) != NULL; i++) {     /* reposition all */
                 return sim_messagef (SCPE_IERR, "Can't seek to %u in %s for %s\n", (unsigned)uptr->pos, uptr->filename, sim_uname (uptr));
         }
     }
-stop_cpu = 0;
-sim_is_running = 1;                                     /* flag running */
-if ((r = sim_ttrun ()) != SCPE_OK) {                          /* set console mode */
-    sim_is_running = 0;                                 /* flag idle */
+if ((r = sim_ttrun ()) != SCPE_OK) {                    /* set console mode */
     sim_ttcmd ();
     return sim_messagef (SCPE_TTYERR, "sim_ttrun() returned: %s\n", sim_error_text (r));
     }
 if ((r = sim_check_console (30)) != SCPE_OK) {          /* check console, error? */
-    sim_is_running = 0;                                 /* flag idle */
     sim_ttcmd ();
     sim_messagef (r, "sim_check_console () returned: %s\n", sim_error_text (r));
     }
 #ifdef SIGHUP
 if (signal (SIGHUP, int_handler) == SIG_ERR) {          /* set WRU */
-    sim_is_running = 0;                                 /* flag idle */
     sim_ttcmd ();
     return sim_messagef (SCPE_SIGERR, "Can't establish SIGHUP");
     }
 #endif
 if (signal (SIGTERM, int_handler) == SIG_ERR) {         /* set WRU */
-    sim_is_running = 0;                                 /* flag idle */
     sim_ttcmd ();
     return sim_messagef (SCPE_SIGERR, "Can't establish SIGTERM");
     }
+stop_cpu = FALSE;
+sim_is_running = TRUE;                                  /* flag running */
 if (sim_step)                                           /* set step timer */
     sim_activate (&sim_step_unit, sim_step);
 fflush(stdout);                                         /* flush stdout */
@@ -7278,15 +7274,15 @@ do {
         }
     else
         sim_step = 1;
-    if (sim_step)                                           /* set step timer */
+    if (sim_step)                                       /* set step timer */
         sim_activate (&sim_step_unit, sim_step);
     } while (1);
 
-if ((SCPE_BARE_STATUS(r) == SCPE_STOP) &&                   /* WRU exit from sim_instr() */
-    (sim_on_actions[sim_do_depth][SCPE_STOP] == NULL) &&    /* without a handler for a STOP condition */
+if ((SCPE_BARE_STATUS(r) == SCPE_STOP) &&               /* WRU exit from sim_instr() */
+    (sim_on_actions[sim_do_depth][SCPE_STOP] == NULL) &&/* without a handler for a STOP condition */
     (sim_on_actions[sim_do_depth][0] == NULL))
-    sim_os_ms_sleep (sim_stop_sleep_ms);                    /* wait a bit for SIGINT */
-sim_is_running = 0;                                     /* flag idle */
+    sim_os_ms_sleep (sim_stop_sleep_ms);                /* wait a bit for SIGINT */
+sim_is_running = FALSE;                                 /* flag idle */
 sim_stop_timer_services ();                             /* disable wall clock timing */
 sim_ttcmd ();                                           /* restore console */
 sim_brk_clrall (BRK_TYP_DYN_STEPOVER);                  /* cancel any step/over subroutine breakpoints */
@@ -7451,7 +7447,7 @@ return sim_cancel (&sim_step_unit);
 
 void int_handler (int sig)
 {
-stop_cpu = 1;
+stop_cpu = TRUE;
 return;
 }
 
@@ -9895,7 +9891,7 @@ else
     sim_debug (SIM_DBG_EVENT, sim_dflt_dev, "Processing Queue Complete New Interval = %d(%s)\n", sim_interval, sim_uname(sim_clock_queue));
 
 if ((reason == SCPE_OK) && stop_cpu) {
-    stop_cpu = 0;
+    stop_cpu = FALSE;
     reason = SCPE_STOP;
     }
 sim_processing_event = FALSE;
diff --git a/scp.h b/scp.h
index 12e7b03c..c999c1a1 100644
--- a/scp.h
+++ b/scp.h
@@ -347,12 +347,12 @@ extern struct timespec sim_deb_basetime;                /* debug base time for r
 extern DEVICE **sim_internal_devices;
 extern uint32 sim_internal_device_count;
 extern UNIT *sim_clock_queue;
-extern int32 sim_is_running;
+extern volatile t_bool sim_is_running;
 extern t_bool sim_processing_event;                     /* Called from sim_process_event */
 extern char *sim_prompt;                                /* prompt string */
 extern const char *sim_savename;                        /* Simulator Name used in Save/Restore files */
 extern t_value *sim_eval;
-extern volatile int32 stop_cpu;
+extern volatile t_bool stop_cpu;
 extern uint32 sim_brk_types;                            /* breakpoint info */
 extern uint32 sim_brk_dflt;
 extern uint32 sim_brk_summ;
diff --git a/sim_console.c b/sim_console.c
index ba7e9f88..cdd2c90a 100644
--- a/sim_console.c
+++ b/sim_console.c
@@ -1397,7 +1397,7 @@ for (i=(was_active_command ? sim_rem_cmd_active_line : 0);
             continue;
             }
         else {
-            sim_is_running = 0;
+            sim_is_running = FALSE;
             sim_rem_collect_all_registers ();
             sim_stop_timer_services ();
             if (rem->act == NULL) {
@@ -1422,7 +1422,7 @@ for (i=(was_active_command ? sim_rem_cmd_active_line : 0);
             if (rem->single_mode) {
                 if (c == sim_int_char) {            /* ^E (the interrupt character) must start continue mode console interaction */
                     rem->single_mode = FALSE;       /* enter multi command mode */
-                    sim_is_running = 0;
+                    sim_is_running = FALSE;
                     sim_rem_collect_all_registers ();
                     sim_stop_timer_services ();
                     stat = SCPE_STOP;
@@ -1787,7 +1787,7 @@ for (i=(was_active_command ? sim_rem_cmd_active_line : 0);
                     tmxr_linemsg (lpj, "Simulator Running...");
                     tmxr_send_buffered_data (lpj);
                     }
-                sim_is_running = 1;
+                sim_is_running = TRUE;
                 sim_start_timer_services ();
                 }
             if (cmdp && (cmdp->action == &x_continue_cmd))
@@ -2768,7 +2768,7 @@ if (!sim_rem_master_mode) {
     else
         c = SCPE_OK;
     if (c == SCPE_STOP) {                                   /* ^E */
-        stop_cpu = 1;                                       /* Force a stop (which is picked up by sim_process_event */
+        stop_cpu = TRUE;                                    /* Force a stop (which is picked up by sim_process_event */
         return SCPE_OK;
         }
     if ((sim_con_tmxr.master == 0) &&                       /* not Telnet? */
@@ -2958,7 +2958,6 @@ extern pthread_mutex_t     sim_tmxr_poll_lock;
 extern pthread_cond_t      sim_tmxr_poll_cond;
 extern int32               sim_tmxr_poll_count;
 extern t_bool              sim_tmxr_poll_running;
-extern int32 sim_is_running;
 
 pthread_t           sim_console_poll_thread;       /* Keyboard Polling Thread Id */
 t_bool              sim_console_poll_running = FALSE;