From 26fa91a335f0e09dff26f1220e766f2f579973a9 Mon Sep 17 00:00:00 2001
From: Mark Pizzolato <mark@infocomm.com>
Date: Sun, 23 Feb 2020 00:31:45 -0800
Subject: [PATCH] TIMER: Avoid clocks with changing tick rates as the primary
 calibrated timer

Some simulators have clocks that have dynamically programmable tick
rates. Such a clock is only a reliable candidate to be the calibrated
clock if it uses a single tick rate rather than changing the tick rate
on the fly.  Generally most systems like this, under normal conditions
don't change their tick rates unless they're running something that is
examining the behavior of the clock system (like a diagnostic).  Under
these conditions this clock is removed from the potential selection as
"the" calibrated clock that all others are relative to and if necessary,
an internal calibrated clock is selected.
---
 sim_timer.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/sim_timer.c b/sim_timer.c
index b1207165..4377b811 100644
--- a/sim_timer.c
+++ b/sim_timer.c
@@ -943,9 +943,9 @@ if (rtc->hz != ticksper) {                          /* changing tick rate? */
         (rtc->last_hz != ticksper) && 
         (ticksper != 0))
         rtc->currd = (int32)(sim_timer_inst_per_sec () / ticksper);
+    _rtcn_configure_calibrated_clock (tmr);
     rtc->last_hz = rtc->hz;
     rtc->hz = ticksper;
-    _rtcn_configure_calibrated_clock (tmr);
     if (ticksper != 0) {
         RTC *crtc = &rtcs[sim_calb_tmr];
 
@@ -2451,6 +2451,16 @@ return SCPE_OK;
   clock with an appropriate tick rate, an internal clock is run that meets 
   this requirement, OR when asynch clocks are enabled, the internal clock
   is always run.
+
+  Some simulators have clocks that have dynamically programmable tick 
+  rates. Such a clock is only a reliable candidate to be the calibrated 
+  clock if it uses a single tick rate rather than changing the tick rate 
+  on the fly.  Generally most systems like this, under normal conditions
+  don't change their tick rates unless they're running something that is
+  examining the behavior of the clock system (like a diagnostic).  Under 
+  these conditions this clock is removed from the potential selection as
+  "the" calibrated clock all others are relative to and if necessary, an
+  internal calibrated clock is selected.
  */
 static void _rtcn_configure_calibrated_clock (int32 newtmr)
 {
@@ -2463,7 +2473,8 @@ for (tmr=0; tmr<SIM_NTIMERS; tmr++) {
     rtc = &rtcs[tmr];
     if ((rtc->hz) &&
         (rtc->hz <= (uint32)sim_os_tick_hz) &&
-        (rtc->clock_unit))
+        (rtc->clock_unit) &&
+        ((rtc->last_hz == 0) || (rtc->last_hz == rtc->hz)))
         break;
     }
 if (tmr == SIM_NTIMERS) {                   /* None found? */
@@ -2499,7 +2510,7 @@ if (tmr == SIM_NTIMERS) {                   /* None found? */
         SIM_INTERNAL_UNIT.action = &sim_timer_clock_tick_svc;
         SIM_INTERNAL_UNIT.flags = UNIT_IDLE;
         sim_register_internal_device (&sim_int_timer_dev);      /* Register Internal timer device */
-        sim_rtcn_init_unit (&SIM_INTERNAL_UNIT, (int32)((CLK_INIT*CLK_TPS)/sim_int_clk_tps), SIM_INTERNAL_CLK);
+        sim_rtcn_init_unit_ticks (&SIM_INTERNAL_UNIT, (int32)((CLK_INIT*CLK_TPS)/sim_int_clk_tps), SIM_INTERNAL_CLK, sim_int_clk_tps);
         SIM_INTERNAL_UNIT.action (&SIM_INTERNAL_UNIT);          /* Force tick to activate timer */
         }
     return;