SCP: Add timer logic description comments to sim_timer

sim_timer.c

@@ -827,7 +827,6 @@ static uint32 cyc_ms = 0;
 uint32 w_ms, w_idle, act_ms;
 int32 act_cyc;
 
-//sim_idle_idled = TRUE;                                  /* record idle attempt */
 if ((!sim_idle_enab)                             ||     /* idling disabled */
     ((sim_clock_queue == QUEUE_LIST_END) &&             /* or clock queue empty? */
 #if defined(SIM_ASYNCH_IO) && defined(SIM_ASYNCH_CLOCKS)
@@ -842,6 +841,29 @@ if ((!sim_idle_enab)                             ||     /* idling disabled */
         sim_interval = sim_interval - 1;
     return FALSE;
     }
+/*
+   When a simulator is in an instruction path (or under other conditions 
+   which would indicate idling), the countdown of sim_interval will not 
+   be happening at a pace which is consistent with the rate it happens 
+   when not in the ‘idle capable’ state.  The consequence of this is that 
+   the clock calibration may produce calibrated results which vary much 
+   more than they do when not in the idle able state.  Sim_idle also uses 
+   the calibrated tick size to approximate an adjustment to sim_interval
+   to reflect the number of instructions which would have executed during 
+   the actual idle time, so consistent calibrated numbers produce better 
+   adjustments. 
+   
+   To negate this effect, we set a flag (sim_idle_idled) here and the 
+   sim_rtcn_calb routine checks this flag before performing an actual 
+   calibration and skips calibration if the flag was set and then clears 
+   the flag.  Thus recalibration only happens if things didn’t idle.
+
+   we also check check sim_idle_enab above so that all simulators can avoid
+   directly checking sim_idle_enab before calling sim_idle so that all of 
+   the bookkeeping on sim_idle_idled is done here in sim_timer where it 
+   means something, while not idling when it isn’t enabled.  
+   */
+//sim_idle_idled = TRUE;                                  /* record idle attempt */
 sim_debug (DBG_TRC, &sim_timer_dev, "sim_idle(tmr=%d, sin_cyc=%d)\n", tmr, sin_cyc);
 if (cyc_ms == 0)                                        /* not computed yet? */
     cyc_ms = (rtc_currd[tmr] * rtc_hz[tmr]) / 1000;     /* cycles per msec */