BESM6: Schedule the serial line timer by instruction count when I/O is pending, and by wallclock time otherwise. This achieves a good TTY output rate even when the CPU is busy with a user process.

BESM6/besm6_cpu.c

@@ -708,6 +708,12 @@ void check_initial_setup ()
     /* 7 р. яч. ЗАНЯТА - разр любые приказы */
     const t_value ALL_REQS_ENABLED = 1 << 6;
 
+    if (!vt_is_idle()) {
+        /* Avoid sending setup requests while the OS
+         * is still printing boot-up messages.
+         */
+        return;
+    }
     if ((memory[TAKEN] & SETUP_REQS_ENABLED) == 0 ||
         (memory[TAKEN] & ALL_REQS_ENABLED) != 0 ||
         (MGRP & GRP_PANEL_REQ) == 0) {
@@ -1427,15 +1433,8 @@ void cpu_one_inst ()
 
     /* Не находимся ли мы в цикле "ЖДУ" диспака? */
     if (RUU == 047 && PC == 04440 && RK == 067704440) {
-        /* Если периферия простаивает, освобождаем процессор
-         * до следующего тика таймера. */
-        if (vt_is_idle()) {
-          check_initial_setup ();
-          sim_idle (0, TRUE);
-        } else if (sim_activate_time(tty_unit) > 1000*MSEC/300) {
-            /* Insert a TTY interrupt if a regular one is too far away */
-            sim_activate_abs(tty_unit, 0);
-        }
+        check_initial_setup ();
+        sim_idle(0, TRUE);
     }
 }
 

BESM6/besm6_defs.h

@@ -128,6 +128,7 @@ enum {
 
 extern UNIT cpu_unit;
 extern UNIT tty_unit[];
+extern UNIT clocks[];
 extern t_value memory [MEMSIZE];
 extern t_value pult [8];
 extern uint32 PC, RAU, RUU;
@@ -326,7 +327,6 @@ int disk_errors (void);
  */
 void printer_control (int num, uint32 cmd);
 void printer_hammer (int num, int pos, uint32 mask);
-int printer_is_idle (void);
 
 /*
  * Терминалы (телетайпы, видеотоны, "Консулы").
@@ -344,7 +344,6 @@ int vt_is_idle (void);
  */
 void fs_control (int num, uint32 cmd);
 int fs_read (int num);
-int fs_is_idle (void);
 
 /*
  * Отладочная выдача.

BESM6/besm6_printer.c

@@ -317,15 +317,3 @@ offset_gost_write (int num, FILE *fout)
     memset(dev->line, 0, sizeof (dev->line));
     dev->length = dev->strikes = 0;
 }
-
-/*
- * Выясняем, остановлены ли АЦПУ. Нужно для входа в "спящий" режим.
- */
-int printer_is_idle ()
-{
-    if (sim_is_active(&printer_unit[0]))
-        return 0;
-    if (sim_is_active(&printer_unit[1]))
-        return 0;
-    return 1;
-}

BESM6/besm6_punch.c

@@ -279,11 +279,6 @@ int fs_read(int num) {
     return FS[num];
 }
 
-int fs_is_idle (void)
-{
-    return fs_state[0] == FS_IDLE && fs_state[1] == FS_IDLE;
-}
-
 unsigned char
 unicode_to_gost (unsigned short val)
 {

BESM6/besm6_tty.c

@@ -238,12 +238,19 @@ t_stat vt_clk (UNIT * this)
 
     /* Polling sockets for transmission. */
     tmxr_poll_tx (&tty_desc);
-
-    /* We'll get a guaranteed interrupt rate of 250 Hz (wallclock)
-     * plus additional interrupts during the idle loop for speedup
-     * (see besm6_cpu.c).
+    /* If the TTY system is not idle, schedule the next interrupt
+     * by instruction count using the target interrupt rate of 300 Hz;
+     * otherwise we can wait for a roughly equivalent wallclock time period,
+     * e.g. until the next 250 Hz wallclock interrupt, but making sure 
+     * that the model time interval between GRP_SERIAL interrupts 
+     * is never less than expected.
+     * Using sim_activate_after() would be more straightforward (no need for a check
+     * as the host is faster than the target), but likely less efficient for idling.
      */
-    return sim_clock_coschedule (this, tmr_poll);
+    if (vt_is_idle() && sim_activate_time(clocks) > 1000*MSEC/300)
+        return sim_clock_coschedule (this, tmr_poll);
+    else
+        return sim_activate(this, 1000*MSEC/300);
 }
 
 t_stat tty_setmode (UNIT *u, int32 val, char *cptr, void *desc)

BESM6/dispak.ini

@@ -29,7 +29,7 @@ attach -e disk2 alt2048.bin
 ;
 ; Подключаем АЦПУ.
 ;
-attach prn0 /dev/tty
+attach prn0 output.txt
 
 ;
 ; Активируем операторский терминал,