Clean up timer handling to make sure at all stages, the output signal…

… correctly asserts or de-asserts the timer interrupt. Some modes during mode byte or counter program assert OUT high and would therefore trigger an interrupt. This allows the really weird interrupt handling in Tyrian 2000 to work and the game music and sound effects no longer stall during title, menu, and setup screens. It still does not fix the crackly Sound Blaster audio but I think some rework of the DMA mask/unmask events in sblaster.cpp can help. This also fixes the issue with Tyrian 2000 where the game speed also seems to control the speed of the music.

src/hardware/timer.cpp

@@ -92,9 +92,13 @@ struct PIT_Block {
     }
     void set_active_counter(Bitu new_cntr) {
         assert(new_cntr != 0);
+        if (mode == 2) { assert(new_cntr != 1); }
 
         cntr_cur = new_cntr;
-        delay = ((pic_tickindex_t)(1000ul * cntr_cur)) / PIT_TICK_RATE;
+        if (mode == 2)
+            delay = ((pic_tickindex_t)(1000ul * (cntr_cur-1u))) / PIT_TICK_RATE; /* counts down to ONE, not ZERO */
+        else
+            delay = ((pic_tickindex_t)(1000ul * cntr_cur)) / PIT_TICK_RATE;
 
         /* Make sure the new counter value is returned if read back even if the gate is off!
          * Some games like to constantly reprogram PIT 2 with precise event timey stuff and
@@ -123,7 +127,10 @@ struct PIT_Block {
         pic_tickindex_t c_delay;
 
         /* NTS: Remember, the counter counts DOWN, not up, so the delay is how long it takes to get there */
-        c_delay = ((pic_tickindex_t)(1000ull * (0x10000u - counter))) / PIT_TICK_RATE;
+        if (mode == 2)
+            c_delay = ((pic_tickindex_t)(1000ull * (0xFFFFu - counter))) / PIT_TICK_RATE; /* counts down to ONE, not ZERO */
+        else
+            c_delay = ((pic_tickindex_t)(1000ull * (0x10000u - counter))) / PIT_TICK_RATE;
 
         start = (t - c_delay);
     }
@@ -576,13 +583,13 @@ static void write_latch(Bitu port,Bitu val,Bitu /*iolen*/) {
 			if (p->bcd == false)
 				p->set_next_counter(0x10000);
 			else
-				p->set_next_counter(9999/*check this*/);
+				p->set_next_counter(10000/*check this*/);
 		}
-		else if (p->write_latch == 1 && p->mode == 3/*square wave, count by 2*/) { /* counter==1 and mode==3 makes a low frequency buzz (Paratrooper) */
+		else if (p->write_latch == 1 && (p->mode == 2/*rate generator*/ || p->mode == 3/*square wave, count by 2*/)) { /* counter==1 and mode==3 makes a low frequency buzz (Paratrooper) */
 			if (p->bcd == false)
 				p->set_next_counter(0x10001);
 			else
-				p->set_next_counter(10000/*check this*/);
+				p->set_next_counter(10001/*check this*/);
 		}
 		else {
 			p->set_next_counter(p->write_latch);
@@ -595,6 +602,12 @@ static void write_latch(Bitu port,Bitu val,Bitu /*iolen*/) {
 			if (counter == 0) {
 				PIC_RemoveEvents(PIT0_Event);
 				PIC_AddEvent(PIT0_Event,p->delay);
+
+				counter_output(counter);
+				if(pit[counter].output)
+					PIC_ActivateIRQ(0);
+				else
+					PIC_DeActivateIRQ(0);
 			}
 		}
 		else {
@@ -668,7 +681,12 @@ static void write_latch(Bitu port,Bitu val,Bitu /*iolen*/) {
 		if (p->mode == 0 || p->mode == 4) {
 			if (counter == 0) {
 				PIC_RemoveEvents(PIT0_Event);
-				PIC_DeActivateIRQ(0);
+
+				counter_output(counter);
+				if(pit[counter].output)
+					PIC_ActivateIRQ(0);
+				else
+					PIC_DeActivateIRQ(0);
 			}
 			p->update_count = false;
 		}
@@ -803,11 +821,12 @@ static void write_p43(Bitu /*port*/,Bitu val,Bitu /*iolen*/) {
 
 			if (latch == 0) {
 				PIC_RemoveEvents(PIT0_Event);
-				if((mode != 0)&& (pit[latch].reltime() > pit[latch].delay)) {
+
+				counter_output(latch);
+				if(pit[latch].output)
 					PIC_ActivateIRQ(0);
-				} else {
+				else
 					PIC_DeActivateIRQ(0);
-				}
 			}
 			pit[latch].new_mode = true;
 			if (latch == (IS_PC98_ARCH ? 1 : 2)) {