M CHANGES → CHANGES

@@ -92,6 +92,7 @@ - ARM: Partially fix LDM/STM writeback with empty register list
  - ARM: Fix stepping when events are pending
  - GBA DMA: Fix case where DMAs could get misaligned (fixes mgba.io/i/1092)
  - GBA Memory: Fix open bus from IWRAM (fixes mgba.io/i/1575)
+ - GBA Timers: Improve timer startup emulation
  - GBA Video: Fix OpenGL renderer 512x512 backgrounds (fixes mgba.io/i/1572)
  - GBA Video: Fix BLDY for semitransparent sprite on non-target-2 backgrounds
  - GBA Video: Fix effects blending improperly in some non-last windows

M src/gba/io.c → src/gba/io.c

@@ -712,16 +712,16 @@
 	switch (address) {
 	// Reading this takes two cycles (1N+1I), so let's remove them preemptively
 	case REG_TM0CNT_LO:
-		GBATimerUpdateRegister(gba, 0, 4);
+		GBATimerUpdateRegister(gba, 0, 2);
 		break;
 	case REG_TM1CNT_LO:
-		GBATimerUpdateRegister(gba, 1, 4);
+		GBATimerUpdateRegister(gba, 1, 2);
 		break;
 	case REG_TM2CNT_LO:
-		GBATimerUpdateRegister(gba, 2, 4);
+		GBATimerUpdateRegister(gba, 2, 2);
 		break;
 	case REG_TM3CNT_LO:
-		GBATimerUpdateRegister(gba, 3, 4);
+		GBATimerUpdateRegister(gba, 3, 2);
 		break;
 
 	case REG_KEYINPUT:

M src/gba/timer.c → src/gba/timer.c

@@ -8,9 +8,6 @@
 #include <mgba/internal/gba/gba.h>
 #include <mgba/internal/gba/io.h>
 
-#define TIMER_RELOAD_DELAY 0
-#define TIMER_STARTUP_DELAY 2
-
 #define REG_TMCNT_LO(X) (REG_TM0CNT_LO + ((X) << 2))
 
 static void GBATimerUpdate(struct GBA* gba, int timerId, uint32_t cyclesLate) {
@@ -18,7 +15,7 @@ struct GBATimer* timer = &gba->timers[timerId];
 	if (GBATimerFlagsIsCountUp(timer->flags)) {
 		gba->memory.io[REG_TMCNT_LO(timerId) >> 1] = timer->reload;
 	} else {
-		GBATimerUpdateRegister(gba, timerId, TIMER_RELOAD_DELAY + cyclesLate);
+		GBATimerUpdateRegister(gba, timerId, cyclesLate);
 	}
 
 	if (GBATimerFlagsIsDoIrq(timer->flags)) {
@@ -150,14 +147,14 @@ if (!wasEnabled && GBATimerFlagsIsEnable(currentTimer->flags)) {
 		mTimingDeschedule(&gba->timing, &currentTimer->event);
 		gba->memory.io[REG_TMCNT_LO(timer) >> 1] = currentTimer->reload;
 		int32_t tickMask = (1 << prescaleBits) - 1;
-		currentTimer->lastEvent = (mTimingCurrentTime(&gba->timing) - TIMER_STARTUP_DELAY) & ~tickMask;
-		GBATimerUpdateRegister(gba, timer, TIMER_STARTUP_DELAY);
+		currentTimer->lastEvent = mTimingCurrentTime(&gba->timing) & ~tickMask;
+		GBATimerUpdateRegister(gba, timer, 0);
 	} else if (wasEnabled && !GBATimerFlagsIsEnable(currentTimer->flags)) {
 		mTimingDeschedule(&gba->timing, &currentTimer->event);
 	} else if (GBATimerFlagsIsEnable(currentTimer->flags) && GBATimerFlagsGetPrescaleBits(currentTimer->flags) != oldPrescale && !GBATimerFlagsIsCountUp(currentTimer->flags)) {
 		mTimingDeschedule(&gba->timing, &currentTimer->event);
 		int32_t tickMask = (1 << prescaleBits) - 1;
-		currentTimer->lastEvent = (mTimingCurrentTime(&gba->timing) - TIMER_STARTUP_DELAY) & ~tickMask;
-		GBATimerUpdateRegister(gba, timer, TIMER_STARTUP_DELAY);
+		currentTimer->lastEvent = mTimingCurrentTime(&gba->timing) & ~tickMask;
+		GBATimerUpdateRegister(gba, timer, 0);
 	}
 }