GBA Timer: Improve accuracy of timers

CHANGES

@@ -75,6 +75,8 @@ Bugfixes:
  - GB Timer: Fix sub-M-cycle DIV reset timing and edge triggering
  - Core: Fix interrupting a thread while on the thread (fixes mgba.io/i/692)
  - Core: Fix directory sets crashing on close if base isn't properly detached
+ - Qt: Fix window icon being stretched
+ - Qt: Fix data directory path
 Misc:
  - SDL: Remove scancode key input
  - GBA Video: Clean up unused timers
@@ -145,6 +147,7 @@ Misc:
  - GB MBC: New MBC7 implementation
  - Qt: Better highlight active key in control binding
  - Core: Improved threading interrupted detection
+ - GBA Timer: Improve accuracy of timers
 
 0.6 beta 2: (Future)
 Bugfixes:
@@ -154,6 +157,7 @@ Bugfixes:
  - Qt: Fix data directory path
 Misc:
  - Qt: Add language selector
+ - GBA Timer: Improve accuracy of timers
 
 0.6 beta 1: (2017-06-29)
  - Initial beta for 0.6

src/gba/timer.c

@@ -103,20 +103,22 @@ void GBATimerWriteTMCNT_HI(struct GBA* gba, int timer, uint16_t control) {
 	GBATimerUpdateRegister(gba, timer);
 
 	unsigned oldPrescale = GBATimerFlagsGetPrescaleBits(currentTimer->flags);
+	unsigned prescaleBits;
 	switch (control & 0x0003) {
 	case 0x0000:
-		currentTimer->flags = GBATimerFlagsSetPrescaleBits(currentTimer->flags, 0);
+		prescaleBits = 0;
 		break;
 	case 0x0001:
-		currentTimer->flags = GBATimerFlagsSetPrescaleBits(currentTimer->flags, 6);
+		prescaleBits = 6;
 		break;
 	case 0x0002:
-		currentTimer->flags = GBATimerFlagsSetPrescaleBits(currentTimer->flags, 8);
+		prescaleBits = 8;
 		break;
 	case 0x0003:
-		currentTimer->flags = GBATimerFlagsSetPrescaleBits(currentTimer->flags, 10);
+		prescaleBits = 10;
 		break;
 	}
+	currentTimer->flags = GBATimerFlagsSetPrescaleBits(currentTimer->flags, prescaleBits);
 	currentTimer->flags = GBATimerFlagsTestFillCountUp(currentTimer->flags, timer > 0 && (control & 0x0004));
 	currentTimer->flags = GBATimerFlagsTestFillDoIrq(currentTimer->flags, control & 0x0040);
 	currentTimer->overflowInterval = (0x10000 - currentTimer->reload) << GBATimerFlagsGetPrescaleBits(currentTimer->flags);
@@ -125,7 +127,7 @@ void GBATimerWriteTMCNT_HI(struct GBA* gba, int timer, uint16_t control) {
 	if (!wasEnabled && GBATimerFlagsIsEnable(currentTimer->flags)) {
 		mTimingDeschedule(&gba->timing, &currentTimer->event);
 		if (!GBATimerFlagsIsCountUp(currentTimer->flags)) {
-			mTimingSchedule(&gba->timing, &currentTimer->event, currentTimer->overflowInterval);
+			mTimingSchedule(&gba->timing, &currentTimer->event, currentTimer->overflowInterval + 7 - 6 * prescaleBits);
 		}
 		gba->memory.io[(REG_TM0CNT_LO + (timer << 2)) >> 1] = currentTimer->reload;
 		currentTimer->oldReload = currentTimer->reload;