From 3a03d180d2f7c7081f42d2ada5582c17ad2032cd Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Thu, 22 Jun 2017 01:20:22 -0700
Subject: [PATCH] GB Timer: Fix sub-M-cycle DIV reset timing and edge
 triggering

---
 CHANGES        | 1 +
 src/gb/timer.c | 9 ++++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/CHANGES b/CHANGES
index 360f7a5e0..0f6f062dd 100644
--- a/CHANGES
+++ b/CHANGES
@@ -72,6 +72,7 @@ Bugfixes:
  - GBA Savedata: Update and fix Sharkport importing (fixes mgba.io/i/658)
  - OpenGL: Fix some shaders causing offset graphics
  - Qt: Fix game unpausing after frame advancing and refocusing
+ - GB Timer: Fix sub-M-cycle DIV reset timing and edge triggering
 Misc:
  - SDL: Remove scancode key input
  - GBA Video: Clean up unused timers
diff --git a/src/gb/timer.c b/src/gb/timer.c
index 1533792eb..ba34d4bb7 100644
--- a/src/gb/timer.c
+++ b/src/gb/timer.c
@@ -5,6 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 #include <mgba/internal/gb/timer.h>
 
+#include <mgba/internal/lr35902/lr35902.h>
 #include <mgba/internal/gb/gb.h>
 #include <mgba/internal/gb/io.h>
 #include <mgba/internal/gb/serialize.h>
@@ -63,11 +64,17 @@ void GBTimerReset(struct GBTimer* timer) {
 }
 
 void GBTimerDivReset(struct GBTimer* timer) {
+	if (timer->internalDiv & (timer->timaPeriod >> 1)) {
+		++timer->p->memory.io[REG_TIMA];
+		if (!timer->p->memory.io[REG_TIMA]) {
+			mTimingSchedule(&timer->p->timing, &timer->irq, 4 - ((timer->p->cpu->executionState + 1) & 3));
+		}
+	}
 	timer->p->memory.io[REG_DIV] = 0;
 	timer->internalDiv = 0;
 	timer->nextDiv = GB_DMG_DIV_PERIOD;
 	mTimingDeschedule(&timer->p->timing, &timer->event);
-	mTimingSchedule(&timer->p->timing, &timer->event, timer->nextDiv);
+	mTimingSchedule(&timer->p->timing, &timer->event, timer->nextDiv - ((timer->p->cpu->executionState + 1) & 3));
 }
 
 uint8_t GBTimerUpdateTAC(struct GBTimer* timer, GBRegisterTAC tac) {