From a85ae6563c1008685ccd91fcedc1bb2846139ad6 Mon Sep 17 00:00:00 2001
From: Jeffrey Pfau <jeffrey@endrift.com>
Date: Mon, 22 Jun 2015 22:32:46 -0700
Subject: [PATCH] GBA: Fix timing of reading from timer registers

---
 CHANGES       | 1 +
 src/gba/gba.c | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/CHANGES b/CHANGES
index 65246e8ec..717b6616d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -48,6 +48,7 @@ Bugfixes:
  - GBA Video: Fix windows not affecting sprites
  - VFS: Fix line-reading to return proper values
  - GBA Memory: Fix load/store multiple video memory waitstates
+ - GBA: Fix timing of reading from timer registers
 Misc:
  - Qt: Handle saving input settings better
  - Debugger: Free watchpoints in addition to breakpoints
diff --git a/src/gba/gba.c b/src/gba/gba.c
index 7994f03fa..f99c3a549 100644
--- a/src/gba/gba.c
+++ b/src/gba/gba.c
@@ -451,7 +451,8 @@ void GBAApplyPatch(struct GBA* gba, struct Patch* patch) {
 void GBATimerUpdateRegister(struct GBA* gba, int timer) {
 	struct GBATimer* currentTimer = &gba->timers[timer];
 	if (currentTimer->enable && !currentTimer->countUp) {
-		gba->memory.io[(REG_TM0CNT_LO + (timer << 2)) >> 1] = currentTimer->oldReload + ((gba->cpu->cycles - currentTimer->lastEvent) >> currentTimer->prescaleBits);
+		// Reading this takes two cycles (1N+1I), so let's remove them preemptively
+		gba->memory.io[(REG_TM0CNT_LO + (timer << 2)) >> 1] = currentTimer->oldReload + ((gba->cpu->cycles - currentTimer->lastEvent - 2) >> currentTimer->prescaleBits);
 	}
 }