From f1a9ebec8a34295ebeb0a4c2ba2d285009a868ed Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Sat, 23 Jun 2018 16:34:35 -0700
Subject: [PATCH] GB I/O: Fix JOYP IRQs

---
 src/gb/io.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/src/gb/io.c b/src/gb/io.c
index 62f8e5b32..14e3b3621 100644
--- a/src/gb/io.c
+++ b/src/gb/io.c
@@ -104,6 +104,8 @@ static const uint8_t _registerMask[] = {
 	[REG_IE]   = 0xE0,
 };
 
+static uint8_t _readKeys(struct GB* gb);
+
 static void _writeSGBBits(struct GB* gb, int bits) {
 	if (!bits) {
 		gb->sgbBit = -1;
@@ -394,10 +396,12 @@ void GBIOWrite(struct GB* gb, unsigned address, uint8_t value) {
 		}
 		break;
 	case REG_JOYP:
+		gb->memory.io[REG_JOYP] = value | 0x0F;
+		_readKeys(gb);
 		if (gb->model == GB_MODEL_SGB) {
 			_writeSGBBits(gb, (value >> 4) & 3);
 		}
-		break;
+		return;
 	case REG_TIMA:
 		if (value && mTimingUntil(&gb->timing, &gb->timer.irq) > 1) {
 			mTimingDeschedule(&gb->timing, &gb->timer.irq);
@@ -522,7 +526,8 @@ static uint8_t _readKeys(struct GB* gb) {
 	if (gb->sgbCurrentController != 0) {
 		keys = 0;
 	}
-	switch (gb->memory.io[REG_JOYP] & 0x30) {
+	uint8_t joyp = gb->memory.io[REG_JOYP];
+	switch (joyp & 0x30) {
 	case 0x30:
 		keys = gb->sgbCurrentController;
 		break;
@@ -535,7 +540,12 @@ static uint8_t _readKeys(struct GB* gb) {
 		keys |= keys >> 4;
 		break;
 	}
-	return (0xC0 | (gb->memory.io[REG_JOYP] | 0xF)) ^ (keys & 0xF);
+	gb->memory.io[REG_JOYP] = (0xCF | joyp) ^ (keys & 0xF);
+	if (joyp & ~gb->memory.io[REG_JOYP] & 0xF) {
+		gb->memory.io[REG_IF] |= (1 << GB_IRQ_KEYPAD);
+		GBUpdateIRQs(gb);
+	}
+	return gb->memory.io[REG_JOYP];
 }
 
 uint8_t GBIORead(struct GB* gb, unsigned address) {
@@ -639,10 +649,7 @@ uint8_t GBIORead(struct GB* gb, unsigned address) {
 }
 
 void GBTestKeypadIRQ(struct GB* gb) {
-	if (_readKeys(gb)) {
-		gb->memory.io[REG_IF] |= (1 << GB_IRQ_KEYPAD);
-		GBUpdateIRQs(gb);
-	}
+	_readKeys(gb);
 }
 
 struct GBSerializedState;