From 4bca59daa5123a01df43f6216e6dd0b3d9005c81 Mon Sep 17 00:00:00 2001
From: CasualPokePlayer <50538166+CasualPokePlayer@users.noreply.github.com>
Date: Mon, 31 Mar 2025 18:41:03 -0700
Subject: [PATCH] Mask away unused GPIO bits

Unused GPIO bits are not writable and always return 0. This is documented on gbatek and I've confirmed this is the correct behavior on my own Emerald cartridge.
---
 src/gba/cart/gpio.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/gba/cart/gpio.c b/src/gba/cart/gpio.c
index a44bab294..5798a636e 100644
--- a/src/gba/cart/gpio.c
+++ b/src/gba/cart/gpio.c
@@ -78,15 +78,15 @@ void GBAHardwareGPIOWrite(struct GBACartridgeHardware* hw, uint32_t address, uin
 			hw->pinState &= ~hw->direction;
 			hw->pinState |= value & hw->direction;
 		} else {
-			hw->pinState = value;
+			hw->pinState = value & 0xF;
 		}
 		_readPins(hw);
 		break;
 	case GPIO_REG_DIRECTION:
-		hw->direction = value;
+		hw->direction = value & 0xF;
 		break;
 	case GPIO_REG_CONTROL:
-		hw->readWrite = value;
+		hw->readWrite = value & 0x1;
 		break;
 	default:
 		mLOG(GBA_HW, WARN, "Invalid GPIO address");
@@ -514,6 +514,8 @@ void GBAHardwareDeserialize(struct GBACartridgeHardware* hw, const struct GBASer
 	hw->readWrite = GBASerializedHWFlags1GetReadWrite(flags1);
 	LOAD_16(hw->pinState, 0, &state->hw.pinState);
 	LOAD_16(hw->direction, 0, &state->hw.pinDirection);
+	hw->pinState &= 0xF;
+	hw->direction &= 0xF;
 	hw->devices = state->hw.devices;
 
 	if ((hw->devices & HW_GPIO) && hw->gpioBase) {