From a74fe13506d1f7adfc3b4d2074830417b5b3e750 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Mon, 8 Jan 2018 18:24:29 -0800
Subject: [PATCH] GB MBC: Fix MBC2 saves (fixes #954)

---
 CHANGES         |  1 +
 src/gb/mbc.c    | 19 +++++++++++++++++--
 src/gb/memory.c |  2 +-
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/CHANGES b/CHANGES
index 72d9bc232..206f877b2 100644
--- a/CHANGES
+++ b/CHANGES
@@ -13,6 +13,7 @@ Bugfixes:
  - GBA Audio: Increase PSG volume (fixes mgba.io/i/932)
  - GBA BIOS: Fix incorrect exit condition in LZ77
  - 3DS: Fix opening files in directory names with trailing slashes
+ - GB MBC: Fix MBC2 saves (fixes mgba.io/i/954)
 Misc:
  - GBA: Improve multiboot image detection
  - GB MBC: Remove erroneous bank 0 wrapping
diff --git a/src/gb/mbc.c b/src/gb/mbc.c
index 2f2d1216e..a1c56574f 100644
--- a/src/gb/mbc.c
+++ b/src/gb/mbc.c
@@ -30,6 +30,7 @@ static void _GBMBC7(struct GB*, uint16_t address, uint8_t value);
 static void _GBHuC3(struct GB*, uint16_t address, uint8_t value);
 static void _GBPocketCam(struct GB* gb, uint16_t address, uint8_t value);
 
+static uint8_t _GBMBC2Read(struct GBMemory*, uint16_t address);
 static uint8_t _GBMBC7Read(struct GBMemory*, uint16_t address);
 static uint8_t _GBPocketCamRead(struct GBMemory*, uint16_t address);
 
@@ -183,7 +184,8 @@ void GBMBCInit(struct GB* gb) {
 		break;
 	case GB_MBC2:
 		gb->memory.mbcWrite = _GBMBC2;
-		gb->sramSize = 0x200;
+		gb->memory.mbcRead = _GBMBC2Read;
+		gb->sramSize = 0x100;
 		break;
 	case GB_MBC3:
 		gb->memory.mbcWrite = _GBMBC3;
@@ -355,6 +357,7 @@ void _GBMBC1(struct GB* gb, uint16_t address, uint8_t value) {
 
 void _GBMBC2(struct GB* gb, uint16_t address, uint8_t value) {
 	struct GBMemory* memory = &gb->memory;
+	int shift = (address & 1) * 4;
 	int bank = value & 0xF;
 	switch (address >> 13) {
 	case 0x0:
@@ -364,7 +367,6 @@ void _GBMBC2(struct GB* gb, uint16_t address, uint8_t value) {
 			break;
 		case 0xA:
 			memory->sramAccess = true;
-			GBMBCSwitchSramBank(gb, memory->sramCurrentBank);
 			break;
 		default:
 			// TODO
@@ -378,6 +380,13 @@ void _GBMBC2(struct GB* gb, uint16_t address, uint8_t value) {
 		}
 		GBMBCSwitchBank(gb, bank);
 		break;
+	case 0x5:
+		if (!memory->sramAccess) {
+			return;
+		}
+		address &= 0x1FF;
+		memory->sramBank[(address >> 1)] &= 0xF0 >> shift;
+		memory->sramBank[(address >> 1)] |= (value & 0xF) << shift;
 	default:
 		// TODO
 		mLOG(GB_MBC, STUB, "MBC2 unknown address: %04X:%02X", address, value);
@@ -385,6 +394,12 @@ void _GBMBC2(struct GB* gb, uint16_t address, uint8_t value) {
 	}
 }
 
+static uint8_t _GBMBC2Read(struct GBMemory* memory, uint16_t address) {
+	address &= 0x1FF;
+	int shift = (address & 1) * 4;
+	return (memory->sramBank[(address >> 1)] >> shift) | 0xF0;
+}
+
 void _GBMBC3(struct GB* gb, uint16_t address, uint8_t value) {
 	struct GBMemory* memory = &gb->memory;
 	int bank = value & 0x7F;
diff --git a/src/gb/memory.c b/src/gb/memory.c
index 953588990..071f11e0f 100644
--- a/src/gb/memory.c
+++ b/src/gb/memory.c
@@ -289,7 +289,7 @@ void GBStore8(struct LR35902Core* cpu, uint16_t address, int8_t value) {
 	case GB_REGION_EXTERNAL_RAM + 1:
 		if (memory->rtcAccess) {
 			memory->rtcRegs[memory->activeRtcReg] = value;
-		} else if (memory->sramAccess && memory->sram) {
+		} else if (memory->sramAccess && memory->sram && memory->mbcType != GB_MBC2) {
 			memory->sramBank[address & (GB_SIZE_EXTERNAL_RAM - 1)] = value;
 		} else if (memory->mbcType == GB_MBC7) {
 			GBMBC7Write(memory, address, value);