M CHANGES → CHANGES

@@ -13,6 +13,7 @@ - LR35902: Fix watchpoints not reporting new value
  - 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

M src/gb/mbc.c → 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 @@ gb->memory.mbcWrite = _GBMBC1;
 		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 _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 @@ memory->sramAccess = false;
 			break;
 		case 0xA:
 			memory->sramAccess = true;
-			GBMBCSwitchSramBank(gb, memory->sramCurrentBank);
 			break;
 		default:
 			// TODO
@@ -378,11 +380,24 @@ ++bank;
 		}
 		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);
 		break;
 	}
+}
+
+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) {

M src/gb/memory.c → src/gb/memory.c

@@ -289,7 +289,7 @@ case GB_REGION_EXTERNAL_RAM:
 	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);