GB MBC: Fix MBC1 mode changing behavior

CHANGES

@@ -5,6 +5,7 @@ Features:
 Emulation fixes:
  - ARM: Fix ALU reading PC after shifting
  - ARM: Fix STR storing PC after address calculation
+ - GB MBC: Fix MBC1 mode changing behavior
  - GB Video: Fix state after skipping BIOS (fixes mgba.io/i/1715 and mgba.io/i/1716)
  - GBA: Fix timing advancing too quickly in rare cases
  - GBA BIOS: Implement dummy sound driver calls

cinema/gb/mooneye-gb/emulator-only/mbc1/bits_mode/config.ini

@@ -1,2 +0,0 @@
-[testinfo]
-fail=1

include/mgba/internal/gb/memory.h

@@ -108,6 +108,8 @@ enum GBTAMA5Register {
 struct GBMBC1State {
 	int mode;
 	int multicartStride;
+	uint8_t bankLo;
+	uint8_t bankHi;
 };
 
 struct GBMBC6State {

include/mgba/internal/gb/serialize.h

@@ -362,6 +362,8 @@ struct GBSerializedState {
 			struct {
 				uint8_t mode;
 				uint8_t multicartStride;
+				uint8_t bankLo;
+				uint8_t bankHi;
 			} mbc1;
 			struct {
 				uint64_t lastLatch;

src/gb/mbc.c

@@ -419,10 +419,27 @@ static void _latchRtc(struct mRTCSource* rtc, uint8_t* rtcRegs, time_t* rtcLastL
 	}
 }
 
+static void _GBMBC1Update(struct GB* gb) {
+	struct GBMBC1State* state = &gb->memory.mbcState.mbc1;
+	int bank = state->bankLo;
+	bank &= (1 << state->multicartStride) - 1;
+	bank |= state->bankHi << state->multicartStride;
+	if (state->mode) {
+		GBMBCSwitchBank0(gb, state->bankHi << state->multicartStride);
+		GBMBCSwitchSramBank(gb, state->bankHi & 3);
+	} else {
+		GBMBCSwitchBank0(gb, 0);
+		GBMBCSwitchSramBank(gb, 0);
+	}
+	if (!(state->bankLo & 0x1F)) {
+		++bank;
+	}
+	GBMBCSwitchBank(gb, bank);
+}
+
 void _GBMBC1(struct GB* gb, uint16_t address, uint8_t value) {
 	struct GBMemory* memory = &gb->memory;
 	int bank = value & 0x1F;
-	int stride = 1 << memory->mbcState.mbc1.multicartStride;
 	switch (address >> 13) {
 	case 0x0:
 		switch (value) {
@@ -440,28 +457,17 @@ void _GBMBC1(struct GB* gb, uint16_t address, uint8_t value) {
 		}
 		break;
 	case 0x1:
-		if (!bank) {
+		memory->mbcState.mbc1.bankLo = bank;
-			++bank;
+		_GBMBC1Update(gb);
-		}
-		bank &= stride - 1;
-		GBMBCSwitchBank(gb, bank | (memory->currentBank & (3 * stride)));
 		break;
 	case 0x2:
 		bank &= 3;
-		if (memory->mbcState.mbc1.mode) {
+		memory->mbcState.mbc1.bankHi = bank;
-			GBMBCSwitchBank0(gb, bank << gb->memory.mbcState.mbc1.multicartStride);
+		_GBMBC1Update(gb);
-			GBMBCSwitchSramBank(gb, bank);
-		}
-		GBMBCSwitchBank(gb, (bank << memory->mbcState.mbc1.multicartStride) | (memory->currentBank & (stride - 1)));
 		break;
 	case 0x3:
 		memory->mbcState.mbc1.mode = value & 1;
-		if (memory->mbcState.mbc1.mode) {
+		_GBMBC1Update(gb);
-			GBMBCSwitchBank0(gb, memory->currentBank & ~((1 << memory->mbcState.mbc1.multicartStride) - 1));
-		} else {
-			GBMBCSwitchBank0(gb, 0);
-			GBMBCSwitchSramBank(gb, 0);
-		}
 		break;
 	default:
 		// TODO

src/gb/memory.c

@@ -733,6 +733,8 @@ void GBMemorySerialize(const struct GB* gb, struct GBSerializedState* state) {
 	case GB_MBC1:
 		state->memory.mbc1.mode = memory->mbcState.mbc1.mode;
 		state->memory.mbc1.multicartStride = memory->mbcState.mbc1.multicartStride;
+		state->memory.mbc1.bankLo = memory->mbcState.mbc1.bankLo;
+		state->memory.mbc1.bankHi = memory->mbcState.mbc1.bankHi;
 		break;
 	case GB_MBC3_RTC:
 		STORE_64LE(gb->memory.rtcLastLatch, 0, &state->memory.rtc.lastLatch);
@@ -801,8 +803,15 @@ void GBMemoryDeserialize(struct GB* gb, const struct GBSerializedState* state) {
 	case GB_MBC1:
 		memory->mbcState.mbc1.mode = state->memory.mbc1.mode;
 		memory->mbcState.mbc1.multicartStride = state->memory.mbc1.multicartStride;
+		memory->mbcState.mbc1.bankLo = state->memory.mbc1.bankLo;
+		memory->mbcState.mbc1.bankHi = state->memory.mbc1.bankHi;
+		if (!(memory->mbcState.mbc1.bankLo || memory->mbcState.mbc1.bankHi)) {
+			// Backwards compat
+			memory->mbcState.mbc1.bankLo = memory->currentBank & ((1 << memory->mbcState.mbc1.multicartStride) - 1);
+			memory->mbcState.mbc1.bankHi = memory->currentBank >> memory->mbcState.mbc1.multicartStride;
+		}
 		if (memory->mbcState.mbc1.mode) {
-			GBMBCSwitchBank0(gb, memory->currentBank >> memory->mbcState.mbc1.multicartStride);
+			GBMBCSwitchBank0(gb, memory->mbcState.mbc1.bankHi);
 		}
 		break;
 	case GB_MBC3_RTC: