From f84af911739274c9e4ea24b37d8b4f784f040f84 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Tue, 30 May 2017 22:28:23 -0700
Subject: [PATCH] GB MBC: Add MBC1-M support with basic heuristic

---
 CHANGES                           |  1 +
 include/mgba/internal/gb/memory.h |  1 +
 src/gb/gb.c                       |  2 ++
 src/gb/io.c                       |  2 +-
 src/gb/mbc.c                      | 45 +++++++++++++++++++++++++++----
 src/gb/memory.c                   |  8 +++++-
 6 files changed, 52 insertions(+), 7 deletions(-)

diff --git a/CHANGES b/CHANGES
index 891977d01..f9edb5bb6 100644
--- a/CHANGES
+++ b/CHANGES
@@ -20,6 +20,7 @@ Features:
  - Library view
  - Debugger: Segment/bank support
  - GB: Symbol table support
+ - GB MBC: Add MBC1 multicart support
 Bugfixes:
  - LR35902: Fix core never exiting with certain event patterns
  - GB Timer: Improve DIV reset behavior
diff --git a/include/mgba/internal/gb/memory.h b/include/mgba/internal/gb/memory.h
index 7a6bd4630..11f1e929f 100644
--- a/include/mgba/internal/gb/memory.h
+++ b/include/mgba/internal/gb/memory.h
@@ -85,6 +85,7 @@ enum GBMBC7MachineState {
 
 struct GBMBC1State {
 	int mode;
+	int multicartStride;
 };
 
 struct GBMBC7State {
diff --git a/src/gb/gb.c b/src/gb/gb.c
index 8e227a816..4c83984f5 100644
--- a/src/gb/gb.c
+++ b/src/gb/gb.c
@@ -385,7 +385,9 @@ bool GBIsBIOS(struct VFile* vf) {
 
 void GBReset(struct LR35902Core* cpu) {
 	struct GB* gb = (struct GB*) cpu->master;
+	gb->memory.romBase = gb->memory.rom;
 	GBDetectModel(gb);
+
 	if (gb->biosVf) {
 		if (!GBIsBIOS(gb->biosVf)) {
 			gb->biosVf->close(gb->biosVf);
diff --git a/src/gb/io.c b/src/gb/io.c
index b7e234ca7..0f386aaae 100644
--- a/src/gb/io.c
+++ b/src/gb/io.c
@@ -382,7 +382,7 @@ void GBIOWrite(struct GB* gb, unsigned address, uint8_t value) {
 		value = gb->video.stat;
 		break;
 	case 0x50:
-		if (gb->memory.romBase != gb->memory.rom) {
+		if (gb->memory.romBase < gb->memory.rom && gb->memory.romBase > &gb->memory.rom[gb->memory.romSize - 1]) {
 			free(gb->memory.romBase);
 			gb->memory.romBase = gb->memory.rom;
 		}
diff --git a/src/gb/mbc.c b/src/gb/mbc.c
index efc294419..ae69defc6 100644
--- a/src/gb/mbc.c
+++ b/src/gb/mbc.c
@@ -46,6 +46,33 @@ void GBMBCSwitchBank(struct GB* gb, int bank) {
 	}
 }
 
+static void _switchBank0(struct GB* gb, int bank) {
+	size_t bankStart = bank * GB_SIZE_CART_BANK0 << gb->memory.mbcState.mbc1.multicartStride;
+	if (bankStart + GB_SIZE_CART_BANK0 > gb->memory.romSize) {
+		mLOG(GB_MBC, GAME_ERROR, "Attempting to switch to an invalid ROM bank: %0X", bank);
+		bankStart &= (gb->memory.romSize - 1);
+	}
+	gb->memory.romBase = &gb->memory.rom[bankStart];
+	if (gb->cpu->pc < GB_SIZE_CART_BANK0) {
+		gb->cpu->memory.setActiveRegion(gb->cpu, gb->cpu->pc);
+	}
+}
+
+static bool _isMulticart(const uint8_t* mem) {
+	bool success = true;
+	struct VFile* vf;
+
+	vf = VFileFromConstMemory(&mem[GB_SIZE_CART_BANK0 * 0x10], 1024);
+	success = success && GBIsROM(vf);
+	vf->close(vf);
+
+	vf = VFileFromConstMemory(&mem[GB_SIZE_CART_BANK0 * 0x20], 1024);
+	success = success && GBIsROM(vf);
+	vf->close(vf);
+
+	return success;
+}
+
 void GBMBCSwitchSramBank(struct GB* gb, int bank) {
 	size_t bankStart = bank * GB_SIZE_EXTERNAL_RAM;
 	GBResizeSram(gb, (bank + 1) * GB_SIZE_EXTERNAL_RAM);
@@ -83,6 +110,11 @@ void GBMBCInit(struct GB* gb) {
 			case 2:
 			case 3:
 				gb->memory.mbcType = GB_MBC1;
+				if (gb->memory.romSize >= GB_SIZE_CART_BANK0 * 0x31 && _isMulticart(gb->memory.rom)) {
+					gb->memory.mbcState.mbc1.multicartStride = 4;
+				} else {
+					gb->memory.mbcState.mbc1.multicartStride = 5;
+				}
 				break;
 			case 5:
 			case 6:
@@ -233,6 +265,7 @@ static void _latchRtc(struct mRTCSource* rtc, uint8_t* rtcRegs, time_t* rtcLastL
 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) {
@@ -253,21 +286,23 @@ void _GBMBC1(struct GB* gb, uint16_t address, uint8_t value) {
 		if (!bank) {
 			++bank;
 		}
-		GBMBCSwitchBank(gb, bank | (memory->currentBank & 0x60));
+		bank &= stride - 1;
+		GBMBCSwitchBank(gb, bank | (memory->currentBank & (3 * stride)));
 		break;
 	case 0x2:
 		bank &= 3;
-		if (!memory->mbcState.mbc1.mode) {
-			GBMBCSwitchBank(gb, (bank << 5) | (memory->currentBank & 0x1F));
-		} else {
+		if (memory->mbcState.mbc1.mode) {
+			_switchBank0(gb, bank);
 			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) {
-			GBMBCSwitchBank(gb, memory->currentBank & 0x1F);
+			_switchBank0(gb, memory->currentBank >> memory->mbcState.mbc1.multicartStride);
 		} else {
+			_switchBank0(gb, 0);
 			GBMBCSwitchSramBank(gb, 0);
 		}
 		break;
diff --git a/src/gb/memory.c b/src/gb/memory.c
index 6a2d84f55..46c8dcd78 100644
--- a/src/gb/memory.c
+++ b/src/gb/memory.c
@@ -165,7 +165,13 @@ void GBMemoryReset(struct GB* gb) {
 	memset(&gb->memory.rtcRegs, 0, sizeof(gb->memory.rtcRegs));
 
 	memset(&gb->memory.hram, 0, sizeof(gb->memory.hram));
-	memset(&gb->memory.mbcState, 0, sizeof(gb->memory.mbcState));
+	switch (gb->memory.mbcType) {
+	case GB_MBC1:
+		gb->memory.mbcState.mbc1.mode = 0;
+		break;
+	default:
+		memset(&gb->memory.mbcState, 0, sizeof(gb->memory.mbcState));
+	}
 
 	GBMBCInit(gb);
 	gb->memory.sramBank = gb->memory.sram;