From f23f221d49b82007aeb030babadec6071da40f8b Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Tue, 17 Apr 2018 22:44:37 -0700
Subject: [PATCH] GB MBC: Fix MBC6 bank switching

---
 src/gb/mbc.c    |  4 +++-
 src/gb/memory.c | 21 ++++++++++++++++++---
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/gb/mbc.c b/src/gb/mbc.c
index 16d3e01f8..1097bf781 100644
--- a/src/gb/mbc.c
+++ b/src/gb/mbc.c
@@ -531,7 +531,7 @@ void _GBMBC5(struct GB* gb, uint16_t address, uint8_t value) {
 
 void _GBMBC6(struct GB* gb, uint16_t address, uint8_t value) {
 	struct GBMemory* memory = &gb->memory;
-	int bank = value & 0x7F;
+	int bank = value;
 	switch (address >> 10) {
 	case 0:
 		switch (value) {
@@ -548,9 +548,11 @@ void _GBMBC6(struct GB* gb, uint16_t address, uint8_t value) {
 			break;
 		}
 		break;
+	case 0x8:
 	case 0x9:
 		GBMBCSwitchHalfBank(gb, 0, bank);
 		break;
+	case 0xC:
 	case 0xD:
 		GBMBCSwitchHalfBank(gb, 1, bank);
 		break;
diff --git a/src/gb/memory.c b/src/gb/memory.c
index 089558011..d69f7329d 100644
--- a/src/gb/memory.c
+++ b/src/gb/memory.c
@@ -73,9 +73,20 @@ static void GBSetActiveRegion(struct LR35902Core* cpu, uint16_t address) {
 	case GB_REGION_CART_BANK1 + 2:
 	case GB_REGION_CART_BANK1 + 3:
 		cpu->memory.cpuLoad8 = GBFastLoad8;
-		cpu->memory.activeRegion = memory->romBank;
-		cpu->memory.activeRegionEnd = GB_BASE_VRAM;
-		cpu->memory.activeMask = GB_SIZE_CART_BANK0 - 1;
+		if (gb->memory.mbcType != GB_MBC6) {
+			cpu->memory.activeRegion = memory->romBank;
+			cpu->memory.activeRegionEnd = GB_BASE_VRAM;
+			cpu->memory.activeMask = GB_SIZE_CART_BANK0 - 1;
+		} else {
+			cpu->memory.activeMask = GB_SIZE_CART_HALFBANK - 1;
+			if (address & 0x2000) {
+				cpu->memory.activeRegion = memory->mbcState.mbc6.romBank1;
+				cpu->memory.activeRegionEnd = GB_BASE_VRAM;
+			} else {
+				cpu->memory.activeRegion = memory->romBank;
+				cpu->memory.activeRegionEnd = GB_BASE_CART_BANK1 + 0x2000;
+			}
+		}
 		break;
 	default:
 		cpu->memory.cpuLoad8 = GBLoad8;
@@ -169,6 +180,10 @@ void GBMemoryReset(struct GB* gb) {
 	case GB_MBC1:
 		gb->memory.mbcState.mbc1.mode = 0;
 		break;
+	case GB_MBC6:
+		GBMBCSwitchHalfBank(gb, 0, 2);
+		GBMBCSwitchHalfBank(gb, 1, 3);
+		break;
 	default:
 		memset(&gb->memory.mbcState, 0, sizeof(gb->memory.mbcState));
 	}