diff --git a/src/arm.h b/src/arm.h
index 8be46dec7..5136b76f4 100644
--- a/src/arm.h
+++ b/src/arm.h
@@ -76,6 +76,10 @@ struct ARMMemory {
 	void (*store32)(struct ARMMemory*, uint32_t address, int32_t value);
 	void (*store16)(struct ARMMemory*, uint32_t address, int16_t value);
 	void (*store8)(struct ARMMemory*, uint32_t address, int8_t value);
+
+	uint32_t* activeRegion;
+	uint32_t activeMask;
+	void (*setActiveRegion)(struct ARMMemory*, uint32_t address);
 };
 
 struct ARMBoard {
diff --git a/src/gba.c b/src/gba.c
index 3935c77e7..57d9e6596 100644
--- a/src/gba.c
+++ b/src/gba.c
@@ -5,6 +5,8 @@
 
 static const char* GBA_CANNOT_MMAP = "Could not map memory";
 
+static void GBASetActiveRegion(struct ARMMemory* memory, uint32_t region);
+
 void GBAInit(struct GBA* gba) {
 	gba->errno = GBA_NO_ERROR;
 	gba->errstr = 0;
@@ -45,6 +47,10 @@ void GBAMemoryInit(struct GBAMemory* memory) {
 		memory->p->errno = GBA_OUT_OF_MEMORY;
 		memory->p->errstr = GBA_CANNOT_MMAP;
 	}
+
+	memory->d.activeRegion = 0;
+	memory->d.activeMask = 0;
+	memory->d.setActiveRegion = GBASetActiveRegion;
 }
 
 void GBAMemoryDeinit(struct GBAMemory* memory) {
@@ -71,6 +77,38 @@ void GBALoadROM(struct GBA* gba, int fd) {
 	// TODO: error check
 }
 
+static void GBASetActiveRegion(struct ARMMemory* memory, uint32_t address) {
+	struct GBAMemory* gbaMemory = (struct GBAMemory*) memory;
+
+	switch (address & ~OFFSET_MASK) {
+	case BASE_BIOS:
+		memory->activeRegion = gbaMemory->bios;
+		memory->activeMask = 0;
+		break;
+	case BASE_WORKING_RAM:
+		memory->activeRegion = gbaMemory->wram;
+		memory->activeMask = SIZE_WORKING_RAM - 1;
+		break;
+	case BASE_WORKING_IRAM:
+		memory->activeRegion = gbaMemory->iwram;
+		memory->activeMask = SIZE_WORKING_IRAM - 1;
+		break;
+	case BASE_CART0:
+	case BASE_CART0_EX:
+	case BASE_CART1:
+	case BASE_CART1_EX:
+	case BASE_CART2:
+	case BASE_CART2_EX:
+		memory->activeRegion = gbaMemory->rom;
+		memory->activeMask = SIZE_CART0 - 1;
+		break;
+	default:
+		memory->activeRegion = 0;
+		memory->activeMask = 0;
+		break;
+	}
+}
+
 int32_t GBALoad32(struct ARMMemory* memory, uint32_t address) {
 	struct GBAMemory* gbaMemory = (struct GBAMemory*) memory;
 
diff --git a/src/isa-arm.c b/src/isa-arm.c
index 9248a51da..7bffbe139 100644
--- a/src/isa-arm.c
+++ b/src/isa-arm.c
@@ -29,7 +29,7 @@ static inline void _immediate(struct ARMCore* cpu, uint32_t opcode) {
 static const ARMInstruction _armTable[0x1000];
 
 static ARMInstruction _ARMLoadInstructionARM(struct ARMMemory* memory, uint32_t address, uint32_t* opcodeOut) {
-	uint32_t opcode = memory->load32(memory, address);
+	uint32_t opcode = memory->activeRegion[(address & memory->activeMask) >> 2];
 	*opcodeOut = opcode;
 	return _armTable[((opcode >> 16) & 0xFF0) | ((opcode >> 4) & 0x00F)];
 }
@@ -127,7 +127,8 @@ void ARMStep(struct ARMCore* cpu) {
 // Beware pre-processor antics
 
 #define ARM_WRITE_PC \
-	cpu->gprs[ARM_PC] = (cpu->gprs[ARM_PC] & -WORD_SIZE_ARM) + WORD_SIZE_ARM
+	cpu->gprs[ARM_PC] = (cpu->gprs[ARM_PC] & -WORD_SIZE_ARM) + WORD_SIZE_ARM; \
+	cpu->memory->setActiveRegion(cpu->memory, cpu->gprs[ARM_PC]);
 
 #define ARM_ADDITION_S(M, N, D) \
 	if (rd == ARM_PC && _ARMModeHasSPSR(cpu->cpsr.priv)) { \
diff --git a/src/main.c b/src/main.c
index fccdbdc17..dc5445e2d 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2,8 +2,9 @@
 #include "gba.h"
 #include "isa-arm.h"
 
-#include <sys/stat.h>
 #include <fcntl.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
 int main(int argc, char** argv) {
 	struct GBA gba;
@@ -11,9 +12,13 @@ int main(int argc, char** argv) {
 	int fd = open("test.rom", O_RDONLY);
 	GBALoadROM(&gba, fd);
 	gba.cpu.gprs[ARM_PC] = 0x08000004;
-	ARMStep(&gba.cpu);
-	ARMStep(&gba.cpu);
+	gba.memory.d.setActiveRegion(&gba.memory.d, gba.cpu.gprs[ARM_PC]);
+	int i;
+	for (i = 0; i < 1024 * 1024 * 16; ++i) {
+		ARMStep(&gba.cpu);
+	}
 	GBADeinit(&gba);
+	close(fd);
 
 	return 0;
 }