diff --git a/CHANGES b/CHANGES
index 4c29f1696..5371cb06a 100644
--- a/CHANGES
+++ b/CHANGES
@@ -5,6 +5,7 @@ Bugfixes:
 Misc:
  - Qt: Window size command line options are now supported
  - Qt: Increase usability of key mapper
+ - GBA Memory: Use a dynamically sized mask for ROM memory
 
 0.3.0: (2015-08-16)
 Features:
diff --git a/src/gba/gba.c b/src/gba/gba.c
index 2ae273760..f10ec8c6c 100644
--- a/src/gba/gba.c
+++ b/src/gba/gba.c
@@ -17,6 +17,7 @@
 
 #include "util/crc32.h"
 #include "util/memory.h"
+#include "util/math.h"
 #include "util/patch.h"
 #include "util/vfs.h"
 
@@ -156,6 +157,7 @@ void GBAReset(struct ARMCore* cpu) {
 
 	if (gba->yankedRomSize) {
 		gba->memory.romSize = gba->yankedRomSize;
+		gba->memory.romMask = toPow2(gba->memory.romSize) - 1;
 		gba->yankedRomSize = 0;
 	}
 	GBAMemoryReset(gba);
@@ -403,6 +405,7 @@ void GBALoadROM(struct GBA* gba, struct VFile* vf, struct VFile* sav, const char
 	gba->memory.rom = gba->pristineRom;
 	gba->activeFile = fname;
 	gba->memory.romSize = gba->pristineRomSize;
+	gba->memory.romMask = toPow2(gba->memory.romSize) - 1;
 	gba->romCrc32 = doCrc32(gba->memory.rom, gba->memory.romSize);
 	GBASavedataInit(&gba->memory.savedata, sav);
 	GBAHardwareInit(&gba->memory.hw, &((uint16_t*) gba->memory.rom)[GPIO_REG_DATA >> 1]);
@@ -412,6 +415,7 @@ void GBALoadROM(struct GBA* gba, struct VFile* vf, struct VFile* sav, const char
 void GBAYankROM(struct GBA* gba) {
 	gba->yankedRomSize = gba->memory.romSize;
 	gba->memory.romSize = 0;
+	gba->memory.romMask = 0;
 	GBARaiseIRQ(gba, IRQ_GAMEPAK);
 }
 
@@ -452,6 +456,7 @@ void GBAApplyPatch(struct GBA* gba, struct Patch* patch) {
 		return;
 	}
 	gba->memory.romSize = patchedSize;
+	gba->memory.romMask = SIZE_CART0 - 1;
 	gba->romCrc32 = doCrc32(gba->memory.rom, gba->memory.romSize);
 }
 
diff --git a/src/gba/memory.c b/src/gba/memory.c
index d736141a8..056058af2 100644
--- a/src/gba/memory.c
+++ b/src/gba/memory.c
@@ -50,6 +50,7 @@ void GBAMemoryInit(struct GBA* gba) {
 	gba->memory.iwram = 0;
 	gba->memory.rom = 0;
 	gba->memory.romSize = 0;
+	gba->memory.romMask = 0;
 	gba->memory.hw.p = gba;
 
 	int i;
@@ -269,7 +270,7 @@ static void GBASetActiveRegion(struct ARMCore* cpu, uint32_t address) {
 	case REGION_CART2:
 	case REGION_CART2_EX:
 		cpu->memory.activeRegion = memory->rom;
-		cpu->memory.activeMask = SIZE_CART0 - 1;
+		cpu->memory.activeMask = memory->romMask;
 		if ((address & (SIZE_CART0 - 1)) < memory->romSize) {
 			break;
 		}
@@ -893,6 +894,7 @@ void GBAPatch32(struct ARMCore* cpu, uint32_t address, int32_t value, int32_t* o
 		_pristineCow(gba);
 		if ((address & (SIZE_CART0 - 4)) >= gba->memory.romSize) {
 			gba->memory.romSize = (address & (SIZE_CART0 - 4)) + 4;
+			gba->memory.romMask = toPow2(gba->memory.romSize) - 1;
 		}
 		LOAD_32(oldValue, address & (SIZE_CART0 - 4), gba->memory.rom);
 		STORE_32(value, address & (SIZE_CART0 - 4), gba->memory.rom);
@@ -960,6 +962,7 @@ void GBAPatch16(struct ARMCore* cpu, uint32_t address, int16_t value, int16_t* o
 		_pristineCow(gba);
 		if ((address & (SIZE_CART0 - 1)) >= gba->memory.romSize) {
 			gba->memory.romSize = (address & (SIZE_CART0 - 2)) + 2;
+			gba->memory.romMask = toPow2(gba->memory.romSize) - 1;
 		}
 		LOAD_16(oldValue, address & (SIZE_CART0 - 2), gba->memory.rom);
 		STORE_16(value, address & (SIZE_CART0 - 2), gba->memory.rom);
@@ -1017,6 +1020,7 @@ void GBAPatch8(struct ARMCore* cpu, uint32_t address, int8_t value, int8_t* old)
 		_pristineCow(gba);
 		if ((address & (SIZE_CART0 - 1)) >= gba->memory.romSize) {
 			gba->memory.romSize = (address & (SIZE_CART0 - 2)) + 2;
+			gba->memory.romMask = toPow2(gba->memory.romSize) - 1;
 		}
 		oldValue = ((int8_t*) memory->rom)[address & (SIZE_CART0 - 1)];
 		((int8_t*) memory->rom)[address & (SIZE_CART0 - 1)] = value;
diff --git a/src/gba/memory.h b/src/gba/memory.h
index dd9d4066b..e0f059730 100644
--- a/src/gba/memory.h
+++ b/src/gba/memory.h
@@ -118,6 +118,7 @@ struct GBAMemory {
 	struct GBACartridgeHardware hw;
 	struct GBASavedata savedata;
 	size_t romSize;
+	uint32_t romMask;
 	uint16_t romID;
 	int fullBios;
 
diff --git a/src/util/math.h b/src/util/math.h
index 06dca22e5..31709e20a 100644
--- a/src/util/math.h
+++ b/src/util/math.h
@@ -14,4 +14,46 @@ static inline uint32_t popcount32(unsigned bits) {
 	return (((bits + (bits >> 4)) & 0xF0F0F0F) * 0x1010101) >> 24;
 }
 
+static inline unsigned clz32(uint32_t bits) {
+#if defined(__GNUC__) || __clang__
+	return __builtin_clz(bits);
+#else
+	static const int table[256] = {
+		8, 7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
+		3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
+		2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
+		2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
+		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+	};
+
+	if (bits & 0xFF000000) {
+		return table[bits >> 24];
+	} else if (bits & 0x00FF0000) {
+		return table[bits >> 16] + 8;
+	} else if (bits & 0x0000FF00) {
+		return table[bits >> 8] + 16;
+	}
+	return table[bits] + 24;
+#endif
+}
+
+static inline uint32_t toPow2(uint32_t bits) {
+	if (!bits) {
+		return 0;
+	}
+	unsigned lz = clz32(bits - 1);
+	return 1 << (32 - lz);
+}
+
 #endif