diff --git a/CHANGES b/CHANGES
index 804b6bc18..db9fe99c6 100644
--- a/CHANGES
+++ b/CHANGES
@@ -77,6 +77,7 @@ Misc:
  - Qt: Don't save window size when entering fullscreen
  - Qt: Make the default fullscreen binding for Windows be Alt-Enter
  - GBA Video: Refactor software renderer into separate files
+ - ARM7: Add emulation for Undefined CPU mode
 
 0.2.1: (2015-05-13)
 Bugfixes:
diff --git a/src/arm/arm.c b/src/arm/arm.c
index b75274aa3..118383cd7 100644
--- a/src/arm/arm.c
+++ b/src/arm/arm.c
@@ -190,6 +190,26 @@ void ARMRaiseSWI(struct ARMCore* cpu) {
 	cpu->cycles += currentCycles;
 }
 
+void ARMRaiseUndefined(struct ARMCore* cpu) {
+	union PSR cpsr = cpu->cpsr;
+	int instructionWidth;
+	if (cpu->executionMode == MODE_THUMB) {
+		instructionWidth = WORD_SIZE_THUMB;
+	} else {
+		instructionWidth = WORD_SIZE_ARM;
+	}
+	ARMSetPrivilegeMode(cpu, MODE_UNDEFINED);
+	cpu->cpsr.priv = MODE_UNDEFINED;
+	cpu->gprs[ARM_LR] = cpu->gprs[ARM_PC] - instructionWidth;
+	cpu->gprs[ARM_PC] = BASE_UNDEF;
+	int currentCycles = 0;
+	ARM_WRITE_PC;
+	_ARMSetMode(cpu, MODE_ARM);
+	cpu->spsr = cpsr;
+	cpu->cpsr.i = 1;
+	cpu->cycles += currentCycles;
+}
+
 static inline void ARMStep(struct ARMCore* cpu) {
 	uint32_t opcode = cpu->prefetch[0];
 	cpu->prefetch[0] = cpu->prefetch[1];
diff --git a/src/arm/arm.h b/src/arm/arm.h
index 2242bf1fb..ec04f5d72 100644
--- a/src/arm/arm.h
+++ b/src/arm/arm.h
@@ -172,6 +172,7 @@ void ARMReset(struct ARMCore* cpu);
 void ARMSetPrivilegeMode(struct ARMCore*, enum PrivilegeMode);
 void ARMRaiseIRQ(struct ARMCore*);
 void ARMRaiseSWI(struct ARMCore*);
+void ARMRaiseUndefined(struct ARMCore*);
 
 void ARMRun(struct ARMCore* cpu);
 void ARMRunLoop(struct ARMCore* cpu);
diff --git a/src/gba/gba.c b/src/gba/gba.c
index 3613adbec..c1a620b58 100644
--- a/src/gba/gba.c
+++ b/src/gba/gba.c
@@ -650,7 +650,7 @@ void GBAGetGameTitle(struct GBA* gba, char* out) {
 
 void GBAHitStub(struct ARMCore* cpu, uint32_t opcode) {
 	struct GBA* gba = (struct GBA*) cpu->master;
-	enum GBALogLevel level = GBA_LOG_FATAL;
+	enum GBALogLevel level = GBA_LOG_ERROR;
 	if (gba->debugger) {
 		level = GBA_LOG_STUB;
 		struct DebuggerEntryInfo info = {
@@ -671,6 +671,8 @@ void GBAIllegal(struct ARMCore* cpu, uint32_t opcode) {
 			.opcode = opcode
 		};
 		ARMDebuggerEnter(gba->debugger, DEBUGGER_ENTER_ILLEGAL_OP, &info);
+	} else {
+		ARMRaiseUndefined(cpu);
 	}
 }