From dd1d1a0a9d28d50e96f05539a27cd1b04911458d Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Wed, 3 Mar 2021 03:06:04 -0800
Subject: [PATCH] GBA Memory: Fix masking of misaligned jumps

---
 CHANGES                                 | 1 +
 include/mgba/internal/arm/isa-inlines.h | 4 +++-
 src/gba/memory.c                        | 1 +
 src/gba/serialize.c                     | 5 ++---
 4 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/CHANGES b/CHANGES
index ba37f80f4..a42382559 100644
--- a/CHANGES
+++ b/CHANGES
@@ -48,6 +48,7 @@ Emulation fixes:
  - GBA Memory: Mark Famicom Mini games 22 through 28 as non-mirroring
  - GBA Memory: Return correct byte for odd ROM open bus addresses
  - GBA Memory: Improved AGBPrint emulation of edge cases (fixes mgba.io/i/1867)
+ - GBA Memory: Fix masking of misaligned jumps
  - GBA Serialize: Fix alignment check when loading states
  - GBA SIO: Fix copying Normal mode transfer values
  - GBA SIO: Fix Normal mode being totally broken (fixes mgba.io/i/1800)
diff --git a/include/mgba/internal/arm/isa-inlines.h b/include/mgba/internal/arm/isa-inlines.h
index d9b8c9528..2ee6aee7c 100644
--- a/include/mgba/internal/arm/isa-inlines.h
+++ b/include/mgba/internal/arm/isa-inlines.h
@@ -56,7 +56,7 @@
 #define ARM_ILL cpu->irqh.hitIllegal(cpu, opcode)
 
 static inline int32_t ARMWritePC(struct ARMCore* cpu) {
-	uint32_t pc = cpu->gprs[ARM_PC] & -WORD_SIZE_ARM;
+	uint32_t pc = cpu->gprs[ARM_PC] & -WORD_SIZE_THUMB;
 	cpu->memory.setActiveRegion(cpu, pc);
 	LOAD_32(cpu->prefetch[0], pc & cpu->memory.activeMask, cpu->memory.activeRegion);
 	pc += WORD_SIZE_ARM;
@@ -88,9 +88,11 @@ static inline void _ARMSetMode(struct ARMCore* cpu, enum ExecutionMode execution
 	switch (executionMode) {
 	case MODE_ARM:
 		cpu->cpsr.t = 0;
+		cpu->memory.activeMask &= ~2;
 		break;
 	case MODE_THUMB:
 		cpu->cpsr.t = 1;
+		cpu->memory.activeMask |= 2;
 	}
 	cpu->nextEvent = cpu->cycles;
 }
diff --git a/src/gba/memory.c b/src/gba/memory.c
index 1e6455ef7..e6e3a6b2d 100644
--- a/src/gba/memory.c
+++ b/src/gba/memory.c
@@ -345,6 +345,7 @@ static void GBASetActiveRegion(struct ARMCore* cpu, uint32_t address) {
 	cpu->memory.activeSeqCycles16 = memory->waitstatesSeq16[memory->activeRegion];
 	cpu->memory.activeNonseqCycles32 = memory->waitstatesNonseq32[memory->activeRegion];
 	cpu->memory.activeNonseqCycles16 = memory->waitstatesNonseq16[memory->activeRegion];
+	cpu->memory.activeMask &= -(cpu->executionMode == MODE_THUMB ? WORD_SIZE_THUMB : WORD_SIZE_ARM);
 }
 
 #define LOAD_BAD \
diff --git a/src/gba/serialize.c b/src/gba/serialize.c
index f6eafa6a4..022d7b92f 100644
--- a/src/gba/serialize.c
+++ b/src/gba/serialize.c
@@ -154,10 +154,9 @@ bool GBADeserialize(struct GBA* gba, const struct GBASerializedState* state) {
 		LOAD_32(gba->cpu->bankedSPSRs[i], i * sizeof(gba->cpu->bankedSPSRs[0]), state->cpu.bankedSPSRs);
 	}
 	gba->cpu->privilegeMode = gba->cpu->cpsr.priv;
-	uint32_t pcMask = (gba->cpu->cpsr.t ? WORD_SIZE_THUMB : WORD_SIZE_ARM) - 1;
-	if (gba->cpu->gprs[ARM_PC] & pcMask) {
+	if (gba->cpu->gprs[ARM_PC] & 1) {
 		mLOG(GBA_STATE, WARN, "Savestate has unaligned PC and is probably corrupted");
-		gba->cpu->gprs[ARM_PC] &= ~pcMask;
+		gba->cpu->gprs[ARM_PC] &= ~1;
 	}
 	gba->cpu->memory.setActiveRegion(gba->cpu, gba->cpu->gprs[ARM_PC]);
 	if (state->biosPrefetch) {