M CHANGES → CHANGES

@@ -8,6 +8,7 @@ - ARM: Fix ALU reading PC after shifting
  - ARM: Fix STR storing PC after address calculation
  - ARM: Fix LDM^ writeback to user-mode register
  - ARM: Fix LDM^ {pc} differences (fixes mgba.io/i/1698)
+ - ARM: Fix edge case with Thumb SBC flags (fixes mgba.io/i/1818)
  - GB: Partially fix timing for skipped BIOS
  - GB Memory: Fix OAM DMA from top 8 kB
  - GB MBC: Fix MBC1 mode changing behavior

M src/arm/isa-thumb.c → src/arm/isa-thumb.c

@@ -25,6 +25,13 @@ cpu->cpsr.z = !(D); \
 	cpu->cpsr.c = ARM_BORROW_FROM(M, N, D); \
 	cpu->cpsr.v = ARM_V_SUBTRACTION(M, N, D);
 
+#define THUMB_SUBTRACTION_CARRY_S(M, N, D, C) \
+	cpu->cpsr.flags = 0; \
+	cpu->cpsr.n = ARM_SIGN(D); \
+	cpu->cpsr.z = !(D); \
+	cpu->cpsr.c = ARM_BORROW_FROM_CARRY(M, N, D, C); \
+	cpu->cpsr.v = ARM_V_SUBTRACTION(M, N, D);
+
 #define THUMB_NEUTRAL_S(M, N, D) \
 	cpu->cpsr.n = ARM_SIGN(D); \
 	cpu->cpsr.z = !(D);
@@ -203,10 +210,11 @@ cpu->gprs[rd] = d + n + cpu->cpsr.c;
 	THUMB_ADDITION_S(d, n, cpu->gprs[rd]);)
 
 DEFINE_DATA_FORM_5_INSTRUCTION_THUMB(SBC,
-	int n = cpu->gprs[rn] + !cpu->cpsr.c;
+	int n = cpu->gprs[rn];
 	int d = cpu->gprs[rd];
-	cpu->gprs[rd] = d - n;
-	THUMB_SUBTRACTION_S(d, n, cpu->gprs[rd]);)
+	cpu->gprs[rd] = d - n - !cpu->cpsr.c;
+	THUMB_SUBTRACTION_CARRY_S(d, n, cpu->gprs[rd], !cpu->cpsr.c);)
+
 DEFINE_DATA_FORM_5_INSTRUCTION_THUMB(ROR,
 	int rs = cpu->gprs[rn] & 0xFF;
 	if (rs) {