LR35902: Fix decoding LD r, $imm and 0-valued immediates (fixes #735)

CHANGES

@@ -56,6 +56,7 @@ Bugfixes:
  - GBA Hardware: Fix crash if a savestate lies about game hardware
  - Test: Fix crash when fuzzing fails to load a file
  - Qt: Disable "New multiplayer window" when MAX_GBAS is reached (fixes mgba.io/i/107)
+ - LR35902: Fix decoding LD r, $imm and 0-valued immediates (fixes mgba.io/i/735)
 Misc:
  - SDL: Remove scancode key input
  - GBA Video: Clean up unused timers

src/lr35902/decoder.c

@@ -66,8 +66,7 @@ DEFINE_DECODER_LR35902(NOP, info->mnemonic = LR35902_MN_NOP;)
 	DEFINE_LD_DECODER_LR35902_MEM(NAME, HL) \
 	DEFINE_LD_DECODER_LR35902_MEM_2(NAME, HL) \
 	DEFINE_DECODER_LR35902(LD ## NAME ## _, info->mnemonic = LR35902_MN_LD; \
-		info->op1.reg = LR35902_REG_A; \
-		info->op1.flags = LR35902_OP_FLAG_IMPLICIT; \
+		info->op1.reg = LR35902_REG_ ## NAME; \
 		return 1;) \
 	DEFINE_LD_DECODER_LR35902_NOHL(NAME)
 
@@ -500,18 +499,17 @@ static int _decodeOperand(struct LR35902Operand op, char* buffer, int blen) {
 		strncpy(buffer, "(", blen - 1);
 		ADVANCE(1);
 	}
-	if (op.immediate) {
+	if (op.reg) {
+		int written = snprintf(buffer, blen - 1, "%s", _lr35902Registers[op.reg]);
+		ADVANCE(written);
+	} else {
 		int written = snprintf(buffer, blen - 1, "$%02X", op.immediate);
 		ADVANCE(written);
 		if (op.reg) {
 			strncpy(buffer, "+", blen - 1);
 			ADVANCE(1);
 		}
 	}
-	if (op.reg) {
-		int written = snprintf(buffer, blen - 1, "%s", _lr35902Registers[op.reg]);
-		ADVANCE(written);
-	}
 	if (op.flags & LR35902_OP_FLAG_INCREMENT) {
 		strncpy(buffer, "+", blen - 1);
 		ADVANCE(1);
@@ -546,10 +544,12 @@ int LR35902Disassemble(struct LR35902InstructionInfo* info, char* buffer, int bl
 		}
 	}
 
-	written = _decodeOperand(info->op1, buffer, blen);
-	ADVANCE(written);
+	if (info->op1.reg || info->op1.immediate) {
+		written = _decodeOperand(info->op1, buffer, blen);
+		ADVANCE(written);
+	}
 
-	if (info->op2.reg || info->op2.immediate) {
+	if (info->op2.reg || (!info->op1.immediate && info->opcodeSize > 1 && info->opcode[0] != 0xCB)) {
 		if (written) {
 			strncpy(buffer, ", ", blen - 1);
 			ADVANCE(2);