1 files changed, 29 insertions, 60 deletions

diff --git a/cpu.c b/cpu.c
index 6e44c7f..df9c852 100644
--- a/cpu.c
+++ b/cpu.c
@@ -99,55 +99,6 @@ memwrite16(uint16_t addr, uint16_t word)
 	memory[addr + 1] = (word & 0xFF00) >> 8;
 }
 
-static uint8_t
-opcode_arg(enum addressing_mode mode)
-{
-	uint16_t arg, val;
-
-	if (mode != AM_ABS && mode != AM_ABS_X && mode != AM_ABS_Y) {
-		arg = peek(regs.pc++);
-	} else {
-		arg = peek16(regs.pc);
-		regs.pc += 2;
-	}
-
-	switch (mode) {
-	case AM_IMM:
-	case AM_REL:
-		val = arg;
-		break;
-	case AM_ZP:
-		val = peek(arg % 256);
-		break;
-	case AM_ZP_X:
-		val = peek((arg + regs.x) % 256);
-		break;
-	case AM_ZP_Y:
-		val = peek((arg + regs.y) % 256);
-		break;
-	case AM_ABS:
-		val = peek16(arg);
-		break;
-	case AM_ABS_X:
-		val = peek16(arg + regs.x);
-		break;
-	case AM_ABS_Y:
-		val = peek16(arg + regs.y);
-		break;
-	case AM_IND_X:
-		val = peek(peek((arg + regs.x) % 256) + peek((arg + regs.x + 1) % 256) * 256);
-		break;
-	case AM_IND_Y:
-		val = peek(peek(arg) + peek((arg + 1) % 256) * 256 + regs.y);
-		break;
-	default:
-		fprintf(stderr, "INVALID ADDRESSING MODE %i\n", mode);
-		abort();
-	}
-
-	return val;
-}
-
 static uint16_t
 opcode_mem(enum addressing_mode mode)
 {
@@ -193,7 +144,7 @@ opcode_mem(enum addressing_mode mode)
 		val = peek(arg) + peek((arg + 1) % 256) * 256 + regs.y;
 		break;
 	default:
-		fprintf(stderr, "opcode_mem INVALID ADDRESSING MODE %i\n", mode);
+		fprintf(stderr, "INVALID ADDRESSING MODE %i\n", mode);
 		abort();
 	}
 
@@ -271,7 +222,8 @@ BEQ(uint16_t arg)
 void
 BIT(uint16_t arg)
 {
-	uint8_t tmp = peek(arg);
+	uint8_t tmp = arg;
+
 	regs.status.zero = (regs.a & tmp) == 0;
 	regs.status.overflow = (tmp & (1 << 6)) != 0;
 	STATUS_UPDATE_NEGATIVE(tmp);
@@ -628,8 +580,18 @@ RTS(uint16_t arg)
 void
 SBC(uint16_t arg)
 {
-	/* SBC is described online as ADC with argument as two's complement */
-	ADC(~(uint8_t)arg);
+	uint8_t tmp = arg & 0xFF;
+	uint16_t diff;
+
+	diff = regs.a - tmp - !regs.status.carry;
+
+	regs.status.carry = diff > 0xFF;
+	regs.status.carry = !regs.status.carry;
+	/* overflow flag formula: https://stackoverflow.com/a/29224684 */
+	regs.status.overflow = ((regs.a ^ arg) & (regs.a ^ diff) & 0x80) != 0;
+	regs.a = diff & 0xFF;
+
+	STATUS_UPDATE_NZ(regs.a);
 }
 
 void
@@ -759,10 +721,10 @@ interpret(void)
 			printf("$%04X", arg);
 			break;
 		case AM_ABS_X:
-			printf("$%04X,X", arg);
+			printf("$%04X,X @ %04X", arg - regs.x, arg);
 			break;
 		case AM_ABS_Y:
-			printf("$%04X,Y", arg);
+			printf("$%04X,Y @ %04X", arg - regs.y, arg);
 			break;
 		case AM_IND:
 			printf("($%04X) = %04X\t\t", arg, peek16(arg));
@@ -776,13 +738,20 @@ interpret(void)
 			break;
 		}
 
-		if (opcodes[op].didmemory) printf(" = %02X", peek(arg));
-		printf("\t\t\t");
+		if (opcodes[op].memread || opcodes[op].memwrite)
+			printf(" = %02X", peek(arg));
+
+		if (mode != AM_ABS_X && mode != AM_ABS_Y)
+			putchar('\t');
+		printf("\t\t");
 
-		printf("A:%02X X:%02X Y:%02X P:%02X SP:%02X CYC:%d %08b\n",
-		    regs.a, regs.x, regs.y, STATUS_TO_INT(), regs.sp, cycles, STATUS_TO_INT());
+		printf("A:%02X X:%02X Y:%02X P:%02X SP:%02X CYC:%d\n",
+		    regs.a, regs.x, regs.y, STATUS_TO_INT(), regs.sp, cycles);
 
-		opcodes[op].instr(arg);
+		if (opcodes[op].memread)
+			opcodes[op].instr(peek(arg));
+		else
+			opcodes[op].instr(arg);
 		cycles += opcodes[op].cycles;
 	}
 loop_exit: