emu_nes/cpu.c

#include <libgen.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "cpu.h"
#include "opcodes.h"
#include "rom.h"

#define MAX(a, b)	((a > b) ? a : b)

#define STATUS_UPDATE_ZERO(r)		\
	(regs.status.zero = r == 0)
#define STATUS_UPDATE_NEGATIVE(r)	\
	(regs.status.negative = ((r & (1 << 7)) != 0))
#define STATUS_UPDATE_NZ(r)		\
	{STATUS_UPDATE_ZERO(r);		\
	 STATUS_UPDATE_NEGATIVE(r);}
#define STATUS_TO_INT()			\
	(regs.status.carry | (regs.status.zero << 1) \
	    | (regs.status.interrupt_disable << 2) | (regs.status.decimal_mode << 3) \
	    | (regs.status.brk << 4) | (regs.status.unused << 5) \
	    | (regs.status.overflow << 6) | (regs.status.negative << 7))

#define MEMORY_MIRROR(addr)	\
	if (addr < 0x2000)	\
		addr &= 0x07FF;	\
	else if (addr < 0x4000)	\
		addr &= 0x2007;

#define PUSH(b)	\
	(memwrite(0x0100 + regs.sp--, b))
#define PULL()	\
	(peek(0x0100 + ++regs.sp))

struct cpu_flags {
	uint8_t carry : 1;
	uint8_t zero : 1;
	uint8_t interrupt_disable : 1;
	uint8_t decimal_mode : 1;
	uint8_t brk : 1;
	uint8_t unused : 1;
	uint8_t overflow : 1;
	uint8_t negative : 1;
};

struct registers {
	uint8_t a, x, y, sp;
	struct cpu_flags status;
	uint16_t pc;
};
struct registers regs = {0};

struct Rom rom = {0};

/* 64K address space, 16bit words */
uint8_t memory[0x16000];

uint32_t cycles = 0;

static uint8_t
peek(uint16_t addr)
{
	MEMORY_MIRROR(addr);

	if (addr > 0x7FFF) {
		if (rom.prg_rom_size == 0x4000)
			return rom.prg_rom[(addr - 0x8000) % 0x4000];
		else if (rom.prg_rom_size == 0x8000)
			return rom.prg_rom[addr - 0x8000];
	} else {
		return memory[addr];
	}
}

static uint16_t
peek16(uint16_t addr)
{
	/* bytes are stored in little-endian (low then high) */
	return peek(addr) | (peek(addr + 1) << 8);
}

static void
memwrite(uint16_t addr, uint8_t byte)
{
	MEMORY_MIRROR(addr);

	memory[addr] = byte;
}

static void
memwrite16(uint16_t addr, uint16_t word)
{
	MEMORY_MIRROR(addr);

	/* bytes are stored in little-endian (low then high) */
	memory[addr] = word & 0xFF;
	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++);
		printf(" %02X\t", arg);
	} else {
		arg = peek16(regs.pc);
		printf(" %02X %02X\t", peek(regs.pc), peek(regs.pc + 1));
		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)
{
	uint16_t arg, val;

	if (mode != AM_ABS && mode != AM_ABS_X && mode != AM_ABS_Y && mode != AM_IND && mode) {
		arg = peek(regs.pc++);
		printf(" %02X\t", arg);
	} else {
		arg = peek16(regs.pc);
		printf(" %02X %02X\t", peek(regs.pc), peek(regs.pc + 1));
		regs.pc += 2;
	}

	switch (mode) {
	case AM_ZP:
		val = arg % 256;
		break;
	case AM_ZP_X:
		val = (arg + regs.x) % 256;
		break;
	case AM_ZP_Y:
		val = (arg + regs.y) % 256;
		break;
	case AM_IMM:
	case AM_ABS:
		val = arg;
		break;
	case AM_ABS_X:
		val = arg + regs.x;
		break;
	case AM_ABS_Y:
		val = arg + regs.y;
		break;
	case AM_IND:
		val = peek(arg) | (peek(((arg + 1) & 0xFF) | (arg & 0xFF00)) << 8);
		break;
	case AM_IND_X:
		val = peek((arg + regs.x) % 256) + peek((arg + regs.x + 1) % 256) * 256;
		break;
	case AM_IND_Y:
		val = peek(arg) + peek((arg + 1) % 256) * 256 + regs.y;
		break;
	default:
		fprintf(stderr, "INVALID ADDRESSING MODE\n");
		abort();
	}

	return val;
}

static void
adc(uint8_t arg)
{
	uint16_t sum;	// 16-bit sum makes it easier to determine carry flag

	sum = regs.a + arg + regs.status.carry;

	regs.status.carry = sum > 0xFF;
	/* overflow flag formula: https://stackoverflow.com/a/29224684 */
	regs.status.overflow = (~(regs.a ^ arg) & (regs.a ^ sum) & 0x80) != 0;
	regs.a = sum & 0xFF;

	STATUS_UPDATE_NZ(regs.a);
}

static void
and(uint8_t arg)
{
	regs.a &= arg;

	STATUS_UPDATE_NZ(regs.a);
}

static void
asl_acc(void)
{
	uint16_t tmp;

	tmp = regs.a << 1;
	regs.a = tmp & 0xFF;

	regs.status.carry = tmp > 0xFF;
	STATUS_UPDATE_NZ(regs.a);
}

static void
asl(uint16_t mem)
{
	uint16_t tmp;

	tmp = peek(mem) << 1;
	memwrite(mem, tmp & 0xFF);

	regs.status.carry = tmp > 0xFF;
	STATUS_UPDATE_NZ(tmp);
}

static void
bcc(uint8_t arg)
{
	if (regs.status.carry == 0)
		regs.pc += arg;
}

static void
bcs(uint8_t arg)
{
	if (regs.status.carry == 1)
		regs.pc += arg;
}

static void
beq(uint8_t arg)
{
	if (regs.status.zero == 1)
		regs.pc += arg;
}

static uint8_t
bit(uint8_t arg)
{
	regs.status.zero = (regs.a & arg) == 0;
	regs.status.overflow = (arg & (1 << 6)) != 0;
	STATUS_UPDATE_NEGATIVE(arg);

	return regs.a & arg;
}

static void
bmi(uint8_t arg)
{
	if (regs.status.negative == 1)
		regs.pc += arg;
}

static void
bne(uint8_t arg)
{
	if (regs.status.zero == 0)
		regs.pc += arg;
}

static void
bpl(uint8_t arg)
{
	if (regs.status.negative == 0)
		regs.pc += arg;
}

static void
brk(void)
{
	/* TODO: push regs.pc and regs.status to stack and load IRQ vector */
	regs.status.brk = 1;
	// exit(0);
}

static void
bvc(uint8_t arg)
{
	//regs.status.overflow = (STATUS_TO_INT() & (1 << 6)) == 0;
	//if (regs.status.overflow == 0)
	if ((STATUS_TO_INT() & (1 << 6)) == 0)
		regs.pc += arg;
}

static void
bvs(uint8_t arg)
{
	regs.status.overflow = (STATUS_TO_INT() & (1 << 6)) != 0;
	if (regs.status.overflow == 1)
		regs.pc += arg;
}

static void
clc(void)
{
	regs.status.carry = 0;
}

static void
cld(void)
{
	regs.status.decimal_mode = 0;
}

static void
cli(void)
{
	regs.status.interrupt_disable = 0;
}

static void
clv(void)
{
	regs.status.overflow = 0;
}

static void
cmp(uint8_t arg)
{
	uint8_t tmp;

	tmp = regs.a - arg;

	regs.status.carry = regs.a >= arg;
	regs.status.zero = regs.a == arg;
	STATUS_UPDATE_NEGATIVE(tmp);
}

static void
cpx(uint8_t arg)
{
	uint8_t tmp;

	tmp = regs.x - arg;

	regs.status.carry = regs.x >= arg;
	regs.status.zero = regs.x == arg;
	STATUS_UPDATE_NEGATIVE(tmp);
}

static void
cpy(uint8_t arg)
{
	uint8_t tmp;

	tmp = regs.y - arg;

	regs.status.carry = regs.y >= arg;
	regs.status.zero = regs.y == arg;
	STATUS_UPDATE_NEGATIVE(tmp);
}

static void
dec(uint16_t mem)
{
	memwrite(mem, peek(mem) - 1);

	STATUS_UPDATE_NZ(peek(mem));
}

static void
dex(void)
{
	regs.x--;

	STATUS_UPDATE_NZ(regs.x);
}

static void
dey(void)
{
	regs.y--;

	STATUS_UPDATE_NZ(regs.y);
}

static void
eor(uint8_t arg)
{
	regs.a ^= arg;

	STATUS_UPDATE_NZ(regs.a);
}

static void
inc(uint16_t mem)
{
	memwrite(mem, peek(mem) + 1);

	STATUS_UPDATE_NZ(peek(mem));
}

static void
inx(void)
{
	regs.x++;

	STATUS_UPDATE_NZ(regs.x);
}

static void
iny(void)
{
	regs.y++;

	STATUS_UPDATE_NZ(regs.y);
}

static void
jmp(uint16_t arg)
{
	regs.pc = arg;
}

static void
jsr(uint16_t arg)
{
	uint16_t tmp = regs.pc - 1;

	/*
	 * first push high-byte of return address then low-byte
	 * https://www.masswerk.at/6502/6502_instruction_set.html
	 */
	PUSH((tmp & 0xFF00) >> 8);
	PUSH(tmp & 0xFF);
	regs.pc = arg;
}

static void
lda(uint8_t arg)
{
	regs.a = arg;

	STATUS_UPDATE_NZ(regs.a);
}

static void
ldx(uint8_t arg)
{
	regs.x = arg;

	STATUS_UPDATE_NZ(regs.x);
}

static void
ldy(uint8_t arg)
{
	regs.y = arg;

	STATUS_UPDATE_NZ(regs.y);
}

static void
lsr_acc(void)
{
	regs.status.carry = regs.a & 1;	// bit 0 in carry
	regs.a >>= 1;
	regs.a &= ~(1 << 7);		// bit 7 cleared

	STATUS_UPDATE_NZ(regs.a);
}

static void
lsr(uint16_t mem)
{
	uint8_t tmp;

	tmp = peek(mem);

	regs.status.carry = tmp & 1;	// bit 0 in carry
	tmp >>= 1;
	tmp &= ~(1 << 7);		// bit 7 cleared

	memwrite(mem, tmp);

	STATUS_UPDATE_NZ(tmp);
}

static void
nop(void)
{
	return;
}

static void
ora(uint8_t arg)
{
	regs.a |= arg;

	STATUS_UPDATE_NZ(regs.a);
}

static void
pha(void)
{
	PUSH(regs.a);
}

static void
php(void)
{
	PUSH(STATUS_TO_INT() | (1 << 4));
}

static void
pla(void)
{
	regs.a = PULL();

	STATUS_UPDATE_NZ(regs.a);
}

static void
plp(void)
{
	uint8_t status;

	status = PULL();

	regs.status.carry = (status & 1) != 0;
	regs.status.zero = (status & (1 << 1)) != 0;
	regs.status.interrupt_disable = (status & (1 << 2)) != 0;
	regs.status.decimal_mode = (status & (1 << 3)) != 0;
	regs.status.brk = 0;
	regs.status.unused = 1;
	regs.status.overflow = (status & (1 << 6)) != 0;
	regs.status.negative = (status & (1 << 7)) != 0;
}

static void
rol_acc(void)
{
	uint8_t carry;
	carry = (regs.a & (1 << 7)) != 0;

	regs.a <<= 1;
	regs.a |= regs.status.carry;

	regs.status.carry = carry;
	STATUS_UPDATE_NZ(regs.a);
}

static void
rol(uint16_t mem)
{
	uint8_t carry, tmp;
	carry = (peek(mem) & (1 << 7)) != 0;

	tmp = (peek(mem) << 1) | regs.status.carry;
	memwrite(mem, tmp);

	regs.status.carry = carry;
	STATUS_UPDATE_NZ(regs.a);
}

static void
ror_acc(void)
{
	uint8_t carry;
	carry = regs.a & 1;

	regs.a >>= 1;
	regs.a |= regs.status.carry << 7;

	regs.status.carry = carry;
	STATUS_UPDATE_NZ(regs.a);
}

static void
ror(uint16_t mem)
{
	uint8_t carry, tmp;
	carry = peek(mem) & 1;

	tmp = (peek(mem) >> 1) | (regs.status.carry << 7);
	memwrite(mem, tmp);

	regs.status.carry = carry;
	STATUS_UPDATE_NZ(tmp);
}

static void
rti(void)
{
	plp();
	regs.pc = PULL() | (PULL() << 8);
}

static void
rts(void)
{
	regs.pc = (PULL() | (PULL() << 8)) + 1;
}

static void
sbc(uint8_t arg)
{
	/* SBC is described online as ADC with argument as two's complement */
	adc(~arg);
}

static void
sec(void)
{
	regs.status.carry = 1;
}

static void
sed(void)
{
	regs.status.decimal_mode = 1;
}

static void
sei(void)
{
	regs.status.interrupt_disable = 1;
}

static uint8_t
sta(uint16_t mem)
{
	uint8_t tmp = peek(mem);
	memwrite(mem, regs.a);

	return tmp;
}

static uint8_t
stx(uint16_t mem)
{
	uint8_t tmp = peek(mem);
	memwrite(mem, regs.x);

	return tmp;
}

static uint8_t
sty(uint16_t mem)
{
	uint8_t tmp = peek(mem);
	memwrite(mem, regs.y);

	return tmp;
}

static void
tax(void)
{
	regs.x = regs.a;

	STATUS_UPDATE_NZ(regs.x);
}

static void
tay(void)
{
	regs.y = regs.a;

	STATUS_UPDATE_NZ(regs.y);
}

static void
tsx(void)
{
//	regs.x = PULL();
	regs.x = regs.sp;

	STATUS_UPDATE_NZ(regs.x);
}

static void
txa(void)
{
	regs.a = regs.x;

	STATUS_UPDATE_NZ(regs.a);
}

static void
txs(void)
{
//	PUSH(regs.x);
	regs.sp = regs.x;
}

static void
tya(void)
{
	regs.a = regs.y;

	STATUS_UPDATE_NZ(regs.a);
}

static void
interpret(void)
{
	uint8_t opcode, did_memwrite, ret, status;
	uint16_t arg, mem, deref;
	uint32_t cycles_;
	enum addressing_mode mode;
	struct registers regs_;

	for (;;) {
		opcode = peek(regs.pc++);
		cycles_ = cycles;
		did_memwrite = 0;
		status = STATUS_TO_INT();
		regs_ = regs;

		printf("%04X  %02X", regs.pc - 1, opcode);

		switch (opcode) {
		case 0x69:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 2;
			printf("ADC");
			break;
		case 0x65:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 3;
			printf("ADC");
			break;
		case 0x75:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 4;
			printf("ADC");
			break;
		case 0x6d:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 4;
			printf("ADC");
			break;
		case 0x7d:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 4;
			printf("ADC");
			break;
		case 0x79:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 4;
			printf("ADC");
			break;
		case 0x61:
			mode = AM_IND_X;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 6;
			printf("ADC");
			break;
		case 0x71:
			mode = AM_IND_Y;
			arg = opcode_arg(mode);
			adc(arg);
			cycles += 5;
			printf("ADC");
			break;
		case 0x29:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 2;
			printf("AND");
			break;
		case 0x25:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 3;
			printf("AND");
			break;
		case 0x35:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 4;
			printf("AND");
			break;
		case 0x2d:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 4;
			printf("AND");
			break;
		case 0x3d:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 4;
			printf("AND");
			break;
		case 0x39:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 4;
			printf("AND");
			break;
		case 0x21:
			mode = AM_IND_X;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 6;
			printf("AND");
			break;
		case 0x31:
			mode = AM_IND_Y;
			arg = opcode_arg(mode);
			and(arg);
			cycles += 5;
			printf("AND");
			break;
		case 0x0a:
			mode = AM_ACC;
			asl_acc();
			cycles += 2;
			printf("\tASL");
			break;
		case 0x06:
			mode = AM_ZP;
			arg = opcode_mem(mode);
			asl(arg);
			cycles += 5;
			printf("ASL");
			break;
		case 0x16:
			mode = AM_ZP_X;
			arg = opcode_mem(mode);
			asl(arg);
			cycles += 6;
			printf("ASL");
			break;
		case 0x0e:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			asl(arg);
			cycles += 6;
			printf("ASL");
			break;
		case 0x1e:
			mode = AM_ABS_X;
			arg = opcode_mem(mode);
			asl(arg);
			cycles += 6;
			printf("ASL");
			break;
		case 0x90:
			mode = AM_REL;
			arg = opcode_arg(mode);
			bcc(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BCC");
			break;
		case 0xb0:
			mode = AM_REL;
			arg = opcode_arg(mode);
			bcs(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BCS");
			break;
		case 0xf0:
			mode = AM_REL;
			arg = opcode_arg(mode);
			beq(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BEQ");
			break;
		case 0x89:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			did_memwrite = 1;
			ret = bit(arg);
			cycles += 2;
			printf("BIT");
			break;
		case 0x24:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			did_memwrite = 1;
			ret = bit(arg);
			cycles += 3;
			printf("BIT");
			break;
		case 0x34:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			did_memwrite = 1;
			ret = bit(arg);
			cycles += 4;
			printf("BIT");
			break;
		case 0x2c:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			did_memwrite = 1;
			ret = bit(arg);
			cycles += 4;
			printf("BIT");
			break;
		case 0x3c:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			did_memwrite = 1;
			ret = bit(arg);
			cycles += 4;
			printf("BIT");
			break;
		case 0x30:
			mode = AM_REL;
			arg = opcode_arg(mode);
			bmi(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BMI");
			break;
		case 0xd0:
			mode = AM_REL;
			arg = opcode_arg(mode);
			bne(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BNE");
			break;
		case 0x10:
			mode = AM_REL;
			arg = opcode_arg(mode);
			bpl(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BPL");
			break;
		case 0x00:
			mode = AM_IMPLICIT;
			brk();
			cycles += 7;
			printf("\tBRK");
			goto loop_exit;
		case 0x50:
			mode = AM_REL;
			arg = opcode_arg(mode);
			bvc(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BVC");
			break;
		case 0x70:
			mode = AM_REL;
			arg = opcode_arg(mode);
			bvs(arg);
			arg += regs_.pc + 1;
			cycles += 2;
			printf("BVS");
			break;
		case 0x18:
			mode = AM_IMPLICIT;
			clc();
			cycles += 2;
			printf("\tCLC");
			break;
		case 0xd8:
			mode = AM_IMPLICIT;
			cld();
			cycles += 2;
			printf("\tCLD");
			break;
		case 0x58:
			mode = AM_IMPLICIT;
			cli();
			cycles += 2;
			printf("\tCLI");
			break;
		case 0xb8:
			mode = AM_IMPLICIT;
			clv();
			cycles += 2;
			printf("\tCLV");
			break;
		case 0xc9:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 2;
			printf("CMP");
			break;
		case 0xc5:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 3;
			printf("CMP");
			break;
		case 0xd5:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 4;
			printf("CMP");
			break;
		case 0xcd:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 4;
			printf("CMP");
			break;
		case 0xdd:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 4;
			printf("CMP");
			break;
		case 0xd9:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 4;
			printf("CMP");
			break;
		case 0xc1:
			mode = AM_IND_X;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 6;
			printf("CMP");
			break;
		case 0xd1:
			mode = AM_IND_Y;
			arg = opcode_arg(mode);
			cmp(arg);
			cycles += 5;
			printf("CMP");
			break;
		case 0xe0:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			cpx(arg);
			cycles += 2;
			printf("CPX");
			break;
		case 0xe4:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			cpx(arg);
			cycles += 3;
			printf("CPX");
			break;
		case 0xec:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			cpx(arg);
			cycles += 4;
			printf("CPX");
			break;
		case 0xc0:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			cpy(arg);
			cycles += 2;
			printf("CPY");
			break;
		case 0xc4:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			cpy(arg);
			cycles += 3;
			printf("CPY");
			break;
		case 0xcc:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			cpy(arg);
			cycles += 4;
			printf("CPY");
			break;
		case 0xc6:
			mode = AM_ZP;
			arg = opcode_mem(mode);
			dec(arg);
			cycles += 5;
			printf("DEC");
			break;
		case 0xd6:
			mode = AM_ZP_X;
			arg = opcode_mem(mode);
			dec(arg);
			cycles += 6;
			printf("DEC");
			break;
		case 0xce:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			dec(arg);
			cycles += 6;
			printf("DEC");
			break;
		case 0xde:
			mode = AM_ABS_X;
			arg = opcode_mem(mode);
			dec(arg);
			cycles += 7;
			printf("DEC");
			break;
		case 0xca:
			mode = AM_IMPLICIT;
			dex();
			cycles += 2;
			printf("\tDEX");
			break;
		case 0x88:
			mode = AM_IMPLICIT;
			dey();
			cycles += 2;
			printf("\tDEY");
			break;
		case 0x49:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 2;
			printf("EOR");
			break;
		case 0x45:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 3;
			printf("EOR");
			break;
		case 0x55:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 4;
			printf("EOR");
			break;
		case 0x4d:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 4;
			printf("EOR");
			break;
		case 0x5d:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 4;
			printf("EOR");
			break;
		case 0x59:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 4;
			printf("EOR");
			break;
		case 0x41:
			mode = AM_IND_X;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 6;
			printf("EOR");
			break;
		case 0x51:
			mode = AM_IND_Y;
			arg = opcode_arg(mode);
			eor(arg);
			cycles += 5;
			printf("EOR");
			break;
		case 0xe6:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			inc(arg);
			cycles += 5;
			printf("INC");
			break;
		case 0xf6:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			inc(arg);
			cycles += 6;
			printf("INC");
			break;
		case 0xee:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			inc(arg);
			cycles += 6;
			printf("INC");
			break;
		case 0xfe:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			inc(arg);
			cycles += 7;
			printf("INC");
			break;
		case 0xe8:
			mode = AM_IMPLICIT;
			inx();
			cycles += 2;
			printf("\tINX");
			break;
		case 0xc8:
			mode = AM_IMPLICIT;
			iny();
			cycles += 2;
			printf("\tINY");
			break;
		case 0x4c:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			jmp(arg);
			cycles += 3;
			printf("JMP");
			break;
		case 0x6c:
			mode = AM_IND;
			mem = peek16(regs.pc);
			deref = peek16(mem);
			arg = opcode_mem(mode);
			deref = arg;
			jmp(arg);
			cycles += 6;
			printf("JMP");
			break;
		case 0x7c:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			jmp(arg);
			cycles += 6;
			printf("JMP");
			break;
		case 0x20:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			jsr(arg);
			cycles += 6;
			printf("JSR");
			break;
		case 0xa9:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 2;
			printf("LDA");
			break;
		case 0xa5:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 3;
			printf("LDA");
			break;
		case 0xb5:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 4;
			printf("LDA");
			break;
		case 0xad:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 4;
			printf("LDA");
			break;
		case 0xbd:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 4;
			printf("LDA");
			break;
		case 0xb9:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 4;
			printf("LDA");
			break;
		case 0xa1:
			mode = AM_IND_X;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 6;
			printf("LDA");
			break;
		case 0xb1:
			mode = AM_IND_Y;
			arg = opcode_arg(mode);
			lda(arg);
			cycles += 5;
			printf("LDA");
			break;
		case 0xa2:
			mode = AM_IMM;
			arg = opcode_mem(mode);
			ldx(arg);
			cycles += 2;
			printf("LDX");
			break;
		case 0xa6:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			ldx(arg);
			cycles += 3;
			printf("LDX");
			break;
		case 0xb6:
			mode = AM_ZP_Y;
			arg = opcode_arg(mode);
			ldx(arg);
			cycles += 4;
			printf("LDX");
			break;
		case 0xae:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			ldx(arg);
			cycles += 4;
			printf("LDX");
			break;
		case 0xbe:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			ldx(arg);
			cycles += 4;
			printf("LDX");
			break;
		case 0xa0:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			ldy(arg);
			cycles += 2;
			printf("LDY");
			break;
		case 0xa4:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			ldy(arg);
			cycles += 3;
			printf("LDY");
			break;
		case 0xb4:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			ldy(arg);
			cycles += 4;
			printf("LDY");
			break;
		case 0xac:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			ldy(arg);
			cycles += 4;
			printf("LDY");
			break;
		case 0xbc:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			ldy(arg);
			cycles += 4;
			printf("LDY");
			break;
		case 0x4a:
			mode = AM_ACC;
			lsr_acc();
			cycles += 2;
			printf("\tLSR");
			break;
		case 0x46:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			lsr(arg);
			cycles += 5;
			printf("LSR");
			break;
		case 0x56:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			lsr(arg);
			cycles += 6;
			printf("LSR");
			break;
		case 0x4e:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			lsr(arg);
			cycles += 6;
			printf("LSR");
			break;
		case 0x5e:
			mode = AM_ABS_X;
			arg = opcode_mem(mode);
			lsr(arg);
			cycles += 6;
			printf("LSR");
			break;
		case 0xea:
			mode = AM_IMPLICIT;
			nop();
			cycles += 2;
			printf("\tNOP");
			break;
		case 0x09:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 2;
			printf("ORA");
			break;
		case 0x05:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 3;
			printf("ORA");
			break;
		case 0x15:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 4;
			printf("ORA");
			break;
		case 0x0d:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 4;
			printf("ORA");
			break;
		case 0x1d:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 4;
			printf("ORA");
			break;
		case 0x19:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 4;
			printf("ORA");
			break;
		case 0x01:
			mode = AM_IND_X;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 6;
			printf("ORA");
			break;
		case 0x11:
			mode = AM_IND_Y;
			arg = opcode_arg(mode);
			ora(arg);
			cycles += 5;
			printf("ORA");
			break;
		case 0x48:
			mode = AM_IMPLICIT;
			pha();
			cycles += 3;
			printf("\tPHA");
			break;
		case 0x08:
			mode = AM_IMPLICIT;
			php();
			cycles += 3;
			printf("\tPHP");
			break;
		case 0x68:
			mode = AM_IMPLICIT;
			pla();
			cycles += 4;
			printf("\tPLA");
			break;
		case 0x28:
			mode = AM_IMPLICIT;
			plp();
			cycles += 4;
			printf("\tPLP");
			break;
		case 0x2a:
			mode = AM_ACC;
			rol_acc();
			cycles += 2;
			printf("\tROL");
			break;
		case 0x26:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			rol(arg);
			cycles += 5;
			printf("ROL");
			break;
		case 0x36:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			rol(arg);
			cycles += 6;
			printf("ROL");
			break;
		case 0x2e:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			rol(arg);
			cycles += 6;
			printf("ROL");
			break;
		case 0x3e:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			rol(arg);
			cycles += 6;
			printf("ROL");
			break;
		case 0x6a:
			mode = AM_ACC;
			ror_acc();
			cycles += 2;
			printf("\tROR");
			break;
		case 0x66:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			ror(arg);
			cycles += 5;
			printf("ROR");
			break;
		case 0x76:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			ror(arg);
			cycles += 6;
			printf("ROR");
			break;
		case 0x6e:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			ror(arg);
			cycles += 6;
			printf("ROR");
			break;
		case 0x7e:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			ror(arg);
			cycles += 6;
			printf("ROR");
			break;
		case 0x40:
			mode = AM_IMPLICIT;
			rti();
			cycles += 6;
			printf("\tRTI");
			break;
		case 0x60:
			mode = AM_IMPLICIT;
			rts();
			cycles += 6;
			printf("\tRTS");
			break;
		case 0xe9:
			mode = AM_IMM;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 2;
			printf("SBC");
			break;
		case 0xe5:
			mode = AM_ZP;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 3;
			printf("SBC");
			break;
		case 0xf5:
			mode = AM_ZP_X;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 4;
			printf("SBC");
			break;
		case 0xed:
			mode = AM_ABS;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 4;
			printf("SBC");
			break;
		case 0xfd:
			mode = AM_ABS_X;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 4;
			printf("SBC");
			break;
		case 0xf9:
			mode = AM_ABS_Y;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 4;
			printf("SBC");
			break;
		case 0xe1:
			mode = AM_IND_X;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 6;
			printf("SBC");
			break;
		case 0xf1:
			mode = AM_IND_Y;
			arg = opcode_arg(mode);
			sbc(arg);
			cycles += 5;
			printf("SBC");
			break;
		case 0x38:
			mode = AM_IMPLICIT;
			sec();
			cycles += 2;
			printf("\tSEC");
			break;
		case 0xf8:
			mode = AM_IMPLICIT;
			sed();
			cycles += 2;
			printf("\tSED");
			break;
		case 0x78:
			mode = AM_IMPLICIT;
			sei();
			cycles += 2;
			printf("\tSEI");
			break;
		case 0x85:
			mode = AM_ZP;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sta(arg);
			cycles += 4;
			printf("STA");
			break;
		case 0x95:
			mode = AM_ZP_X;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sta(arg);
			cycles += 5;
			printf("STA");
			break;
		case 0x8d:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sta(arg);
			cycles += 5;
			printf("STA");
			break;
		case 0x9d:
			mode = AM_ABS_X;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sta(arg);
			cycles += 6;
			printf("STA");
			break;
		case 0x99:
			mode = AM_ABS_Y;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sta(arg);
			cycles += 6;
			printf("STA");
			break;
		case 0x81:
			mode = AM_IND_X;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sta(arg);
			cycles += 7;
			printf("STA");
			break;
		case 0x91:
			mode = AM_IND_Y;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sta(arg);
			cycles += 7;
			printf("STA");
			break;
		case 0x86:
			mode = AM_ZP;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = stx(arg);
			cycles += 4;
			printf("STX");
			break;
		case 0x96:
			mode = AM_ZP_Y;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = stx(arg);
			cycles += 5;
			printf("STX");
			break;
		case 0x8e:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = stx(arg);
			cycles += 5;
			printf("STX");
			break;
		case 0x84:
			mode = AM_ZP;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sty(arg);
			cycles += 4;
			printf("STY");
			break;
		case 0x94:
			mode = AM_ZP_X;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sty(arg);
			cycles += 5;
			printf("STY");
			break;
		case 0x8c:
			mode = AM_ABS;
			arg = opcode_mem(mode);
			did_memwrite = 1;
			ret = sty(arg);
			cycles += 5;
			printf("STY");
			break;
		case 0xaa:
			mode = AM_IMPLICIT;
			tax();
			cycles += 2;
			printf("\tTAX");
			break;
		case 0xa8:
			mode = AM_IMPLICIT;
			tay();
			cycles += 2;
			printf("\tTAY");
			break;
		case 0xba:
			mode = AM_IMPLICIT;
			tsx();
			cycles += 2;
			printf("\tTSX");
			break;
		case 0x8a:
			mode = AM_IMPLICIT;
			txa();
			cycles += 2;
			printf("\tTXA");
			break;
		case 0x9a:
			mode = AM_IMPLICIT;
			txs();
			cycles += 2;
			printf("\tTXS");
			break;
		case 0x98:
			mode = AM_IMPLICIT;
			tya();
			cycles += 2;
			printf("\tTYA");
			break;
		default:
			printf("opcode $%02X not implemented\n", opcode);
			break;
		}

		switch (mode) {
		case AM_IMM:
			printf(" #$%02X", arg);
			if (did_memwrite) printf(" = %02X\t\t\t", ret);
			else printf("\t\t\t");
			break;
		case AM_ZP:
			printf(" $%02X", arg);
			if (did_memwrite) printf(" = %02X\t\t\t", ret);
			else printf("\t\t\t\t");
			break;
		case AM_ZP_X:
			printf(" $%02X,X", arg);
			if (did_memwrite) printf(" = %02X\t\t\t", ret);
			else printf("\t\t\t");
			break;
		case AM_ZP_Y:
			printf(" $%02X,Y", arg);
			if (did_memwrite) printf(" = %02X\t\t\t", ret);
			else printf("\t\t\t\t");
			break;
		case AM_REL:
		case AM_ABS:
			printf(" $%04X", arg);
			if (did_memwrite) printf(" = %02X\t\t\t", ret);
			else printf("\t\t\t");
			break;
		case AM_ABS_X:
			printf(" $%04X,X", arg);
			if (did_memwrite) printf(" = %02X\t\t", ret);
			else printf("\t\t\t");
			break;
		case AM_ABS_Y:
			printf(" $%04X,Y", arg);
			if (did_memwrite) printf(" = %02X\t\t\t", ret);
			else printf("\t\t\t");
			break;
		case AM_IND:
			printf(" ($%04X) = %04X\t\t", mem, deref);
			break;
		case AM_ACC:
		case AM_IMPLICIT:
		default:
			printf("\t\t\t\t");
			break;
		}

		printf("A:%02X X:%02X Y:%02X P:%02X SP:%02X CYC:%d\n",
		    regs_.a, regs_.x, regs_.y, status, regs_.sp, cycles_);
	}
loop_exit:
}

/* https://www.nesdev.org/wiki/CPU_power_up_state */
void
cpu_init(void)
{
	regs.a = regs.x = regs.y = 0;
	regs.pc = 0xFFFC;
	regs.sp = 0xFD;

	//memset(&regs.status, 0, sizeof(regs.status));
	regs.status.interrupt_disable = 1;
	regs.status.unused = 1;

	cycles += 7;
}

int
main(int argc, char *argv[])
{
	FILE *fp;
	uint8_t *buf;
	size_t buflen;

	if (argc != 2) {
		fprintf(stderr, "Usage: %s rom.nes\n", basename(argv[0]));
		return 1;
	}

	fp = fopen(argv[1], "r");
	fseek(fp, 0, SEEK_END);
	buflen = ftell(fp);
	buf = calloc(1, buflen);
	fseek(fp, 0, SEEK_SET);
	if (fread(buf, 1, buflen, fp) != buflen && ferror(fp)) {
		fprintf(stderr, "file %s was not read properly\n", argv[1]);
		clearerr(fp);
		fclose(fp);
		return 1;
	}
	fclose(fp);

	parse_rom(buf, buflen, &rom);
	free(buf);

	cpu_init();

	/* TODO: move to separate file? */
	if (rom.mapper != 0) {
		fprintf(stderr, "Only iNES ROMs using Mapper 0 are supported for now.\n");
		return 1;
	}

	memwrite16(0xFFFC, 0xC000);
	regs.pc = 0xC000;

	interpret();

	printf("\n$02 = %02X\n", memory[0x02]);
	printf("$03 = %02X\n", memory[0x03]);

	free_rom(&rom);

	return 0;
}