emu_nes/cpu.c

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

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;

enum addressing_mode {
	AM_ACC,
	AM_IMM,
	AM_ZP,
	AM_ZP_X,
	AM_ZP_Y,
	AM_REL,
	AM_ABS,
	AM_ABS_X,
	AM_ABS_Y,
	AM_IND,
	AM_IND_X,
	AM_IND_Y,
};

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

/* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */
uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };

uint8_t
peek(uint16_t addr)
{
	return memory[addr];
}

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

uint8_t
opmode_arg(enum addressing_mode mode)
{
	uint8_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:
		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\n");
		abort();
	}

	return val;
}

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.a = sum & 0xFF;

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

void
brk(void)
{
	/* $00 */
	/* TODO: push regs.pc and regs.status to stack and load IRQ vector */
	regs.status.brk = 1;
	return;
}

void
tax(void)
{
	/* $AA */
	regs.x = regs.a;

	STATUS_UPDATE_ZERO(regs.x);
	STATUS_UPDATE_NEGATIVE(regs.x);
}

void
inx(void)
{
	/* $E8 */
	regs.x++;

	STATUS_UPDATE_ZERO(regs.x);
	STATUS_UPDATE_NEGATIVE(regs.x);
}

void
lda(uint8_t arg)
{
	printf("arg1 $%02X\n", arg);
	regs.a = arg;

	STATUS_UPDATE_ZERO(regs.a);
	STATUS_UPDATE_NEGATIVE(regs.a);
}

void
interpret(void)
{
	uint8_t opcode;

	for (;;) {
		opcode = peek(regs.pc++);

		printf("opcode: $%02X\n", opcode);

		switch (opcode) {
		case 0x00:
			brk();
			return;
		case 0x61:
			adc(opmode_arg(AM_IND_X));
			break;
		case 0x65:
			adc(opmode_arg(AM_ZP));
			break;
		case 0x69:
			adc(opmode_arg(AM_IMM));
			break;
		case 0x6D:
			adc(opmode_arg(AM_ABS));
			break;
		case 0x71:
			adc(opmode_arg(AM_IND_Y));
			break;
		case 0x75:
			adc(opmode_arg(AM_ZP_X));
			break;
		case 0x79:
			adc(opmode_arg(AM_ABS_Y));
			break;
		case 0x7D:
			adc(opmode_arg(AM_ABS_X));
			break;
		case 0xa1:
			lda(opmode_arg(AM_IND_X));
			break;
		case 0xa5:
			lda(opmode_arg(AM_ZP));
			break;
		case 0xa9:
			lda(opmode_arg(AM_IMM));
			break;
		case 0xaa:
			tax();
			break;
		case 0xad:
			lda(opmode_arg(AM_ABS));
			break;
		case 0xb1:
			lda(opmode_arg(AM_IND_Y));
			break;
		case 0xb5:
			lda(opmode_arg(AM_ZP_X));
			break;
		case 0xb9:
			lda(opmode_arg(AM_ABS_Y));
			break;
		case 0xbd:
			lda(opmode_arg(AM_ABS_X));
			break;
		case 0xe8:
			inx();
			break;
		default:
			printf("opcode $%02X not implemented\n", opcode);
			break;
		}

		printf("status: %i%i%i%i%i%i%i%i\n", regs.status.carry,
		    regs.status.zero, regs.status.interrupt_disable,
		    regs.status.decimal_mode, regs.status.brk,
		    regs.status.unused, regs.status.overflow,
		    regs.status.negative);
		printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
		    regs.a, regs.x, regs.y, regs.sp, regs.pc);
	}
}

/* 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.unused = 1;
}

int
main(void)
{
	cpu_init();

	if (sizeof(program) > (0x10000 - 0x8000)) {
		fprintf(stderr, "program is too big for memory\n");
		return 1;
	}

	memcpy(memory + 0x8000, program, sizeof(program));
	regs.pc = 0x8000;

	printf("Initial State:\n");
	printf("status: %i%i%i%i%i%i%i%i\n", regs.status.carry,
	    regs.status.zero, regs.status.interrupt_disable,
	    regs.status.decimal_mode, regs.status.brk,
	    regs.status.unused, regs.status.overflow,
	    regs.status.negative);
	printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
	    regs.a, regs.x, regs.y, regs.sp, regs.pc);

	putchar('\n');

	interpret();

	return 0;
}