emu_nes/cpu.c

#include <stdint.h>

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 acc, x, y, sp;
	struct cpu_flags status;
	uint16_t pc;
};
struct registers regs;

enum addressing_mode {
	AM_ACCUMULATOR,
	AM_IMMEDIATE,
	AM_ZERO_PAGE,
	AM_ZERO_PAGE_X,
	AM_ZERO_PAGE_Y,
	AM_RELATIVE,
	AM_ABSOLUTE,
	AM_ABSOLUTE_X,
	AM_ABSOLUTE_Y,
	AM_INDIRECT,
	AM_INDIRECT_X,
	AM_INDIRECT_Y,
	AM_INDEXED_INDIRECT,
	AM_INDIRECT_INDEXED,
};

uint8_t addrbus[0x10000];

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

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

void lda(enum addressing_mode mode)
{
	uint8_t val;

	switch (mode) {
	case AM_IMMEDIATE:	/* $A9 */
		val = program[regs.pc++];
		regs.acc = val;
		break;
	case AM_ZERO_PAGE:	/* $A5 */
	case AM_ZERO_PAGE_X:	/* $B5 */
	case AM_ABSOLUTE:	/* $AD */
	case AM_ABSOLUTE_X:	/* $BD */
	case AM_ABSOLUTE_Y:	/* $B9 */
	case AM_INDIRECT_X:	/* $A1 */
	case AM_INDIRECT_Y:	/* $B1 */
		/* TODO */
		break;
	default:
		return;
	}

	regs.status.zero = regs.acc == 0;
	regs.status.negative = (regs.acc & (1 << 7)) != 0;
}

void interpret(void)
{
	uint8_t opcode;
	regs.pc = 0;

	for (;;) {
		opcode = program[regs.pc++];

		switch (opcode) {
		case 0x00:
			brk();
			return;
		case 0xa9:
			lda(AM_IMMEDIATE);
			break;
		case 0xa5:
			lda(AM_ZERO_PAGE);
			break;
		case 0xb5:
			lda(AM_ZERO_PAGE_X);
			break;
		case 0xad:
			lda(AM_ABSOLUTE);
			break;
		case 0xbd:
			lda(AM_ABSOLUTE_X);
			break;
		case 0xb9:
			lda(AM_ABSOLUTE_Y);
			break;
		case 0xa1:
			lda(AM_INDIRECT_X);
			break;
		case 0xb1:
			lda(AM_INDIRECT_Y);
			break;
		default:
			break;
		}
	}
}