emu_nes/ppu.c

#include "ppu.h"

struct ppu ppu = {0};

static void
vram_addr_inc(void)
{
	ppu.regs.v += (ppu.ctrl.inc_mode) ? 32 : 1;
}

static uint16_t
vram_addr_mirror(uint16_t addr)
{
	uint8_t nametable;

	addr &= 0x2fff;
	addr -= 0x2000;
	nametable = addr / 0x400;

	switch (ppu.rom->mirror) {
	case M_HORIZONTAL:
		if (nametable == 1 || nametable == 2)
			return addr - 0x400;
		else if (nametable == 3)
			return addr - 0x800;
		break;
	case M_VERTICAL:
		if (nametable == 2 || nametable == 3)
			return addr - 0x800;
	default:
		break;
	};

	return addr;
}

void
ppu_tick(void)
{
	ppu.cycles++;

	if (ppu.cycles > 256 && ppu.cycles <= 320)
		ppu.oam_addr = 0;

	if (ppu.cycles >= 341) {
		ppu.cycles -= 341;
		ppu.scanlines++;
	}
	if (ppu.scanlines >= 262)
		ppu.scanlines -= 262;
}

uint8_t
ppu_read(uint16_t addr)
{
	uint8_t tmp;

	switch (addr) {
	case 0x2002:
		tmp = (ppu.status.vblank << 7)
		    | (ppu.status.sprite0_hit << 6)
		    | (ppu.status.sprite_overflow << 5);

		ppu.status.vblank = 0;
		ppu.regs.write_toggle = 0;

		return tmp;
	case 0x2004:
		return ppu.oam[ppu.oam_addr];
	case 0x2007:
		tmp = ppu.last_read;

		if (addr <= 0x1fff)
			ppu.last_read = ppu.rom->chr_rom[ppu.regs.v];
		else if (addr <= 0x2fff)
			ppu.last_read = ppu.vram[vram_addr_mirror(addr)];

		vram_addr_inc();

		return tmp;
	default:
		fprintf(stderr, "Invalid PPU read at address $%04\n", addr);
		return 0;
	}
}

void
ppu_write(uint16_t addr, uint8_t byte)
{
	switch (addr) {
	case 0x2000:
		ppu.ctrl.nametable_base = byte & 3;
		ppu.ctrl.inc_mode = byte & (1 << 2);
		ppu.ctrl.sprite_tile_sel = byte & (1 << 3);
		ppu.ctrl.bg_tile_sel = byte & (1 << 4);
		ppu.ctrl.sprite_height = byte & (1 << 5);
		ppu.ctrl.master_slave = byte & (1 << 6);
		ppu.ctrl.nmi_enable = byte & (1 << 7);

		if (ppu.ctrl.nametable_base & 1)
			ppu.regs.scroll_x += 256;
		if (ppu.ctrl.nametable_base & 2)
			ppu.regs.scroll_y += 240;

		/*
		 * TODO: send NMI if vblank flag still set and nmi_enable
		 * changes from 0 to 1
		 */
		break;
	case 0x2001:
		ppu.mask.grayscale = byte & 1;
		ppu.mask.bg_left_enable = byte & (1 << 1);
		ppu.mask.sprite_left_enable = byte & (1 << 2);
		ppu.mask.bg_enable = byte & (1 << 3);
		ppu.mask.sprite_enable = byte & (1 << 4);
		ppu.mask.colour_emphasis = byte & (1 << 5);

		break;
	case 0x2003:
		ppu.oam_addr = byte;

		break;
	case 0x2004:
		ppu.oam[ppu.oam_addr++] = byte;

		break;
	case 0x2005:
		if (ppu.regs.write_toggle == 0)
			ppu.regs.scroll_x = byte;
		else
			ppu.regs.scroll_y = byte;
		ppu.regs.write_toggle = !ppu.regs.write_toggle;

		break;
	case 0x2006:
		if (ppu.regs.write_toggle == 0)
			ppu.regs.v = (byte << 8);
		else {
			ppu.regs.v |= byte;
			ppu.regs.v &= 0x3fff;
		}
		ppu.regs.write_toggle = !ppu.regs.write_toggle;

		break;
	case 0x2007:
		ppu.vram[ppu.regs.v] = byte;
		vram_addr_inc();

		break;
	case 0x4014:
		break;
	default:
		fprintf(stderr, "Invalid PPU write at address $%04X\n", addr);
		return;
	}
}