path: root/ppu.c

static uint8_t memory_read_dma(struct nes_state *state, uint32_t offset);

static void ppu_reset_scroll_latch(struct nes_state *state) {
	state->ppu.write_toggle = 0;
}

static uint32_t compute_nt_address(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;
	return 0x2000 | ppu->nt_base_y | ppu->nt_base_x | ppu->coarse_y_offs | ppu->coarse_x_offs;
}


static uint32_t compute_at_address(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;
	uint32_t nt_offset = ppu->nt_base_y | ppu->nt_base_x;
	uint32_t attr_x = (ppu->coarse_x_offs >> 2) & 0x07;
	uint32_t attr_y = (ppu->coarse_y_offs >> 2) & 0x38;
	return 0x23c0 | nt_offset | attr_y | attr_x;
}

static void increment_x(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;

	if(ppu->coarse_x_offs == 31 << 0) {
		ppu->coarse_x_offs = 0;
		ppu->nt_base_x ^= 0x400;
	} else {
		ppu->coarse_x_offs += 1 << 0;
	}
}

static void increment_y(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;

	if(ppu->fine_y < 7) {
		ppu->fine_y++;
		return;
	}

	ppu->fine_y = 0;

	uint32_t coarse_y = ppu->coarse_y_offs >> 5;

	if(coarse_y == 29) {
		ppu->coarse_y_offs = 0;
		ppu->nt_base_y ^= 0x800;
	} else if(coarse_y == 31) {
		ppu->coarse_y_offs = 0;
		// Do not toggle nt_base_y — stays the same
	} else {
		ppu->coarse_y_offs += 1 << 5;
	}
}


static void ppu_vram_write(struct nes_state *state, uint32_t addr, uint8_t value) {
	// PPU address bus is 14 bits, wraps every 0x4000
	addr &= 0x3fff;

	if(addr < 0x2000) {
		// Pattern table (CHR ROM is read-only, ignore write)
		return;
	}

	if(addr < 0x3f00) {
		// Nametable RAM (CIRAM), mirrored every 0x1000
		uint32_t nt_index = addr & 0x0fff;
		state->ciram[nt_index] = value;
		return;
	}

	if(addr < 0x4000) {
		// Palette RAM (0x3F00–0x3FFF), mirrored every 32 bytes
		addr = 0x3f00 + (addr & 0x1f);

		// Palette mirroring: $3F10/$3F14/$3F18/$3F1C mirror $3F00/$3F04/$3F08/$3F0C
		if((addr & 0x13) == 0x10) {
			addr &= ~0x10;
		}

		state->palette[addr & 0x1f] = value;
	}
}

static uint8_t ppu_vram_read(struct nes_state *state, uint32_t addr) {
	// PPU address bus is 14 bits, wraps every 0x4000
	addr &= 0x3fff;

	if(addr < 0x2000) {
		// Pattern table (CHR ROM)
		return state->chrrom[addr];
	}

	if(addr < 0x3f00) {
		// Nametables (0x2000–0x2FFF mirrored every 0x1000)
		uint32_t nt_index = addr & 0x0fff;
		return state->ciram[nt_index];
	}

	if(addr < 0x4000) {
		// Palette RAM (0x3F00–0x3FFF, mirrored every 32 bytes)
		addr = 0x3f00 + (addr & 0x1f);

		// Palette mirroring: $3F10/$3F14/$3F18/$3F1C mirror $3F00/$3F04/$3F08/$3F0C
		if((addr & 0x13) == 0x10) {
			addr &= ~0x10;
		}

		return state->palette[addr & 0x1f];
	}

	return 0;
}

static uint8_t ppu_read_2002(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;
	uint8_t result = ppu->status;

	ppu->status &= ~PPU_STATUS_VBLANK;
	ppu->write_toggle = 0;

	return result;
}

static void ppu_write_2003(struct nes_state *state, uint8_t value) {
	state->ppu.oam_addr = value;
}

static uint8_t ppu_read_2004(struct nes_state *state) {
	return state->ppu.oam[state->ppu.oam_addr];
}

static void ppu_write_2004(struct nes_state *state, uint8_t value) {
	state->ppu.oam[state->ppu.oam_addr] = value;
	state->ppu.oam_addr++;
}


static void ppu_write_2005(struct nes_state *state, uint8_t value)
{
	struct ppu_state *ppu = &state->ppu;

	if(ppu->write_toggle == 0) {
		ppu->temp_fine_x = value & 7;
		ppu->temp_coarse_x_offs = (value >> 3) & 0x1f;
		ppu->temp_coarse_x_offs <<= 0;

		ppu->write_toggle = 1;
	} else {
		ppu->temp_fine_y = value & 7;
		uint8_t coarse_y = (value >> 3) & 0x1f;
		ppu->temp_coarse_y_offs = coarse_y << 5;

		if(coarse_y >= 30) {
			// vertical nametable select
			ppu->temp_nt_base_y = 0x800;
		} else {
			ppu->temp_nt_base_y = 0x000;
		}

		ppu->write_toggle = 0;
	}
}

static void ppu_write_2006(struct nes_state *state, uint8_t value)
{
	struct ppu_state *ppu = &state->ppu;

	if(ppu->write_toggle == 0) {
		// High byte: bits 14–8 of VRAM address
		ppu->temp_nt_base_y = (value & 0x0c) << 8;
		ppu->temp_nt_base_x = (value & 0x01) << 10;
		ppu->temp_coarse_y_offs = (value & 0x03) << 3;
		ppu->temp_coarse_y_offs <<= 5;

		ppu->write_toggle = 1;
	} else {
		// Low byte: bits 7–0 of VRAM address
		ppu->temp_coarse_x_offs = (value & 0x1f) << 0;
		ppu->temp_fine_y = (value >> 2) & 7;

		// On second write, transfer temp -> current scroll state
		ppu->coarse_x_offs = ppu->temp_coarse_x_offs;
		ppu->coarse_y_offs = ppu->temp_coarse_y_offs;
		ppu->fine_x = ppu->temp_fine_x;
		ppu->fine_y = ppu->temp_fine_y;
		ppu->nt_base_x = ppu->temp_nt_base_x;
		ppu->nt_base_y = ppu->temp_nt_base_y;

		ppu->write_toggle = 0;
	}
}

static uint8_t ppu_read_2007(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;

	uint32_t addr = compute_nt_address(state);
	if(ppu->fine_y) {
		addr += ppu->fine_y;
	}

	uint8_t result;

	if(addr < 0x3f00) {
		// Return buffered value, load new one
		result = ppu->read_buffer;
		ppu->read_buffer = ppu_vram_read(state, addr);
	} else {
		// Palette reads bypass buffer
		result = ppu_vram_read(state, addr);
		ppu->read_buffer = ppu_vram_read(state, addr & 0x2fff); // mirror to pattern/nametable area
	}

	// Auto-increment address
	if(ppu->ctrl & PPU_CTRL_INCREMENT) {
		increment_y(state);
	} else {
		increment_x(state);
	}

	return result;
}


static void ppu_write_2007(struct nes_state *state, uint8_t value) {
	struct ppu_state *ppu = &state->ppu;
	uint32_t addr = compute_nt_address(state);

	if(ppu->fine_y)
		addr += ppu->fine_y;

	ppu_vram_write(state, addr, value);

	if(ppu->ctrl & PPU_CTRL_INCREMENT) {
		increment_y(state);
	} else {
		increment_x(state);
	}
}

static uint8_t ppu_pattern_read(struct nes_state *state, uint8_t tile_index, uint8_t fine_y, uint8_t plane) {
	struct ppu_state *ppu = &state->ppu;
	uint32_t base = (ppu->ctrl & PPU_CTRL_BG_TABLE) ? 0x1000 : 0x0000;
	uint32_t addr = base + (tile_index << 4) + fine_y;

	if(plane) {
		addr += 8;
	}

	return ppu_vram_read(state, addr);
}

static void ppu_eval_sprites(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;

	uint8_t scanline = ppu->scanline;
	uint8_t height = (ppu->ctrl & PPU_CTRL_SPRITE_HEIGHT) ? 16 : 8;

	uint8_t count = 0;

	for(uint8_t i = 0; i < 64; i++) {
		uint8_t *sprite = &ppu->oam[i * 4];
		uint8_t sprite_y = sprite[0];

		if(scanline >= sprite_y && scanline < sprite_y + height) {
			if(count < 8) {
				uint8_t *dest = &ppu->sec_oam[count * 4];
				dest[0] = sprite[0];
				dest[1] = sprite[1];
				dest[2] = sprite[2];
				dest[3] = sprite[3];
				count++;
			} else {
				ppu->status |= PPU_STATUS_OVERFLOW;
				break;
			}
		}
	}
}

static void ppu_clear_sec_oam(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;
	for(int i=0; i<32; i++) {
		ppu->sec_oam[i] = 0xff;
	}
}

static void ppu_render_sprites(struct nes_state *state)
{
	struct ppu_state *ppu = &state->ppu;

	// Clear sprite pixel buffer
	for(int x=0;x<256;x++) {
		ppu->sprite_pixels[x] = 0;
		ppu->sprite_zero_flags[x] = 0;
	}

	for(int i=0;i<8;i++) {
		uint8_t *sprite = &ppu->sec_oam[i * 4];
		uint8_t y = sprite[0];
		uint8_t tile = sprite[1];
		uint8_t attr = sprite[2];
		uint8_t x = sprite[3];

		uint8_t scanline = ppu->scanline;
		uint8_t height = (ppu->ctrl & PPU_CTRL_SPRITE_HEIGHT) ? 16 : 8;
		int sprite_y = scanline - y;

		if(attr & 0x80) {
			// Vertical flip
			sprite_y = height - 1 - sprite_y;
		}

		uint32_t base = (ppu->ctrl & PPU_CTRL_SPRITE_TABLE) ? 0x1000 : 0x0000;
		uint32_t addr = base + tile * 16 + sprite_y;

		uint8_t lo = ppu_vram_read(state, addr);
		uint8_t hi = ppu_vram_read(state, addr + 8);

		for(int j=0;j<8;j++) {
			int sx = x + (attr & 0x40 ? j : (7 - j)); // horizontal flip
			if(sx >= 256)
				continue;

			uint8_t bit = 1 << j;
			uint8_t p0 = (lo & (1 << (7 - j))) ? 1 : 0;
			uint8_t p1 = (hi & (1 << (7 - j))) ? 1 : 0;
			uint8_t color = (attr & 0x03) << 2 | (p1 << 1) | p0;

			if((p0 | p1) == 0)
				continue;

			if(ppu->sprite_pixels[sx] == 0) {
				ppu->sprite_pixels[sx] = color | 0x10;
				if(i == 0)
					ppu->sprite_zero_flags[sx] = 1;
			}
		}
	}
}


static void ppu_tick(struct nes_state *state) {
	struct ppu_state *ppu = &state->ppu;

	if(ppu->scanline == 241 && ppu->dot == 1) {
		ppu->status |= PPU_STATUS_VBLANK;
		ppu->nmi_occurred = 1;
	}

	if(ppu->scanline == 261 && ppu->dot == 1) {
		ppu->status &= ~PPU_STATUS_VBLANK;
		ppu->status &= ~PPU_STATUS_SPRITE0_HIT;
		ppu->status &= ~PPU_STATUS_OVERFLOW;
		ppu->nmi_occurred = 0;
	}

	if((ppu->scanline < 240 || ppu->scanline == 261) && (ppu->dot >= 1 && ppu->dot <= 256)) {
		uint32_t cycle = (ppu->dot - 1) % 8;

		switch(cycle) {
			case 0: {
				ppu->bg_tile_lsb = (ppu->bg_tile_lsb & 0xffff0000) | ppu->tile_lsb;
				ppu->bg_tile_msb = (ppu->bg_tile_msb & 0xffff0000) | ppu->tile_msb;
				ppu->bg_attr_lsb = (ppu->attr_byte & 1) ? 0xff : 0x00;
				ppu->bg_attr_msb = (ppu->attr_byte & 2) ? 0xff : 0x00;
			} break;

			case 1: {
				ppu->nt_byte = ppu_vram_read(state, compute_nt_address(state));
			} break;

			case 3: {
				ppu->attr_byte = ppu_vram_read(state, compute_at_address(state));
			} break;

			case 5: {
				ppu->tile_lsb = ppu_pattern_read(state, ppu->nt_byte, ppu->fine_y, 0);
			} break;

			case 7: {
				ppu->tile_msb = ppu_pattern_read(state, ppu->nt_byte, ppu->fine_y, 1);
			} break;
		}

		ppu->bg_tile_lsb <<= 1;
		ppu->bg_tile_msb <<= 1;
		ppu->bg_attr_lsb <<= 1;
		ppu->bg_attr_msb <<= 1;

		if(ppu->scanline < 240) {
			uint8_t p0 = (ppu->bg_tile_lsb & 0x8000) ? 1 : 0;
			uint8_t p1 = (ppu->bg_tile_msb & 0x8000) ? 1 : 0;
			uint8_t a0 = (ppu->bg_attr_lsb & 0x80) ? 1 : 0;
			uint8_t a1 = (ppu->bg_attr_msb & 0x80) ? 1 : 0;
			uint8_t color = (a1 << 3) | (a0 << 2) | (p1 << 1) | p0;

			uint32_t index = ppu->scanline * 256 + (ppu->dot - 1);

			uint8_t bg_color = color;
			uint8_t sprite_color = ppu->sprite_pixels[ppu->dot - 1];

			uint8_t final_color = bg_color;

			if(sprite_color && ((bg_color & 3) == 0 || !(ppu->mask & 0x04))) {
				// Sprite above BG, or BG is transparent
				final_color = sprite_color;
			}

			if(ppu->sprite_zero_flags[ppu->dot - 1] &&
				(bg_color & 3) && (sprite_color & 0x1f) &&
				(ppu->mask & 0x18)) {
				ppu->status |= PPU_STATUS_SPRITE0_HIT;
			}

			ppu->pixels[index] = final_color;

		}
	}

	if(ppu->scanline < 240 && ppu->dot == 257) {
		ppu_render_sprites(state);
	}

	if(ppu->scanline < 240 || ppu->scanline == 261) {
		if(ppu->dot == 256) {
			increment_x(state);
			increment_y(state);
		} else if(ppu->dot == 328) {
			increment_x(state);
		}
	}

	ppu->dot++;

	if(ppu->scanline == 261 && ppu->dot == 339 && ppu->frame_even) {
		if(ppu->mask & (PPU_MASK_SHOW_BG | PPU_MASK_SHOW_SPRITES)) {
			ppu->dot++;
		}
	}

	if(ppu->dot > 340) {
		ppu->dot = 0;
		ppu->scanline++;

		if(ppu->scanline > 261) {
			ppu->scanline = 0;
			ppu->frame_even ^= 1;
		}
	}

	// Clear secondary OAM at start of scanline
	if(ppu->scanline < 240 || ppu->scanline == 261) {
		if(ppu->dot == 1) {
			ppu_clear_sec_oam(state);
		}
	}

	// Perform sprite evaluation
	if(ppu->scanline < 240 && ppu->dot == 65) {
		ppu_eval_sprites(state);
	}
}


static void ppu_dma_4014(struct nes_state *state, uint8_t page) {
	uint32_t base = page << 8;

	for(int i = 0; i < 256; i++) {
		uint8_t value = memory_read_dma(state, base + i);
		state->ppu.oam[i] = value;

		ppu_tick(state);
		ppu_tick(state);
		ppu_tick(state);
		// apu_tick(state);
		state->cycle++;
	}

	// DMA takes 513 cycles on even CPU cycles, 514 on odd
	int extra = (state->cycle & 1) ? 514 : 513;
	for(int i = 0; i < extra; i++) {
		ppu_tick(state);
		ppu_tick(state);
		ppu_tick(state);
		// apu_tick(state);
		state->cycle++;
	}
}