2019-nes-emulator/Source/PPU/PPU.cpp

#include "PPU.hpp"
#include "NES.hpp"


const unsigned int tilesPerWidth = 32u;
const unsigned int tilesPerHeight = 30u;

unsigned char reverse(uint8_t b)
{
	b = (b & 0xF0u) >> 4u | (b & 0x0Fu) << 4u;
	b = (b & 0xCCu) >> 2u | (b & 0x33u) << 2u;
	b = (b & 0xAAu) >> 1u | (b & 0x55u) << 1u;
	return b;
}

void PPU::Cycle(uint8_t cpuCyclesElapsed)
{
	for (int cpuCycle = 0; cpuCycle < cpuCyclesElapsed; ++cpuCycle)
	{
		if (scanline < 240u && cycles < 256u)
		{
			// Draw BG
			uint8_t bgTilePositionX = cycles / 8u;
			uint8_t bgTilePositionY = scanline / 8u;
			uint8_t bgBlockX = bgTilePositionX / 4u;
			uint8_t bgBlockY = bgTilePositionY / 4u;
			uint16_t blockAddress = (bgBlockY * 8 + bgBlockX);
			uint8_t attribute = ReadMemory(0x23C0 + blockAddress);

			uint8_t paletteIndex[4] = {
				static_cast<uint8_t>(attribute & 0x03u), // Upper Left
				static_cast<uint8_t>((attribute & 0x0Cu) >> 2u), // Upper Right
				static_cast<uint8_t>((attribute & 0x30u) >> 4u), // Bottom Left
				static_cast<uint8_t>((attribute & 0xC0u) >> 6u), // Bottom Right
			};

			// Which quadrant of block are we in?
			uint8_t blockX = (bgTilePositionX % 4u) / 2;
			uint8_t blockY = (bgTilePositionY % 4u) / 2;

			uint8_t pIndex = paletteIndex[(blockY << 1u) | blockX];

			uint8_t bgTileIndex = ram[bgTilePositionY * tilesPerWidth + bgTilePositionX];
			uint8_t bgPixelWithinTileX = cycles % 8u;
			uint8_t bgPixelWithinTileY = scanline % 8u;
			uint16_t bgTileAddress = (0x1000u | (bgTileIndex << 4u)) + bgPixelWithinTileY;

			int screenPixelX = (bgTilePositionX * 8) + bgPixelWithinTileX;
			int screenPixelY = (bgTilePositionY * 8) + bgPixelWithinTileY;

			uint8_t bgPixelColorLsb =
				(ReadMemory(bgTileAddress) & (1u << (7u - bgPixelWithinTileX))) >> (7u - bgPixelWithinTileX);
			uint8_t bgPixelColorMsb =
				(ReadMemory(bgTileAddress + 8) & (1u << (7u - bgPixelWithinTileX))) >> (7u - bgPixelWithinTileX);

			uint8_t bgPixelColor = bgPixelColorMsb << 1u | bgPixelColorLsb;

			uint8_t systemPaletteIndex = paletteIndexes[(4 * pIndex) + bgPixelColor];

			uint32_t color = nes->palette[systemPaletteIndex].GetValue();

			video[screenPixelY * VIDEO_WIDTH + screenPixelX] = color;

			// Draw sprites - lower index = higher priority
			for (int sprite = 63; sprite >= 0; --sprite)
			{
				uint8_t spritePositionY = oam[4 * sprite + 0];
				uint8_t spriteTileIndex = oam[4 * sprite + 1];
				uint8_t spriteAttributes = oam[4 * sprite + 2];
				uint8_t spritePositionX = oam[4 * sprite + 3];

				uint8_t spritePalette = spriteAttributes & 0x3u;

				bool spriteWithinScanline = (scanline >= spritePositionY) && (scanline < (spritePositionY + 8));
				bool spriteWithinCycle = (cycles >= spritePositionX) && (cycles < (spritePositionX + 8));

				// Sprite on scanline
				if (spriteWithinScanline && spriteWithinCycle)
				{
					uint8_t spritePixelWithinTileY = scanline - spritePositionY;
					uint8_t spritePixelWithinTileX = cycles - spritePositionX;
					uint16_t spriteTileAddress = (spriteTileIndex << 4u) + spritePixelWithinTileY;

					uint8_t pixelX = ((spritePositionX) + spritePixelWithinTileX) % 256;
					uint8_t pixelY = ((spritePositionY) + spritePixelWithinTileY) % 240;

					uint8_t spriteLsb = ReadMemory(spriteTileAddress);
					uint8_t spriteMsb = ReadMemory(spriteTileAddress + 8);
					uint8_t spriteChr = spriteMsb | spriteLsb;

					// Flip horizontally
					if (spriteAttributes & 0x40u)
					{
						spriteLsb = reverse(spriteLsb);
						spriteMsb = reverse(spriteMsb);
					}

					uint8_t spritePixelColorLsb =
						(spriteLsb & (1u << (7u - spritePixelWithinTileX))) >> (7u - spritePixelWithinTileX);
					uint8_t spritePixelColorMsb =
						(spriteMsb & (1u << (7u - spritePixelWithinTileX))) >> (7u - spritePixelWithinTileX);

					uint8_t spritePixelColor = spritePixelColorMsb << 1u | spritePixelColorLsb;

					uint8_t spriteSystemPaletteIndex = paletteIndexes[0x10 + (4 * spritePalette) + spritePixelColor];

					uint32_t spriteColor = nes->palette[spriteSystemPaletteIndex].GetValue();

					// Do not draw empty pixels of sprite
					if (spriteChr != 0u)
					{
						if (sprite == 0)
						{
							if (bgPixelColor != 0u)
							{
								ppuStatus.sprite0Hit = 1u;
							}
						}

						video[pixelY * VIDEO_WIDTH + pixelX] = spriteColor;
					}
				}
			}
		}
		else if (scanline == 241u && cycles == 0u)
		{
			ppuStatus.vblankStarted = 1u;

			if (ppuCtrl.nmiEnable)
			{
				nes->nmi = true;
			}
		}
		else if (scanline == 261u)
		{
			ppuStatus.sprite0Hit = 0u;
			ppuStatus.vblankStarted = 0;
		}

		++cycles;

		if (cycles == 341u)
		{
			cycles = 0;

			++scanline;

			if (scanline == 262u)
			{
				scanline = 0;
			}
		}
	}
}

void PPU::WriteRegister(PpuRegister reg, uint8_t value)
{
	switch (reg)
	{
		case PpuRegister::PPUCTRL:
		{
			ppuCtrl.SetByte(value);
			regT |= (value & 0x3u) << 10u;

			break;
		}
		case PpuRegister::PPUMASK:
		{
			ppuMask.SetByte(value);

			break;
		}
		case PpuRegister::PPUSTATUS:
		{
			ppuStatus.SetByte(value);
			regW = 0u;

			break;
		}
		case PpuRegister::OAMADDR:
		{
			oamAddr = value;

			break;
		}
		case PpuRegister::OAMDATA:
		{
			oamData = value;

			break;
		}
		case PpuRegister::PPUSCROLL:
		{
			ppuScroll = value;

			if (regW == 0u)
			{
				regT |= (value & 0xF8u) >> 3u;
				regX = value & 0x07u;

				regW = 1u;
			}
			else if (regW == 1u)
			{
				regT |= (value & 0x07u) << 12u;
				regT |= (value & 0x38u) << 2u;
				regT |= (value & 0xC0u) << 2u;

				regW = 0u;
			}

			break;
		}
		case PpuRegister::PPUADDR:
		{
			// MSB
			if (!ppuAddrW)
			{
				ppuAddr = value << 8u;

				ppuAddrW = true;
			}

			// LSB
			else
			{
				ppuAddr |= value;

				ppuAddrW = false;
			}

			if (regW == 0u)
			{
				regT |= (value & 0x3Fu) << 8u;
				regT &= 0x7Fu;
				regW = 1u;
			}
			else if (regW == 1u)
			{
				regT |= value;
				regV = regT;
				regW = 0u;
			}

			break;
		}
		case PpuRegister::PPUDATA:
		{
			WriteMemory(ppuAddr, value);

			ppuAddr += (ppuCtrl.vramAddrIncrement) ? 32u : 1u;

			break;
		}
	}
}

uint8_t PPU::ReadRegister(PpuRegister reg)
{
	uint8_t byte{};

	switch (reg)
	{
		case PpuRegister::PPUCTRL:
		{
			byte = ppuCtrl.GetByte();

			break;
		}
		case PpuRegister::PPUMASK:
		{
			byte = ppuMask.GetByte();

			break;
		}
		case PpuRegister::PPUSTATUS:
		{
			byte = ppuStatus.GetByte();
			ppuStatus.vblankStarted = 0u;

			break;
		}
		case PpuRegister::OAMADDR:
		{
			byte = oamAddr;

			break;
		}
		case PpuRegister::OAMDATA:
		{
			byte = oamData;

			break;
		}
		case PpuRegister::PPUSCROLL:
		{
			byte = ppuScroll;

			break;
		}
		case PpuRegister::PPUADDR:
		{
			byte = ppuAddr;

			break;
		}
		case PpuRegister::PPUDATA:
		{
			byte = ppuData;

			break;
		}
	}

	return byte;
}

void PPU::WriteMemory(uint16_t address, uint8_t value)
{
	nes->Write(BusSource::PPU, address, value);
}

uint8_t PPU::ReadMemory(uint16_t address)
{
	return nes->Read(BusSource::PPU, address);
}