pub const VRAM_SIZE: usize = 8192;
pub const HRAM_SIZE: usize = 128;
pub const PALETTE_SIZE: usize = 4;
pub const RGBA_SIZE: usize = 4;
/// The number of tiles that can be store in Game Boy's
/// VRAM memory according to specifications.
pub const TILE_COUNT: usize = 384;
/// The width of the Game Boy screen in pixels.
pub const SCREEN_WIDTH: usize = 160;
/// The height of the Game Boy screen in pixels.
pub const SCREEN_HEIGHT: usize = 154;
/// Represents the Game Boy PPU (Pixel Processing Unit) and controls
/// all of the logic behind the graphics processing and presentation.
/// Should store both the VRAM and HRAM together with the internal
/// graphic related registers.
/// Outputs the screen as a RGBA 8 bit frame buffer.
///
/// # Basic usage
/// ```rust
/// let ppu = Ppu::new();
/// ppu.tick();
/// ```
pub struct Ppu {
/// The 8 bit based RGBA frame buffer with the
/// processed set of pixels ready to be displayed on screen.
pub frame_buffer: Box<[u8; SCREEN_WIDTH * SCREEN_HEIGHT * RGBA_SIZE]>,
/// Video dedicated memory (VRAM) where both the tiles and
/// the sprites are going to be stored.
pub vram: [u8; VRAM_SIZE],
/// High RAM memory that should provide extra speed for regular
/// operations.
pub hram: [u8; HRAM_SIZE],
/// The current set of processed tiles that are store in the
/// PPU related structures.
tiles: [[[u8; 8]; 8]; TILE_COUNT],
/// The palette of colors that is currently loaded in Game Boy.
palette: [[u8; RGBA_SIZE]; PALETTE_SIZE],
/// The scroll Y register that controls the Y offset
/// of the background.
scy: u8,
/// The scroll X register that controls the X offset
/// of the background.
scx: u8,
/// The current scan line in processing, should
/// range between 0 (0x00) and 153 (0x99), representing
/// the 154 lines plus 10 extra v-blank lines.
line: u8,
switch_bg: bool,
bg_map: bool,
bg_tile: bool,
switch_lcd: bool,
/// The current execution mode of the PPU, should change
/// between states over the drawing of a frame.
mode: PpuMode,
/// Internal clock counter used to control the time in ticks
/// spent in each of the PPU modes.
mode_clock: u16,
}
pub enum PpuMode {
OamRead,
VramRead,
Hblank,
VBlank,
}
impl Ppu {
pub fn new() -> Ppu {
Ppu {
frame_buffer: Box::new([0u8; SCREEN_WIDTH * SCREEN_HEIGHT * RGBA_SIZE]),
vram: [0u8; VRAM_SIZE],
hram: [0u8; HRAM_SIZE],
tiles: [[[0u8; 8]; 8]; TILE_COUNT],
palette: [[0u8; RGBA_SIZE]; PALETTE_SIZE],
scy: 0x0,
scx: 0x0,
line: 0x0,
switch_bg: false,
bg_map: false,
bg_tile: false,
switch_lcd: false,
mode: PpuMode::OamRead,
mode_clock: 0,
}
}
pub fn clock(&mut self, cycles: u8) {
self.mode_clock += cycles as u16;
match self.mode {
PpuMode::OamRead => {
if self.mode_clock >= 204 {
self.mode_clock = 0;
self.mode = PpuMode::VramRead;
}
}
PpuMode::VramRead => {
if self.mode_clock >= 172 {
self.render_line();
self.mode_clock = 0;
self.mode = PpuMode::Hblank;
}
}
PpuMode::Hblank => {
if self.mode_clock >= 204 {
self.line += 1;
// in case we've reached the end of the
// screen we're now entering the v-blank
if self.line == 143 {
self.mode = PpuMode::VBlank;
// self.drawData @todo implement this one
} else {
self.mode = PpuMode::OamRead;
}
self.mode_clock = 0;
}
}
PpuMode::VBlank => {
if self.mode_clock >= 456 {
self.line += 1;
// in case the end of v-blank has been reached then
// we must jump again to the OAM read mode and reset
// the scan line counter to the zero value
if self.line == 153 {
self.mode = PpuMode::OamRead;
self.line = 0;
}
self.mode_clock = 0;
}
}
}
}
pub fn read(&mut self, addr: u16) -> u8 {
match addr & 0x00ff {
0x0040 => {
let value = if self.switch_bg { 0x01 } else { 0x00 }
| if self.bg_map { 0x08 } else { 0x00 }
| if self.bg_tile { 0x10 } else { 0x00 }
| if self.switch_lcd { 0x80 } else { 0x00 };
value
}
0x0042 => self.scy,
0x0043 => self.scx,
0x0044 => self.line,
addr => panic!("Reading from unknown PPU location 0x{:04x}", addr),
}
}
pub fn write(&mut self, addr: u16, value: u8) {
match addr & 0x00ff {
0x0040 => {
self.switch_bg = value & 0x01 == 0x01;
self.bg_map = value & 0x08 == 0x08;
self.bg_tile = value & 0x10 == 0x10;
self.switch_lcd = value & 0x80 == 0x80;
}
0x0042 => self.scy = value,
0x0043 => self.scx = value,
0x0047 => {
for index in 0..PALETTE_SIZE {
match (value >> (index * 2)) & 3 {
0 => self.palette[index] = [255, 255, 255, 255],
1 => self.palette[index] = [192, 192, 192, 255],
2 => self.palette[index] = [96, 96, 96, 255],
3 => self.palette[index] = [0, 0, 0, 255],
color_index => panic!("Invalid palette color index {:04x}", color_index),
}
}
}
addr => panic!("Writing in unknown PPU location 0x{:04x}", addr),
}
}
/// Updates the tile structure with the value that has
/// just been written to a location on the VRAM associated
/// with tiles.
pub fn update_tile(&mut self, addr: u16, _value: u8) {
let addr = addr & 0x1ffe;
let tile_index = (addr >> 4) & 0x01ff;
let y = (addr >> 1) & 0x0007;
let mut mask;
for x in 0..8 {
mask = 1 << (7 - x);
self.tiles[tile_index as usize][y as usize][x as usize] =
if self.vram[addr as usize] & mask > 0 {
0x1
} else {
0x0
} | if self.vram[(addr + 1) as usize] & mask > 0 {
0x2
} else {
0x0
}
}
}
fn render_line(&mut self) {
let mut map_offset: usize = if self.bg_map { 0x1c00 } else { 0x1800 };
map_offset += (((self.line + self.scy) & 0xff) >> 3) as usize;
// calculates the sprite line offset by using the SCX register
// shifted by 3 meaning as the tiles are 8x8
let mut line_offset: usize = (self.scx >> 3) as usize;
let y = ((self.scy + self.line) & 0x07) as usize;
let mut x = (self.scx & 0x07) as usize;
// calculates the index of the initial tile in drawing,
// if the tile data set in use is #1, the indices are
// signed, then calculates a real tile offset
let mut tile_index = self.vram[map_offset + line_offset] as usize;
if self.bg_tile && tile_index < 128 {
tile_index += 256;
}
// calculates the frame buffer offset position assuming the proper
// Game Boy screen width and RGBA pixel (4 bytes) size
let mut frame_offset = self.line as usize * SCREEN_WIDTH * RGBA_SIZE;
for _index in 0..SCREEN_WIDTH {
// in case the end of tile width has been reached then
// a new tile must be retrieved for plotting
if x == 8 {
// resets the tile X position to the base value
// as a new tile is going to be drawn
x = 0;
// calculates the new line tile offset making sure that
// the maximum of 32 is not overflown
line_offset = (line_offset + 1) & 31;
// calculates the tile index nad makes sure the value
// takes into consideration the bg tile value
tile_index = self.vram[map_offset + line_offset] as usize;
if self.bg_tile && tile_index < 128 {
tile_index += 256;
}
}
// obtains the current pixel data from the tile and
// re-maps it according to the current palette
let pixel = self.tiles[tile_index][y][x];
let color = self.palette[pixel as usize];
// set the color pixel in the frame buffer
self.frame_buffer[frame_offset] = color[0];
self.frame_buffer[frame_offset + 1] = color[1];
self.frame_buffer[frame_offset + 2] = color[2];
self.frame_buffer[frame_offset + 3] = color[3];
frame_offset += RGBA_SIZE;
// increments the current tile X position in drawing
x += 1;
}
}
}