Picture Processing Unit
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The PPU (Picture Processing Unit), more specifically known as Ricoh RP2C02 (NTSC version) / RP2C07 (PAL version), is the integrated circuit in the Nintendo Entertainment System responsible for generating video signals from graphic data stored in memory.
The chip is known for its effective use of memory, using very little memory to store graphical data. It was rather advanced for its time when the Famicom (Japanese version of the Nintendo Entertainment System) was released, having full sprite support, movable backgrounds, and many colors on screen at the same time. To compete with other video game systems, like the graphically superior Sega Master System, Nintendo also extended the PPU's technical capabilities through the use of mappers, which were placed on the game cartridge. The mappers added more memory or could bank switch data into the PPU's address space, making it possible to create more advanced graphics, using more colors and bigger tile sets.
- 2 kB of external RAM to store tile layout and auxiliary color information for background graphics (commonly referred to as nametables)
- 288 (256+32) bytes of internal DRAM for sprite attribute storage. This is measured in the amount of address space consumed; not every bit of every byte exists in the PPU.
- 32 bytes of internal SRAM for palette storage. As for sprite attribute storage, not all bits exist in the PPU.
- 8 × 8 or 8 × 16 (selectable) sized sprites
- Two external 4 kB tile sets with space for 256 tiles each, usually stored in either RAM or ROM on the game cartridge
- Up to 64 sprites (movable objects) on screen simultaneously (only 8 visible per scan line)
- 25 colors simultaneously (although more colors are possible using programming tricks) from a hardware color palette of 54 colors
- Picture resolution of 256 × 240 pixels (fully visible on PAL, but cropped to 256 × 224 on most NTSC television sets)
The PPU is controlled via eight registers visible in the CPU's address space in the addresses $2000 through $2007. All data and information is passed to the PPU through these, except the raw tile data (there are exceptions, as some games had RAM instead of ROM to store the tile data, and the tiles had to be written each time), which is hardwired to the PPU's address space. The PPU uses the tile graphics data together with information stored by the program in the PPU's RAM, such as color and position, to render the final graphical output to the screen.
The lowest graphical components the PPU operates with are tiles, which are blocks of 8×8 or 8×16 pixels. The tiles are stored in a ROM chip on the game cartridge. They are the basic building blocks, used to create larger moving objects, or large static backgrounds.
Due to the small size of NES sprites, most moving objects are made of multiple sprites. Only 8 sprites can be drawn per scanline, and so the PPU contains an "overflow" flag that is set if more than 8 sprites appear on a scanline. However, this flag is buggy and generates both false positives and false negatives, so it was seldom used.
As noted above, some games (mostly early MMC1 titles such as Legend of Zelda and Castlevania) store their graphics data in the main PRG ROM. These have a CHR RAM chip instead of a ROM and pass the data from the PRG ROM to the CHR RAM, the main purpose of this being to produce animated background tiles. The arrival of the MMC3 mapper in 1988 eliminated the need for this as animated tiles could now be banked from the CHR ROM on the fly. The PPU can access up to 8 KB of CHR ROM or RAM at once.
Essentially, the PPU supports two different kinds of drawable objects: movable (sprites) and non-movable (background). Both kinds of objects are composed of tiles, and moreover a sprite and background object can use the same tile. The difference is that a tile used as a sprite can move around, whereas a tile used as a background cannot. There are no collision detection registers for sprites as was common on most game systems of the era.
Sprite data is stored in a special memory called the "Object Attribute Memory", or "OAM" for short, which is a 256-byte memory built into the PPU core. The data stored here is 4 bytes: the position, color and tile, for each of the 64 sprites. This data is used by the PPU to place the sprite when it renders the frame. Background objects, however, are stored in a much less exclusive way, which is more like the way characters are stored in text mode on PCs. A background is defined by a simple data structure called a nametable, which is essentially a two dimensional array. The integer value in each array slot corresponds to a tile number, and the index values of this slot correspond to the tile's intended x/y position on screen. The PPU has, without the use of memory mappers, two nametables, so smooth scrolling between backgrounds is possible.
Once tile data is set up in the nametable, it is a simple matter of adjusting the PPU's X/Y scrolling registers to move the screen around.
A color palette must be defined in order to show graphics on the screen. It is stored in a separate 32 byte location in RAM, known as "palette RAM". Each entry here picks a color from the hardware color palette, which are the predefined colors to choose from. There are four background palettes and four sprite palettes, with three colors each, plus transparency. There is also a background color, which is normally seen behind transparent areas of background tiles. Thus, the PPU can normally draw 25 unique colors on the screen.
- Nintendo Entertainment System technical specifications
- Super Nintendo Entertainment System technical specifications § Video, describing the Super NES's upgraded S-PPU
- List of video game console palettes