Picture Processing Unit: Difference between revisions

Revision as of 03:34, 31 March 2007

Revision as of 03:36, 31 March 2007

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==Technical information==

The PPU is controlled via eight [[Processor register|registers]] visible in the [[CPU|CPU's]]'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, 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 [[tile]]s, which are blocks of 8×8 or 8×16 pixels. The tiles are stored in a [[ROM]]-chip on the game cartridge. The tiles are the basic building blocks, used to create larger moving objects, or large static backgrounds.

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Essentially, the PPU supports two different kinds of objects: movable (sprites) and non-movable (background). Both kind of objects are basically a tile, 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.

Sprite data is stored in a special memory called the "Sprite-RAM" or "SPR-RAM" 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 [[rendering|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 [[PCPersonal computer|PCs]]s. A background is defined by a simple data structure called a nametable. It's basically a two dimensional array where each entry is the tile number of one tile in the background, and the entry's position is the tile's place on the screen. The PPU has, without the use of memory mappers, two nametables, so smooth scrolling between backgrounds is possible.

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 between. 16 colors can be chosen for sprites, and 16 colors for backgrounds. However, bytes 4, 8 and 12 of the sprite palette, and bytes 0, 4, 8, and 12 of the background palette, are not in use by the PPU. Therefore, the number of actually usable colors is reduced to 25 instead of 32. The first byte of the sprite palette also defines the global background color for both sprites and the background.