Picture Processing Unit: Difference between revisions

Revision as of 19:49, 20 November 2009

Revision as of 03:17, 17 June 2010

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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 (computer graphics)|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 [[Personal computer|PCs]]. A background is defined by a simple data structure called a nametable., It'swhich is basicallyessentially a two dimensional array. where eachThe entryinteger isvalue thein tileeach numberarray ofslot onecorresponds tileto ina thetile backgroundnumber, and the entry'sindex positionvalues isof this slot correspond to the tile's placeintended onx/y theposition on 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.