User:Blargg/Blargg PPU

Just a rough attempt at striking the right detail level for the main PPU documentation. Posting here (rather than just previewing a local copy) in case someone wants to get an idea of what I'm trying.

Pattern tables

The background and sprites are drawn using small 8x8 pixel images called tiles. There are 512 tiles total, arranged in two pattern tables, each holding 256 tiles. A tile takes 16 bytes of memory, with 2 bits per pixel. The pixels are stored as two bit planes, each 8 bytes. Each byte of a plane represents one row of the tile, with the most significant bit representing the left-hand pixel. The first plane specifies the least significant bit of each pixel. A pixel value of 0 is transparent, while 1, 2, and 3 specify a color from one of the sub-palettes.

Palette

The colors of bckground and sprite pixels are determined by the palette. It consists of 8 sub-palettes; the first four for the background and the remaining four for the sprites. Each sub-palette specifies the colors for tile pixel values of 1 to 3.

The palette uses 32 bytes of VRAM starting at $3F00, with each sub-palette using 4 bytes. Normally the first entry of each sub-palette is ignored, except for the first entry of the first sub-palette, which specifies the backdrop color.

$3F00 Backdrop	$3F01 Color 1	$3F02 Color 2	$3F03 Color 3	Background sub-palette 0
$3F04 Unused	$3F05 Color 1	$3F06 Color 2	$3F07 Color 3	Background sub-palette 1
$3F08 Unused	$3F09 Color 1	$3F0A Color 2	$3F0B Color 3	Background sub-palette 2
$3F0C Unused	$3F0D Color 1	$3F0E Color 2	$3F0F Color 3	Background sub-palette 3
$3F10 Backdrop mirror	$3F11 Color 1	$3F12 Color 1	$3F13 Color 1	Sprite sub-palette 0
$3F14 Unused	$3F15 Color 1	$3F16 Color 2	$3F17 Color 3	Sprite sub-palette 1
$3F18 Unused	$3F19 Color 1	$3F1A Color 2	$3F1B Color 3	Sprite sub-palette 2
$3F1C Unused	$3F1D Color 1	$3F1E Color 2	$3F1F Color 3	Sprite sub-palette 3

Note that $3F00 and $3F10 both set the backdrop color; these two memory locations actually access the same byte of memory. This must be kept in mind when writing to sprite sub-palette 0.

Each palette entry is only 6 bits. The upper 2 bits specify brightness, and the lower 4 hue:

Palette entry	--BB HHHH	Brightness and Hue
bits 0-3	---- HHHH	Hue
bits 4-5	--BB ----	Brightness

[palette goes here, which shows palette and hex values for each color]

Note that a value of $0D should not be used (brightness=0, hue=13), as this results in a "blacker than black" color that causes stability problems on some televisions.

Background

The background is composed of 64x60 tiles, divided into four 32x30 nametables. Each nametable is 960 bytes, with 8-bit tile indicies into one of the two pattern tables as selected by PPUCTRL. Each nametable also has a corresponding 64-byte attribute table, which breaks the tiles into blocks of 2x2 and specifies which background sub-palette to use for each.

Normally there is only enough VRAM for two nametables and attribute tables; the other two are duplicates of these. Some mappers support other schemes, like having a single nametable duplicated in all four positions, or extra VRAM to allow four unique nametables. In the following table, A, B, C, and D refer to the underlying memory the nametables are stored in.

Arrangement	Name
A A B B	Horizontal
A B A B	Vertical
A A A A	One-screen
A B C D	Four-screen