PPU attribute tables

The attribute table is a 64-byte array at the end of each nametable that controls which palette is assigned to each part of the background.

Each attribute table, starting at $23C0, $27C0, $2BC0, or $2FC0, is arranged as an 8x8 byte array:

       2xx0    2xx1    2xx2    2xx3    2xx4    2xx5    2xx6    2xx7
     ,-------+-------+-------+-------+-------+-------+-------+-------.
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
2xC0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
2xC8:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
2xD0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
2xD8:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
2xE0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
2xE8:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
2xF0:| - + - | - + - | - + - | - + - | - + - | - + - | - + - | - + - |
     |   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     +-------+-------+-------+-------+-------+-------+-------+-------+
2xF8:|   .   |   .   |   .   |   .   |   .   |   .   |   .   |   .   |
     `-------+-------+-------+-------+-------+-------+-------+-------'

,---+---+---+---.
|   |   |   |   |
+ D1-D0 + D3-D2 +
|   |   |   |   |
+---+---+---+---+
|   |   |   |   |
+ D5-D4 + D7-D6 +
|   |   |   |   |
`---+---+---+---'

Each byte controls the palette of a 32x32 pixel part of the nametable and is divided into four 2-bit areas. Each area covers four tiles or 16x16 pixels, the size of a [?] block in Super Mario Bros. Given palette numbers topleft, topright, bottomleft, bottomright, each in the range 0 to 3, the value of the byte is

value = (topleft << 0) | (topright << 2) | (bottomleft << 4) | (bottomright << 6)

Most games for the NES use 16x16 pixel metatiles (size of Super Mario Bros. ? block) or 32x32 pixel metatiles (width of SMB pipe) in order to align the map with the attribute areas.

Nametable tiles are 8x8 pixels, and the left-side clipping window in PPUMASK ($2001) is 8 pixels wide, but attribute table tiles are 16x16 pixels. This is why games that use the horizontal or vertical mirroring mode for diagonal scrolling often have color artifacts on one side of the screen (on the right side in Super Mario Bros. 3; on the trailing side of the scroll in Kirby's Adventure; at the top and bottom in Super C).