PPU pattern tables

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Revision as of 03:30, 31 January 2023 by Rainwarrior (talk | contribs) (should probably mention CHR here, especially since it seems to be the preferred terminology for this in NES circles)
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The pattern table is an area of memory connected to the PPU that defines the shapes of tiles that make up backgrounds and sprites. This data is also known as CHR, and the memory attached to the PPU which contains it may either be CHR-ROM or CHR-RAM. CHR comes from "character", as related to computer text displays where each tile might represent a single letter character.

Each tile in the pattern table is 16 bytes, made of two planes. The first plane controls bit 0 of the color; the second plane controls bit 1. Any pixel whose color is 0 is background/transparent (represented by '.' in the following diagram):

Bit Planes            Pixel Pattern
$0xx0=$41  01000001
$0xx1=$C2  11000010
$0xx2=$44  01000100
$0xx3=$48  01001000
$0xx4=$10  00010000
$0xx5=$20  00100000         .1.....3
$0xx6=$40  01000000         11....3.
$0xx7=$80  10000000  =====  .1...3..
$0xx8=$01  00000001  =====  ...3.22.
$0xx9=$02  00000010         ..3....2
$0xxA=$04  00000100         .3....2.
$0xxB=$08  00001000         3....222
$0xxC=$16  00010110
$0xxD=$21  00100001
$0xxE=$42  01000010
$0xxF=$87  10000111

The pattern table is divided into two 256-tile sections: $0000-$0FFF, nicknamed "left", and $1000-$1FFF, nicknamed "right". The nicknames come from how emulators with a debugger display the pattern table. Traditionally, they are displayed as two side-by-side 128x128 pixel sections, each representing 16x16 tiles from the pattern table, with $0000-$0FFF on the left and $1000-$1FFF on the right.

An important aspect of a mapper's capability is how finely it allows bank switching parts of the pattern table.


PPU addresses within the pattern tables can be decoded as follows:

DCBA98 76543210
|||||| |||||+++- T: Fine Y offset, the row number within a tile
|||||| ||||+---- P: Bit plane (0: "lower"; 1: "upper")
|||||| ++++----- C: Tile column
||++++---------- R: Tile row
|+-------------- H: Half of pattern table (0: "left"; 1: "right")
+--------------- 0: Pattern table is at $0000-$1FFF

The value written to PPUCTRL ($2000) controls whether the background and sprites use the left half ($0000-$0FFF) or the right half ($1000-$1FFF) of the pattern table. PPUCTRL bit 4 applies to backgrounds, bit 3 applies to 8x8 sprites, and bit 0 of each OAM entry's tile number applies to 8x16 sprites.

For example, if rows of a tile are numbered 0 through 7, row 1 of tile $69 in the left pattern table is stored with plane 0 in $0691 and plane 1 in $0699.