PPU pattern tables

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.

Encoding of a ½ tile

Each tile in the pattern table is 16 bytes, made of two planes. Each bit in the first plane controls bit 0 of a pixel's color index; the corresponding bit in the second plane controls bit 1.

• If neither bit is set to 1: The pixel is background/transparent.
• If only the bit in the first plane is set to 1: The pixel's color index is 1.
• If only the bit in the second plane is set to 1: The pixel's color index is 2.
• If both bits are set to 1: The pixel's color index is 3.

This diagram depicts how a tile for ½ (one-half fraction) is encoded, with . representing a transparent pixel.

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..
                            .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.

Addressing

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

DCBA98 76543210
---------------
0HNNNN NNNNPyyy
|||||| |||||+++- T: Fine Y offset, the row number within a tile
|||||| ||||+---- P: Bit plane (0: less significant bit; 1: more significant bit)
||++++-++++----- N: Tile number from name table
|+-------------- 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 first pattern table ($0000-$0FFF) or the second pattern table ($1000-$1FFF). 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.

Display convention

It is conventional for debugging emulators' video memory viewers to draw the pattern table as two 16x16-tile grids side by side. They draw the pattern table at $0000-$0FFF on the left and the pattern table at $1000-$1FFF on the right. Each pattern table is commonly represented as a 128 by 128 pixel square, with 16 rows of 16 tiles. Usually the tiles are drawn in left to right, top to bottom, in Western reading order: $00 in the top left, $01 to the right of that, through $0F at the top right, then $10 through $1F on the second row, all the way through $FF at the bottom right. Some emulators have an option to rearrange the view for 8x16 sprites, where the first two rows are $00, $02, $04, ..., $1E, and $01, $03, $05, ..., $1F, and then each pair of rows below that shows another 16 pairs of tiles.