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.
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: a first pattern table at $0000-$0FFF and a second pattern table at $1000-$1FFF. Occasionally, these are nicknamed the "left" and "right" pattern tables based on how emulators with a debugger display them. (See #Display convention below.)
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 display 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 shown 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.
