PPU OAM

OAM (Object Attribute Memory) contains a display list of up to 64 sprites, where each sprite's information occupies 4 bytes.

Byte 0

Y position of top of sprite

Sprite data is delayed by one scanline; you must subtract 1 from the sprite's Y coordinate before writing it here. Hide a sprite by writing any values in $EF-$FF here.

Byte 1

Tile index number

For 8x8 sprites, the tile number of this sprite. For 8x16 sprites:

76543210
||||||||
|||||||+- Bank ($0000 or $1000) of tiles
+++++++-- Tile number of top of sprite (0 to 254; bottom half gets the next tile)

Thus, the pattern table memory map for 8x16 sprites looks like this:

• $00: $0000-$001F
• $01: $1000-$101F
• $02: $0020-$003F
• $03: $1020-$103F
• $04: $0040-$005F
  [...]
• $FE: $0FE0-$0FFF
• $FF: $1FE0-$1FFF

Byte 2

Attributes

76543210
||||||||
||||||++- Palette (4 to 7) of sprite
|||+++--- Unimplemented
||+------ Priority (0: in front of background; 1: behind background)
|+------- Flip sprite horizontally
+-------- Flip sprite vertically

Byte 3

X position of left side of sprite

X-scroll values of F9-FF do NOT result in the sprite wrapping around to the left side of the screen.

DMA

Most programs write to a copy of OAM somewhere in CPU addressable RAM (often $0200-$02FF) and then copy it to OAM each frame using the OAM_DMA ($4014) register. This takes 513 cycles to copy 256 bytes from this memory into $2004, where an unrolled LDA/STA loop would usually take four times as long.

Sprite overlapping

Priority between sprites is determined by their address inside OAM. So to have a sprite displayed in front of another sprite in a scanline, the sprite data that occurs first will overlap any other sprites before it. For example, when sprites at OAM $0C and $28 overlap, the sprite at $0C will appear in front.