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)

Byte 2

Attributes

76543210
||||||||
||||||++- Palette (4 to 7) of sprite
|||+++--- Unimplemented, reads back as 0
||+------ 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.

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.