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
Flipping does not change the position of the sprite's bounding box, just the position of pixels within the sprite. In 8x16 mode, vertical flip affects the whole sprite as a unit, not the individual tiles. This behavior differs from the behavior of the unofficial 16x32 and 32x64 pixel sprite sizes on the Super NES, which vertically flip each square sub-region.
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 (+1 on odd 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.
Notes
Each OAM entry is 29 bits wide. The unimplemented bits of each sprite's byte 2 do not exist in the PPU. On PPU revisions that allow reading PPU OAM through $2004, the unimplemented bits of each sprite's byte 2 always read back as 0. This can be emulated by ANDing byte 2 with $E3, either when writing to OAM or when reading back. It has not been determined whether the PPU actually drives these bits low or whether this is the effect of data bus capacitance from reading the last byte of the instruction (LDA $2004, which assembles to AD 04 20).
