PPU OAM: Difference between revisions
(→Byte 1: just in case it wasn't clear) |
(→Byte 3: Why use DMA and not STA $2004) |
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X-scroll values of F9-FF do NOT result in the sprite wrapping around to the left side of the screen. | 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. | 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. | |||
Revision as of 02:03, 23 March 2010
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, 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.
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.
