Sprite Status/Slot Select

Number	$303B
Decimal
Short desc.	Selects active sprite slot and palette address, and returns sprite status.
Bit Mask
Readable	Yes
Writable	Yes
Subsystem	Sprites

Used to select the sprite slot configured by Sprite Attribute Upload ($xx57 / 87) and Sprite Pattern Upload ($xx5B / 91). Once written, these ports may auto-increment the sprite number they are addressing. This auto-increment happens independently and so the two ports may end up addressing different sprites as a result. A write to this port will resynchronise them to the same sprite.

Also "initializes the address of the palette".

When read, returns sprite status information as follows:

Bit	Effect
2-7	Reserved
1	Max sprites per line flag (set if more than 12 sprites share a line)
0	Collision flag (set if non transparent areas of any 2 sprites overlap)

It is not possible to obtain from the sprite hardware the numbers or IDs of the sprites which overlapped. The collision flag should thus be seen as initial broad phase collision detection.