Tilemap Transparency Index Register

Next Register Number	$4C
Readable	Yes
Writable	Yes
Short Description	Index into Tilemap palette (of "transparent" colour).

bits 7-4 = Reserved, must be 0

bits 3-0 = Set the index value (0xF after reset)

The pixel index (bottom 4 bits) from tile graphics is compared before the palette offset is applied to upper 4 bits, so there is always one index of the 0-15 working as "transparent" colour.