Tilemap Transparency Index Register
From SpecNext official Wiki
|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.