Block mappings
From Sega Retro
Block mappings are used to arrange large amounts of 8x8 tiles into larger tiles (16x16, followed by 128x128 or 256x256). These larger tiles are then arranged into the level layout.
16x16 tiles
Games
Format
Each 8x8 tile in the 16x16 block mappings has a two-byte value in the format XYYY, where YYY is the number of the tile to be loaded from the VRAM, and X is the display method for the tile. These are all the possible values for X:
=================================== value pallet orientation =================================== 0 1 normal 1 1 y 2 2 normal 3 2 y 4 3 normal 5 3 y 6 4 normal 7 4 y 8 1 normal 9 1 x A 2 normal B 2 x C 3 normal D 3 x E 4 normal F 4 x