Default tiling in C&C maps

[BOfH]

Hi all, for my own entertainment, I've been digging into C&C and trying to draw the maps by extracting the various files from the game.  It passes the time.  I see in C&C there is a bin file describing the tiles to be layed out but a lot of the tiles are empty (ID 0, tile 255).  I understand that there's a 4x4 tile default background that's layered before the tilesets are applied?  What I can't seem to figure out is where the data comes from to draw the default background?