I
Well, when looking at a map file, near the beginning of it, you will see lots of 00 80 bytes, sometimes a few others. like 00 00. Those are the movement permissions. They are layed out Left to Right, Top to Bottom.
I have found some more info here: http://projectpokemon.org/wiki/Map_structure
I have found some more info here: http://projectpokemon.org/wiki/Map_structure