http://wiki.qhimm.com/view/FF7/SavemapI'll also send you a list of bytes that Aali sent me detailing what the world map uses. Battles can only affect bank 1 (field 1/2). Field can be seen with Makou.
Remember that Field vars:
1/2 are Bank 1
3/4 are Bank 2
5/6 are temporary vars in game and not saved. They reset to 0 every time you enter new field.
8/9 & A cannot be written to.
B/C is Bank 3
D/E is Bank 4
7/F is Bank 5
Field vars 1,3,5,7,B,D are for one-byte writes
Field vars 2,4,6,C,E,F are for two-byte writes
Remember that 1/2, 3/4, 5/6, B/C, D/E and 7/F share the SAME 256 bytes.
So if you write to var [1][10] you are ALSO writing to [2][10].
And if you write two bytes to [2][10] you are ALSO writing to [1][10] and [1][11]
From my own tests, Field B/C (Bank 3) is the safest bet. I am using [B/C][243] through to [B/C][255]
For my Soldier mod. So if you want your mod to remain compatible with mine, avoid those and
start at [B/C][242] I am pretty sure a lot of those bytes down are unused.
edit.
Oh... PSX again. I keep assuming PC. Well, you are free to use B/C[255] down really. Can't see my Soldier mod being used in PSX game, though you never know.