Oh sorry. It uses Romdiskfs (yeah from GPF who is home at this site ;) )
It will support allmost all flashcards which goes trough gba-slot and have 32MB sram for holding a gba rom...So: MK2/3,NinjaDS otr GBAMP wouldnt work for sure...last one just because romsize > 4MB.
The reason for that is its much faster then fat :)
But i will make a hybrid type between KOS and FAT which means it has a filesystem inside a FAT. Then it will work more devices. Sources are free so i just need time. When i finished the FS i will post source. i think fat isnt best idea for games because u can easily change content. And KOSFS or GBFS is much more handy but worse supported. So a mix must be done.
