The "OS" is pogoshell, and it is a very handy program for the gba. in addition to.mod playback, it also has loopynes built into the program, so you can just load nes roms directly from pogoshell. It supports save compression, so you can have multiple saves for each rom you have on your flash card. It even saves the sram so if you flash a game save it, and delete it, you can reload the rom and still have your save file there. It is very handy for flash linkers and is pretty much a must have for flash cart owners.