Well I couldn't tell you how it exactly works, but from what I am to understand, all the native resolutions of the MAME games are lower than 800 x 600 as it is, even 640x480 (it think). If you are looking for a fuller screen, you can use the GUI (this one I don't know how to do in the mame.ini file) and their is an option for 'Keep Aspect Ratio.' I turned this off for some games. I did this to Missile Command to fill my 34" Widescreen. I don't think you want to do this with all the games though. Some one else? Know anything about details on Resolutions?