Sorry for this "looks-like-clueless" post, but I'm stuck in a thought: in GBA you are able to play GB games, right? The cartriges match in physics and system (Though GBA carts are far more advanced, of course). But when it comes to emulation, most of GBA emulators don't allow GB games to be played, either because of no compatibility or system differences. If a GBA emulator is to emulate every aspect of the console, why most of them just cannot run simple GB games in addition to the GBA ones? I think the only emulator capable of doing such is VisualBoyAdvance.