On the title screen, press 'Up, Down, Left, Right' on the d-pad. You will hear a chime. While holding the button that is the Genesis A button, press start. Go into the options menu. From there, play ...

Beats the hell out of the GBA port of Sonic 1. Example, this one is playable. They should have used oldskool early 90s Sonic on the cover, it would have sold more that way.