Out of curiousity, I looked into the Sega CDs mode 1 and tried to implement it. I failed horrendously, but I was able to detect the presence of a Sega CD properly, which is good enough for me. My problem, however, is that when you boot the Sega CD in Kega Fusion alongside any revision of Sonic 2 (rev00, rev01, rev02, and my hack), the graphics mess up. Sprites become invisible, animated level art doesn't work, the menu backgrounds don't look right, etc. The screenshots here are from a vanilla Rev01 Sonic 2 rom, with the Sega CD booted alongside it in Kega. Spoiler: Screenshots I was hoping to add an easter egg or something if the Sega CD was detected, but it kind of defeats the point if said hardware makes the game unplayable That said, I can only test this in Kega Fusion, as I don't have any actual hardware to test it on, and Kegas' the only emulator that supports running the Sega CD and a Cartridge at the same time (that I know of). S3K seems to run into trouble here as well. It seems that all uncompressed artwork is scrambled when the Sega CD is running... Spoiler: S3K screenshots Sonic 1 seems to work just fine though... Just to clarify, I'm not actually using the Sega CD here. All the screenshots are from unmodified Sonic 2 and S3K roms.