It's quite simple; the "expansion port" is almost identical to the cartridge slot, as far as the presence and behavior of main signals go (the main cart slot has a bunch of stuff that almost never gets used). When the MD runs the BIOS, it pretty much sees the ROM as it would any cart ROM. Put a game there instead of a BIOS program, and it runs the game.Arakon, how the hell did you manage to get it to take a megadrive game in the bios chip?
It can be done, but it was Columns for a reason - putting three BIOS images on a single ROM leaves a fourth of the ROM unused. Since it's a 4Mb ROM, that leaves a big 1Mb for the game. There aren't many 1Mb MD games (fewer than ten I think, and presumably most of them aren't that great). Using a bigger ROM would require soldering additional connections, and would waste more of the ROM. An SMS game can't be done because the M3/SMS mode pin isn't available at the expansion slot, and you'd be limited in your selection there too because SMS games bigger than 48KB require a mapper chip.any way you could add a different game?