For one reason or another, [Dragao] has an old Sonic The Hedgehog cartridge that throws an illegal instruction somewhere in the Marble Zone stage. While the cause of this illegal instruction is probably cosmic rays, how to repair this cartridge isn’t quite as clear. It can be done, though, using BIOS chips from an old computer.
[Dragao] got the idea of repairing this cartridge from Game Boy flash carts. These cartridges use chips that are a simple parallel interface to the address and data lines of the Game Boy’s CPU, and Sega Genesis / Mega Drive flash cart would work the same way. The problem was finding old DIP flash chips that would work. He eventually found some 8-bit wide chips on the motherboard of an old computer, and by stacking the chips, he had a 16-bit wide Flash chip.
To program the chips, [Dragao] wired everything up to an Arduino Mega, put a ROM on the chip, and wired it up to the old Sega cartridge. Surprisingly or unsurprisingly, everything worked, and now [Dragao] has a fully functioning copy of Sonic The Hedgehog.