The arcade games "Satan's Hollow" and "Tron" both use the same hardware, the Bally/Midway MCR2 boardset, but expect various things to be in different places. Most notably, the joystick and buttons are not wired to the same ports on both games.
It was just a matter of reverse engineering enough of the roms to disable the checksum check, and finding all of the locations where those ports are checked, and patching that code. It sounds simple, but it actually required a good deal of effort... even though the resulting patch is only 87 bytes large.
This was a response to a previous hack I had done, where I put an unmodified Satan's Hollow boardset in a Tron Mini arcade cabinet, and rewired the cabinet to match the game. This was the wrong solution to the problem, since I was then unable to put Tron back into it easily, and I had to rewire the whole thing.
This project is notable since it was the first non-Pac-Man game I hacked.