Copy Protection

If you didn’t do anything to prevent it, 400/800 game cartridges could be copied to disk and then just re-loaded at the same address and run from RAM. Cartridges were starting to have some simple copy protection — mostly just code that pretty blatantly scribbled to the ROM space, trying to wipe out other code and cause a crash. In Super Pac-Man I had time to be more clever. There were several levels of protection, including some “bait” code that stomped the ROM, but if you modified the bait then game would notice that and collapse in more subtle ways.

The clever stuff involved decrypting some code in an interrupt handler over several minutes, which (when finally run) changed some constants around in the ROM that in turn caused a crash a while later. It took about two weeks to get all of this working and tested. I’d gotten my start in the games industry by cracking protection on a few disk-based titles, so this was fun to do.