Karateka is not just a classic game, but one of the most well-documented thanks to Jordan Mechner's memoirs and his habit for maintaining archives. 34 years after its release, Charles Mangin studied the game's source code and patched it to allow a second player to control the enemies—effectively adding a vs. battle mode.
I've taught myself 6502 assembly after getting back into the Apple II, through the thriving community online. The idea of a two player version of Karateka came back to me while at KansasFest a couple of years ago. I noodled a little on it back then, getting distracted by finding the code that created the unique music in the game. Long story short: I finally found the places in the game code that needed patching to allow a second player to control the enemies in the game, and create a functioning two player version of Karateka. The resulting patch is only 42 bytes long
42, the meaning of life! You can play the two-player Karateka at the Internet Archive.
I'd love to see this done to Great Gurianos (sometimes renamed Gladiator), another 80s' fighter with an interesting combat system whose attract mode suggested vs. battles that were not in the game itself.
