Karateka is not just a classic game, but one of the most well-documented thanks to Jordan Mechner's diaries and his archives. 30 years after its release, Charles Mangin studied the game's source code and patched it to add a two-player vs. battle.
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.
