There's a whole fun game to played yet today trying to defeat copy protection on old DOS games. Programmers of the time resorted to a lot of complicated tricks

There's a whole fun game to played yet today trying to defeat copy protection on old DOS games. Programmers of the time resorted to a lot of complicated tricks to keep what you're describing from working properly, but assembly of the era is pretty easy to follow and having a 30 year advantage in tooling helps a lot. Hyperspeed was an interesting one. The crack that comes with the Steam version doesn't actually work since it triggers a lockup the first time you use the spindrive, so players just disable the crack and play the game normally with a PDF manual. I spent a long time trying to bypass that copy protection screen and all the checks to see if I'd bypassed it, but every time I thought I'd finally nailed it I ran into a new place the game would intentionally lock up. Ultimately I just truncated the word match table to a single entry so the word would always be the same. That was a single-byte change.