With Skate or Die, it’d pull a byte off the genuine disk and use it to decrypt (xor) a loop immediately after the current PC.

I think it went through about five iterations.

My crack was to step it with the original disk to decrypt the encrypted part and modify the code to jump over the decryption. It was pretty sophisticated for the day.

To another extreme, Ocean disk games had a “IsGenuineDisk” style function. It returned 1 or 0 in the accumulator. I could crack it in under a minute with a disk sector editor.

Oddly, I ended up doing Mac copy protection years later.