Copy Protection Used on PC-Compatible Computers
Background
The PC floppy disk system is based on the off-the-shelf NEC uPD765A (or compatibles, such as the Intel 8272A) floppy disk controller chip. This chip controls all communication between the disk drive and the computer, which limits what the PC is able to write to the floppy disk to the standards provided by this chip. These limitations are what is exploited for copy protection. The disk drives most commonly found on the PC are 5.25" and 3.5" in double-density or high-density. Only the original IBM PC (1981) used a single-sided 5.25 disk drive - all following models had double-sided drives.
IBM INTERLOCK
Electronic Arts used "IBM INTERLOCK"
SuperLok
C64 Serial Registry