Copy Protection Used on Commodore Amiga Computers
Background
The Amiga floppy controller is unusually flexible since it is implemented in the custom chips of the Amiga itself. It can read and write raw bit sequences directly from and to the disk via DMA or programmed I/O at 500 (double density) or 250 kbit/s (single density or GCR). MFM or GCR were the two most commonly used formats though in theory any run-length limited code could be used. It also provides a number of convenient features, such as sync-on-word (in MFM coding, $4489 is usually used as the sync word). MFM encoding/decoding is usually done with the blitter - one pass for decode, three passes for encode. Normally the entire track is read or written in one shot, rather than sector-by-sector; this made it possible to get rid of most of the inter-sector gaps that most floppy disk formats need to safely prevent the "bleeding" of a written sector into the previously-existing header of the next sector due to speed variations of the drive. If all sectors and their headers are always written in one go, such bleeding is only an issue at the end of the track (which still must not bleed back into its beginning), so that only one gap per track is needed. This way, for the native Amiga disk format, the raw storage capacity of 3.5 inch DD disks was increased from the typical 720 KB to 880 KB, although the less-than-ideal file system of the earlier Amiga models reduced this again to ca. 830 KB of actual payload data.
The Amiga is limited to writing disks at a single bitrate, though, so this is the most commonly exploited method of copy protection.
CopyLock - Rob Northen Computing
This protection was very prolific, used by many publishers on hundreds of titles. When the protection is called, a sector is read and the time it takes to do so is measured. This will be different on a copy vs. the original because the sector will be recorded at a different bit rate. It takes this and some other checksum information and generates a key. This "key" is returned and acted on in the protected program to varying degrees.
Earlier versions of the protection don't have the timing changes, but may instead contain a short or long track that is checked for length, as well as checks for exact gap length.
It is used on the Amiga, and Atari ST.
C64 Serial Registry