Floppy Format and Structure

Tracks and Sectors

Copy programs have been referred to by these two different types since the first nibblers came out on the Apple II. A little more background is required in how the disk is structured to explain the difference. A track is broken up into a number or sectors, which are further broken up into a header and data portion, and each of these has a filler "gap" at the end to give the drive software time to change modes.

The actual GCR data is structured like this:

  • SYNC (at least 10 bits of '1' in a row)
  • header 0 (8 bytes in DOS)
  • header gap (usually 5 filler bytes, longer for nibbled copies)
  • SYNC
  • sector data 0 (CBM DOS uses 260 bytes)
  • sector gap (usually 5 filler bytes, longer for nibbled copies)
  • SYNC
  • header1... (and on until the end of the track)
  • tail gap.. (unused bytes until it wraps back around to header 0)

Fast copiers

A "fast copier" reads the source disk on a sector-by-sector, normal CBM DOS level, and recreates them onto a new formatted disk. This method produces a "clean" copy of the disk, but does not copy any protection since it will ignore any sectors with errors, or any sectors that don't conform to the format explained above. It was really only used when we weren't going to try to copy the disk protection, but rather apply a "parameter" to the disk. A "parameter" is nothing more than a patch program that was run on the backup disk to remove the check for copy protection. Fast Hack'em, Kracker Jax, and later Maverick were all very popular copy programs that utilized parameters.


A "nibbler" works at a lower level and will duplicate the actual headers and data, whether or not they are in CBM DOS format. It does have limitations, though. First, they don't duplicate the header or tail gaps or the original sync length, but rather creates its own, so protections that hide (seemingly unused) bytes in the gaps or count sync lengths won't be duplicated. Secondly, the drive only has 2k of RAM, but a whole track is up to 8k, so it must be broken down and read in several passes, then stitched together on the destination disk. Protections that exploited this would use a sector or track larger than would fit in RAM at once, making it impossible to copy. Later, there were hardware solutions to this, such as extra drive RAM and parallel ports.