Apple II 16 sector disk

The Apple II 16 sector disk was the floppy-disk format for the Apple II computer line starting with DOS 3.3, superseding the Apple II 13 sector disk used in DOS 3.2.1 and earlier. It was a 5.25" disk (single-sided, soft-sectored, meaning that the index hole was ignored in finding sector locations) with 35 tracks each containing 16 sectors, storing 256 bytes of data in each sector for a total of 143,360 bytes (140K). A ROM upgrade was necessary for older disk drives to support the new format.

Three tracks of the disk were taken up by the DOS software (on bootable disks), and one more track with the catalog (directory) for the Apple DOS file system, leaving 124K for file storage. Some other filesystems also used with this format were ProDOS file system, SOS file system, and CP/M file system.

While the disks were single-sided, users often punched an extra notch on the opposite side to permit the disks to be flipped over for use of the other side (the notch was necessary to prevent the disk from being seen by the drive as write-protected). This was done despite being discouraged by disk manufacturers, since the disks weren't designed to work well this way (only one side was certified for data storage, and also running it in both directions could cause debris to build up in ways unanticipated by the designers). However, some disk manufacturers eventually did produce double-notched disks for this use.

When the 16-sector format was introduced, a utility called MUFFIN was distributed on the system disk to transfer files from 13-sector disks into the new format. An "unofficial" utility called NIFFUM transferred files in the opposite direction.

Links

 * Apple II History: DOS 3.3
 * Wikipedia article on Apple DOS
 * Apple II system master disk images