Atari 8-bit Line
The combination of DOS and hardware (810, 1050 and XF551 disk drives) for Atari 8-bit floppy usage allowed sectors numbered from 1 to 720. The DOS' 2.0 disk bitmap provides information on sector allocation, counts from 0 to 719. As a result, sector 720 could not be written to by the DOS. Some companies used a copy protection scheme where "hidden" data was put in sector 720 that could not be copied through the DOS copy option. Another more-common early copy-protected scheme simply did not record important sectors as "used" in the FAT, so the DOS Utility Package (DUP) did not duplicate them. All of these early techniques were thwarted by the first program that simply duplicated all 720 sectors.
Later DOS versions (3.0 and later 2.5) and DOS systems by third parties (i.e. OSS) accepted (and formatted) disks with up to 960 and 1020 sectors, resulting in 127KB storage capacity per disk side on drives equipped with double-density heads (i.e. not the Atari 810) vs. previous 90KB. That unusual 127K format allowed sectors 1-720 to still be read on a single-density 810 disk drive, and was introduced by Atari with the 1050 drive with the introduction of DOS 3.0 in 1983.
A true 180K double-density Atari floppy format used 128 byte sectors for sectors 1-3, then 256 byte sectors for 4-720. The first three sectors typically contain boot code as used by the onboard ROM OS; it is up to the resulting boot program (such as SpartaDOS) to recognize the density of the formatted disk structure. While this 180K format was developed by Atari for their DOS 2.0D and their (canceled) Atari 815 Floppy Drive, that double-density DOS was never widely released and the format was generally used by third-party DOS products. Under the Atari DOS scheme, sector 360 was the FAT sector map, and sectors 361-367 contained the file listing. The Atari-brand DOS versions and compatible used three bytes per sector for housekeeping and to link-list to the next sector.
Third-party DOS systems added features such as double-sided drives, subdirectories, and drive types such as 1.2 MByte and 8". Well-known 3rd party Atari DOS products included SmartDOS (distributed with the Rana disk drive), TopDos, MyDos and SpartaDOS.
Read more about this topic: Floppy Disk Variants
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