The Atari 810 was a floppy disk drive for the Atari 400 and 800 computers, using a 5 1/4" single-density soft-sector disk format consisting of 720 sectors (numbered from 1 to 720 decimal, or 1 to 2D0 hex) each containing 128 bytes (numbered from 0 to 127 decimal, or 0 to 7F hex). This resulted in a total capacity of 90 kilobytes. The sectors were arranged in 40 tracks of 18 sectors each. Data was stored with FM encoding.
The contents of the disk were organized using the Atari File Management Subsystem (FMS) as the filesystem. This allocated sector 360 (168 hex) for the VTOC (Volume Table of Contents) and sectors 361-368 (169-170 hex) for the directory. Sector 720 (2D0 hex) was unused (due to it not being addressable by the file system because of a conflict in its zero-based numbering from 0 to 719 being unable to address it, while it can address a nonexistent zero sector), and bootable disks had the boot record in sector 1 (followed by a DOS.SYS file in the next few sectors), leaving the remaining sectors free for user files.
The FORMAT command on the Atari prepares a disk for use by writing all-zero values for each of its sectors.