Floppy disk formats and DOS systems

Atari 810: SD single density

The 810 records one side of a 5¼-inch disk at single density. A typical format is 40 tracks of 18 sectors of 128 bytes: 92,160 bytes, or 90 KiB, before file system and DOS files. Many later drives also read this format. In an SIO chain, the drive number must match the device name, such as D1:. Booting normally uses the first drive.

Atari 1050: SD and enhanced density

A standard 1050 supports SD and enhanced density, ED. ED has 40 tracks of 26 sectors of 128 bytes: 133,120 bytes, or 130 KiB, before DOS overhead. It is not DD with 256-byte sectors.

DOS 2.5 supports ED and SD. US Doubler and Happy upgrades add capabilities absent from a standard 1050.

XF551: two sides and DD

The XF551 has two heads and uses both disk sides without flipping. Its full DD format is 40 × 18 × 256 × 2 = 368,640 bytes, or 360 KiB. The file system occupies some of that space.

Full capacity requires a suitable system such as DOS XE. The bundled DOS 2.5 supports only the smaller SD and ED formats.

What DOS handles

The ROM operating system handles low-level drive communication. DOS adds a directory, filenames and file commands. In DOS 2.5, DOS.SYS handles disk access, while DUP.SYS contains the utility menu.

A disk can also have its own loader and boot without a conventional DOS. Copying requires matching media and file system formats. Formatting erases previous contents, so use it only on media whose data can be discarded.