Editing Atari DOS (section)

==={{anchor|QDOS}}DOS 4.0===
:Codename during production: ''QDOS''
DOS 4.0 was designed for the [[Atari 1450XLD|1450XLD]]. It was designed to operate with larger disk formats, adding double density and double sided support while also supporting the older single and enhanced density formats from DOS 2 and 2.5. To support the newer modes, it returned to the blocks concept used in DOS 3.0 when used to format enhanced or double density disks, but this time using slightly smaller 6-sector format, which held 768 bytes in enhanced density and 1536 bytes in double density.

Like DOS 3, disks formatted in the new higher-density formats were not compatible with older drives and DOSes. It could read and write to those disks to retain compatibility. However, it could not determine the format of a disk automatically, and the user had to select the format of the disks from the DOS menu.

The 1450XLD was never released, and the rights for DOS 4 were returned to the author, Michael Barall, who placed it in the [[public domain]]. It was published by [[Antic Software]] in 1984, and is sometimes referred to as "Antic DOS" for this reason.