Atari DOS

Atari DOS

Infobox OS
name = Atari DOS
logo =

caption = Atari DOS version 2.5, main menu
developer = Atari, Inc., Atari Corporation
source_model = Closed source
kernel_type =
supported_platforms = Atari 8-bit family
ui = A menu or Command line interface
family =
released = 1979
latest_release_version = XE 1.0
latest_release_date = 1987
latest_test_version =
latest_test_date =
marketing_target =
programmed_in =
prog_language =
language = English
updatemodel =
package_manager =
working_state = Historic
license =
website =

Atari DOS is the disk operating system used with the Atari 8-bit family of computers. Operating system extensions loaded into memory were required in order for an Atari computer to access a disk drive. These extensions to the operating system added the disk handler and other file management features.

The most important extension is the disk handler. In Atari DOS 2.0, this was the File Management System (FMS), an implementation of a file system loaded from a floppy disk. This meant at least an additional 32K RAM memory was needed to run with DOS loaded,.


There were several versions of Atari DOS available, with the first version released in 1979. [ [;isa=Category;op=show Atari ] ]

DOS 1.0

In the first version of DOS from Atari all commands were only accessible from the menu. It was bundled with the "810" disk drives. This version was entirely memory resident, which made it fast but occupied memory space.

DOS 2.0

:Also known as "DISK OPERATING SYSTEM II VERSION 2.0S"The second, more popular version of DOS from Atari was bundled with the "810" disk drives and some early "1050" disk drives. It is considered to be the lowest common denominator for ATARI DOSes, as any Atari-compatible disk drive can read a disk formatted with "DOS 2.0S".

"DOS 2.0S" consisted of DOS.SYS and DUP.SYS. DOS.SYS was loaded into memory, while DUP.SYS contained the disk utilities and was loaded only when the user exited to DOS.

In addition to bug fixes, "DOS 2.0S" featured improved NOTE/POINT support and the ability to automatically run an Atari executable file named AUTORUN.SYS. Since user memory was erased when DUP.SYS was loaded, an option to create a MEM.SAV file was added. This stored user memory in a temporary file (MEM.SAV) and restored it after DUP.SYS was unloaded. The previous menu option from "DOS 1.0", N. DEFINE DEVICE, was replaced with N. CREATE MEM.SAV in "DOS 2.0S".

Version 2.0S was for single-density disks, 2.0D was for double-density disks. 2.0D was for the 815 Dual Disk Drive.

DOS 3.0

A new version of DOS that came bundled with the "1050" disk drives from Atari. This made use of the new "Enhanced Density" capability (referred to by Atari as "Dual Density") offered by the "1050".

By organizing sectors into blocks, Atari was anticipating larger capacity floppy disks, but this resulted in incompatibility with "DOS 2.0S". Files converted to DOS 3 could not be converted back to "DOS 2.0". As a result, DOS 3.0 was extremely unpopular and did not gain widespread acceptance amongst the Atari user community. Due to complaints and bugs, Atari released "DOS 2.5", which was actually released after DOS 3.0, in contravention of standard version naming practices.

"DOS 3.0" provided built-in help via the Atari HELP key and/or the inverse key. Help files needed to be present on the dos disk to function properly.

DOS 2.5

:Also known as "DISK OPERATING SYSTEM II VERSION 2.5"After listening to complaints by their customers, Atari released an improved version of their previous DOS. This allowed the use of Enhanced Density disks, and there was a utility to read DOS 3 disks. An additional option was added to the menu (P. FORMAT SINGLE) to format single-density disks. DOS 2.5 was shipped with "1050" disk drives and some early "XF551" disk-drives.

Included utilities were DISKFIX.COM, COPY32.COM, SETUP.COM and RAMDISK.COM.


:Codename during production: "ADOS"When the Atari "XF551" drive came out, not only was it Atari's first drive that could read double-density disks, it was also double-sided. So support was added in the DOS for double-density and double-sided disks.

A new proprietary file format made "DOS XE" incompatible with "DOS 2.0S" or "DOS 2.5". A separate utility was needed for reading older 2.0 files.

Only XL/XE computers were supported, "DOS XE" did not work with the older 400/800 computers.

"DOS XE" also supported date-stamping of files, sub-directories and burst I/O for the XF551 drive.

Last DOS made by Atari for the Atari 8-bit family.

DOS 4.0

:Codename during production: "QDOS"Designed for the never-released 1450XLD, the rights were returned to the author who placed it in the public domain. It was later published by ANTIC Software. Using blocks instead of sectors, it supported single, double, enhanced and double-sided drives. Not compatible with DOS 2 or 3 disks but could read files from them. Did not automatically switch densities, it was necessary to go to the menu and manually select the correct density.

Written by Michael Barall.

Third-party DOS programs

Many of these DOSes were released by manufacturers of third-party drives, anyone who made drive modifications, or anyone who was dissatisfied with the available DOSes. Often, these DOSes could read disks in higher densities, and could set the drive to read disks faster (using "Warp Speed" or "Ultra-Speed" techniques). Most of these DOSes (except Sparta DOS) were DOS 2.0 compatible.


Menu driven DOS that was compatible with DOS 2.0. Among the first third-party DOS programs to support double-density drives.

Many enhancements including sector copying and verifying, speed checking, turning on/off file verifying and drive reconfiguration.

Written by John Chenoweth and Ron Beiber.

OS/A+ and DOS XL

DOS produced by Optimized Systems Software. Compatible with DOS 2.0 - Allowed the use of Double Density floppies. Unlike most ATARI DOSses, this used a command line instead of a menu. DOS XL provided a menu program in addition to the command line.

uper DOS

This DOS could read SS/ED/DD/DS disks, and made use of all known methods of speeding up disk-reads supported by the various 3rd party drive-manufacturers.


Menu driven DOS with enhanced features. Sorts disk directory listings and can set display options. File directory can be compressed. Can display deleted files and undelete them. Some advanced features required a proprietary TOPDOS format.

Published by Eclipse Software.


This DOS adds the ability to use sub-directories, and supports hard-drives.

Published by Wordmark Systems, includes complete source code.


This DOS used a command-line interface. Was not compatible with DOS 2.0, but could read DOS 2.0 disks. Supports subdirectories and hard drives being capable of handling filesystems sized up to 16 MB. Included the capability to create primitive batch files.

partaDOS X

A more sophisticated version of SpartaDOS, which strongly resembles MS-DOS in its look and feel. It was shipped on a 64K ROM cartridge.


A SpartaDOS compatible DOS, the last version 1.30 was released in December 1995. It has a much lower memory footprint compared to the original SpartaDOS and is not using the RAM under the ROM of XL/XE machines, so it can be used on the older Atari 400/800 models.

BW-DOS is Freeware by Jiří Bernasek.

DOS 2.6

Someone in the Atari hacker community modified DOS 2.0 to add a few features and allow the use of dual density disk drives, with the "look and feel" of DOS 2.0. One new feature added was "RADIX", which one could use to translate hexidecimal numbers to base 10 or base 10 to hex.


A SpartaDOS compatible DOS (in fact, a renamed version of SpartaDOS 3.x, due to legal reasons).

RealDOS is Shareware by Stephen J. Carden and Ken Ames.

Disk formats

A number of different formats existed for Atari disks. The standard Atari single-sided, single-density disk had 720 sectors divided into 40 tracks. After formatting with an Atari 2 compatible DOS, 707 sectors were free. Each 128 byte sector used the last 3 bytes for housekeeping data (bytes used, file number, next sector), leaving 125 bytes for data.

* Single-Sided, Single-Density: 40 tracks with 18 sectors per track, 128 bytes per sector. 90K capacity.
* Single-Sided, Double-Density: 40 tracks with 18 sectors per track, 256 bytes per sector. 180K capacity.
* Single-Sided, Enhanced-Density: 40 tracks with 26 sectors per track, 128 bytes per sector. 127K capacity.
* Double-Sided, Double-Density: 80 tracks (40 tracks per side) with 18 sectors per track, 256 bytes per sector. 360K capacity.


* [ Inside Atari DOS] by Bill Wilkinson
* [ Mapping the Atari, Revised Edition] by Ian Chadwick

External links

* [ What are Atari DOS 1, DOS 2.0S, DOS 3, DOS 2.5, and DOS XE?] — From the [ Atari 8-bit FAQ] .
* [ Atari DOS Reference Manual] — Reference manual for DOS 3.
* [ Antic Vol.4 No.3] Everything You Wanted To Know About Every DOS
* [ Atari Dos 4] Documentation on Atari DOS 4
* [ BEWE-DOS 1.30] BEWE-DOS Manual
* [ XFormer, a free Atari Emulator which needs no ROM's and other items]

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