This FAQ section describes the various DOS versions produced by Atari for use
with their 8-bit computers.

The Atari Operating System includes a simple "resident disk handler" that
supports four functions for communicating with a disk drive connected via the
SIO hardware port:
  FORMAT - Issue a format command to the disk controller
  READ SECTOR - Read a specified sector
  WRITE/VERIFY SECTOR - Write sector; check sector to see if written
  STATUS - Ask the disk controller for its status

The resident disk handler is used to load a full "file management system"
(FMS) from disk into RAM at power-up.

The FMS may be distributed with additional programs that are optionally loaded
from disk into memory after the FMS is loaded.

On the Atari, then, a "Disk Operating System" (DOS) consists of:
  1) The OS-resident disk handler
  2) A FMS
  3) Possible software extensions to the FMS

However, in practice it is much more common to think only of a FMS and any
additional programs distributed with the FMS as a "DOS" on the Atari.

In 1978 Atari contracted with Shepardson Microsystems, Inc. (SMI) to create a
FMS (along with a version of BASIC) for the upcoming Atari personal computers.
The following worked together on the project, which resulted in the original
Atari DOS (along with Atari BASIC):

Paul Laughton (author of Apple DOS) - project leader, co-primary contributor
Kathleen O'Brien - co-primary contributor
Bill Wilkinson - floating point scheme design
Paul Krasno - implemented the math library routines following guidelines
              supplied by Fred Ruckdeschel (author of the acclaimed text, 
              BASIC Scientific Subroutines)
Bob Shepardson - Modified IMP-16 Assembler to accept special syntax tables
                 Paul invented
Mike Peters - keypuncher/computer operator/junior programmer/troubleshooter

The original Atari DOS shipped with 810 disk drives until 1981.  It consists
of a single file, DOS.SYS, which is loaded into memory from disk on startup.
At the top of the menu screen it reads:
   DISK OPERATING SYSTEM    9/24/79
   COPYRIGHT 1979 ATARI
With the planned release of DOS II in 1981, Atari referred to this first
release of DOS as DOS I.  Nearly all users quickly abandoned DOS I in favor of
DOS II.
Trivia: The DOS I "N. DEFINE DEVICE" menu option does not work.
Also, DOS I is not compatible with the 850 Interface Module.
The Atari DOS I disk is labeled: Atari 810 Master Diskette (CX8101).

DOS II Version 2.0S was shipped with 810 disk drives, and early 1050 disk
drives, from 1981-1983.  It also shipped with the CX77 Touch Tablet.
It consists of two files:
 - DOS.SYS is loaded into memory from disk on startup
 - DUP.SYS, which contains the DOS menu, is loaded only when needed.
By splitting the menu screen off into a separate program that is not loaded
into memory until needed, more memory remained available for user programs in
comparison to the single-file approach of DOS I.  MEM.SAV can be employed to
preserve the contents of memory to disk when DUP.SYS is loaded, so that the
data can be restored to memory when exiting from the DOS menu.  A file named
AUTORUN.SYS is launched on startup after DOS.SYS is loaded.  DOS 2.0S supports
Atari's proprietary single-sided, single density 90K 5.25" floppy disk format
only.  DOS 2.0S represents the lowest common denominator of Atari DOS
versions--you can be assured than any Atari disk drive for the 8-bit Atari can
work with disks formatted with DOS 2.0S.  DOS 2.0S can read disks written with
DOS I; the reverse is not the case.  The DOS 2.0S disk (CX8104) is labeled:
"Atari 810 Master Diskette II" (most) or "Atari 810/1050 Master Diskette II"
(later production).  DOS 2.0S was delivered by Optimized Systems Software
(OSS), headed by Bill Wilkinson, for Atari.

DOS II Version 2.0D was shipped with the rare Atari 815 Dual Disk Drive.
Supports double-density disk drives; also supports single-density disk drives.
The DOS 2.0D disk is labeled: Atari 815 Master Diskette (CX8201).

For much more about DOS II see Inside Atari DOS by Bill Wilkinson (1982),
online at: http://www.atariarchives.org/iad/

DOS 3 shipped with 1050 disk drives from 1983-1985.  It was created in part to
take advantage of the 1050's Dual-Density capability, by employing a single-
sided, enhanced-density 130K 5.25" floppy disk drive format.  Atari called
this format "dual-density," but the Atari community quickly came to refer to
this format as "enhanced-density" to better differentiate it from widely
available 3rd-party truly double density disk drives and supporting versions
of DOS.  DOS 3 consists of multiple files: FMS.SYS (the FMS), KCP.SYS,
KCPOVER.SYS, COPY.UTL, DUPDISK.UTL, INIT.UTL, CONVERT.UTL (converts files from
DOS 2.0S to DOS 3, but not back again) and HELP.UTL.  It also has support for
MEM.SAV and AUTORUN.SYS.  DOS 3 uses a disk format incompatible with, and less
efficient than, DOS 2.0S (DOS 3: 1024-byte "blocks"; DOS 2: 128-byte
"sectors").  For these reasons and others, DOS 3 was not widely accepted by
the Atari community, and like DOS I is not generally used except for
curiosity's sake.  The DOS 3 disk is labeled: Master Diskette 3 (DX5052).

DOS II Version 2.5 (DOS 2.5) shipped with 1050 disk drives and early XF551
disk drives from 1985-1988.  DOS 2.5 represented Atari's relenting to the
masses, returning to DOS 2.0S compatibility.  DOS 2.5 very closely resembles
DOS 2.0S, with just a few features added.  It supports both DOS 2.0S single-
density 90K formats, as well as an enhanced density 130K format for use with
the 1050 disk drive.  In addition to the two main files DOS.SYS and DUP.SYS,
DOS 2.5 also includes a RAMdisk utility for use with the 128K 130XE computer,
a utility to convert files from DOS 3 disks back to DOS 2.5, and other disk
utilities.  DOS 2.5 is just about as universal among Atari users as DOS 2.0S. 
The DOS 2.5 disk is labeled simply: DOS 2.5 (DX5075).

DOS XE shipped with XF551 disk drives after 1988.  Like DOS 3, DOS XE
introduced a whole new format for Atari floppy disks; but unlike DOS 3, DOS XE
also preserved general compatibility with DOS 2.0S/2.5.  DOS XE supports the
full capabilities of the double-sided, double density 360K per 5.25" floppy
disk XF551 disk drive, including that drive's high-speed burst mode.  DOS XE
also fully supports the 90K SS/SD capability of the 810 disk drive, the 130K
SS/ED capability of the 1050 disk drive, the 180K SS/DD capability of most
3rd-party disk drives for the Atari, and a RAMdisk for use with the 130XE.
Date-stamping of files is supported.  DOS XE requires an XL or XE computer; it
is not compatible with the 400/800 computer models.  Even though DOS XE was
critically well-received, and represented a substantial jump in capabilities
over DOS 2.5, its arrival came so late in the crowded realm of Atari and 3rd-
party DOS versions that it never achieved much acceptance among real users,
and is now relegated to the same status as DOS I and DOS 3 before it.  Before
its release, DOS XE was widely known as "ADOS."  It was developed by Bill
Wilkinson for Atari.  The DOS XE disk is labeled: DOS XE Master Diskette
(DX5090).