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{{short description|Operating system focused on disk-based file operations}}
{{about|disk operating systems in the broadest sense|the disk operating system used by IBM PC compatibles|DOS|IBM mainframe operating systems|DOS/360 and successors}}
{{refimprove|date=August 2009}}
A '''disk operating system''' ('''DOS''') is a computer [[operating system]] that requires a disk or other [[direct-access storage device]] as [[secondary storage]]. A DOS provides a [[file system]] and a means for loading and running [[computer program|programs]] stored on the disk.

The term is now historical, as most if not all operating systems for general-purpose computers now require direct-access storage devices as secondary storage.

==History==
Before modern storage such as the disk drive, floppy disk, and [[flash storage]], early computers used storage such as [[Analog delay line|delay line]], [[core memory]], [[punched card]], [[punched tape]], [[magnetic tape]], and [[magnetic drum]]. Early microcomputers and [[home computer]]s used paper tape, [[audio cassette tape]] (such as [[Kansas City standard]]), or no permanent storage at all. Without permanent storage, programs and data are input directly into memory using [[front panel]] switches, or is input through a [[computer terminal]] or keyboard, sometimes controlled by a [[BASIC]] interpreter in [[read-only memory|ROM]]. When power is turned off, all information is lost.

In the early 1960s, as disk drives became larger and more affordable, various mainframe and minicomputer vendors introduced disk operating systems and modified existing operating systems to use disks.

[[Hard disk]]s and floppy disk drives require software to manage rapid access to block storage of sequential and other data. For most microcomputers, a disk drive of any kind was an optional peripheral. Systems could be used with a tape drive or booted without a storage device at all. The disk operating system component of the operating system was only needed when a disk drive was used.

By the time IBM announced the [[System/360]] mainframes, the concept of a disk operating system was well established. Although IBM did offer [[IBM Basic Programming Support|Basic Programming Support]] (BPS/360) and TOS/360 for small systems, they were out of the mainstream and most customers used either DOS/360 or OS/360.

Most home and personal computers of the late 1970s and 1980s used a disk operating system, most often with "DOS" in the name and simply referred to as "DOS" in the context of its user community. For example, [[Commodore DOS|CBM DOS]], [[Atari DOS]], [[TRS-DOS]], [[Apple DOS]], [[Apple ProDOS]], and [[MS-DOS]]. [[CP/M]] is also a disk operating system, despite not having "DOS" in the name.<ref name="byte1983_07">{{cite magazine |author-last=Dahmke | author-first=Mark |title=CP/M Plus: The new disk operating system is faster and more efficient than CP/M |magazine=BYTE Magazine | url= https://archive.org/details/byte-magazine-1983-07/page/n359/mode/2up?view=theater |date=1983-07-01 |volume=8 |issue=7 |pages=360 |issn= }}</ref>

A DOS is usually loaded from a disk, but there are exceptions, such as [[Commodore 1541|Commodore's disk drive]] for the [[Commodore 64]] and [[VIC-20]] which contain the DOS in [[ROM]]. Some versions of [[AmigaDOS]] mostly resides in ROM, as a part of a [[Kickstart (Amiga)|Kickstart]] firmware.

==OS extensions==
*[[Commodore DOS]] is on 8-bit [[Commodore International|Commodore]] computers such as the [[Commodore 64]]. Unlike most other DOS systems, it is integrated into the disk drives, not loaded into the computer's own memory.
*[[Atari DOS]] is used by the [[Atari 8-bit computers]]. The Atari OS only offers low-level disk-access, so an extra layer called DOS can be booted from a floppy for higher level functions such as filesystems.<ref name=wilkinson/> Third-party replacements for Atari DOS include [[DOS XL]], [[SpartaDOS X|SpartaDOS]], [[MyDOS]], [[TurboDOS]], and [[Top-DOS]].
*[[MSX-DOS]] is for the [[MSX]] computer standard. The initial version, released in 1984, is MS-DOS 1.0 ported to [[Zilog Z80|Z80]]. In 1988, version 2 has facilities such as subdirectories, memory management, and environment strings. The MSX-DOS kernel resides in ROM (built-in on the disk controller) so basic file access capacity is available even without the command interpreter, by using [[BASIC]] extended commands.
*[[Disc Filing System]] (DFS) is an optional component for the Acorn [[BBC Micro]], as a kit with a disk controller chip, a ROM chip, and a few logic chips, to be installed inside the computer.
*[[Advanced Disc Filing System]] (ADFS) is a successor to Acorn's DFS.
*[[AMSDOS]] is for the [[Amstrad CPC]] computers.
*GDOS and G+DOS is for the [[+D]] and [[DISCiPLE]] disk interfaces for the [[ZX Spectrum]].

==Main OSes==
Some disk operating systems are the operating systems for the entire computer system.
*The [[Burroughs Corporation|Burroughs]] (now Unisys) [[Burroughs MCP|Master Control Program]]<ref name=MCP/> (MCP) for the [[Burroughs large systems|B5000]] originally runs from a drum, but starting with the B5500 it runs from a disk. It is the basis for the MCP on the B6500, B7500, and successors.
*The SIPROS, [[Chippewa Operating System]] (COS), [[CDC SCOPE|SCOPE]], MACE and [[CDC Kronos|KRONOS]] operating systems<ref name=CDC/> on the [[Control Data Corporation ]] (CDC) [[CDC 6000 series|6000 series]] and [[CDC 7600|7600]] are all disk operating systems. KRONOS became [[NOS (operating system)|NOS]] and SCOPE became NOS/BE.
*The [[GECOS]]<ref name=GECOS/> operating system for the [[GE]] (later [[Honeywell]] and [[Groupe Bull]]) [[GE-600 series|600]] family of [[mainframe computer]]s (it later became GCOS).
*The IBM Basic Operating System/360<ref name=BPS/> ([[BOS/360]]), [[DOS/360 and successors|Disk Operating System/360]]<ref name=DOS/> (DOS/360) and [[OS/360 and successors |Operating System/360]]<ref name=OS/> (OS/360) are standard for all but the smallest [[IBM System/360|System/360]] installations; the [[360/67]] also has Control Program-67 /Cambridge Monitor System<ref name=CP-67 /> ([[CP-67/CMS]]) and Time Sharing System/360 <ref name=TSS /> ([[TSS/360]]). BOS is gone, CP-67/CMS has evolved into [[z/VM]], DOS has evolved into [[z/VSE]], OS has evolved into [[z/OS]] and TSS/360 evolved into TSS/370 PRPQ, which is now gone.
*The [[UNIVAC EXEC II|EXEC II]] operating system for the [[UNIVAC 1107]] and [[UNIVAC 1108|1108]], and the [[EXEC 8]] operating system for the 1108, which has evolved into OS 2200 for the [[Unisys]] [[UNIVAC 1100/2200 series#UNISYS ClearPath IX series|ClearPath Dorado Series]].
*The [[DEC BATCH-11/DOS-11|DOS-11]] operating system for [[Digital Equipment Corporation|DEC]] [[PDP-11]] [[minicomputer]]s.
*[[CP/M]] is a disk operating system, as the main or alternate operating system for numerous microcomputers of the 1970s and 1980s.
*[[Apple DOS]] is the primary operating system for the [[Apple II]], from 1979 with the introduction of the [[floppy disk drive]], until 1983 when it was replaced by [[ProDOS]].
*[[TRSDOS]] is the operating system for the [[TRS-80]] line of computers from Tandy.<ref name=trsdos/>
*[[MS-DOS]] for [[IBM PC]] compatibles with [[Intel]] [[x86]] CPUs. [[86-DOS]] was modeled on [[CP/M]], and then was adapted as the basis for [[Microsoft]]'s [[MS-DOS]]. It was rebranded by IBM as [[IBM PC DOS|PC DOS]] until 1993. Various compatible systems were later produced by different organizations, starting with [[DR-DOS]] in 1988.

==See also==
* [[List of disk operating systems called DOS]]
* [[Live CD]]

==References==
{{Reflist|refs=
<ref name=MCP>{{cite book
 |     title = A Narrative Description of the Burroughs B5500 Disk File Master Control Program
 |        id = 1023579
 |      date = October 1966
 |       url = http://bitsavers.org/pdf/burroughs/LargeSystems/B5000_5500_5700/1023579_Narrative_Description_Of_B5500_MCP_196610.pdf
 |    series = Systems Documentation
 | publisher = Burroughs 
 }}
</ref>
<ref name=CDC>{{cite web
 |     title = CDC Operating System History
 |       url = http://bitsavers.org/pdf/cdc/cyber/CDC_Operating_System_History_Mar76.pdf
 | publisher = CDC
 }}
</ref>
<ref name=GECOS>{{cite book
 |     title = GE-635 Comprehensive Operating Supervisor (GECOS)
 |        id = CPB-1002
 |      date = July 1964
 | publisher = General Electric
 |       url = http://bitsavers.org/pdf/ge/GE-6xx/GECOS/CPB-1002_GECOS_Jul64.pdf
 }}
</ref>
<ref name=BPS>{{cite book
 |     title = IBM System/360 Basic Programming Support and IBM Basic Operating System/360 Programming Systems Summary
 |       url = http://bitsavers.org/pdf/ibm/360/bos_bps/C24-3420-0_BPS_BOS_Programming_Systems_Summary_Aug65.pdf
 | publisher = IBM
 |    series = Systems Reference Library 
 }}
</ref>
<ref name=CP-67>{{cite manual
 | title       = Control Program-67 /Cambridge Monitor System - (CP-67 /CMS) Version .3.1 - Program Number 3600-05.2.005 - System Description Manual
 | id          = GH20-0802-2
 | date        = September 1971
 | edition     = Third
 | url         = http://bitsavers.org/pdf/ibm/360/cp67/GH20-0802-2_CP-67_CMS_Version_3.1_System_Description_Manual_197109.pdf
 | series      = Type III Class A Program 
 | publisher   = [[IBM]]
 | access-date = January 9, 2023
 }}
</ref>
<ref name=DOS>{{cite book
 |     title = IBM System/360 Disk and Tape Operating Systems Concepts and Facilities
 |        id = GC24-5030-8
 |   edition = Ninth
 |      date = October 1970
 |       url = http://bitsavers.org/pdf/ibm/360/dos/GC24-5030-8_Disk_and_Tape_Operating_Systems_Concepts_and_Facilities_Oct70.pdf
 |    series = Systems Reference Library
 | publisher = IBM
 }}
</ref>
<ref name="OS">{{cite book
 |     title = IBM Operating System/360 Concepts and Facilities
 |        id = C28-6535-0
 |      year = 1965
 |       url = http://bitsavers.org/pdf/ibm/360/os/R01-08/C28-6535-0_OS360_Concepts_and_Facilities_1965.pdf
 | publisher = IBM
 |    series = Systems Reference Library
 }}
</ref>
<ref name=TSS>{{cite manual
 | title       = IBM System/360 Time Sharing System - Concepts and Facilities 
 | id          = C28-2003-3 
 | date        = September 1968
 | edition     = Fourth
 | url         = http://bitsavers.org/pdf/ibm/360/tss/C28-2003-3_TSS360Concepts.pdf
 | series      = Systems Reference Library  
 | publisher   = [[IBM]]
 | access-date = January 9, 2023
 }}
</ref>
<ref name=wilkinson>{{cite book|last1=Wilkinson|first1=Bill|title=Inside Atari DOS|date=1982|publisher=COMPUTE! Books|location=Greensboro, NC|isbn=0-942386-02-7|url=http://atariarchives.org/iad/|url-status=live|archive-url=https://web.archive.org/web/20171002120523/http://atariarchives.org/iad/|archive-date=2017-10-02}}</ref>
<ref name="trsdos">{{cite web|title=Definitive List of TRS-80 Model II Operating Systems|url=https://pski.net/definitive-list-of-trs-80-model-ii-operating-systems/}}</ref>
}}

{{Disk operating systems}}
{{Operating system}}

[[Category:Disk operating systems|Disk operating systems]]