FreeDOS screenshot showing the command line interface, directory structure and version information.

DOS, short for "Disk Operating System",[1] is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.

Related systems include MS-DOS, PC-DOS, DR-DOS, FreeDOS, PTS-DOS, ROM-DOS, Novell DOS, OpenDOS and several others.

In spite of the common usage, none of these systems were simply named "DOS" (a name given only to an unrelated IBM mainframe operating system in the 1960s). A number of unrelated, non-x86 microcomputer disk operating systems had "DOS" in their name, and are often referred to simply as "DOS" when discussing machines that use them (e.g. AmigaDOS, AMSDOS, ANDOS, Apple DOS, Atari DOS, Commodore DOS, CSI-DOS, ProDOS, and TRS-DOS). While providing many of the same operating system functions for their respective computer systems, programs running under any one of these operating systems would not run under others.



All MS-DOS-type operating systems run on machines with the Intel x86 or compatible CPUs, mainly the IBM PC and compatibles. Machine-dependent versions of MS-DOS were produced for many non-IBM-compatible x86-based machines, with variations from relabelling of the Microsoft distribution under the manufacturer's name, to versions specifically designed to work with non-IBM-PC-compatible hardware. For as long as application programs used DOS APIs instead of direct hardware access, they could thereby also run on non-IBM-PC compatible machines. In 1985, Digital Research also had a version of Concurrent DOS 68K for use on Motorola 68000 CPUs, and the original FreeDOS kernel DOS-C derived from DOS/NT, also for Motorola CPUs, in the early 1990s. While these systems resembled the DOS architecture, applications were not binary compatible due to the incompatible instruction sets of these non-x86-CPUs. However, applications written in high-level languages could be ported easily.

DOS is a single-user, single-task operating system with basic kernel functions that are non-reentrant: only one program at a time can use them. There is an exception with Terminate and Stay Resident (TSR) programs, and some TSRs can allow multitasking. However, there is still a problem with the non-reentrant kernel: once a process calls a service inside of operating system kernel (system call), it must not be interrupted with another process calling system call, until the first call is finished.[2]

The DOS kernel provides various functions for programs (an application program interface), like displaying characters on-screen, reading a character from the keyboard, accessing disk files and more.

DOS by default provides a primitive ability for shell scripting, via batch files (with the filename extension .BAT). These are text files that can be created in any text editor. They are executed in the same fashion as compiled programs, and run each line of the batch file as a command. Batch files can also make use of several internal commands, such as GOTO and conditional statements.[3] GOSUB and simple arithmetic is supported with the DR DOS COMMAND.COM as well as some with third-party shells like 4DOS, but can also be faked via strange workarounds; however, no real form of programming is usually enabled.

The operating system offers a hardware abstraction layer that allows development of character-based applications, but not for accessing most of the hardware, such as graphics cards, printers, or mice. This required programmers to access the hardware directly, usually resulting in each application having its own set of device drivers for each hardware peripheral. Hardware manufacturers would release specifications to ensure device drivers for popular applications were available.[4]

Drive naming scheme

In DOS, drives are referred to by identifying letters. Standard practice is to reserve "A" and "B" for floppy drives. On systems with only one floppy drive DOS assigns both letters to the drive, prompting the user to swap disks as programs alternate access between them. This facilitates copying from floppy to floppy or having a program run from one floppy while accessing its data on another. Hard drives were originally assigned the letters "C" and "D". DOS could only support one active partition per drive. As support for more hard drives became available, this developed into first assigning a drive letter to each drive's active primary partition, then making a second pass over the drives to allocate letters to logical drives in the extended partition, then a third pass to give any other non-active primary partitions their names (where such additional partitions existed and contained a DOS-supported file system.) Lastly, DOS allocates letters for optical disc drives, RAM disks, and other hardware. Letter assignments usually occur in the order the drivers are loaded, but the drivers can instruct DOS to assign a different letter; drivers for network drives, for example, typically assign letters nearer the end of the alphabet.[5]

Because DOS applications use these drive letters directly (unlike the /dev directory in Unix-like systems), they can be disrupted by adding new hardware that needs a drive letter. An example is the addition of a new hard drive having a primary partition where a pre-existing hard drive contains logical drives in extended partitions; the new drive will be assigned a letter that was previously assigned to one of the extended partition logical drives. Moreover, even adding a new hard drive having only logical drives in an extended partition would still disrupt the letters of RAM disks and optical drives. This problem persisted through Microsoft's DOS-based 9x versions of Windows until they were replaced by versions based on the NT line, which preserves the letters of existing drives until the user changes them.[5] Under DOS, this problem can be worked around by defining a SUBST drive and installing the DOS program into this logical drive. The assignment of this drive would then be changed in a batchjob whenever the application starts. Under some versions of Concurrent DOS, as well as under Multiuser DOS, System Manager and REAL/32, the reserved drive letter L: will automatically be assigned to the corresponding load drive whenever an application starts.

Reserved device names

There are reserved device names in DOS that cannot be used as filenames regardless of extension; these are used to send application output to hardware peripherals. These restrictions also affect several Windows versions, in some cases causing crashes and security vulnerabilities.[6]

A partial list of these reserved names is: NUL:, CON:, AUX:, PRN:, COM1:, COM2:, COM3:, COM4:, LPT1:, LPT2:, LPT3:,[7] and sometimes LPT4: as well. AUX: typically defaults to COM1:, and PRN: to LPT1:, but these defaults can be changed on some systems.

Colons are not necessary in some cases, for example:

echo This does nothing > nul

It is still possible to create files or directories using these reserved device names, such as through direct editing of directory data structures in disk sectors. Such naming, such as starting a file name with a space, has sometimes been used by viruses or hacking programs to obscure files from users who do not know how to access these locations.

Boot sequence

  • The bootstrap loader on PC-compatible computers (MBR or boot sector) is located at track zero, the first sector on a disk. The ROM-BIOS will load this sector into memory at address 0000h:7C00h, and check for a signature "55h AAh" at position 01FEh. If found, the sector is considered to be valid and executed, if not, the ROM-BIOS will try the next physical disk in the row.
  • If the loaded sector happens to be a MBR, its code will scan the partition table, which is located within this sector, for an active partition (bit7=1 at pos 01BEh+10h*n), and, if found, load the first sector of the corresponding partition into memory in the similar fashion as it had been loaded by the ROM-BIOS itself.
  • The boot sector code loads the start of the DOS-BIOS, which is located in the file IO.SYS on MS-DOS systems, into segment 0000h:0600h[8] In some cases the boot sector instead relocates itself into 0000h:0600h and loads the partition boot code into 0000h:7C00h and executes it.[9] The boot sector will then pass execution to the loaded portion.
  • The loaded portion of the DOS-BIOS will then load the remainder of itself into memory. The DOS-BIOS will then load the DOS kernel, located in MSDOS.SYS on MS-DOS systems, into memory as well. In Windows 9x, the DOS-BIOS and kernel are combined into IO.SYS, and MSDOS.SYS is used as a text configuration file.
  • The DOS-BIOS then loads the \CONFIG.SYS file to parse configuration parameters. The SHELL variable specifies the location of the shell which defaults to \COMMAND.COM.
  • The shell is loaded and executed.
  • The startup batch file AUTOEXEC.BAT is then run by the shell.[10][11] DR-DOS COMMAND.COM and 4DOS allow for the specification of the startup batch file through a parameter in the SHELL statement.[12][13]:392

The DOS-BIOS and kernel files loaded by the boot sector must be contiguous and be the first two directory entries.[14] As such, removing and adding this file is likely to render the media unbootable. It is, however, possible to replace the shell at will, a method that can be used to start the execution of dedicated applications faster. This limitation does not apply to any version of DR DOS, where the system files can be located anywhere in the root directory and do not need to be contiguous. Therefore, system files can be simply copied to a disk provided that the boot sector is DR DOS compatible already. This works because the DR DOS boot sector mounts the FAT filesystem, locates the DOS-BIOS and loads the whole file into memory.

In variation from MS-DOS, the DOS-BIOS and kernel files have different names in some DOS versions, for example, in PC-DOS as well as in DR-DOS 5.0 (and higher) they are named IBMBIO.COM instead of IO.SYS and IBMDOS.COM in place of MSDOS.SYS. Older versions of DR DOS used DRBIOS.SYS and DRBDOS.SYS instead. On systems designed for PC-DOS v1.10 the signature 55h AAh at position 01FEh is not checked.[citation needed]


IBM PC-DOS (and the separately sold MS-DOS) and its predecessor, 86-DOS, were loosely inspired by Digital Research's CP/M, which was the dominant disk operating system for 8-bit Intel 8080 and Zilog Z80 based microcomputers. PC-DOS ran on 8088-family processors.

When IBM introduced the IBM PC, built with the Intel 8088 microprocessor, they needed an operating system. Seeking an 8088-compatible build of CP/M, IBM initially approached Microsoft CEO Bill Gates (possibly believing that Microsoft owned CP/M due to the Microsoft Z-80 SoftCard, which allowed CP/M to run on an Apple II[15]). IBM was sent to Digital Research, and a meeting was set up. However, the initial negotiations for the use of CP/M broke down—Digital Research wished to sell CP/M on a royalty basis, while IBM sought a single license, and to change the name to "PC DOS". DR founder Gary Kildall refused, and IBM withdrew.[15][16]

IBM again approached Bill Gates. Gates in turn approached Seattle Computer Products. There, programmer Tim Paterson had developed a variant of CP/M-80, intended as an internal product for testing SCP's new 16-bit Intel 8086 CPU card for the S-100 bus. The system was initially named "QDOS" (Quick and Dirty Operating System), before being made commercially available as 86-DOS. Microsoft purchased 86-DOS, allegedly for $50,000. This became Microsoft Disk Operating System, MS-DOS, introduced in 1981.[17]

Microsoft also licensed their system to multiple computer companies, who supplied MS-DOS for their own hardware, sometimes under their own names. Microsoft later required the use of the MS-DOS name, with the exception of the IBM variant. IBM continued to develop their version, PC DOS, for the IBM PC.[17] Digital Research became aware that an operating system similar to CP/M was being sold by IBM (under the same name that IBM insisted upon for CP/M), and threatened legal action. IBM responded by offering an agreement: they would give PC consumers a choice of PC DOS or CP/M-86, Kildall's 8086 version. Side-by-side, CP/M cost almost $200 more than PC DOS, and sales were low. CP/M faded, with MS-DOS and PC DOS becoming the marketed operating system for PCs and PC compatibles.[15]

Digital Research attempted to regain the market lost from CP/M-86; initially with Concurrent DOS, FlexOS and DOS Plus (both compatible with both MS-DOS and CP/M-86 software), later with Multiuser DOS (compatible with both MS-DOS and CP/M-86 software) and DR-DOS (compatible with MS-DOS software). Digital Research was bought by Novell, and DR DOS became Novell DOS 7; later, it was part of Caldera (under the names OpenDOS and DR-DOS 7.02/7.03), Lineo, and DeviceLogics.

Microsoft and IBM later had a series of disagreements over two successor operating systems to DOS - Microsoft's Windows and IBM's OS/2.[18] They split development of their DOS systems as a result.[19] MS-DOS was partially transformed into Windows; the last version of PC DOS was PC DOS 7.10, released in 2003.

The FreeDOS project began 26 June 1994, when Microsoft announced it would no longer sell or support MS-DOS. Jim Hall then posted a manifesto proposing the development of an open-source replacement. Within a few weeks, other programmers including Pat Villani and Tim Norman joined the project. A kernel, the command line interpreter (shell) and core utilities were created by pooling code they had written or found available. There were several official pre-release distributions of FreeDOS before the FreeDOS 1.0 distribution was released on 3 September 2006. Made available under the GNU General Public License (GPL), FreeDOS does not require license fees or royalties.[20][21]


Early versions of Microsoft Windows were an application that ran on top of a separate version of DOS.[22] By the early 1990s, Windows saw heavy use on new DOS systems. With MS-Windows for Workgroups 3.11, DOS was almost reduced to the role of a boot loader for the Windows kernel; in 1995, MS-Windows 95 was bundled as a standalone operating system that did not require a separate DOS license. With Windows 95 (and Windows 98 and Me, that followed it), the MS-DOS kernel remained, but with Windows as the system's graphical shell. With Windows 95 and 98, but not ME, the MS-DOS component could be run without starting Windows.[23][24] With DOS no longer required to use Windows, the majority of PC users stopped using it directly.

Continued use

Currently available DOS systems are FreeDOS, DR-DOS (and Enhanced DR-DOS), ROM-DOS, PTS-DOS, RxDOS, Multiuser DOS,[25] REAL/32, and others. Some computer manufacturers, including Dell and HP, sell computers with FreeDOS as the OEM operating system.[26][27] NX-DOS is currently under development. It is 16-bit, real-time, networkable, bootable from a floppy, and has an incomplete USB driver. It dates back to 1992 as a personal project, and was released as GPL in 2005.[28]


DOS' structure of accessing hardware directly makes it ideal for use in embedded devices. The final versions of DR-DOS are still aimed at this market.[29] ROM-DOS was used as the embedded system on the Canon PowerShot Pro 70.[30]


DOSBox running on Windows Vista

Under Linux, it is also possible to run copies of DOS and many of its clones under DOSEMU, a Linux-native virtual machine for running DOS programs at near native speed. There are a number of other emulators for running DOS under various versions of UNIX, even on non-x86 platforms, such as DOSBox.[31][32]

DOS emulators are gaining popularity among Windows XP and Windows Vista users, due to these systems being very incompatible with pure DOS. They can be used to run games or other DOS software. One of the best-known is DOSBox, designed for legacy gaming (e.g. King's Quest, Doom) on modern operating systems.[22][31]

It is possible to run DOS applications in a copy of a DOS operating system on a PC emulator, allowing better compatibility than DOS emulators where the emulation of the DOS operating environment is imperfect.[33]

With Microsoft Windows

CMD.EXE, the DOS-styled command prompt used in Microsoft-NT-based Windows.

True 32-bit versions of Windows, starting with NT and including 2000, XP, and Vista, are not based on DOS. These include the NT Virtual DOS Machine (NTVDM), which runs a modified version of MS-DOS 5 in a virtual machine. While DOS-based versions used the traditional COMMAND.COM for a command line interface, MS-Windows NT and its derivatives use CMD.EXE, a descendant of OS/2's command interpreter which recognizes many DOS commands (although COMMAND.COM is still called and used when DOS .EXE files are run).

64-bit Windows Versions

Windows XP x64, Windows Vista x64 and Windows 7 x64 editions do not contain the NTVDM and cannot run 16-bit DOS applications directly. COMMAND.COM is missing and the only way to run a DOS application under these versions of Windows is by using an emulator.



Arachne web browser

DOS was the dominant PC-Compatible platform and many notable programs were written for it. These included:

Ease of use

User interface

Screenshot of a Windows command shell listing the files in a directory

DOS systems utilize a command line interface. Programs are started by entering their filename at the command prompt. DOS systems include several programs as system utilities, and provides additional commands that don't correspond to programs (internal commands).[35]

In an attempt to provide a more user-friendly environment, numerous software manufacturers wrote file management programs that provided users with menu- and/or icon-based interfaces. Microsoft Windows is a notable example, eventually resulting in Microsoft Windows 9x becoming a self-contained program loader, and replacing DOS as the most-used PC-compatible program loader. Text user interface programs included Norton Commander, Dos Navigator, Volkov Commander, Quarterdesk DESQview, and SideKick. Graphical user interface programs included Digital Research's Graphical Environment Manager (originally written for CP/M) and GEOS.

Eventually, the manufacturers of major DOS systems began to include their own environment managers. MS-DOS/IBM DOS 4 included DOS Shell;[36] DR DOS 5.0, released the next year, included ViewMAX, based upon GEM.[37]


By its original design, MS-DOS and PC DOS were single task operating systems, but Digital Research's Concurrent DOS 3.1, based on their Concurrent CP/M-86 operating system, introduced DOS multi-user and multi-tasking capabilities in March 1984. This system later evolved into FlexOS, Multiuser DOS, System Manager and REAL/32.

MS-DOS and PC DOS 5.0 introduced task switching via DOSSHELL in June 1991,[38] and DR DOS 6.0 with TASKMAX, optionally aided by ViewMAX, in September 1991.[13]:320–324

MS-DOS and PC DOS never had a multitasking capability;[39] whereas pre-emptive multitasking in virtual DOS machines was introduced into the DR DOS line of products with Novell DOS 7 in December 1993 with EMM386 /MULTI and TASKMGR.[40]

Programs such as VMIX or DESQView could multitask even on an 8088, which did not have hardware support for multitasking due to its lack of protected mode.


Several limitations plague the DOS architecture. The original 8088 microprocessor could only address 1 megabyte of physical RAM. With additional hardware devices being mapped into this range, the highest amount of available memory was 640 kilobytes, known as conventional memory. Due to DOS's structure, this was assumed to be the maximum, and DOS could not address more than this. An early workaround was expanded memory; later, extended memory was developed with the 80286. While these provided usable memory to applications, they still had to start in conventional memory, thereby using part of the existing 640 KB. Newer version of DOS starting with 5.0 could relocate parts of itself as well as device drivers into previously unused areas in the adapter range between 640 KB and 1 MB (upper memory) and the HMA (high memory area), thereby freeing up more conventional memory for DOS programs to run within the first 640 KB of the system. With DPMS, it was possible to load and execute drivers in extended memory, leaving only a stub in conventional memory as an interface with the DOS system. With the 80386 microprocessor's redesigned protected mode, DOS extenders and the DOS Protected Mode Interface were able to provide additional memory to extended DOS applications running in protected mode.[41][42] The XMS drivers in many versions of DOS have a limit of 64 MB,[43] but this limit is not in the underlaying architecture, but in specific implementations. Using third-party XMS drivers, extended DOS applications can take advantage of up to theoretical 4 GB of memory.

DOS also has an upper limit to the size of hard disk partitions. This has two causes. First, many DOS-type systems never had support for any file system newer than FAT16, which, by design, does not allow partitions larger than 2 gigabytes.[44] Additionally, DOS accesses the hard disk by calling Interrupt 13h. Older version of DOS utilized the cylinder-head-sector (CHS) system of mapping the disk. Under this system, only the first 8.4 GB or 7.84 GiB[45] are visible to the operating system.[46] Newer versions of DOS also deploy LBA-access through Interrupt 13h. Thereby, they can address hard disks up to 128 GB in size. Newer versions of DOS also support the FAT32 file system (in addition to FAT12 and FAT16) to allow for partitions larger than 2 GB.

Filenames in the FAT filesystem can not be longer than eight characters, and the filename extension cannot be longer than three. The VFAT extension, introduced with Windows 95, however, allows to store them in hidden directory entries on FAT12, FAT16 or FAT32 volumes.

Using the traditional DOS APIs, the maximum file size remains limited to 4 GB, but 3rd-party extensions such as FAT+ exist to work around this problem.

See also

  • COMMAND.COM, the command line interpreter for DOS and MS-Windows 9x
  • VGA compatible text mode, the base of DOS’s TUI on IBM PC compatibles
  • Timeline of x86 DOS operating systems


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  38. ^ Murdock, Everett. DOS the Easy Way. EasyWay Downloadable Books. pp. 83. ISBN 0923178023. 
  39. ^ Perry, Greg; Jill Freeze (2003). Sams Teach Yourself Windows XP Computer Basics All in One. Sams Publishing. pp. 445. ISBN 0672325357. 
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  43. ^ Physical RAM limit for DOS.
  44. ^ Mueller, Scott (2003). Upgrading and Repairing PCs. Que Publishing. p. 812. ISBN 0789729741. 
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  • IBM Corp., IBM, (January 1984). “IBM DOS Release 2.10 Cloth bound retail hard board box”. 1st edition. IBM Corp. Item Number. 6183946
  • IBM Corp., IBM, (January 1984). “Disk Operating System User's guide (DOS Release 2.10)”. 1st edition. Microsoft Corp. (100 pages including colour illustrations) Item Number. 6183947
  • IBM Corp., IBM, (January 1984). “Disk Operating System Manual (DOS Release 2.10)”. 1st edition. Microsoft Corp. (574 looseleaf pages in 3 ring folder) Item No. 6183940
  • IBM Corp., IBM, (May 1984). “BASIC HANDBOOK General Programming Information (BASIC Release 3.0)”. 3rd edition. IBM Corp. (127 pages) Item Number 6361129
  • IBM Corp., IBM, (May 1984). “BASIC Reference (BASIC Release 3.0)”. 3rd edition. IBM Corp. Item Number 6361134

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