screenshot showing the command-line interface, directory structure and version information
"DOS" (, ) is a platform-independent acronym for Disk Operating System which later became a common shorthand for disk-based operating systems on IBM PC compatibles. DOS primarily consists of Microsoft's MS-DOS and a rebranded IBM version under the name PC DOS, both of which were introduced in 1981. Later compatible systems from other manufacturers are DR DOS (1988), ROM-DOS (1989), PTS-DOS (1993), and FreeDOS (1998). MS-DOS dominated the IBM PC compatible market between 1981 and 1995.
Dozens of other operating systems also use the acronym "DOS", beginning with the mainframe DOS/360 and successors from 1966. Others include Apple DOS, Apple ProDOS, Atari DOS, Commodore DOS, TRSDOS, and AmigaDOS.
IBM PC DOS (and the separately sold MS-DOS) and its predecessor, 86-DOS, was based on Digital Research's CP/M—the dominant disk operating system for 8-bit Intel 8080 and Zilog Z80 microcomputers—but instead ran on Intel 8086 16-bit 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). 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". Digital Research founder Gary Kildall refused, and IBM withdrew.
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 Central processing unit card for the S-100 bus. The system was initially named Seattle Computer Products (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.Within a year Microsoft licensed MS-DOS to over 70 other companies, which supplied the operating system 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. This version of DOS is generally referred to as "European MS-DOS 4" because it was developed for International Computers Limited and licensed to several European companies. This version of DOS supports preemptive multitasking, shared memory, device helper services and New Executable ("NE") format executables. None of these features were used in later versions of DOS, but they were used to form the basis of the OS/2 1.0 kernel. This version of DOS is distinct from the widely released PC DOS 4.0 which was developed by IBM and based upon DOS 3.3.
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 PalmDOS and Novell DOS; later, it was part of Caldera (company) (under the names OpenDOS and DR-DOS 7.02/DR-DOS 7.03), Lineo, and DeviceLogics.
Gordon Letwin wrote in 1995 that "DOS was, when we first wrote it, a one-time throw-away product intended to keep IBM happy so that they'd buy our languages". Microsoft expected that it would be an interim solution before Xenix. The company planned to over time improve MS-DOS so it would be almost indistinguishable from single-user Xenix,
XEDOS, which would also run on the Motorola 68000, Zilog Z-8000, and LSI-11; they would be upward compatible with Xenix, which ''BYTE'' in 1983 described as "the multi-user MS-DOS of the future".
IBM, however, did not want to replace DOS. After AT&T began selling Unix, Microsoft and IBM began developing OS/2 as an alternative. The two companies later had a series of disagreements over two successor operating systems to DOS, OS/2 and Windows. They split development of their DOS systems as a result. The last retail version of MS-DOS was MS-DOS 6.22; after this, MS-DOS became part of Windows 95, 98 and Me. The last retail version of PC DOS was PC DOS 2000 (also called PC DOS 7 revision 1), though IBM did later develop PC DOS 7.10 for OEMs and internal use.
The FreeDOS project began on 26 June 1994, when Microsoft announced it would no longer sell
support MS-DOS. Jim Hall (programmer) 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.COM command line interpreter (shell), and core utilities were created by pooling code they had written
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
Early versions of Microsoft Windows ran on a separate version of MS-DOS. With DOS no longer required to use Windows, the majority of users stopped using it directly.
Available DOS systems in 2012 are FreeDOS, DR-DOS, ROM-DOS, PTS-DOS, RxDOS and REAL/32. Some computer manufacturers, including Dell and Hewlett-Packard, sell computers with FreeDOS and DR-DOS as Original equipment manufacturer operating systems.
DOS's 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. ROM-DOS was used as the embedded system on the Canon PowerShot Pro 70.
On Linux, it is possible to run copies of DOS and many of its clones on ''DOSEMU'', a Linux-native virtual machine for running DOS programs at near native speed. There are a number of other emulators for running DOS on various versions of Unix and Microsoft Windows such as DOSBox. DOSBox is designed for legacy gaming (e.g. ''King's Quest'', ''Doom (1993 video game)'') on modern operating systems.
MS-DOS and IBM PC DOS related operating systems are commonly associated with machines using the Intel x86
compatible CPUs, mainly IBM PC 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. As long as application programs used DOS APIs instead of direct hardware access, they could run on both IBM-PC-compatible and incompatible machines. The original FreeDOS kernel, DOS-C, derived from DOS/NT for the Motorola 68000 series of CPUs in the early 1990s. While these systems loosely 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-tasking operating system with basic kernel (computer science) functions that are reentrant (subroutine): only one program at a time can use them and DOS itself has no functionality to allow more than one program to execute at a time. The DOS kernel provides DOS API (an ''application program interface''), like character I/O, file management, memory management, program loading and termination.
DOS provides the ability for shell scripting via batch files (with the filename extension
.BAT). Each line of a batch file is interpreted as a program to run. Batch files can also make use of internal commands, such as GOTO and Conditional (programming).
The operating system offers an application programming interface that allows development of character-based applications, but not for accessing most of the computer hardware, such as Video cards, computer printers,
computer mouse. 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.
The DOS system files loaded by the boot sector must be Fragmentation (computer) and be the first two FAT directory table. 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.
In PC DOS and DR DOS 5.0 and above, the DOS system files are named IBMBIO.COM instead of IO.SYS and IBMDOS.COM instead of MSDOS.SYS. Older versions of DR DOS used DRBIOS.SYS and DRBDOS.SYS instead.
Starting with MS-DOS 7.0 the binary system files IO.SYS and MSDOS.SYS were combined into a single file IO.SYS whilst MSDOS.SYS became a configuration file similar to CONFIG.SYS and AUTOEXEC.BAT. If the MSDOS.SYS BootGUI directive is set to
0, the boot process will stop with the command processor (typically COMMAND.COM) loaded, instead of executing WIN.COM automatically.
DOS uses a filesystem which supports 8.3 filenames: 8 characters for the filename and 3 characters for the extension. Starting with DOS 2 hierarchical directories are supported. Each directory name is also 8.3 format but the maximum directory path length is 64 characters due to the internal current directory structure (CDS) tables that DOS maintains. Including the drive name, the maximum length of a fully qualified filename that DOS supports is 80 characters using the format drive:\path\filename.ext followed by a null byte.
DOS uses the File Allocation Table (FAT) filesystem. This was originally FAT12 which supported up to 4078 clusters per drive. DOS 3.0 added support for FAT16 which used 16-bit allocation entries and supported up to 65518 clusters per drive. Compaq MS-DOS 3.31 added support for FAT16B which removed the 32 MB drive limit and could support up to 512 MB. Finally MS-DOS 7.1 (the DOS component of Windows 9x) added support for FAT32 which used 32-bit allocation entries and could support hard drives up to 137 GB and beyond.
Starting with DOS 3.1, file redirector support was added to DOS. This was initially used to support networking but was later used to support CD-ROM drives with MSCDEX. IBM PC DOS 4.0 also had preliminary installable file system (IFS) support but this was unused and removed in DOS 5.0. DOS also supported Block Devices ("Disk Drive" devices) loaded from CONFIG.SYS that could be used under the DOS file system to support network devices.
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
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.
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.
The reserved names are:
CON (for console),
AUX (for auxiliary),
PRN (for printer) and
LST (for lister), which were introduced with 86-DOS 0.74. 86-DOS 1.10 and PC DOS 1.0 added
NUL. Except for
LST they continued to be supported in all versions of MS-DOS, PC DOS and DR-DOS ever since.
PLT was reconfigurable as well.
In an attempt to provide a more user-friendly environment, numerous software manufacturers wrote file manager that provided users with menu- and/or icon-based interfaces. Microsoft Windows is a notable example, eventually resulting in 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 Borland Sidekick. Graphical user interface programs included Digital Research's Graphics Environment Manager (originally written for CP/M) and GEOS (16-bit operating system).
Eventually, the manufacturers of major DOS systems began to include their own environment managers. MS-DOS/IBM DOS 4 included DOS Shell; DR DOS 5.0, released the following year, included ViewMAX, based upon GEM.
DOS is not a multitasking operating system. DOS did however provide a Terminate and Stay Resident (TSR) function which allowed programs to remain resident in memory. These programs could hook the system timer and/or keyboard interrupts to allow themselves to run tasks in the background
to be invoked at any time preempting the current running program effectively implementing a simple form of multitasking on a program-specific basis. The PRINT command did this to implement background print spooling. Borland Sidekick, a popup personal information manager (PIM), also used this technique.
Terminate and Stay Resident programs were also used to provide additional features not available by default. Programs like CED and DOSKEY provided command line editing facilities beyond what was available in COMMAND.COM. Programs like the Microsoft CD-ROM Extensions (MSCDEX) provided access to files on CD-ROM disks.
Some TSRs could even perform a rudimentary form of task switching. For example, the shareware program Back and Forth (1990) had a hotkey to save the state of the currently-running program to disk, load another program, and switch to it, hence it was possible to switch "back and forth" between programs, albeit slowly due to the disk access required. Back and Forth could not enable background processing however; that needed DESQview (on at least a Intel 80386).
, Arachne (web browser), a 16-bit graphical web browser, dBase, database program, Harvard Graphics, a presentation graphics design program, Lotus 1-2-3, a spreadsheet has been credited with the success of the IBM PC., Norton Commander and XTree, file management utilities, PKZIP, the compression utility that quickly became the standard in file compression, ProComm, Qmodem, and Telix, modem communication programs, Borland Sidekick, personal information manager that could be used from within other programs, WordPerfect, a word processor that was dominant in the 1980s, WordStar, word processor originally for CP/M that became popular on the IBM PC
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