AskDefine | Define DOS

Dictionary Definition



1 the federal department in the UnitedStates that sets and maintains foreign policies; "the Department of State was created in 1789" [syn: Department of State, State Department, State]
2 an operating system that is on a disk [syn: disk operating system]

User Contributed Dictionary

see DoS


  1. Disk Operating System, sometimes MS-DOS for Microsoft-DOS, DR-DOS, etc.
  1. Date[s] of Service
  2. Date of Separation
  3. Department of State
  4. Density of states

Extensive Definition

DOS is a family of closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995 (or until about 2000, if one includes DOS-based Microsoft Windows versions (Windows 95, Windows 98, and Windows ME). Related systems include MS-DOS, PC-DOS, DR-DOS (and Novell DOS and OpenDOS, which were based on DR-DOS), FreeDOS, PTS-DOS, ROM-DOS, JM-OS and several others.
All of these operating systems run on machines with the Intel x86 or compatible CPUs, mainly the IBM PC and compatibles. Initially, DOS was not restricted to these, and machine-dependent versions of DOS and similar operating systems were produced for many non-IBM-compatible x86-based machines.
DOS is a single-user, single-task operating system with basic kernel functions that are non-reentrant code; once a process is begun, it must be allowed to run until finished before the same process can be used again. The DOS kernel provides various functions for programs, like displaying characters on-screen, reading a character from the keyboard, and accessing disk files.
In spite of the common usage, there has never been a microcomputer operating system called simply "DOS" (though there was a 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 (AmigaDOS, AMSDOS, ANDOS, Apple DOS, Atari DOS, Commodore DOS, CSI-DOS, ProDOS, TRS-DOS).


MS-DOS (and rebranded IBM PC-DOS which was licensed therefrom), and its predecessor, 86-DOS, were inspired by CP/M (Control Program / (for) Microcomputers) from Digital Research, which was the dominant disk operating system for 8-bit Intel 8080 and Zilog Z80 based microcomputers.
IBM was introducing their first microcomputer, built with the Intel 8088 microprocessor, and needed an operating system. In 1980, IBM initially approached Microsoft CEO Bill Gates (possibly believing that Microsoft owned CP/M due to the Microsoft 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". DR founder Gary Kildall refused, and IBM withdrew.
Microsoft also licensed their system to multiple computer companies, who supplied MS-DOS for their own hardware and 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. The PC used the Intel 8088 CPU, which used the same instruction set as the 8086. They split development of their DOS systems as a result. MS-DOS was partially transformed into Windows; the last version of PC-DOS was PC-DOS 2000, released in 1998.
The FreeDOS project began June 26, 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 September 3, 2006. FreeDOS does not require license fees or royalties.
The only other DOS-type systems that are actively distributed now are Enhanced DR-DOS, the Russian PTS-DOS, and embeddable ROM-DOS. Only one commercially available DOS system is sold, DR-DOS.
Since 2005, there is a 100% GPL licensed version of DOS, called NX-DOS. Currently under development, 16-bit, real-time, networkable, boots from a floppy, and has an incomplete USB driver. Started on 1992 as a personal project, it was released as GPL on 2005

DOS and Microsoft Windows

Early versions of Microsoft Windows were an application that ran on top of a separate version of DOS. With Windows for Workgroups 3.11, DOS was essentially reduced to the role of a boot loader for the Windows kernel.
With Windows 95, 98, and ME, MS-DOS is included as the boot loader rather than being sold separately. With Windows 95 and 98, but not ME, the MS-DOS component could be run without starting Windows. Often, Windows 9x can be loaded as a version of DOS despite saying "Loading Windows" in lieu of the typical loading message.
The true 32-bit versions of Windows starting with NT and including 2003, XP, and Vista, run entirely independent of DOS. Most versions include a DOS subsystem, NTVDM, that runs a modified version of MS-DOS 5.0 in a virtual machine for the purpose of running DOS software and Windows command-line programs of similar appearance which are not compatible with true MS-DOS.


See Comparison of x86 DOS operating systems for a timeline and comparison of versions.


Accessing hardware under DOS

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, 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.

Reserved device names under DOS

There are reserved device names in DOS that cannot be used as filenames regardless of extension; these restrictions also affect several Windows versions, in some cases causing crashes and security vulnerabilities.
A partial list of these reserved names is: NUL:, COM1: or AUX:, COM2:, COM3:, COM4:, CON:, LPT1: or PRN:, LPT2:, LPT3:, and CLOCK$.
More recent versions of both MS-DOS and IBM-DOS allow reserved device names without the trailing colon; e.g., PRN refers to PRN:.
The NUL filename redirects to a null file, similar in function to the UNIX device /dev/null. It is best suited for being used in batch command files to discard unneeded output. If NUL is copied to a file that already exists, it will truncate the target file; otherwise, a zero byte file will be created. (Thus, copy NUL foo is functionally similar to the UNIX commands cat foo and cp /dev/null foo.) Naming a file as NUL, regardless of extension, could cause unpredictable behavior in most applications. Well-designed applications will generate an error stating that NUL is a DOS reserved filename; others generate the file but whatever the program saves is lost; finally, some applications may hang or leave the computer in an inconsistent state, requiring a reboot.

Drive naming scheme

Under Microsoft's DOS operating system and its derivatives 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 permits the use of both letters for one drive, and DOS will ask to swap disks. This permits copying from floppy to floppy or having a program run from one floppy while having 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 assigning the active primary partition on each drive letters first, then making a second pass over the drives to allocate letters to logical drives in the extended partition, then making a third, which gives the other non-active primary partitions their names. (Always assumed, they exist and contain a DOS-readable file system.) Lastly, DOS allocate letters for optical disc drives, RAM disks, and other hardware. Letter assignments usually occur in the order of the drivers loaded, but the drivers can instruct DOS to assign a different letter. An example is network drives, for which the driver will assign letters nearer the end of the alphabets.
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 with a primary partition to an original hard drive that contains logical drives in extended partitions. As primary partitions have higher priority than the logical drives, it will change drive letters in the configuration. Moreover, attempts to add a new hard drive with only logical drives in an extended partition would still disrupt the letters of RAM disks and optical drives. This problem persisted through the 9x versions of Windows until NT, which preserves the letters of existing drives until the user changes it.

The DOS boot sequence

The boot information for PC-compatible computers is located at track zero. In DOS, this code will read the DOS BIOS into memory and execute it. The BIOS is located in IBMBIO.COM on DR DOS and PC DOS, and IO.SYS on MS DOS. The BIOS will then load the DOS kernel, located in IBMDOS.COM (PC DOS or DR DOS) or MSDOS.SYS (MS DOS). In the Windows DOS versions (MS DOS 7 and 8), the BIOS and kernel are combined in IO.SYS, and MSDOS.SYS is a text configuration file.
The kernel then executes the CONFIG.SYS file. In CONFIG.SYS, the SHELL command specifies the location of the shell (typically COMMAND.COM). The shell will then launch, and open a startup batch file (typically AUTOEXEC.BAT)

DOS emulators

Under Linux it is possible to run copies of DOS and many of its clones under DOSEMU, a Linux-native virtual machine for running real mode programs. There are a number of other emulators for running DOS under various versions of UNIX, even on non-x86 platforms, such as DOSBox.
DOS emulators are gaining popularity among Windows XP users because this system is incompatible with pure DOS. They can be used to run software (often 'abandonware') made for DOS. One of the most famous emulators is DOSBox, designed for game-playing on modern operating systems. Another emulator Tao ExDOS is designed for business & printing solutions. VDMSound is also popular on Windows XP for its GUI and sound support.
It is possible to run DOS applications under a Virtual PC environment, allowing better compatibility than DOS emulators as a legitimate version of MS-DOS can be installed which should allow all but the most stubborn applications to run.


See also

External links

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