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MS-DOS (for Microsoft Disk Operating System) is an operating system commercialized by Microsoft. It was the most widely used member of the DOS family of operating systems and was the dominant operating system for the PC compatible platform during the 1980s. It has gradually been replaced on consumer desktop computers with various generations of the Windows operating system.

MS-DOS was originally released in 1981 and had eight major versions released before Microsoft stopped development in 2000. It was the key product in Microsoft's growth from a programming languages company to a diverse software development firm, providing the company with essential revenue and marketing resources.


MS-DOS began as QDOS (for Quick and Dirty Operating System), written by Tim Paterson for computer manufacturer Seattle Computer Products (SCP) in 1980. It was marketed by SCP as 86-DOS because it was designed to run on the Intel 8086 processor. QDOS function calls were based on the dominant CP/M-80 operating system, written by Digital Research, but it used a different file system. In a sequence of events that would later inspire much folklore, Microsoft negotiated a license for QDOS from SCP in December 1980 for $25,000, then re-licensed QDOS to IBM. Microsoft then acquired all rights to QDOS for only $50,000 from SCP in July, 1981, shortly before the PC's release.

IBM and Microsoft both released versions of DOS; the IBM version was supplied with the IBM PC and known as PC-DOS. Originally, IBM only validated and packaged Microsoft developments, and thus IBM's versions tended to be released shortly after Microsoft's. However, MS-DOS 4.0 was actually based on IBM PC-DOS 4.0, as Microsoft was by then concentrating on OS/2 development. Microsoft released its versions under the name "MS-DOS", while IBM released its versions under the name "PC-DOS." Initially, when Microsoft would license their OEM version of MS-DOS, the computer manufacturer would customize its name (i.e. TandyDOS, Compaq DOS, etc). Most of these versions were identical to the official MS-DOS; however, Microsoft began to insist that OEMs start calling the product MS-DOS. Eventually, only IBM resisted this move.

Computer advertisements of this period often claimed that computers were "IBM-Compatible" or very rarely "MS-DOS compatible." The two terms were not synonyms. There were computers which used MS-DOS which could not run all the software that an IBM-Compatible machine could. An example is the Pivot, which used MS-DOS but was not IBM-Compatible.

Programs written specifically for IBM compatibles could run faster by bypassing slow MS-DOS functions, e.g. by writing video information directly to the area of memory assigned to it.

The versions of MS-DOS and PC DOS and their releases are the following:

  • PC DOS 1.0 , August 1981 , initial release with the first IBM-PC (COMMAND.COM is 4959 bytes)
  • PC DOS 1.1 , May 1982 , support for 320 kB double-sided floppy disk
  • MS-DOS 1.25 , May 1982 , first release for IBM PC compatibles marketed under different brands (COMMAND.COM is 4986 bytes)
  • MS-DOS 2.0 , March 1983 , introduced subdirectories, handle-based file operations, command input/output redirection, and pipes. Microsoft decided to use backslashes as pathname separators rather than slashes as on Unix apparently due to the latter character being used as the switch character in most DOS and CP/M programs. Adds support for hard drives and 360KB floppy disks
  • PC DOS 2.1 , October 1983 , support for IBM PCjr
  • MS-DOS 2.11 , March 1984 , non-English language and date format support (COMMAND.COM is 16229 bytes)
  • MS-DOS 2.25 , October 1985 , better support for Japanese Kanji, and Korean character sets, shipped to western Pacific countries only
  • MS-DOS 3.0 , August 1984 , added support for PC AT: 1.2 MB floppy disks and hard disk partitions of up to 32MB, one primary and one "logical drive" in an "extended partition"
  • MS-DOS 3.1 , November 1984 , support for Microsoft networking
  • MS-DOS 3.2 , January 1986 , support for 3.5 inch, 720 kB floppy disk drives (v 3.21 COMMAND.COM is 23612 bytes)
  • PC DOS 3.3 , April 1987 , support for IBM PS/2: 1.44 MB floppy disk drives, added codepage support (international character sets) (COMMAND.COM is 25307 bytes)
  • MS-DOS 3.3 , August 1987 , supported multiple logical drives (COMMAND.COM is 25276 bytes)
  • MS-DOS 4.0 , June 1988 , derived from IBM's codebase rather than Microsoft's
  • PC DOS 4.0 , July 1988 , added DOS Shell & support for hard disks of >32MB using the format from Compaq DOS 3.31. But it had many bugs and less free conventional memory than before. Generally regarded as an unpopular release
  • MS-DOS 4.01 , December 1988 , bug-fix release (COMMAND.COM is 37557 bytes)
  • MS-DOS 5.0 , June 1991 , memory management, full-screen editor, QBasic programming language, online help, and DOS Shell gains task switcher. Also add file transfer facillity licensed from Rupp Technology (FastLynx) (COMMAND.COM is 47845 bytes)
  • MS-DOS 6.0 , March 1993 , added DoubleSpace disk compression and other features (COMMAND.COM is 52925 bytes)
  • MS-DOS 6.2 , November 1993 , bug fix release (COMMAND.COM is 54619 bytes)
  • MS-DOS 6.21 , February, 1994 , following Stac Electronics lawsuit, removed DoubleSpace disk compression (COMMAND.COM is 54619 bytes)
  • PC DOS 6.3 , April 1994
  • MS-DOS 6.22 , June 1994 , last official stand-alone version. DoubleSpace replaced with non-infringing but compatible DriveSpace tool (COMMAND.COM is 54645 bytes)
  • PC DOS 7.0 , April, 1995 , bundles Stacker in place of DriveSpace
  • MS-DOS 7.0 , August 1995 , shipped embedded in Windows 95. Included Logical block addressing and Long File Name (LFN) support
  • MS-DOS 7.1 , August 1996 , shipped embedded in Windows 95B (OSR2) (and Windows 98 in June 1998). Added support for FAT32 file system
  • MS-DOS 8.0 , September 2000 , shipped embedded in Windows Me. Last version of MS-DOS. Removes SYS command, ability to boot to command line and other features
  • PC DOS 2000 , year 2000-compliant version with minor additional features. Final member of the MS-DOS family


On the IBM PC (and clones) platform, the initial competition to the PC-DOS/MS-DOS line came from Digital Research, whose CP/M-86 operating system had inspired MS-DOS. Digital Research continued to develop CP/M-86 and offer it to computer manufacturers as an alternate to MS-DOS and Microsoft's licensing requirements.

In the business world, the PC platform that MS-DOS was tied to faced competition from the Unix operating system which ran on many different hardware architectures. Microsoft even sold a version of Unix called Xenix. The Pick operating system was another alternative OS for the PC.

In the emerging world of home users, a variety of other hardware platforms were in serious competition with the IBM PC: the Apple II, early Apple Macintosh, the Commodore 64 and others. At first, the competition for these other platforms was with IBM PC computers running MS-DOS. With the advent of IBM PC clones all running on Intel processors, the name IBM became less important to home users. What was important was keeping up with Intel's steadily increasing clock speeds and the ability to run MS-DOS.

Microsoft and IBM together began what was intended as the follow-on to DOS, called OS/2. When OS/2 was released in 1987, Microsoft began an ad campaign announcing that "DOS is Dead", boldly proclaiming version 4 was the last full release.

MS-DOS had grown in spurts, with many significant features being taken (or duplicated) from other products and operating systems, as well as reverse-engineering tools and utilities including Norton Utilities, PC Tools (Microsoft Anti-Virus), QEMM expanded memory manager, Stacker disk compression, and so on. The advent of OS/2, which offered a number of advanced features which had been written together, was seen as the legitimate heir to the "kludgey" DOS platform.

Digital Research, recognizing the need to continue the lower-level platform represented by DOS, then developed DR DOS 5, which mirrored the OS/2 "platform integration" model by adding features which were available only as third-party add-ons for MS-DOS. Unwilling to lose any portion of the market, Microsoft responded by announcing the "pending" release of MS-DOS 5.0 in May of 1990. This effectively killed most DR DOS sales, until the actual release of MS-DOS 5.0 in June 1991. Digital Research brought out DR DOS 6, which sold well until the "pre-announcement" of MS-DOS 6.0 again stifled the sales of DR DOS.

Microsoft has been accused of carefully orchestrating leaks about future versions of MS-DOS in an attempt to create what in the industry is called FUD (fear, uncertainty, and doubt) regarding DR DOS. For example, in October 1990, shortly after the release of DR DOS 5.0, and long before the eventual June 1991 release of MS-DOS 5.0, stories on feature enhancements in MS-DOS started to appear in InfoWorld and PC Week. Brad Silverberg, Vice President of Systems Software at Microsoft and General Manager of its Windows and MS-DOS Business Unit, wrote a forceful letter to PC Week (November 5, 1990), denying that Microsoft was engaged in FUD tactics ("to serve our customers better, we decided to be more forthcoming about version 5.0") and denying that Microsoft cops features from DR DOS: "The feature enhancements of MS-DOS version 5.0 were decided and development was begun long before we heard about DR DOS 5.0. There will be some similar features. With 50 million MS-DOS users, it shouldn't be surprising that DRI has heard some of the same requests from customers that we have." (Schulman et al. 1994).

The pact between Microsoft and IBM to promote OS/2 began to fall apart in 1990 when Windows 3.0 became a marketplace success. Much of Microsoft's further contributions to OS/2 also went in to creating a third GUI replacement for DOS, Windows NT.

IBM, which had already been developing the next version of OS/2, carried on development of the platform without Microsoft and sold it as the alternative to DOS and Windows.

End of MS-DOS

MS-DOS has effectively ceased to exist as a product. It became the bootstrap for Windows 95, Windows 98 and Windows Me but was integrated as a full product, thus ending the days of a standalone DOS. Today it is still used in various embedded x86 systems due to its simplistic architecture, minimal memory requirements, and minimal processor speed requirements.

Legal issues

As a response to Digital Research's DR-DOS 6.0, which bundled SuperStor disk compression, Microsoft opened negotiations with Stac Electronics, vendor of the most popular DOS disk compression tool, Stacker. Stac was unwilling to meet Microsoft's terms for licensing Stacker and withdrew from the negotiations. In the due diligence process, Stac engineers had shown Microsoft some Stacker source code.

Soon, MS-DOS 6.0 was released, including the Microsoft DoubleSpace disk compression utility program. Stac successfully sued Microsoft for patent infringement regarding the compression algorithm used in DoubleSpace. This resulted in the release of MS-DOS 6.21, which had disk-compression removed. Shortly afterwards came version 6.22, with a new version of the disk compression system, DriveSpace, rewritten to avoid the infringing code.

Prior to 1995, Microsoft licensed MS-DOS (and Windows) to computer manufacturers under three types of agreement: per-processor (a fee for each system the company sold), per-system (a fee for each system of a particular model), or per-copy (a fee for each copy of MS-DOS installed). The largest manufacturers used the per-processor arrangement, which had the lowest fee. This arrangement made it expensive for the large manufacturers to migrate to any other operating system, such as DR-DOS. In 1991 the US government Federal Trade Commission began investigating Microsoft's licensing procedures resulting in a 1994 settlement agreement limiting Microsoft to per-copy licensing. Digital Research did not gain by this settlement, and years later its successor in interest Caldera sued Microsoft for damages. This lawsuit was settled with a monetary payment of 150 million dollars.

Microsoft also used a variety of tactics in MS-DOS and several of their applications and development tools that, while operating perfectly when running on genuine MS-DOS (and PC-DOS), would break when run on another vendor's implementation of DOS. Notable examples of this practice included:

  • Microsoft QuickC v2.5, Programmer's Workbench and Microsoft C v6.0 modified the program's Program Segment Prefix using undocumented DOS functions, and then checked whether or not the associated value changed in a fixed position within the DOS data segment (also undocumented).
  • The (once infamous) AARD code, a block of code in Windows 3.1's WIN.COM that was XOR encrypted, self-modifying, and deliberately obfuscated, that used various undocumented DOS structures and functions to determine whether or not Windows really was running on MS-DOS.
  • Interrupt routines called by Windows to inform MS-DOS that Windows is starting/exiting, information that MS-DOS retained in an IN_WINDOWS flag, in spite of the fact that MS-DOS and Windows were supposed to be to separate products.


MS-DOS was not designed to be a multi-user or multitasking operating system, but many attempts were made to add these capabilities. Terminate and Stay Resident (TSR) system calls were originally designed for device drivers and extensible plugins that enhanced or added features. Companies such as Borland began to tap into the TSR design with products like SideKick. Add-on environments like TopView and especially DESQview attempted to provide multitasking, and achieved some success when later combined with the virtual 8086 mode and virtual memory features of the Intel 80386 and later processors.

User interface

MS-DOS employs a command line interface and a batch scripting facility via its command interpreter, command.com. MS-DOS was designed so users could easily substitute a different command line interpreter, for example 4DOS.

Beginning with version 4.0, MS-DOS included a file manager program with a quasi-graphical user interface (the DOS Shell) that featured menus, split windows, and program shortcuts using character mode graphics.

Windows NT

Windows NT, although not based on DOS, provides a command-line interface similar to MS-DOS's character-mode interface. This command line is provided by a native executable, cmd.exe. Many command-line applications (known as console applications) for Windows are incorrectly referred to as DOS applications, when actually they are full Windows applications which use the console for their output rather than a graphical interface, and cannot be run under any version of MS-DOS.

Windows NT can run MS-DOS programs through the use of the NTVDM (NT Virtual DOS Machine), and the 16-bit command.com interpreter from MS-DOS 5.0 is still included to maintain application compatibility with programs that expect it (This is illustrated by the output produced by the command "command.com /k ver", which displays "MS-DOS Version 5.00.500" in the console window). The command "ver" returns the string "Microsoft(R) Windows DOS" when executed under command.com, but "Microsoft Windows XP (Version 5.1.2600)" (or similar depending on the version of NT) when run from cmd.exe.

Recent versions of NT for x64 architectures, including Windows XP Professional x64 Edition, Windows Server 2003 x64 and the upcoming Windows Vista x64, no longer include the NTVDM and can therefore no longer natively run MS-DOS (or 16-bit Windows) applications. For MS-DOS programs, however, there exists an alternative in the form of emulators such as VMWare, Bochs, DOSBox, etc.

MS-DOS compatibility with other Microsoft operating systems

From 1983 onwards various companies have worked on graphical user interfaces capable of running on PC hardware. With DOS being the dominant operating system several companies released alternate shells, e.g. Microsoft Word for DOS, XTree, and the Norton Shell. However, this required duplication of effort and did not provide much consistency in interface design (even between products from the same company).

Later, in 1985, Microsoft Windows was released as Microsoft's first attempt at providing a consistent user interface (for applications). The early versions of Windows ran on top of MS-DOS and its clones. At first Windows met with little success, but this was also true for most other companies' efforts as well, for example GEM. After version 3.0, Windows gained marked acceptance.

Later versions (Windows 95, Windows 98 and Windows Me) used the DOS boot process to launch itself into protected mode. Basic features related to the file system, such as long file names, were only available to DOS when running as a subsystem of Windows. Windows NT ran independently of DOS but included a DOS subsystem so applications could run in a virtual machine under the new OS. With the latest Windows releases, even dual-booting MS-DOS is problematic as DOS can no longer read the basic file system.

Related systems

Several similar products were produced by other companies. In the case of PC-DOS and DR-DOS, it is common but incorrect to call these "clones". Given that Microsoft manufactured PC-DOS for IBM, PC-DOS and MS-DOS were (to continue the genetic analogy) "identical twins" that diverged only in adulthood and eventually became quite different products; DR-DOS was a clone of itself once removed.

  • PC-DOS
  • DR-DOS, Novell DOS, and OpenDOS
  • FreeDOS, FreeDOS 32, and GNU/DOS
These products are collectively referred to as DOS.

source: wikipedia

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