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DOS stands for Disk Operating System and is used as a shorthand term for 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.

In spite of the common usage, there has never been a microcomputer operating system called simply "DOS" (though there was a mainframe operating system with that name 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).

Operating system structure

All DOS-type 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.

History

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.

In 1980, IBM was introducing their first microcomputer, built with the Intel 8088 microprocessor, and 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 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.

IBM again approached Bill Gates. Gates in turn approached Seattle Computer Products- there, Tim Paterson had developed a variant of CP/M-80, intended as an internal product for testing the SCP's new 16-bit 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.

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. 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. CP/M cost almost $200 more than PC-DOS, however, and sales were low. CP/M faded, with MS-DOS and PC-DOS becoming the marketed operating system for PCs and PC compatibles.

Digital Research attempted to regain the market lost from CP/M-86 with DR-DOS, compatible with both MS-DOS and CP/M-86 software. Digital Research was later bought by Novell, and DR DOS became Novell DOS 7, and later became part of Caldera (as OpenDOS and DR DOS 7), 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. 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.

Free systems

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.com 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. Some computer manufactures, including Dell and HP, sell computers with FreeDOS.

A 100% GPL licensed DOS, NX-DOS, also exists. It is currently under development, 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

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.

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.

Versions

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

Operations

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

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.

A partial list of these reserved names is: NUL:, COM1: or AUX:, COM2:, COM3:, COM4:, CON:, LPT1: or PRN:, LPT2:, LPT3:, and CLOCK$.

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 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.

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).

Limitations

Several limitations plagued the DOS architecture. The original 8088 microprocessor could only address 1 mebibyte of physical RAM. With additional hardware devices being mapped into this range, the highest amount of available memory was 640 kibibytes, known as conventional memory. Due to DOS' structure, this was assumed to be the maximum, and DOS could not address more than this. Early workaround included expanded memory and extended memory. Later, with the advent of protected mode in the 80286 and 80386 microprocessor, DOS extenders and the DOS Protected Mode Interface were utilized to provide additional memory to applications.

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 2.1 gibibytes. Additionally, DOS accesses the hard disk by calling Interrupt 13, which utilizes the cylinder-head-sector system of mapping the disk. Under this system, only 8 gibibytes are visible to the operating system. Newer operating systems accomplished disk access via software means, e.g. 32-bit disk access.

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 and Vista users because these systems are 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. Tao ExDOS emulator is used for business applications. VDMSound is also popular in Windows 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

• COMMAND.COM, the command line interpreter for DOS and Windows 9x
• MS-DOS API
• MS-DOS
• IBM PC-DOS
• DR-DOS
• FreeDOS

References

1. See MS-DOS
2. RESIDENT PROGRAMS
3. ^ The rest of the story: How Bill Gates beat Gary Kildall in OS war, Part 1 | ScobleShow: Videoblog about geeks, technology, and developers
4. ^ The Unusual History of MS-DOS The Microsoft Operating System
5. Microsoft Widens Its Split With I.B.M. Over Software
6. I.B.M. Executive Describes Price Pressure by Microsoft
7. Jim Hall (2002-03-25). "The past, present, and future of the FreeDOS Project". Retrieved 2008-06-14.
8. Hall, Jim (September 23, 2006). "History of FreeDOS". freedos.org. Retrieved 2007-05-28.{{cite web}}: CS1 maint: year (link)
9. Jim Hall (2007-07-13). "Jim Hall". Retrieved 2008-06-12.
10. "Dell PCs Featuring FreeDOS". Retrieved 2008-06-14.
11. "GPL'd DOS workalike adds features". 2007-04-01. Retrieved 2008-06-01.
12. ^ James Bannan (2006-13-10). "HOW TO: Coax retro DOS games to play on Vista". Retrieved 2008-07-03. {{cite web}}: Check date values in: |date= (help)
13. http://www.zingtech.com/features/gamedev/gnewprog.htm
14. Juniper.net
15. PC Magazine
16. ^ http://www.storagereview.com/guide/partLetter.html
17. http://www.pcguide.com/ref/hdd/file/structBoot-c.html
18. "DOS: still thriving after all these years". Software Magazine. Findarticles.com. May 1990. Retrieved 2008-07-10.
19. Duncan, Ray (1991). Extending DOS: A Programmer's Guide to Protected-Mode DOS (2 ed.). Addison-Wesley. ISBN 0201567989.
20. Mueller, Scott (2003). Upgrading and Repairing PCs. Que Publishing. p. 812. ISBN 0789729741.
21. "The Int 13 Interface". The PC Guide. storagereview.com. Retrieved 2008-07-10.
22. ^ "DOSBox Information". Retrieved 2008-05-18.
23. "DOSEMU Home". 2007-05-05. Retrieved 2008-07-03.
24. "Tao ExDOS Information". Retrieved 2008-05-18.
25. Installing DOS under Virtual PC

External links

• MS-DOS Reference — Not just for MS-DOS but also for other DOSses on the PC platform.
• DOS and Windows timeline

• DOS on IBM PC compatibles
• Disk operating systems
• Orphan initialisms