The DOS API is an

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

which originated with

86-DOS 86-DOS (known internally as QDOS, for Quick and Dirty Operating System) is a discontinued operating system developed and marketed by Seattle Computer Products (SCP) for its Intel 8086-based computer kit. 86-DOS shared a few of its commands wi ...

and is used in

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few oper ...

/ PC DOS and other DOS-compatible operating systems. Most calls to the DOS API are invoked using

software interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...

21h ( INT 21h). By calling INT 21h with a subfunction number in the AH processor register and other parameters in other registers, various DOS services can be invoked. These include handling keyboard input, video output, disk file access, program execution, memory allocation, and various other activities. In the late 1980s, DOS extenders along with the

DOS Protected Mode Interface In computing, the DOS Protected Mode Interface (DPMI) is a specification introduced in 1989 which allows a DOS program to run in protected mode, giving access to many features of the new PC processors of the time not available in real mode. It ...

(DPMI) allow the programs to run in either 16-bit or 32-bit protected mode and still have access to the DOS API.

History of the DOS API

The original DOS API in 86-DOS and MS-DOS 1.0 was designed to be functionally compatible with

CP/M CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initially ...

. Files were accessed using file control blocks (FCBs). The DOS API was greatly extended in MS-DOS 2.0 with several Unix concepts, including file access using

file handles In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier (handle) for a file or other input/output resource, such as a pipe or network socket. File descriptors typically have ...

, hierarchical directories and device I/O control. In DOS 3.1,

network redirector In DOS and Windows, a network redirector, or redirector, is an operating system driver that sends data to and receives data from a remote device. A network redirector provides mechanisms to locate, open, read, write, and delete files and submit p ...

support was added. In MS-DOS 3.31, the INT 25h/26h functions were enhanced to support hard disks greater than 32 MB. MS-DOS 5 added support for using

upper memory blocks In DOS memory management, the upper memory area (UMA) refers to memory between the addresses of 640 KB and 1024 KB ( 0xA0000–0xFFFFF) in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB a ...

(UMBs). After MS-DOS 5, the DOS API was unchanged for the successive standalone releases of DOS.

The DOS API and Windows

Windows 9x Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced from 1995 to 2000, which were based on the Windows 95 kernel and its underlying foundation of MS-DOS, both of which were updated in subs ...

, DOS loaded the protected-mode system and graphical shell. DOS was usually accessed from a virtual DOS machine (VDM) but it was also possible to boot directly to real mode MS-DOS 7.0 without loading Windows. The DOS API was extended with enhanced internationalization support and long filename support, though the long filename support was only available in a VDM. With

Windows 95 Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturi ...

OSR2, DOS was updated to 7.1, which added FAT32 support, and functions were added to the DOS API to support this.

Windows 98 Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released t ...

and

Windows ME Windows Millennium Edition, or Windows Me (marketed with the pronunciation of the pronoun "me"), is an operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. It is the successor to Win ...

also implement the MS-DOS 7.1 API, though Windows ME reports itself as MS-DOS 8.0. Windows NT and the systems based on it (e.g.

Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Windows 2000 for high-end and ...

and Windows Vista) are not based on MS-DOS, but use a virtual machine, NTVDM, to handle the DOS API. NTVDM works by running a DOS program in

virtual 8086 mode In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running ...

(an emulation of

real mode Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit se ...

within

protected mode In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as virtual memory, paging and safe multi-tasking ...

available on

80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsDOSEMU for Linux uses a similar approach.

Interrupt vectors used by DOS

The following is the list of interrupt vectors used by programs to invoke the DOS API functions.

DOS INT 21h services

The following is the list of functions provided via the DOS API primary software interrupt vector.

Operating systems with native support

– most widespread implementation * PC DOS – IBM OEM version of MS-DOS * OS/2 1.x – Microsoft/IBM successor to MS-DOS and PC DOS * SISNE plus – Clone created by Itautec and Scopus Tecnologia in Brazil * DR-DOS – Digital Research DOS family, including Novell DOS, PalmDOS, OpenDOS, etc. * PTS-DOS – PhysTechSoft & Paragon DOS clone, including S/DOS * ROM-DOS – Datalight ROM DOS version * Embedded DOS –

General Software General Software was a Washington, USA based creator and supplier of system software headquartered in Bellevue, Washington. It was founded in 1989 by Steve Jones and later incorporated in 1990 as General Software, Inc. In 2008, the company was ...

version * FreeDOS – Free, open source DOS clone * ReactOS ( IA-32 and x86-64 versions) *

– contains MS-DOS 7.0 *

– contains MS-DOS 7.1 * Windows 98 SE – contains MS-DOS 7.1 *

– contains MS-DOS 8.0

Operating systems with DOS emulation layer

* Concurrent CP/M-86 (3.1 only) with PCMODE – Digital Research CP/M-86-based OS with optional PC DOS emulator *

Concurrent DOS Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers. An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...

– Digital Research CDOS family with built-in PC DOS emulator *

DOS Plus DOS Plus (erroneously also known as DOS+) was the first operating system developed by Digital Research's OEM Support Group in Newbury, Berkshire, UK, first released in 1985. DOS Plus 1.0 was based on CP/M-86 Plus combined with the PCMODE e ...

– a stripped-down single-user variant of Concurrent PC DOS 4.1–5.0 *

Multiuser DOS Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers. An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...

– Digital Research/Novell MDOS family including Datapac System Manager, IMS REAL/32, etc. * OS/2 (2.x and later) – IBM operating system using a fully-licensed MS-DOS 5.0 in a virtual machine * Windows NT (all versions except 64-bit editions)

Other emulators

* NTVDM for Windows NT * DOSEMU for Linux * DOSBox * ReactOS

References

External links

(a.k.a. RBIL, Ralf Brown's Interrupt List)
ctyme.com - INT Calls by function
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{{Disk operating systems DOS technology Operating system APIs X86 architecture Interrupts