DOS API

The DOS API is an API which originated with 86-DOS and is used in MS-DOS/PC DOS and other DOS-compatible operating systems. Most calls to the DOS API are invoked using software interrupt 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 (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. 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, hierarchical directories and device I/O control. In DOS 3.1, network redirector 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 (UMBs). After MS-DOS 5, the DOS API was unchanged for the successive standalone releases of DOS.

The DOS API and Windows
In Windows 9x, 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 OSR2, DOS was updated to 7.1, which added FAT32 support, and functions were added to the DOS API to support this. Windows 98 and Windows ME 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 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 (an emulation of real mode within protected mode available on 80386 and higher processors). NTVDM supports the DOS 5.0 API. DOSEMU 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

 * MS-DOS – 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 version
 * FreeDOS – Free, open source DOS clone
 * ReactOS (IA-32 and x86-64 versions)
 * Windows 95 – contains MS-DOS 7.0
 * Windows 98 – contains MS-DOS 7.1
 * Windows 98 SE – contains MS-DOS 7.1
 * Windows ME – 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 – Digital Research CDOS family with built-in PC DOS emulator
 * DOS Plus – a stripped-down single-user variant of Concurrent PC DOS 4.1–5.0
 * Multiuser DOS – 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 and 32-bit ARM editions)

Other emulators

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