MS-DOS

MS-DOS (an acronym of Microsoft Disk Operating System) is Microsoft's clone of the disk operating system for the Intel  processor, used by the original IBM. It has also been referred to as PC-DOS or simply DOS, though there had already been other OSes with that name since the mid-1960s, starting with IBM's for the. It has also been called "MS-DOG" and "mess-dos" by its critics.

Description
MS-DOS is a single-user operating system that ran one program at a time and was limited to working with one megabyte of memory, 640 kilobytes of which is usable for the application program. Special add-on (EMS) boards allowed EMS-compliant software to exceed the 1 MB limit. Add-ons to DOS, such as Microsoft Windows and, took advantage of EMS and allowed the user to have multiple applications loaded at once and switch between them.

History
MS-DOS originated as, written in 6 weeks by hacker , who is said to have regretted selling it to Microsoft, which then licensed it to IBM. Many of the original DOS functions were calls to BASIC (in ROM on the ), e.g.  and. People with non-IBM PCs had to buy (which later became ). Most versions of DOS included some compatible version of BASIC.

Numerous features, including vaguely Unix-like but rather broken support for subdirectories, I/O redirection and pipelines, were hacked into MS-DOS 2.0 and subsequent versions. As a result, there are two or more incompatible versions of many system calls, and MS-DOS programmers could never agree on basic things like what character to use as an option switch ("-" or "/"). Despite the various issues, its use by many IBM PC-compatibles with Intel 16 and 32-bit microprocessors led to it becoming the highest-unit-volume operating system in history until its eventual replacement by Microsoft Windows.

MS-DOS on Apple hardware
PowerPC-based Macs running classic Mac OS or Mac OS X can use a software emulator such as Virtual PC, or a hardware emulator like a DOS Compatibility Card, to run MS-DOS or PC-DOS. Macs running Mac OS X or macOS on PowerPC, Intel, or Apple processors can also use DOSBox as an open source alternative to run vintage DOS software and games.