/M S doss/ (Or "MS-DOS", "PC-DOS", "MS-DOG", "mess-dos") Microsoft Corporation's clone of the CP/M disk operating system for the 8088 crufted together in 6 weeks by hacker Tim Paterson, who is said to have regretted it ever since.
MS-DOS is a single user operating system that runs one program at a time and is limited to working with one megabyte of memory, 640 kilobytes of which is usable for the application program. Special add-on EMS memory boards allow EMS-compliant software to exceed the 1 MB limit. Add-ons to DOS, such as Microsoft Windows and DESQview, take advantage of EMS and allow the user to have multiple applications loaded at once and switch between them.
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 can never agree on basic things like what character to use as an option switch ("-" or "/"). The resulting mess became the highest-unit-volume operating system in history. It was used on many Intel 16 and 32 bit microprocessors and IBM PC compatibles.
Many of the original DOS functions were calls to BASIC (in ROM on the original IBM PC), e.g. Format and Mode. People with non-IBM PCs had to buy MS-Basic (later called GWBasic). Most version of DOS came with some version of BASIC.
Also know as PC-DOS or simply DOS, ignoring the fact that there were many other OSes with that name, starting in the mid-1960s with IBM's first disk operating system for the IBM 360.