(Or "DOS", "MS-DOG", "mess-dos") [[Computer Dictionary/Microsoft Corporation|Microsoft Corporation]]'s clone of CP/M 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 whether to be case-sensitive. The resulting mess is now the highest-unit-volume operating system in history. It is 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 as DOS, which annoys people familiar with other similarly abbreviated operating systems (the name goes back to the mid-1960s, when it was attached to IBM's first disk operating system for the IBM 360). Some people like to pronounce DOS like "dose" or to compare it to a dose of brain-damaging drugs (a slogan button in wide circulation among hackers exhorts: "MS-DOS: Just say No!").
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