Also see DOS.
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PC-DOS (Personal Computer - Disk Operating System) was the first widely-installed operating system used in personal computers. It was developed for IBM by Bill Gates and his fledgling Microsoft Corporation for installation in IBM's first lines of PCs. Gates marketed an almost identical version of the operating system called MS-DOS (Microsoft - Disk Operating System). Most users of either DOS system have usually referred to their system as just DOS. Like MS-DOS, PC-DOS was (and still is) a non-graphical line-oriented command-driven operating system, with a relatively simple interface but not overly "friendly" user interface. Its prompt to enter a command looks like this:
The first Microsoft Windows operating system was really an application that ran on top of the MS-DOS operating system. Today, Windows operating systems continue to support DOS (or a DOS-like user interface) for special purposes by emulating the operating system.
In the 1970s before the personal computer was invented, IBM had a different and unrelated DOS (Disk Operating System) that ran on smaller business computers. It was replaced by IBM's VSE operating system.