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DOS stands for disk operating system, a type of operating system for computers that provides the abstraction of a file system resident on the disk. In particular, DOS is often used to refer to MS-DOS, PC-DOS and DR-DOS (and FreeDOS and other compatible DOS products) which between them dominated the market for the decade between 1985 and 1995. The major DOS versions all originated, in one way or another, as outgrowths of the previous dominant disk operating system, CP/M.

Prior to the development of this family of microcomputers, several other operating systems for other architectures were already known as DOS:

• The DOS operating system for IBM 360 computers became DOS/VSE, and was later just called VSE.
• The DOS operating system for DEC PDP-11 computers was nearly obsolete by the time personal computers became common, with various descendents and other replacements.

DOS (or DoS, to distinguish it from the first sense) also stands for denial of service. In the context of computer networking, this is the situation where a system is attacked in such a manner that "normal" network communication is blocked by excessive traffic, or the system itself is rendered non-functional due to overload or caused to crash. This is distinct from a compromised system, which may continue to operate but is put to subversive use or exposed to illicit monitoring

In condensed matter physics, DOS is a common acronym for density of states. The density of states, N(E), for electronic energy levels in a solid is defined as follows: N(E)dE is the number of allowed energy levels per unit volume of the solid, in the energy range E to E+dE.