Dos

DOS is a family of disk-based operating systems for IBM PC compatible computers. It primarily consists of IBM PC DOS and a rebranded version, Microsoft's MS-DOS, both of which were introduced in 1981. Later, compatible systems from other manufacturers are DR-DOS (1988), ROM-DOS (1989), PTS-DOS (1993), and FreeDOS (1994). MS-DOS dominated the IBM PC compatible market between 1981 and 1995. (Wikipedia)