(DOS) disk operating system

MS-DOS (Microsoft Disk Operating System) is a single-user, single-tasking operating system with a command-line interface, developed by Microsoft Corporation. It was the dominant operating system for personal computers during the 1980s and early 1990s, particularly on IBM PC compatibles. MS-DOS was originally released in 1981 and was the first operating system that was widely used on the IBM PC. MS-DOS provided the foundation for the development of Microsoft Windows, which provided a graphical user interface on top of the MS-DOS operating system.

https://en.wikipedia.org/wiki/DOS