Disk Operating System (DOS) is the name of many operating systems starting from 1960s mainframe OSes, but most commonly it refers to Microsoft's MS-DOS and compatible systems running on IBM PC compatible hardware. ?FreeDOS is an actively maintained DOS-compatible operating system that supports things like USB memory sticks. DOSBox is a popular emulator for running DOS-based IBM PC programs in modern operating systems.

DOS is designed to only run one program at a time with minimal external dependencies. It is thus quite immune to many problems of more modern OSes, such as software rot. It may well be used as a bedrock platform or as a "bare-bones" system that may work for basic tasks even when everything else fails. The main compatibility issue in DOS is that a lot of hardware is commonly used by "banging the bare metal", which makes e.g. old sound applications incompatible with modern hardware.

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