A number of such systems exist; the Commodore 64 home computer had a DOS, as did (on the other end of the spectrum) IBM's [System 370]? and [System 390]? series of mainframes (DOS/VSE, Disk Operating System/Virtual? Storage Extended).
The most popular family of operating systems called "DOS" is that running on IBM PCs. The original version was provided to IBM by Microsoft, and marketed as "PC-DOS". Later versions were also sold as "MS-DOS". [Digital Research]? produced a compatible variant known as "DR-DOS", and (after a buyout) "OpenDOS". There is also a free version named "FreeDOS".
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