MS DOS
A class Microsoft Operating System that is command line based. Predates Windows, and set some of the standards for Windows today.
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OS/2 Warp
A IBM Operating System that was meant to compete with Micosoft Windows, but never gained the traction it needed to be successful.
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Macintosh "System 1"
This was the GUI interface for the Mac that was ripped off from Xerox Corporation after Steve Jobs visited.