Xenix was Microsoft's first operating system, it predates DOS.
Admittedly, Microsoft did not actually write Xenix. They bought a System3 source license from AT&T and used that as a base, Their main service model was to port it to various systems.
Fun Fact: Xenix was the main reason a partition table was included when the PC first got hard disk support.
Microsoft published its own UNIX from 1980 to 1987: https://en.wikipedia.org/wiki/Xenix?wprov=sfla1
Xenix was Microsoft's first operating system, it predates DOS.
Admittedly, Microsoft did not actually write Xenix. They bought a System3 source license from AT&T and used that as a base, Their main service model was to port it to various systems.
Fun Fact: Xenix was the main reason a partition table was included when the PC first got hard disk support.
They developed something called M-DOS or MIDAS in 1979, but by that time CP/M was already established, so they decided against releasing it.
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