[somat]

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.

[whobre]

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.

[lproven]

This was news to me, so I went digging.

« M-DOS

    During 1977 and 1978, Microsoft adapted both BASIC and Microsoft
    FORTRAN for an increasingly popular 8-bit operating system called
    CP/M. At the end of 1978, Gates and Allen moved Microsoft from
    Albuquerque to Bellevue, Washington. The company continued to
    concentrate on programming languages, producing versions of BASIC for
    the 6502 and the TI9900.

    During this same period, Marc McDonald also worked on developing an 8-
    bit operating system called M-DOS (usually pronounced "Midas" or "My
    DOS"). Although it never became a real part of the Microsoft product
    line, M-DOS was a true multitasking operating system modeled after the
    DEC TOPS-10 operating system. M-DOS provided good performance and,
    with a more flexible FAT than that built into BASIC, had a better
    file-handling structure than the up-and-coming CP/M operating system.
    At about 30 KB, however, M-DOS was unfortunately too big for an 8-bit
    environment and so ended up being relegated to the back room. As Allen
    describes it, "Trying to do a large, full-blown operating system on
    the 8080 was a lot of work, and it took a lot of memory. The 8080
    addresses only 64 K, so with the success of CP/M, we finally concluded
    that it was best not to press on with that."

»

https://www.pcjs.org/documents/books/mspl13/msdos/encycloped...

[somat]

I was always curious on why their original product, BASIC, did not evolve into a full operating system. Looking at it with the sensibility of a unix affectionado. basic is sort of analogous to the unix shell, a unified interactive user interface and scripting language. This would appear to be a far superior interface than the relatively crippled CP/M inspired DOS command interpreter.

My best guess, memory, on those early microcomputers that consideration trumped any user interface ergonomics and DOS(cough CP/M) used less memory than the BASIC interpreter.

[whobre]

There was a rumor of MS and DRI having a non-aggression pact before 1981: https://nemanjatrifunovic.substack.com/p/gary-kildall-bill-g...