86-DOS vs MSDOS:
http://www.os2museum.com/wp/dos/dos-beginnings/
But lest you think that Microsoft’s software contribution was minimal:
Around 1980, Microsoft was the premier supplier of microcomputer language products. First and foremost, that was Microsoft’s BASIC, which IBM decided to build into the ROMs of nearly all of its PC and PS/2 machines built before 1990. IBM also contracted Microsoft to supply an assembler, linker, plus Pascal, FORTRAN, and COBOL compilers.
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On September 28th, 1980 Microsoft (Bill Gates, Paul Allen, and Microsoft VP Kay Nishi) decided to supply an operating system to IBM as well. The thinking was that the tools they already signed up to deliver would total about 400KB of (binary) code, and an OS would only add at most another 20KB. Microsoft already had Paterson’s QDOS in mind, now sold under the name 86-DOS. Microsoft was also worried that if it took IBM much longer to find a suitable operating system, it would make Microsoft’s job of porting its language products to the PC much harder and perhaps impossible to complete on time.
