As I recall, they licenced MS-DOS to IBM for use in their PC hardware for like, $USD 1 -- provided IBM included Microsoft interpreted BASIC. Orgininal IBM PC's boot the interpreter if they can't find the MSDOS.
IBM got to keep that sweet deal even up to the PS/2 -- the low end 286 models have MS interpreted BASIC(with cassette commands
) in their roms.
It's a pretty widely know rumor that Bill Gates
hates IBM because of their original deal.