[LocalH]

IBM DOS was basically MS-DOS (except IBM did do some of their own work of course). Pretty sure Zenith and Compaq DOSes were OEM MS-DOS (Zenith did have a "Z-DOS" but it was just another OEM version of MS-DOS for a non-IBM-compatible system). DR-DOS was of course a true third party DOS (one of the very few contemporary ones IIRC)

[ksaj]

Reading the source code (which is available on archive.org) you can see exactly this. Lots of IFDEF statements for compiling MSDOS and PCDOS differences. When I first saw that, I was really surprised.