[simonsays2]

Windows and MSDOS teams were very close, no issue there.

The real reason someone might need to do this hack is that there were in fact many versions of "DOS" not made by Microsoft. These included Zenith, Compaq, Digital Research and IBM. Each of these each had their own subversions.

[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.