> I left it out on purpose, because DOS was a Microsoft platform as well.

MS-DOS was a Microsoft platform, but not DOS itself:

> http://en.wikipedia.org/wiki/DR-DOS

Also Glide was not a Microsoft API.

I just wrote DOS as I always abbreviated MS-DOS as DOS.

DR-DOS actually came with the first PC I bought and it was for all effects a MS-DOS clone, hence a DOS clone, like PC-DOS from IBM and a few others on those days.

> DR-DOS actually came with the first PC I bought and it was for all effects a MS-DOS clone, hence a DOS clone, like PC-DOS from IBM and a few others on those days.

Under this argumentation GNU/Linux or even OS X is a UNIX clone and both should be abbreviated to UNIX (or UN*X to avoid trademark violations ;-) ). Indeed both originate from UNIX clones - but then new features were added that made them better than the original in a sense. The same happened to DR-DOS. Read DR-DOS' wikipedia article to read about features that DR-DOS added over Microsoft's original.

I was a DR-DOS user, too (Novell DOS 7).

But I believe Glide wasn't a Microsoft API, so the example is still fine to defend your case.