[acdha]

Not only was it based on OS/2 it even supported some of the OS/2 APIs for awhile, along with things like the HPFS filesystem:

https://web.archive.org/web/20090210125723/http://www.micros...

https://support.microsoft.com/en-us/help/100108/overview-of-...

See https://en.wikipedia.org/wiki/OS/2#1990:_Breakup for more information about how they diverged over time.

[Aloha]

I think it still does support OS/2 console applications.

[acdha]

I thought that was removed in the XP/2003 era:

https://web.archive.org/web/20041019061658/http://support.mi...