[hulitu]

Raymond Chen stated that there were different OSs running inside win3.1.

https://devblogs.microsoft.com/oldnewthing/20100517-00/?p=14...

There was a lot of HW which only had DOS drivers which were loaded prior to starting Windows. Yes, the Windows kernel took the whole thing further but it still relied on MS-DOS for some things.

[zamalek]

TSRs (which amount to "DOS drivers") were notoriously flaky with Win3.1. Some requiring installation prior, some requiring installation inside of a guest DOS shell, some just not working at all. More reliable ones (such as MSCDEX) were explicitly compatible with Win3.1 and its IDT.

https://www-user.tu-chemnitz.de/~heha/vxd/vxd.htm