Myth: Windows 3.1 was just a shell on top of DOS

I always thought this was true.

MS-DOS was not really what people would today consider to be a "true" kernel, as it didn't implement any sort of access controls, boundaries, or other process management functionality. It was more of a set of shared libraries, and also included some standards such as a file system, and program loader format.

Even Windows 1.0, which more directly sat on top of MS-DOS, had to implement considerable amounts of functionality that would be considered more of a kernel than what MS-DOS offered.

DOS provided a tiny fraction of what Windows provided?

A full kernel acts as gatekeeper to system resources. While DOS provided access to some resources, it was single-process and so had no gatekeeping. No concept of different pools of resources, different lifetimes of activity. No comprehensive cleanup.

Half of a kernel (or more) is taking things from one list and putting them on another list, so when something exits or dies or faults, it can release resources that were allocated to the dying thing.

The hardware access was the lowest, simplest part of that. And DOS used the BIOS anyway, for half of what it did (floppy drivers etc). Which meant it had not a chance in hell of performing well (executing out of ROM or whatever).

This is a fun little bite-sized post, but I wish it tackled more of the nitty-gritty.

Does anyone know of any write-ups on the architecture of early-90s Windows that are more in depth, now that this has whet my appetite? (Or other OSes of the era, come to think of it?)

It was not just a shell, but all other stuff is presented like microsoft sales memo. It had basic supervisor code that used intel v86 in protected mode and you can hang whole system with single call anytime. Guess what else can do it - Novell DOS 7. Win32s was much more of OS than windows 3.1