MSDOS did have MMU support since 80386, IIRC. So I think GPs claim might be inaccurate. But it wasn't a security feature, it was a productivity feature. It protected your programs from defects in other programs and vice versa. This helps programmers find defects during design and helps users for defects that escape testing.

[citation required] for the MMU support of MS-DOS.

DR and MS DOS 5 and later have a 386 memory manager which can put an x86-32 CPU into Virtual 86 mode, remap RAM over any unused blocks in the I/O and ROM area (between 641kB-1024kB) to make Upper Memory Blocks, and load small resident parts of the OS into those blocks in order to keep as much base memory (0kb-640kB) free.

These memory managers could also turn the extended memory (above 1024kB) into managed XMS or EMS memory. LIM EMS was accessible to DOS apps and XMS to DOS-extender apps.

These were killer features in 1990 or so, and I made good money from optimising DOS memory for clients around that time.

Just replying with a web page showing the 80386 has having control registers doesn't imply that MS-DOS used them in any meaningful way.

The MMU is inside the processor. Any operating system running on an 80386 or higher has set registers/tables and gets hardware provided memory management.

If it uses it. Later versions of DOS did very little with the features the 386 offered, which is why there were MS-DOS extenders.

https://en.m.wikipedia.org/wiki/DOS_extender

You can make your own minimal DOS Extender just by setting those registers. This is what some games did.

I agree with the sentiment but note there were still viruses being circulated through floppy disks. So, 3rd party code may still be executed on a non-networked machine.

Indeed, it would be awesome to see how much a machine can do without all the layers. Even the Intel CPU security fixes could be bypassed and gain some extra speed.

Stuxnet is the modern variant.

Regardless of the qualities of DOS, the user interfaces of the graphical programs was in many cases superior compared to today, everything keyboard controlled, clearly labeled shortcut keys, views that usually only did one thing, distinctive coloring.

I took the time a few years ago to learn a bit about how to use Autodesk Animator (it was released with a BSD license some ~10 years ago and can be downloaded legally for free these days). Was really impressed with the GUI. Just press a single key to open the menu that begins with that letter, then the first letter of the menu-item you want to use. They managed to use only words that begin with unique letters while still making a lot of sense. Plus some other single-key shortcuts. And many, to me, unusual design choices everywhere, but it all makes sense and is consistent in a way that after a few hours I was not bothered at all by the fact that nothing was like a modern GUI, and there was definitely nothing about using more modern GUI conventions I can think of that would make it more pleasant to work with.

https://github.com/AnimatorPro/Animator-Pro

Stuxnet happened even with all of those protection mechanisms in place.

To my knowledge the targeted PLC lacked any form of protection when infected.

It spread first as windows worm that exploited several 0-days. If the host didn't have the Siemens PLC device drivers with a configuration that matched the exact configuration of what they were expecting for the centrifuges, they payload was dormant and it just replicated. Only if it matched would it patch the PLC's programming upon re-connection to fake reporting that the speed was normal while it was actually spinning out of control.

Thus the security model of DOS is still a problem even without internet connection.

Thus window's security theater did nothing.

You made a claim that it was unnecessary to have any security features on a non-network computer.

I have not made any claims about security features on a network computer.

My computer was 'Stoned.' Yes. I got a boot sector virus.

Why did you invent the constraint "no networking"?

I had coax network on MS-DOS even back then.

DR DOS 7 did have built-in peer-to-peer networking: Netware Lite.

MS DOS did not, because that was the main selling point of Windows for Workgroups.

Yea, there was alot of features that one is used to in modern OS that was missing in MS-DOS. It does not mean those features didnt exist or wasnt being used.

We used 3rd party TSR drivers for it.

Examples:

- MOUSE.COM driver https://www.vogons.org/viewtopic.php?t=58777

- CDROM driver http://manmrk.net/tutorials/DOS/cdrom.htm

- Network stack https://en.wikipedia.org/wiki/IPX/SPX

- Graphics driver https://dosdriver.de/graph.php

- Sound card drivers https://dosdriver.de/sound.php

Some of these things were included later on.

For instance:

DR DOS 6 had Netware Lite; DR DOS 7 had Personal Netware.

https://en.wikipedia.org/wiki/Personal_NetWare

I think several versions of DOS included MSCDEX.EXE (or DR's equivalent), but not the device driver, because back then there were lots of different CD interfaces: Mitusmi, Panasonic and Sony on sound cards, some early EIDE ones, and SCSI on higher-end kit.

Mouse drivers came with your mouse. ;-)

MS did have a network stack bundled for free on the NT Server installation CD, and some versions of it support server mode for peer-to-peer operation.

DOS could do all this stuff, but you had to assemble the bits from different sources, and this was sometimes for good reasons, sometimes technical (a profusion of incompatible CD-ROM interfaces) and sometimes commercial (P2P LAN support).