Probably QNX or VxWorks.

QnX is correct.

Is it feasible for mortals to get hold of and run QnX as a desktop OS?

You might find an old archived copy of it but nothing that would be very useful if you intend to run a modern browser and other tools like that. For curiosity's sake you could.

There used to be a one-floppy installer with the desktop on it, I'm sure you could get that to work in a VM.

Lots of screenshots:

https://www.operating-system.org/betriebssystem/_english/bs-...

Docs:

https://www.mikecramer.com/qnx/qnx_6.1_docs/sysadmin/intro.h...

Unfortunately I can't seem to locate any disk images, rumor has it there was a VMWare image floating around.

You can find QNX 6.5 and suitable license keys by digging around on the web a bit. 6.5 is probably the last version suitable for use as a desktop OS—I believe 6.6 is where they started to remove the ability to self-host, and the GUI was completely removed by QNX 7. (It's a shame, because IMO the Photon GUI is really clever.)

Unfortunately QNX 6.5 is pretty old and not too well-supported (QNX is fully POSIX-compliant so you can sometimes get things working on it without too much trouble however) but it's damn fantastic to play around with in a VM. It's actually so fun.

Apparently not really: https://news.ycombinator.com/item?id=24346411

It was once. I still have a machine in the closet with QNX 6.1 and the Photon GUI. It has a version of Firefox so old it's called Firebird. But after Blackberry bought QNX, they made it closed source and no longer self-hosting. Today you have to cross-compile from Windows.

Grr.

Absolutely. The thing you need to be asking for is a hobbyist license, which might not always be advertised but is very much a thing!

I emailed QNX a couple years ago about hobbyist or non-commercial licenses and they replied that they no longer offer them.

Also RTLinux kernel: https://en.wikipedia.org/wiki/RTLinux

RTLinux is soft real time, FWIW.

Can you elaborate? The wiki says it combines hard-realtime with soft-realtime. It sounds like it depends how you configure it but this is the first I’ve read of it so I could be mistaken.

They're just mistaken, RTLinux is hard realtime. It works by running a tiny real time OS on the bare metal, and then running a Linux kernel as a (lightly) paravirtualized lowest priority task. You write your hard realtime code to run on the RTOS, and then your soft/non realtime code runs on Linux.

In this model, your compositor, input pipeline and (at least the UI latency sensitive part of) your applications would have to be ported to the RTOS, which makes this pretty infeasible. But it works really well if you have a some hard-realtime control loop talking to a bunch of non-realtime network IO or UI code. Would be a fun way to build a wifi controlled quadcopter.

These days you probably want to use Xenomai, or possibly RTAI.

Yes, I was mistaken. I was thinking of the preempt_rt patches.

Thanks for the great writeup correcting me!

RTLinux is a special supervisor below the kernel that allows you to run "hard-realtime" programs outside of the Linux environment with a lot of restrictions. Everything within the Linux environment is soft. As far as I can tell, it's not feasible to run the GUI stack in the more primitive environment.