LiteBSD

MMU is also responsible for translating between physical and virtual memory addresses. Making virtual memory support optional is a non-trivial design goal; you're not only allowing userspace to peek at (or straight up overwrite) kernel memory, you also need every executable to be a PIE, or to swap it out to disk as a part of a context switch. Check out RetroBSD <https://github.com/RetroBSD/retrobsd> and LiteBSD <https://github.com/sergev/LiteBSD>; there's a PID 0 (or 1? IIRC) that is called the "swapper" process, which is in charge of implementing the context switching. Fascinating stuff!

LiteBSD runs in only 512KB, RetroBSD runs in 128KB. https://github.com/sergev/LiteBSD http://retrobsd.org/wiki/doku.php

There are BSD clones for smallish (small memory or even no real memory management unit) systems - RetroBSD and LiteBSD: http://retrobsd.org/wiki/doku.php , https://github.com/sergev/LiteBSD/wiki .

yeah, there was also LiteBSD https://github.com/sergev/LiteBSD but Retro is the more active project https://github.com/RetroBSD/retrobsd

I have this running on a Fubarino. It's nice to fiddle with but not quite useful to be serious. I have an ethernet module that at some point I need to hook up to it. What I really want is a DIP version of the PIC32 with enough RAM and flash to run this or the 4.4-based LiteBSD[1] by the same people. [1] - https://github.com/sergev/LiteBSD