Heikki Kultala (heikki.NOSPAM.kultala.delete@this.gmail.com) on September 28, 2021 3:53 am wrote:
> ⚛ (0xe2.0x9a.0x9b.delete@this.gmail.com) on September 27, 2021 7:37 am wrote:

> There is no such thing as "paging unit". So you are having a quite huge straw man here.
>
> > - Protected mode segments (80286 and later), with two privilege levels (kernel-space
> > and user-space), are fully sufficient to implement a Unix-like operating system.
>
> ... how do you handle fork?
>
> With really terrible performance overhead?
>


I'm not sure fork() really presents much trouble for 286-style segmentation. The new process gets a new LDT, just duplicate all the segment descriptors from the old LDT, mark all of them (new and old) read-only, and do COW as usual (obviously on the whole segment). IOW, basically the same process as duplicating the page tables for fork() in the conventional case.

Obviously the OS needs to be able to handle the resulting aliased segments, but that's not really different that having to handle aliased pages in the conventional case.

There were at least a couple of *nix implementation for the 286, but I don't know how they handled that.