[TUHS] Re: [COFF] Re: Supervisor mode on ye olde PDP-11

[Warner Losh <imp@bsdimp.com>]

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On Wed, Jun 19, 2024 at 10:56 AM Phil Budne <phil@ultimate.com> wrote:

> JNC wrote:
> > Is there a document for 2.11 which explains in detail why they did that?
> I
> > suspect it's actually a little more complicated than just "more address
> > space".
> ...
> > ... Switching overlays took a certain amount of
> > overhead (since mapping registers had to be re-loaded); if all the
> networking
> > code ran in supervisor mode, the supervisor mode mapping registers could
> be
> > loaded with the right thing and just left.
>
> That's my understanding... It allows mbufs to be mapped only
> in supervisor mode...
>

Yea. that's a much better explanation than my glossed over 'more address
space'. It's both to get more text space (overlays weren't infinite, and
being a
separate image allowed more selective communication across the interface
boundary) and to provide some separation and allow for more data to be
around easily (the BSD kernel didn't overlay data, which is technically
possible
on PDP-11, but the linker didn't support it).


> https://minnie.tuhs.org/PUPS/Setup/2.11bsd_setup.html says:
>
>        The networking in 2.11BSD, runs in supervisor mode, separate
>        from the mainstream kernel. There is room without overlaying to
>        hold both a SL/IP and ethernet driver. This is a major win, as
>        it allows the networking to maintain its mbufs in normal data
>        space, among other things. The networking portion of the kernel
>        resides in ``/netnix'', and is loaded after the kernel is
>        running. Since the kernel only looks for the file ``/netnix'',
>        it will not run if it is unable to load ``/netnix'' , sites
>        should build and keep a non-networking kernel in ``/'' at all
>        times, as a backup. NOTE: The ``/unix'' and ``/netnix''
>        imagines must have been created at the same time, do not
>        attempt to use mismatched images. The ability to have boot tell
>        the kernel which network image to load is on the wish list (had
>        to have something take the place of wishing for disklabels
>        ;-)).
>
> https://wfjm.github.io/home/ouxr/ shows the code path for the socket(2)
> syscall
>

Oh, that's nice.

Warner

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