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

[Phil Budne <phil@ultimate.com>]

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...

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