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

[jnc@mercury.lcs.mit.edu (Noel Chiappa)]

    > From: Warner Losh

    > 2.11BSD used a mode between kernel and user for the TCP stack to get
    > more effective address space...

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".

The thing is that PDP-11 Unix had been using overlays in the kernel for quite
a while to provide more address space. I forget where they first came in (I
suspect there were a number of local hacks, before everyone started using the
BSD approach), but by 2.9 BSD they were a standard part of the system. (See:

  https://minnie.tuhs.org/cgi-bin/utree.pl?file=2.9BSD/usr/src/sys/conf/Ovmakefile

for some clues about how this works. There is unfortunately no documentation
that I know of which explains clearly how it works; if anyone knows of any,
can you please let me know? Otherwise you'll have to read the sources.)

I can think of two possible reasons they started using supervisor mode: i)
There were a limited number of the 2.9-type overlays, and they were not
large; trying to support all the networking code with the existing overlay
system may have been too hard. ii) I think this one is unlikely, but I'll
list it as a possibility. 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.

	Noel