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From: wilsonj@alum01.its.rpi.edu (John Wilson)
Newsgroups: comp.unix.ultrix,comp.unix.bsd,comp.sys.dec,comp.sys.dec.micro
Subject: Re: UNIX (Ultrix, BSD?) for DEC Micro PDP-11?
Date: 19 Dec 1994 20:42:28 GMT
Organization: Rensselaer Polytechnic Institute, Troy NY
Lines: 17
Message-ID: <3d4r7k$70d@usenet.rpi.edu>
References: <taubman.787470030@spot.Colorado.EDU> <arog.787839518@BIX.com> <3d4872$r81@topaz.sensor.com>
NNTP-Posting-Host: alum01.its.rpi.edu

I haven't looked at the code so I'm on thin ice here, but my impression is
that the current crop of UNIX clones is intended to be portable across
machines that have fixed-size pages (i.e. no fragmentation, ever) and
restartable instructions (for continuing after a page fault).  The PDP-11
has variable sized pages -- sure, you could just define them all to be
4KW but then you'd eat through your PDP's tiny memory in no time -- and
none but the top of the line machines have restartable instructions (and
it's a pain even on them, you need to undo register autoinc/dec by hand
using MMR1).

The PDP-11 is really designed for swapping OSes, not demand-paged ones.
So it would be a very non-trivial port (supposing you found some way to
fit the kernel in in the first place) and performance might not be so hot.
My impression is that the older unices know about swapping and defragging
core and can plan ahead a little more.

John Wilson