*BSD News Article 39721


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From: ron@topaz.sensor.com (Ron Natalie)
Newsgroups: comp.unix.ultrix,comp.unix.bsd,comp.sys.dec,comp.sys.dec.micro
Subject: Re: UNIX (Ultrix, BSD?) for DEC Micro PDP-11?
Followup-To: comp.unix.ultrix,comp.unix.bsd,comp.sys.dec,comp.sys.dec.micro
Date: 19 Dec 1994 15:17:54 GMT
Organization: Sensor Systems
Lines: 33
Message-ID: <3d4872$r81@topaz.sensor.com>
References: <taubman.787470030@spot.Colorado.EDU> <arog.787839518@BIX.com>
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Well, UNIX came from PDP-11's, but it's been a long time.  First off,
if you don't have memory management (11/23,73...) it is hopeless.
UNIX never really did run well on those things.  There were a few
aborted attempts (MiniUnix, LSX) but nobody gave them much credence
even back when UNIX was small and I doubt that you could find one
of those around these days (and they were WE licensed and I don't
even thing you could obtain such a license anymore).

Even with real memory management, it's hard to do things with a
PDP-11 anymore.  The problems is that programs have just gotten
too big.  The PDP-11 memory system is permanently stuck in what
386 people would call small model.  Overlays were hacked in to
give a lease on life, but you had to make hokey interfaces in
the user mode as unlike the 8086, there is no segment facility.

What finally killed the 11's for us was that we ran out of
segment registers to even play the overlay games.  PDP-11's
only have 8 (or if your lucky 8 code + 8 data) to muck around
with.  Each one banks 8K of your 64K address space.  Of course,
now, they are tremendously slow and under-memoried compared to
a PC (they only have a 22 bit physical address bus under the best of
configurations).

I have great fondness for them.  They continued to live on at BRL under my
care even after they were replaced by Goulds and Suns for UNIX work as IP
routers running my own little operating system (I think that I was the only
one who was using an 11/70 as a standalone internet router, but it had the
distinction of being the busiest host on the MILNET for quite a period of
time, kick-ass performance in those early Internet days) and one even
survived as an I/O controller for the HEP supercomputer.

-Ron