[pups] PDP-9?

Norman Wilson norman at nose.cs.utoronto.ca
Mon Sep 2 11:54:57 AEST 2002

Dave Horsfall:

  AFAIK, Unix never ran on the 11/20 (no MM unit); did you mean a DEC-20?

I don't know if it was called an 11/20 at the time (I seem to recall
some model-number upheaval in the early days of the -11), but the first
PDP-11 UNIX system was certainly one without memory management:

  By the beginning of 1970, PDP-7 UNIX was a going concern ... In early
  1970 we proposed acquisition of a PDP-11, which had just been introduced
  by Digital ... to create a system specifically designed for editing and
  formatting text, what might today be called a `word-processing system.'
  ... During the last half of 1971, we supported three typists from the
  Patent Department, who spent the day busily typing, editing, and formatting
  patent applications, and meanwhile tried to carry on our own work.  UNIX
  has a reputation for supplying interesting services on modest hardware,
  and this period may mark a high point in the benefit/equipment ratio;
  on a machine with no memory protection and a single 0.5-MB disk, every
  test of a new program required care and boldness, because it could easily
  crash the system, and every few hours' work by the typists meant pushing
  out more information onto DECtape, because of the very small disk.

  The experiment was trying but successful.  Not only did the Patent
  Department adopt UNIX, and thus become the first of many groups at the
  Laboratories to ratify our work, but we achieved sufficient credibility
  to convince our own management to acquire one of the first PDP-11/45
  systems made.

Dennis M. Ritchie, Evolution of the UNIX Time-Sharing System; AT&T Bell
Labs Technical Journal, Vol. 63 No. 8 Part 2, October 1984.

Maybe Dennis will chime in with further memories.

Certainly there's nothing odd about UNIX running without memory protection,
though, especially in that era.  The PDP-7 had none.  The trick was that
every context switch was also a swap.  The scheme was revived in the late
1970s for the early, no-memory-map versions of the LSI-11 (called LSX and
later Mini-UNIX; paper by Lycklama et al in the 1978 all-UNIX BLTJ, I believe).

I suppose next some whippersnapper will express disbelief that UNIX
could have run on a system with no Ethernet interface.  You mean there
was life before 10BaseT, spam, and pornographic web sites?

(Not, to be fair, that Dave Horsfall is a whippersnapper.)

Norman Wilson
Toronto ON
(Still on the shelf, but crawling toward the edge)

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