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From: jgd@convex.csd.uwm.edu (John G Dobnick)
Newsgroups: alt.folklore.computers,comp.arch
Subject: Re: XDS940 computer (or Xerox Sigma 9)
Message-ID: <13016@uwm.edu>
Date: 12 Jun 91 05:11:19 GMT
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Re: the SDS Sigma 9

> |> The machine has 16 general registers, registers 1-7 were used as index
> |> registers.  Instructions addressed bytes, halfwords (16 bits), words (32 
> |> bits), and doublewords (64 bits).  Included stack instructions that used a 
> |> stack descriptor (multiple stacks).

Reaching back into infrequently accessed memory recesses [rummage, rummage...]

I seem to recall on feature of this machine (or perhaps the Sigma 7) that
has not been mentioned.   The machine had multiple *sets* of general
registers.  When a context switch occurred, the _register set_ was switched.
(Interrupts count as a context switch, of course.) No saving/restoring 
registers -- just "window in" another register set.

If memory serves, the basic machine came with two sets -- one "supervisor
set", and one "user set".  Additional "user sets" could be added, up to,
I think, 15 or 31 user sets.  This certainly beats the IBM model -- with its
constant LM/STM activity.

[Of current general purpose machines, how many use multiple register sets?
 Unisys 1100/2200 (formerly Univac and Sperry) systems do -- two sets,
 one OS only, and one for everything else.  Any others?]
-- 
John G Dobnick  (JGD2)
Computing Services Division @ University of Wisconsin - Milwaukee
INTERNET: jgd@uwm.edu                      ATTnet: (414) 229-5727
UUCP: uunet!uwm!jgd

"Knowing how things work is the basis for appreciation,
and is thus a source of civilized delight."  -- William Safire