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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.arch,comp.unix.wizards
Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?)
Message-ID: <8723@utzoo.UUCP>
Date: Wed, 7-Oct-87 13:52:52 EDT
Article-I.D.: utzoo.8723
Posted: Wed Oct  7 13:52:52 1987
Date-Received: Wed, 7-Oct-87 13:52:52 EDT
References: <1755@ncr-sd.SanDiego.NCR.COM> <275@usl>, <29933@sun.uucp>
Organization: U of Toronto Zoology
Lines: 29

> ... Other PDP-11s, however, did not; a backup routine
> that was sufficient to handle the case of faults taken by references past the
> end of the stack was provided for those machines.  I don't think this routine
> was sufficient to handle *all* the cases of faults taken in midstream, at
> least not in a machine-independent manner.

Correct.  There were cases that simply could not be handled at all:  when
the same instruction modified the same register twice, in general one could
not tell how many modifications had been done.  The compiler largely avoided
such instructions, fortunately.  Making this work for floating point (another
case where registers could get modified repeatedly, since e.g. pushing a
double onto the stack required four one-word push operations) had at least
the potential of requiring model-specific code; I don't know whether this
was ever actually necessary, or whether the processors in question were all
sufficiently similar to avoid it.  I worked on dumb-MMU 11s and floating-
point 11s, but never seriously on a machine that was both.

(In case anyone is interested, the way the MMU differences came about was
that the 11/45 MMU, the first for the 11 series, was kind of a kitchen-sink
design with all sorts of vaguely useful-looking things.  For the mid-range
11/40 a bit later, DEC simplified the design by taking out all the things
that they had never bothered to use.  Unfortunately, some of those things
*were* in use by others.  They eventually more-or-less fixed this; the 11/44
and [I think] all the 11s that followed it have an MMU that is only a slight
subset of the 11/45 one, omitting only a few features that are *truly* more
or less useless.)
-- 
PS/2: Yesterday's hardware today.    |  Henry Spencer @ U of Toronto Zoology
OS/2: Yesterday's software tomorrow. | {allegra,ihnp4,decvax,utai}!utzoo!henry