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From: tony@wldrdg.UUCP (Tony Andrews)
Newsgroups: comp.sys.intel
Subject: Re: comp.sys.intel (segmentation discussion)
Message-ID: <113@wldrdg.UUCP>
Date: Thu, 20-Nov-86 10:42:59 EST
Article-I.D.: wldrdg.113
Posted: Thu Nov 20 10:42:59 1986
Date-Received: Fri, 21-Nov-86 02:55:58 EST
References: <110@wldrdg.UUCP> <13772@amdcad.UUCP> <111@wldrdg.UUCP> <13801@amdcad.UUCP>
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Organization: Wildridge Consulting,  Boulder, CO
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phil@amdcad.UUCP (Phil Ngai) writes:
> It does sound like SVR3 doesn't use segments for anything useful. I
> don't think there is any version of Unix which does. But a long time
> ago, an operating system named Multics did some neat things with
> segments. That Unix doesn't use them doesn't mean segments are a
> useless idea.

It's hard to do within UNIX. I've thought about the problem a lot.
If you take the approach of putting each data structure in its own
segment then you wind up doing a lot of segment register loads which
are slow. The two new segment registers in the 386 will help
alleviate this problem.

A couple of days ago I ran a quick benchmark to see how much a small
model program would slow down when compiled large model. I used the
UNIX "od" command and measured a performance decrease of 35%. I ran
the same experiment with "sum" and it slowed down by 50%. In both
cases I used an optimizer which eliminates redundant segment loads
for a single symbol, but not for different symbols. So this corresponds
fairly well to your example of putting each data structure in its own
segment. The advantages you discussed may be worth the performance
penalty, but there is a penalty.

Another problem with making better use of segments in UNIX is that
the brk and sbrk primitives don't work out very well. You really
need to be able to have a finer level of control over the allocation
and deallocation of segments.

> Well, I think a lot of so called Intel propaganda has actually been
> people putting words in Intel's mouth. I've never seen Intel refer to
> the 80286 as a 32 bit machine. All the flames I have seen have been
> from people trying to use it as such. Now who's fault is that?

Intel representatives (yes, actual employees) have repeatedly stated
that the 286 is a good UNIX machine, in comparison to 32-bit processors
like the 68K and 32032. (And if you only support small model, and really
use it as a 16 bit machine, that's actually true.) This may not be a
globally accepted view within Intel, but at least some Intel people
have marketed the 286 as a 32-bit machine. Granted, this was all a
couple of years ago. I'm sure their strategy has changed now that the
386 is around.

Tony Andrews
...!ihnp4!onecom!wldrdg!tony
Wildridge Consulting, Inc.
Boulder, CO