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From: dswartz@bigbootay.sw.stratus.com (Dan Swartzendruber)
Newsgroups: comp.arch
Subject: Re: Segmented Architectures ( formerly Re: 48-bit computers)
Message-ID: <5051@lectroid.sw.stratus.com>
Date: 12 Apr 91 14:59:23 GMT
References: <1991Apr04.023845.3501@kithrup.COM> <1991Apr6.211320.18594@athena.mit.edu> <7YKAMBE@xds13.ferranti.com> <1991Apr12.021609.5340@athena.mit.edu>
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Reply-To: dswartz@bigbootay.sw.stratus.com (Dan Swartzendruber)
Organization: Stratus Computer, Software Engineering.
Lines: 32

In article <1991Apr12.021609.5340@athena.mit.edu> jfc@athena.mit.edu (John F Carr) writes:
:I don't think the tradeoffs between segments and a flat address space are
:the same now for >32 bit machines than they were for >16 bit machines.
:
:In the past decade, memory cost has dropped by about 2^8.  The 32 bit
:address space that some find too small costs 2^8 times as much to fill as
:the 16 bit address space did 12 years ago.
:

Oh come on!  No one here has been seriously requesting 4GB of real
physical memory! (Well, not many anyway :))  The point that most of
the anti-segmentation folks, including myself have been trying to
make is that internal segmentation (visible only to the OS) is fine;
external segmentation, defined as ANY type of segmentation which prevents
my application from playing with a flat address space, isn't.  Intel's
brain-damaged 64K segments were admittedly the worst, but so what?
All of the new machines which supposedly offer >32 bit virtual address
space are an optical illusion, because the application is now responsible
for using the actual 32-bit virtual address space as a cache, reloading
some segment register or other when it needs to play with object X (can
you say overlays?  I knew you could!)  And I don't really care if IBM
has made loading a segment register on the RS/6000 so fast I can do it
in 30 instructions.  My point is that my application has to know to do
this B.S.  Can you say non-portable?

:--
:    John Carr (jfc@athena.mit.edu)


--

Dan S.