Path: utzoo!utgpu!cunews!bnrgate!bwdls58!mlord
From: mlord@bwdls58.bnr.ca (Mark Lord)
Newsgroups: comp.arch
Subject: Re: Segmented Architectures ( formerly Re: 48-bit computers)
Message-ID: <6435@bwdls58.bnr.ca>
Date: 10 Apr 91 11:43:20 GMT
References: <1991Apr04.023845.3501@kithrup.COM> <1991Apr04.230953.15294@kithrup.COM> <VFIA832@xds13.ferranti.com> <1991Apr06.030330.1533@kithrup.COM> <572@lysator.liu.se>
Sender: mlord@bwdls58.bnr.ca
Reply-To: mlord@bwdls58.bnr.ca (Mark Lord)
Organization: Bell-Northern Research, Ottawa, Canada
Lines: 39

In article <572@lysator.liu.se> bellman@lysator.liu.se (Thomas Bellman) writes:
<
<Substitute "create/attach a segment" for "open file" and substitute
<"index in the segment" for "seek in the file", and you have a
<segmented memory model.  In the file system, you have (say) 8 bits of
<file descriptor, and 32 bits of offset, but even though you have 40
<bits of "pointer", you can't address more than 32 bits.  People
<doesn't seem to have any problem with doing this in a file system, so
<why the dislike for doing this with the memory too?

Not at all the same thing.  Files are read in large chunks to negate the
performance impact somewhat.  Accessing largish data items in main memory
requires segment prefixing (or whatever one calls it) on *each* access,
barring loop optimisations.  Quite the performance hit, in addition to being
a kludge to overcome limited addressing capability.

For that matter, even the analogy is not correct.  How about..
Substitute "access variable" for "open file", and from then on the file itself
looks like one huge flatly addressed table.  No segment boundaries to worry on.
I can seek directly (on most OS's) to exactly the part I want (read "indexing"
the table).  In memory, this *could* be equally clean on a segmented system,
provided we can fit each large data item completely within one segment, and
provided there are enough segment registers to accomodate *all* of the large
data items simultaneously.  Anything else requires extra logic to maintain
segment registers.  The closest counterexample in file systems is having to
put up with multiple disk volumes, where we treat each drive as a "segment".
And yes, we hate that, which is why drives keep getting MUCH larger and larger
(bigger flatter addressing segments).

Computers have plenty of die space to provide enough addressing bits to send
segments back to the dark ages NOW, so why kludge around?

-- 
MLORD@BNR.CA  Ottawa, Ontario *** Personal views only ***
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