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From: trebor@lkbreth.foretune.co.jp (Robert J Woodhead)
Newsgroups: comp.arch
Subject: Re: Adding fire to the segmentation flamefest...
Keywords: segments: do they really suck?
Message-ID: <9256@lkbreth.foretune.co.jp>
Date: 9 Apr 91 00:56:33 GMT
References: <9234@lkbreth.foretune.co.jp> <45180@super.ORG> <9244@lkbreth.foretune.co.jp> <2275@cluster.cs.su.oz.au>
Distribution: comp
Organization: Foretune Co., Ltd.  Tokyo Japan
Lines: 44

rex@cs.su.oz (Rex Di Bona) writes:
>In article <9244@lkbreth.foretune.co.jp> trebor@lkbreth.foretune.co.jp (aka ME) wrote:
>> For example, consider the proposal that was made in a recent message on this
>> topic, for a 32 bit segment and 32 bit address.  If that was implemented in
>> the memory system (so that the cpu goes "duhh, here is a 64 bit address,
>> gimme my memory, dude!") then you get the best of both worlds; you can treat
>> your memory as a 64 bit address space, or you can have up to 2**32 segments
>> 2**32 bytes long, or you can have segments >2**32 bytes long that just happen
>> to be set up as 2 or more contiguous segments by the OS.
>The only problem is what happens when you move your pointer from
>0x0000 0000 ffff ffff (spaces for clarity) up by a character? Does it go
>to 0x0000 0000 0000 0000 or does it go to 0x0000 0001 0000 0000 It is
>the difference in these two answers that differentiate a 'real' 64 bit
>flat address space from a truly segmented address space.

Most assuredly.  If you read my message again, you'll note that the cpu says
"duhh, here is a 64 bit address."  The cpu doesn't care how the memory system
works -- thats the job of the memory hardware and the o/s.    Thus, to the cpu
you have a flat memory space, and the memory hardware and the o/s can cooperate
to provide any attractive architecture they want to.  Segmented, flat space,
multilevel VM, whatever is required by the particular needs of the system.

Now, I'm not a hardware guy -- I have trouble with dimmer switches -- but it
has occurred to me for some time that what would be REALLY nice would be a
chipset that seperates various processor functions (CPU,FPU,MMU,CACHE...)
onto seperate chips (or chips on a carrier) -- and have several flavors of
each chip.  Then the system designer would be able to design in, and pay for,
only those features he or she really wants, and produce a more efficient
design.

But what do I know -- I write games for a living! ;^)


>I don't know, I still can't see anything (besides backward
>compatability :-) that segments provide that cannot be provided with
>flat addressing, and a smarter? virtual to physical translation.
>--------
>Rex di Bona (rex@cs.su.oz.au)
>Penguin Lust is NOT immoral
-- 
+--------------------------------------------------------------------------+
| Robert J. Woodhead, Biar Games / AnimEigo, Incs.   trebor@foretune.co.jp |
| "The Force. It surrounds us; It enfolds us; It gets us dates on Saturday |
| Nights." -- Obi Wan Kenobi, Famous Jedi Knight and Party Animal.         |