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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.arch
Subject: Re: 64 Vs 32
Message-ID: <7863@utzoo.UUCP>
Date: Sat, 4-Apr-87 02:15:01 EST
Article-I.D.: utzoo.7863
Posted: Sat Apr  4 02:15:01 1987
Date-Received: Sat, 4-Apr-87 02:15:01 EST
References: <3810013@nucsrl.UUCP> <985@rpics.RPI.EDU> <2610@phri.UUCP> <528@ima.UUCP>
Organization: U of Toronto Zoology
Lines: 16
Summary: >32 bit addressing on 32 bit machines

> It's already happening.  The ROSETTA memory manager chip in the IBM PC RT
> manages an address space that is 43 bits...

Yes and no, mostly no.  The 43-bit (or 40-bit) address is mostly IBM bull.
The fact is, the instructions use 32-bit addresses.  Not 40, not 43, but 32.
The top 4 bits are used to pick out a segment, which after some slightly
unusual fiddling ends up determining the physical address.  If, repeat if,
you can convince the operating system to change segments for you, you can
of course access more by changing which segments you have in your address
space.  IBM claiming that this is 40-bit addressing is utter bilge.  By
the same standard, most any decent paged machine has INFINITE-bit addresses,
since you can get access to arbitrary amounts of data by having the operating
system change your page tables suitably.
-- 
"We must choose: the stars or	Henry Spencer @ U of Toronto Zoology
the dust.  Which shall it be?"	{allegra,ihnp4,decvax,pyramid}!utzoo!henry