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From: faustus@ucbcad.berkeley.edu (Wayne A. Christopher)
Newsgroups: comp.arch
Subject: Re: 64 Vs 32
Message-ID: <1310@ucbcad.berkeley.edu>
Date: Mon, 16-Mar-87 14:40:48 EST
Article-I.D.: ucbcad.1310
Posted: Mon Mar 16 14:40:48 1987
Date-Received: Tue, 17-Mar-87 03:49:26 EST
References: <3810013@nucsrl.UUCP> <985@rpics.RPI.EDU>
Organization: CAD Group, U.C. Berkeley
Lines: 29

In article <985@rpics.RPI.EDU>, yerazuws@rpics.RPI.EDU (Crah) writes:
> Yep.  Consider- it's now a standard configuration for VAX 8800's
> to come with 512 megs of memory (I've got the part number around
> somewhere).  A VAX has 32 bits- so if we assume (*) that all 32
> can be used as memory address, a VAX (or other 32-bit processor)
> can have AT MOST 4 GIG of memory.
> 	
> If available memory continues to double every 18 months, we have
> about 3.5 doubling periods left, or about 5 years.

I don't know if this is the right question to ask....  A machine could
have over 4G of real memory, but the most one process could address is
4G of virtual memory.  In the case of the VAX, 1/4 of this is "regular"
data and text space, 1/4 is stack space, and the other half is system
space.

What we need to ask is, who will need more than ~1G of memory?  I think
the only applications that currently could use this much memory are
scientific programs that run on Crays (which I think are addressible to
the 64-bit word anyway).  I certainly haven't been running into the 1G
limit too often lately.

I think the tradeoff here is between speed and address space -- from a
32-bit internal datapath to a 64-bit datapath is a big step in terms of
chip complexity, and if you're trying to get the most speed per square
mm, you'd be better off sticking to 32 bits and adding more registers
or whatever.

	Wayne