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From: johnl@esegue.segue.boston.ma.us (John R. Levine)
Newsgroups: comp.arch
Subject: Re: 64-bit addresses
Summary: software
Message-ID: <1990Feb12.025531.3609@esegue.segue.boston.ma.us>
Date: 12 Feb 90 02:55:31 GMT
References: <9708@spool.cs.wisc.edu> <20270@cfctech.cfc.com> <11112@encore.Encore.COM> <753@dgis.dtic.dla.mil>
Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine)
Organization: Segue Software, Cambridge MA
Lines: 27

In article <753@dgis.dtic.dla.mil> jkrueger@dgis.dtic.dla.mil (Jon) writes:
>A question:
>
>Are 64 bit spaces being held back by hardware: complexity, fabrication,
>critical paths?  Or software ...

I suspect it's two things, both mostly software.  Hardware is no problem,
many new chips have 64 bit buses.  One problems is the enormous amount of
software written for 32 bit machines, the other is the lack of perceived
need.  Although the largest systems like IBM 3090s are running out of
address space, it is just these systems that have the largest pile of crufty
old software, so in the case of the 3090 we get ESA mode which is a
impressively kludgy segmented addressing scheme.

For smaller machines, e.g. workstations which are mostly programmed in C and
Fortran and are so less wedded to a particular architecture, most are a long
way away from overflowing a gigabyte address space.  I've never seen as much
as 2^27 bytes of memory on a workstation and that's still 4 or 5 bits away
from running out of address space.

I realize that it'd be nice to map your 4GB disk drive but hacks like the
386's segmented addressing provide address spaces considerably larger than
32 bits without having to go to full 64 bit addressing.
-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 864 9650
johnl@esegue.segue.boston.ma.us, {ima|lotus|spdcc}!esegue!johnl
"Now, we are all jelly doughnuts."