Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!uunet!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: 64 bits--why stop there?
Message-ID: <2437@crdos1.crd.ge.COM>
Date: 21 Aug 90 17:52:01 GMT
References: <5539@darkstar.ucsc.edu> <13285@yunexus.YorkU.CA> <30728@super.ORG> <9660@ganymede.inmos.co.uk> <224@csinc.UUCP> <1263.26cdaecc@waikato.ac.nz> <6106@vanuata.cs.glasgow.ac.uk>
Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen)
Organization: GE Corp R&D Center, Schenectady NY
Lines: 36


  While we're all talking about 64 bits, where is it writ' that word
size shall be a power of two bits? Outside of the prevalence of the
eight bit byte, is there a good technical reason for it? Certainly the
old Honeywells I used to use, with their lovely nine bit bytes and 36
bit words... Conversion to IBM was *REAL* painful. 

    bytes - never had a problem personally, or heard of one
    float - in many cases converted to double
    double - about 10% of the programs were partially rewritten to
        insure valid results. A few were moved to Cray, some dropped.
    int - if it couldn't justify running on the Cray, it got dropped.
        Faking multiple precision in FORTRAN is ugly and not worth it.

  Now this was goping from 36 to 32 bits. If four bits can hurt that
much, how much would 48 bit help, instead of 64? I know there are some
48 bits machines made (or were), what gains were there?

  int is +/- 10^14, assuming 12 bit exponent and 36 mantisa, range
is 10^616, accuracy 1 in 10^10.8. That seems to offer a pretty good
single precision, with double precision even better. The addressing
would be 262144GB, enough for memory mapped databases for a decade or
so.

  Maybe some of the chip designers can give an idea of what the cost
ratio is for 48 vs 64 bits. Obviously there can be a lot of new
applications tackled with 48 bits, is the saving over 64 significant, or
should we expect a leap to 64? 48 bits soon is more useful than 64 bits
eventually, perhaps.

  Points have been made for going to more than 64 bits, now I've put out
a few thoughts on going for less. Since we have some ideas about 64 bit
systems in actual practice, can someone tell us about 48?
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
    VMS is a text-only adventure game. If you win you can use unix.