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From: dricejb@drilex.UUCP (Craig Jackson drilex1)
Newsgroups: comp.arch
Subject: Re: 64 bits--why stop there?
Message-ID: <15249@drilex.UUCP>
Date: 23 Aug 90 13:25:32 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> <2437@crdos1.crd.ge.COM> <41004@mips.mips.COM> <1990Aug22.031911.7376@nlm.nih.go
Organization: DRI/McGraw-Hill, Lexington, MA
Lines: 40

In article <1990Aug22.031911.7376@nlm.nih.gov> states@tech.NLM.NIH.GOV (David States) writes:
>In article <41004@mips.mips.COM> mash@mips.COM (John Mashey) writes:
>
>    [Why not 48-bit processors?]
>
>>1) Software inertia strongly impels people to build machines whose
>>words contain 2**n bytes, for C especially, but also for other languages.

I think C would be the chief offender here--few other languages expose
the characters/word ratio quite as much.  Note, however, that at least
one C compiler has been written for a 48-bit word machine.
>>2) (Some) software inertia and (much) hardware inertia impels people
>>to use 8-bit characters.

The nice thing about 24 and 48 bit words, in years past, was that you
could straddle the six-bits-per-char/eight-bits-per-char argument.

>>3) So, I'd be amazed if a new general-purpose architecture would likely
>>be viable at 48 bits.

I'd agree with this.  In this and several other regards, the industry
has significantly calcified in the last few years.  Similarly, I doubt
if a new general-puropse architecture would be viable if a Unix port
(not just a POSIX interface) wasn't almost trivial.

>In scientific work 64-bit floating point has become standard.  Hydrid
>processors with some 32 and some 64-bit paths/registers are viable, but
>increasing the 32-bit paths to 48 is not going to help much in 64-bit
>transfers, and you would still need two 48-bit registers to store one
>64-bit number.  Finally, I don't think 48-bit FP could totally replace
>64-bit, although it could do alot better than 32-bit FP.

We have been quite successful doing econometric calculations in 48-bit
floating point on a Burroughs for years.  (Econometrics  may not be
scientific, but it's certainly numerically intensive.)  We only rarely
have had to resort to 96-bit double precision.
-- 
Craig Jackson
dricejb@drilex.dri.mgh.com
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