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From: philm@astroatc.UUCP (Phil Mason)
Newsgroups: comp.unix.wizards
Subject: Re: CYBER word length
Message-ID: <614@astroatc.UUCP>
Date: Wed, 12-Nov-86 15:04:59 EST
Article-I.D.: astroatc.614
Posted: Wed Nov 12 15:04:59 1986
Date-Received: Wed, 12-Nov-86 22:10:11 EST
References: <7208@elsie.UUCP> <5142@brl-smoke.ARPA> <2447@hcr.UUCP> <363@yabbie.rmit.oz> <612@astroatc.UUCP> <852@zeus.UUCP>
Reply-To: philm@astroatc.UUCP (Phil Mason)
Organization: Astronautics ATC, Madison, WI
Lines: 30

In article <852@zeus.UUCP> bobr@zeus.UUCP (Robert Reed) writes:
>>The Cyber word length was selected to be 60 bits because of the number of 
>>exact divisors it has : 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30.
>
>That's a great myth.  Almost believable.  But isn't it true that the Cyber
>word length was set a 60 bits to be compatible with the old CDC-6000 series?
>Isn't the operant concern here to be a multiple of 6?  See, when the 6000
>was born, it was a successor of the CDC-3300 series, which used 36 bit words
>(like many contemporaries, such as Univac and Honeywell and DEC).  Back in
>those days, most systems used 6 bit codes for characters, and DEC for
>example, had machines with word sizes of 12 (PDP-8) and 18 (LINC/PDP-15)
>bits.  If anything, the reason for 60 bits is historical precedent.

Yes, that is part of the reason.  One may well ask, "why didn't CDC just double
the word length in the 6600 versus the 3600 machine : 36 to 72 bits? Or
perhaps just bump it up to 42 (a great choice Hitchhiker's Guide Fans!) or
48 bits?  In addition to being a multiple of 6, and having many bits for
floating point precision, you can also pack many different sized data into
60 bit words without slop. 60 bits works out the best of any reasonably
small word length in this regard.


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Kirk  : Bones ?                |  Phil Mason, Astronautics Technology Center
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