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From: kevinw@portia.Stanford.EDU (Kevin Rudd)
Newsgroups: comp.arch
Subject: Re: 64 bits--why stop there?
Message-ID: <1990Aug23.015636.506@portia.Stanford.EDU>
Date: 23 Aug 90 01:56:36 GMT
References: <6106@vanuata.cs.glasgow.ac.uk> <2437@crdos1.crd.ge.COM> <3259@skye.ed.ac.uk>
Sender: Kevin W. Rudd
Organization: Electrical Engineering Department, Stanford University
Lines: 30

In article <3259@skye.ed.ac.uk> richard@aiai.UUCP (Richard Tobin) writes:
>In article <2437@crdos1.crd.ge.COM> davidsen@crdos1.crd.ge.com (bill davidsen) writes:
>>  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?
>
>As you note, the existence of eight bit bytes makes a difference.  A
>risc-like machine on which the number of bytes (by which I mean the
>smallest addressable unit) in a word was not a power of two would be
>very strange.  How big would the pages be?  What would the alignment
>restrictions be? ("doubles must start at an address which is a multiple 
>of six"?).

And who says that machines must have byte addressable memory?  There would
then be no need for alignment restrictions based on bytes.  Of course, as has
been mentioned already, machines with odd sized sub-words (most common
is the byte) can have problems with SW conversion.  But if there is no address
access for a byte except through instructions (such as "get byte n" or
"set byte n") then this shouldn't be a problem.  Page size is only a
problem in terms of mapping the machine page size in words into a
backing store block size.  Since most peripherals are byte or word oriented
it seems that this is where the real problem would lie.  But if there is
a reason for such a machine in a major marketplace, a major manufacturer like
could certainly design custom hardware to match this design.  But
there'd better be a *good* reason for having such an oddball scheme.  For
most applications it doesn't seem too practical...  At least, considering
the marketplace.  Besides, who buys single source, anyway...  386+ and 68k
excepted...

  -- Kevin