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From: hansen@mips.UUCP (Craig Hansen)
Newsgroups: comp.arch
Subject: Re: unaligned accesses [and words-versus bytes] [long, much data]
Message-ID: <307@dumbo.UUCP>
Date: Thu, 16-Apr-87 18:45:03 EST
Article-I.D.: dumbo.307
Posted: Thu Apr 16 18:45:03 1987
Date-Received: Sun, 19-Apr-87 10:02:40 EST
References: <16038@amdcad.AMD.COM> <279@winchester.mips.UUCP> <576@cpocd2.UUCP>
Lines: 41
Summary: it didn't cost much area either

In article <576@cpocd2.UUCP>, howard@cpocd2.UUCP (Howard A. Landman) writes:
> >First, (Geoff Steckel) <466@alliant.UUCP> posted a pretty good overall
> >analysis of the issues, so I won't repeat that, except:
> >"Re bus width, byte extraction, unaligned operands, and memory speed:
> >  1) Byte extraction from words should be free in time; it'll cost a few gates.
> >	Basically this requires one or more cross-couplings in the memory path.

> I don't wish to dispute John's excellent (and voluminous) analysis, but it
> is worth mentioning that Geoff seems to be suffering from the normal logic
> designer's misconception that you measure complexity/cost in gates.  On ICs,
> gates are extremely cheap and small; it is wires (communication over distance)
> that are expensive.  Byte extraction can frequently be performed with the same
> circuitry that does shifts, but fast shifters are area-expensive (expansive?)
> in 2 dimensions.  You don't want to use them without serious consideration.

As it turns out, the load aligner didn't pay an area penalty for the
horizontal shift wires, as the two relevant busses cross each other, for
reasons unrelated to alignment, right next to the load aligner.  The store
alignment is accomplished by the shift unit already, so it didn't cost extra
area either. (It did cost a few gates, but it was well worth it.) Howard's
statement is true in general: wires are expensive, and gates are relatively
cheap, but clever layout makes the best possible use of expensive resources.

I
really
enjoy
trying
to
be
wordy
in
my 
responses
to
articles
posted.
-- 
Craig Hansen
Manager, Architecture Development
MIPS Computer Systems, Inc.
...decwrl!mips!hansen