Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!decwrl!sun-barr!newstop!sun!raptor.Eng.Sun.COM!jputnam
From: jputnam@raptor.Eng.Sun.COM (James M.)
Newsgroups: comp.arch
Subject: Re: Is handling off-alignment important?
Message-ID: <141881@sun.Eng.Sun.COM>
Date: 5 Sep 90 22:06:40 GMT
References: <104037@convex.convex.com> <8840014@hpfcso.HP.COM> <140356@sun.Eng.Sun.COM> <26376@mimsy.umd.edu>
Sender: news@sun.Eng.Sun.COM
Organization: Sun Microsystems, Mt. View, Ca.
Lines: 29

In article <26376@mimsy.umd.edu> chris@mimsy.umd.edu (Chris Torek) writes:
>(Someone is recirculating old news through the net.  The last time this
>came around, I did not comment, but this time I cannot resist :-) )
>
>[Someone asks what machines that require alignment do with unaligned
>addresses in loads.  His `least desirable' scenario is that the
>processor completely ignores the low address bits.]
>
>In article <140356@sun.Eng.Sun.COM> jputnam@raptor.Eng.Sun.COM
>(James M. Putnam) writes:

    Something he shouldn't have about VAX quad-word loads ignoring
    low-order bits, and this being a useful feature for LISP.

>Since there is no mention of this in the VAX architecture handbook, I
>tried it out to make sure.  An unaligned movq reads the unaligned quadword.
>-- 
>In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750)
>Domain:	chris@cs.umd.edu	Path:	uunet!mimsy!chris

    I stand corrected and thanks to Chris for pointing it out. During a 
    conversation I had with Barry Margolin some time ago about this very 
    topic (non-lispm architecture implementations of LISP pointers), I got 
    the impression that VAX quad-word loads ignored bits in the address.

    Sincere apologies for the misinformation. Does anyone know of a machine
    that has this "feature", or I did construct this out of whole cloth?

	Jim