Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cornell!uw-beaver!sumax!quick!srg
From: srg@quick.com (Spencer Garrett)
Newsgroups: comp.arch
Subject: Re: Is handling off-alignment important?
Message-ID: <1990Sep8.225345.745@quick.com>
Date: 8 Sep 90 22:53:45 GMT
References: <104037@convex.convex.com> <8840014@hpfcso.HP.COM> <141881@sun.Eng.Sun.COM>
Organization: Quicksilver Engineering, Seattle USA
Lines: 16

In article <141881@sun.Eng.Sun.COM>, jputnam@raptor.Eng.Sun.COM (James M.) writes:
>     Something he shouldn't have about VAX quad-word loads ignoring
>     low-order bits, and this being a useful feature for LISP.
> 
> In article <26376@mimsy.umd.edu> chris@mimsy.umd.edu (Chris Torek) writes:
> >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.

Jim,
   What may well have happened is that some early LISP implementer just
"tried it" and found that on his vax the low order bits were ignored.
This makes a dandy feature if you're trying to fake a tagged architecture,
so maybe he went ahead and used it.  *BUT* since it isn't in the manual,
DEC is free to change it whenever they feel the need, and they *do* feel
that need from time to time, so the code in question wouldn't even be
portable within the vax line.  You may both have been right.  :-)