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From: chris@mimsy.umd.edu (Chris Torek)
Newsgroups: comp.arch
Subject: Re: Is handling off-alignment important?
Message-ID: <26376@mimsy.umd.edu>
Date: 5 Sep 90 16:15:58 GMT
References: <104037@convex.convex.com> <8840014@hpfcso.HP.COM> <140356@sun.Eng.Sun.COM>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 34

(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:
>The VAX does this for quad word loads. VAX LISP takes advantage of
>this feature to get two free tag bits on LISP pointers that don't 
>need to be masked before being dereferenced.

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.
If r0 contains 0x1003 and memory at 0x1000 looks like this:

	0x1000: 0x00
	0x1001: 0x01
	0x1002: 0x02
  r0 ->	0x1003: 0x03
	0x1004: 0x04
	0x1005: 0x05
	0x1006: 0x06
	0x1007: 0x07
	0x1008: 0x08
	0x1009: 0x09
	0x100a: 0x0a

then after a `movq (r0),r0', r0 contains the value 0x06050403 and
r1 contains the value 0x0a090807.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750)
Domain:	chris@cs.umd.edu	Path:	uunet!mimsy!chris