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From: richard@aiai.ed.ac.uk (Richard Tobin)
Newsgroups: comp.arch
Subject: Re: Is handling off-alignment impor
Message-ID: <3200@skye.ed.ac.uk>
Date: 14 Aug 90 12:02:38 GMT
References: <40711@mips.mips.COM> <3300155@m.cs.uiuc.edu>
Reply-To: richard@aiai.UUCP (Richard Tobin)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 21

In article <3300155@m.cs.uiuc.edu> marick@m.cs.uiuc.edu writes:
>In Lucid's Common Lisp, they used the two low-order bits as the tag.
>On a machine that simply ignored the low-order bits on dereferencing,
>they could avoid masking those bits off.  I don't know whether they
>did so.

It's possible to make use of the fact that most machines *don't*
ignore the low-order bits to get "free" type checking.  Suppose you
make the tag for cons cells 3, and address them something like this:

  move -3(r1), r0

This will cause a trap if the tag is not 3.  Whether the type check is
really free depends on whether you would have been able to use a faster
addressing mode if you didn't want the check.

-- Richard
-- 
Richard Tobin,                       JANET: R.Tobin@uk.ac.ed             
AI Applications Institute,           ARPA:  R.Tobin%uk.ac.ed@nsfnet-relay.ac.uk
Edinburgh University.                UUCP:  ...!ukc!ed.ac.uk!R.Tobin