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From: marick@m.cs.uiuc.edu
Newsgroups: comp.arch
Subject: Re: Is handling off-alignment impor
Message-ID: <3300155@m.cs.uiuc.edu>
Date: 9 Aug 90 16:36:00 GMT
References: <40711@mips.mips.COM>
Lines: 30
Nf-ID: #R:mips.mips.COM:40711:m.cs.uiuc.edu:3300155:000:1284
Nf-From: m.cs.uiuc.edu!marick    Aug  9 11:36:00 1990


Lisp objects are typed at runtime.  This is usually implemented by
using tag bits in the pointers to those objects.  Objects small enough
to fit in a word have tag bits that say, "I'm not a pointer; I'm a
short integer" or whatever.

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.

Another advantage of this scheme is that the tag bits for
short-integers are 0.  No masking is needed before addition, and
overflows can be detected by hardware.  You do have to remember to
shift integers before output.

The Gould Powernode machines had 32 bit words, 24 bit addresses.  The
top 8 bits were ignored.  In our Lisp, we put the tag bits there.
Again, there was no need to mask before dereferences.  The tag bits
for positive short ints were 0x0 and for negative short ints were
0xff, so machine addition could be used without masking.  However, you
had to check for overflow into the tag bits.

(Note:  I didn't work for Lucid.  As I recall, I learned this with
their debugger.  I think the trick has been floating around for a
while.)

Brian Marick
Motorola @ University of Illinois
marick@cs.uiuc.edu, uiucdcs!marick