Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!cs.utexas.edu!samsung!aplcen!haven!uvaarpa!mcnc!decvax!crltrx!max.crl.dec.com!jg
From: jg@max.crl.dec.com (Jim Gettys)
Newsgroups: comp.windows.x
Subject: Re: 29 bit resource id and garbage collection
Message-ID: <1123@crltrx.crl.dec.com>
Date: 2 Nov 89 14:39:25 GMT
References: <67055@philabs.Philips.Com>
Sender: news@crltrx.crl.dec.com
Reply-To: jg@max.crl.dec.com (Jim Gettys)
Distribution: na
Organization: DEC Cambridge Research Lab
Lines: 18

Languages like lisp, CLU, etc. often distinguish an pointer to some
object from a integer internally by
having the high order bit(s) of the integer set.  Among other things,
this means that when a garbage
collector goes rummaging around to find garbage, it can tell objects
that may need garbage collection
from integers on the stack.  Some languages/implementations use as many
as the three bits of a word
for this purpose.

But the real point is that if an ID could have one of these bits on,
then all ID's in lisp might
have to be represented by bignums, causing a substantial performance
penalty for such languages.
Better to just avoid generating problems for people working in those
languages by requiring that
ID's not use those bits.
					- Jim Gettys