Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!sdd.hp.com!hplabs!hpcc05!hpdmd48!sritacco
From: sritacco@hpdmd48.boi.hp.com (Steve Ritacco)
Newsgroups: comp.arch
Subject: Re: Swizzling (very RISC) instead of 64 bits (was Re: 64 bit addresses)
Message-ID: <14900024@hpdmd48.boi.hp.com>
Date: 15 Feb 91 17:07:35 GMT
References: <1991Feb13.170045.16864@uicbert.eecs.uic.edu>
Organization: Hewlett Packard - Boise, ID
Lines: 16

> I don't know if 64-bit addressing is a good idea in general, and I suspect
> it's a *bad* idea for languages like Lisp and Smalltalk, where the standard
> implementation technique is to make everything the size of a pointer,
> so that you can store either a pointer or a tagged "immediate" value
> anywhere.  Going to 64-bit addresses would *double* your memory
> requirements.  There are much better ways to spend transistors.

This doesn't seem very well thought out to me.  64 bits is a nice healthy 
amount of room for a cons cell or an immediate value.  It was pretty common
to steal the top few(or bottom) bits of a word for a tag but with 64 bits
you can do that without sacrificing precision.  What about applications of
clever crd codeing schemes which could give access to a huge address space
but efficiently describe common addresses with smaller offsets.  

I think 64 bits opens up a whole new world of possibilities, and we must be
careful to re-think implementations, not just re-implement them.