Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bbn!apple!rutgers!att!alberta!ubc-cs!van-bc!rsoft!mindlink!a464
From: a464@mindlink.UUCP (Bruce Dawson)
Newsgroups: comp.arch
Subject: Re: Register usage
Message-ID: <259@mindlink.UUCP>
Date: 12 May 89 05:12:08 GMT
Organization: MIND LINK! - British Columbia, Canada
Lines: 23


     One thing that needs to be kept in mind when talking about the advantages
of huge numbers of registers is that some of the advantages of registers go
away when you have a lot (when have you a lot availabel simultaneously I should
say).  In the extreme case of the computer someone mentioned that had 256
registers, a register-register operation would use up sixteen bits just to
specify the two registers involved.  Contrast that with the six bits required
if you only have eight registers.  Given the finite memory speeds that we have
to deal with, an extra ten bits so that you can have 256 instead of eight
registers is probably too big a price to pay and would probably slow programs
down.

     Speaking from my experience on the 68000, I would say that for assembly
language programming, 16 registers is generally enough, with more being
occasionally desirable.  Because C can't do a very good job of comprehending an
entire substantial subroutine or program and deciding what should be in
registers (actually, replace 'C' with HLL) I would think that a compiler could
actually get by with slightly less.  One thing to keep in mind is that the
80386's eight registers are actually eight, whereas the 680x0 families sixteen
are usually more like thirteen or fourteen (by the time you use one for a stack
pointer, one for a data segment pointer and one for a stack frame pointer).

.Bruce.