Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!pt.cs.cmu.edu!MATHOM.GANDALF.CS.CMU.EDU!lindsay
From: lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay)
Newsgroups: comp.arch
Subject: Re: Instruction (dis)continuation
Summary: if simple works why fix it
Message-ID: <6384@pt.cs.cmu.edu>
Date: 3 Oct 89 02:34:02 GMT
References: <2353@oakhill.UUCP> <261500010@S34.Prime.COM> <34701@apple.Apple.COM> <2451@wyse.wyse.com> <477@ctycal.UUCP> <265@ssp1.idca.tds.philips.nl>
Organization: Carnegie-Mellon University, CS/RI
Lines: 34

In article <265@ssp1.idca.tds.philips.nl> roelof@idca.tds.PHILIPS.nl (R. Vuurboom) writes:
>I'ld like to bring back in memory a quote from an execellent posting from 
>Steve Melvin from the University of California Berkeley 2 weeks ago on this 
>subject:
>The bottom
>line for an I/O instruction is that it represents a synchronization point from
>the perspective of the hardware.  That is, all unconfirmed operations have to
>be verified before the I/O operation can take place.  All predicted branches
>have to be confirmed, all pending memory reads and writes have to at least be
>translated to verify that they can be completed and all operations that can
>generate exceptions have to be executed.  Generally this means that the entire
>pipeline has to be drained.  

I'm afraid I don't agree.

Since direct control over a physical IO device implies some level of
privilege, it is reasonable to require each handler to memory-lock
its pages beforehand. It is also reasonable to insist that the
handler not divide by zero (etc) within the immediate vicinity of the
IO action. Handlers don't need a "drain pipeline" instruction, if
they are not going to fault.

As proof, I offer the working systems that are out in the world.

You seem to be asking for processors that are more complicated, and
complication is not free. You cannot justify your case by showing how
it will make handlers possible: they already are.  So, you will have
to show how it makes handlers better, or cheaper. 

Old truism, disguised as an old joke:
	"Doctor, it hurts when I do THIS."
	"Then don't do that."
-- 
Don		D.C.Lindsay 	Carnegie Mellon Computer Science