Path: utzoo!utgpu!news-server.csri.toronto.edu!helios.physics.utoronto.ca!ists!yunexus!davecb
From: davecb@yunexus.YorkU.CA (David Collier-Brown)
Newsgroups: comp.arch
Subject: Re: DEC RISC Architecture?
Message-ID: <16138@yunexus.YorkU.CA>
Date: 13 Oct 90 02:43:06 GMT
References: <4462@trantor.harris-atd.com> <107038@convex.convex.com> <16034@yunexus.YorkU.CA> <1990Oct12.191024.17856@zoo.toronto.edu>
Organization: York U. Computing Services
Lines: 37

In the presence of a Henry, I made the ill-advised comment:
[...]	 Going to a simpler architecture is a straightforward but
| **labourious** problem ...

henry@zoo.toronto.edu (Henry Spencer) replied:
| Unfortunately, there are a couple of big jokers in the deck whenever you
| are talking about emulating one machine on another.
[...]
| (Note that "data formats" includes things like stack frames -- do you
| really want your nice fast RISC to spend half its time emulating the VAX
| calling sequence?)

	I think I can claim that this is a large-scale instance of finding
	the right tranformations or right instructions.  If it's stereotyped
	enough, it is succeptable to transformation, even though it's both
	large and pervasive.  I may be pushing compiler technology fairly hard,
	though.  Certainly I'm well past my state-of-practice.

|	  When talking about kernel code, another big problem will
| be assumptions of atomic execution of instructions.  It Is Rumored,
| to use a non-VMS example, that the NFS code Sun ships is full of
| assumptions that "i++" is an atomic operation, which is true on a
| single-processor 68k but not on most RISCs.

	And that one's a perfect stopper.  If such a semantic for the
	construct is assumed by a kernel developer, we is in deep do-do.
	Even worse, we have to detect it (:-)).  I expect the really good
	``stoppers'' won't be found until after the first few versions of
	the transformed code go into test...  A **real bad** time to go back
	to the compier-design gurus.

--dave (chastened but unbowed) c-b
-- 
David Collier-Brown,  | davecb@Nexus.YorkU.CA, ...!yunexus!davecb or
72 Abitibi Ave.,      | {toronto area...}lethe!dave or just
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