Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpda!motcsd!dms!albaugh
From: albaugh@dms.UUCP (Mike Albaugh)
Newsgroups: comp.arch
Subject: Re: SISC
Message-ID: <767@dms.UUCP>
Date: 8 May 89 17:40:26 GMT
References: <13359@pasteur.Berkeley.EDU>
Distribution: usa
Organization: Atari Games Inc., Milpitas, CA
Lines: 47

From article <13359@pasteur.Berkeley.EDU>, by dean@mars.Berkeley.EDU (R. Drew Dean):
> While this subject started out as a joke, I point the net back to the
> discussion a few months ago about a _real_ one instruction CPU:
> Subtract & Branch Negative.  The instruction looks like
> 	SUBN source1, source2, next
> [detail elided at least three such machines exist, mine is one-address]
> optimal code for this machine would be easy -- just generate the shortest
> sequence of instructions possible -- all (1) instruction(s) take the same
> amount of time, (I'd hope you'd pipeline it (easy) to get it down to 1 CPI),
					       ^^^^
> so the code generator doesn't have to worry about much.

	Not so fast, there... running "traditional" sorts of programs on
such a beast relies heavily on self-modifying code. The problem of forwarding
gets "interesting". Of course, I suppose the sort of scheduling that makes
VLIWs work could make sure there were no dependancies in the pipe...
(Hmmm. maybe I should dust of those plans... :-)

> Of course, to really make this thing scream, it needs to run at about 300 MHz,
> and have a _lot_ of 3 ns memory...:-)  You might want to try microcoding a

	Yes, that turn out to be a key problem for a hobbiest, even at
a mere 20 MHz.

> RISCy instruction set on it, but it would be memory-memory, as the chip
> has no registers other than the PC.

	John Bown took the approach of "emulating" the PDP-11 instruction
set via macro-expansion. This was helped by having a small amount of higher
speed memory for the "registers".

> I remember someone (sorry, I forget who) on the net saying that they had
> started to write a Pascal compiler for this beast....

	One poster had a working VanDerPoels "Zebra", with a Pascal Subset
cross-compiler. I was the one who had "started" to write a small-C compiler,
intending native compilation. No time, no money....

> Drew Dean
> Internet: dean@xcssun.berkeley.edu
> UUCP: ...!ucbvax!xcssun!dean
> FROM Disclaimers IMPORT StandardDisclaimer;

| Mike Albaugh (albaugh@dms.UUCP || {...decwrl!turtlevax!}weitek!dms!albaugh)
| Atari Games Corp (Arcade Games, no relation to the makers of the ST)
| 675 Sycamore Dr. Milpitas, CA 95035		voice: (408)434-1709
| The opinions expressed are my own (Boy, are they ever)