Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!hubcap!jps
From: jps@cat.cmu.edu (James Salsman)
Newsgroups: comp.parallel
Subject: SIMD Extentions
Summary: What We Really Need is a more powerful Processing Element
Message-ID: <5886@hubcap.clemson.edu>
Date: 30 Jun 89 15:44:39 GMT
Sender: fpst@hubcap.clemson.edu
Lines: 46
Approved: parallel@hubcap.clemson.edu

I really hate having to deal with indirect addressing on
most SIMD machines.  I wish someone would build a SIMD array
using PE's with address buffers.  Just one tiny address
buffer per processor is all I want... nothing fancy.  As
long as *ALL* the memory addresses have to come over the
global instruction stream and thus are the *SAME* for each
element, a lot of potential processing power is going to
waste!

For example, on the Connection Machine in *Lisp, indirect
aref!!'s take FOR EVER.  This is SERIOUSLY slowing down the
Production System that I wrote in *Lisp (regardless, it's
faster than CMU/Soar's Production System Machine or any
other implementation of a production system that I've heard
about.)  TMC has added somthing called "sideways arrays" to
help indirect addressing, but the *Lisp manual is totally
obscure (so what else is new) and from what I can tell, it
looks like "sideways" means "spread out over several
physical processors."  Ack/Pft!

The way I would hack an address buffer in to the CM is by
employing a shift register added to each PE.

  (1) Add a new nanoinstruction pin [or two] that selects
      memory input to the ALU between "Address A [or B]"
      from the instruction pins and the contents of the
      indirection register.

  (2) Add a new nanoinstruction pin that causes the output
      from the ALU to be shifted into the indirection register.

That's all there is to it.  Two or three new pins, a shift
register, and PE memory indirection takes 13 nanocycles
instead of a zillion.

I am sure that a similar thing could be done to other SIMD
architectures.  If anybody thinks that indirect addressing
is not worth a register and a couple of new nanopins.

:James

P.S. If anyone wants to use this idea :-) it's free -- I think
patents are morally wrong.
-- 

:James P. Salsman (jps@CAT.CMU.EDU)