Path: utzoo!mnetor!uunet!tektronix!sequent!jjb
From: jjb@sequent.UUCP (Jeff Berkowitz)
Newsgroups: comp.arch
Subject: Re: Wulf's WM
Message-ID: <3342@sequent.UUCP>
Date: 27 Apr 88 01:58:40 GMT
References: <28200131@urbsdc> <1508@pt.cs.cmu.edu>
Organization: Sequent Computer Systems, Beaverton, OR
Lines: 36
Summary: LOAD into a queue: we did it

In article <1508@pt.cs.cmu.edu>, agn@UNH.CS.CMU.EDU (Andreas Nowatzyk) writes:

> Of particular appeal to me was the introduction of fifo's into the
> load/store instructions (decoupling the time when an address is issued
> from the time when the data is accessed) as it has the *potential*  of
> allowing more latency in the memory system without degrading the throughput.

Culler Scientific implemented this feature.  It is visible at the instruction
set level, so the compiler could emit code like
		...
		load	(r2)
		<arbitrary instructions during latency>
		add	fifo, r0, r1
		...
Note that this is psuedocode, not Culler assembler.  The depth of the FIFO
is four.  It is possible to issue three or four reads in succession, then
begin using the data as it arrives from memory.  The performance gain is
substantial; in practice, the compiler is very often able to schedule
reads in advance and make use of the latency.  Making this design
interruptible was complex.  Culler implmented hardware interlocks to stall
the processor on an attempt to read an emptry FIFO.  The machine included
a deadlock detector in hardware which would generate a trap in case of an
infinite stall (I'm not advocating anything, just reporting on a real
implementation.)

I thought this tidbit would be interesting despite the machine's well known
commercial failure.  Late in our design, we also became aware of a group
- maybe they were at U. of Wisconsin? - which had independently designed
a machine called PIPE.  This machine made heavy use of queues, and the
architects had done some worthwhile simulations.  Sorry I can't be more
specific, my article collection has not yet followed me to Oregon.

-- 

Jeff Berkowitz                        ...!tektronix!sequent!jjb
Sequent Computer Systems              Beaverton OR