Path: utzoo!attcan!uunet!husc6!think!ames!lamaster
From: lamaster@ames.arc.nasa.gov (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: dedicated vs general-purpose CPUs
Message-ID: <13022@ames.arc.nasa.gov>
Date: 8 Aug 88 16:02:44 GMT
References: <5254@june.cs.washington.edu> <76700032@p.cs.uiuc.edu> <1988Aug3.180947.12070@utzoo.uucp> <1221@ficc.UUCP> <1173@garth.UUCP>
Reply-To: lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 41

In article <1173@garth.UUCP> smryan@garth.UUCP (Steven Ryan) writes:
>If you want an expensive example of a machine made of diverse processors,
>consider the CDC 6600->170s. A machine consists of 1 or 2 60-bit CPs and
>10 to 20 12-bit PPs. All the number crunching goes in the CP and the
>io in the PPs with all processors in parallel.

It is my belief that this is the primary example of what special purpose
processors are for:  when you have a requirement for at least two high
speed data paths, it is usually cheaper to keep those paths separate
than to try to combine the paths on an even faster common bus.  A separate
processor for that data path may be very useful.  Consider the channel,
for example.  Usually, a channel is a bus with only two interfaces on it,
and typically some means by which the CPU can start up channel commands.

What makes channels useful is that because the channels operate in parallel,
you can use, on a large mainframe, 64 (say) 3 Megabyte/Second channels
instead of trying to build a bus that can operate a 200 MB/second over
a distance of 100 feet while supporting 128 devices.  Graphics is another
tempting place to use a dedicated data path and special purpose hardware,
since the flow of data tends to be unidirectional from program to image.
(But watch out for designs that are so unidirectional that you can't read
your processed image back into main memory.)

Since, as many people have observed, off the shelf components are usually
much better wrt price/performance during periods of rapid technology
advancement, a logical solution seems to be to use general purpose CPUs
as the processors for special purpose systems.  So, for example, the disk
controller with a 68020 in it supporting a system with a faster 68020 in
it as the main CPU.  Or, as I understand the recent Ardent announcement, a
machine which uses the same CPU type as its graphics engine and main CPU,
eschewing custom graphics engine hardware.  At times in the past, when
progress was slower, the motivation to use custom hardware to reduce costs
was there, and I assume that there will be periods like that in the future
as well.  So, I don't assume that the dedicated CPU will disappear- I think
it will just fade rapidly for the next few years.

-- 
  Hugh LaMaster, m/s 233-9,  UUCP ames!lamaster
  NASA Ames Research Center  ARPA lamaster@ames.arc.nasa.gov
  Moffett Field, CA 94035     
  Phone:  (415)694-6117