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From: davidb@titan.inmos.co.uk (David Boreham)
Newsgroups: comp.arch
Subject: Re: Shared Memory vs. Distributed Systems
Message-ID: <2673@ganymede.inmos.co.uk>
Date: 26 Oct 89 08:55:51 GMT
Sender: news@inmos.co.uk
Reply-To: davidb@inmos.co.uk ()
Organization: INMOS Limited, Bristol, UK.
Lines: 50


In article <20764@usc.edu> vorbrueg@bufo.usc.edu (Jan Vorbrueggen) writes (in
response to article <36597@lll-winken.LLNL.GOV> brooks@maddog.llnl.gov (Eugene Brooks):
>Eugene, ever seen a transputer? Overhead for receiving or sending a 
>message is 19 cycles (630 ns for a 30 MHz part). The actual transfer
>is done by a dedicated DMA machine at a maximum rate of 1.7 Mbyte/s

and then  in <1646@ncrcce.StPaul.NCR.COM>  pasek@c10sd3.StPaul.NCR.COM (M. A. Pasek) 
says some things which I don't really understand. 

Anyway, here is what a transputer link actually IS:


                              ------------
                             !            !
           ----------        !   DMA      !        ------------
          !  Memory  !       !   Engine   !       !             !
          ! on CPU A !=======!            !=======!  Serializer !--------
          !          !       !            !       !             !       !
           ----------        !            !        ------------         !
                             !            !                             !
                             !            !                             !
                              ------------                              ! Serial
                                                                        !  Link
                                                                        !
  ----------------------------------------------------------------      !
                                                                        !
                              ------------                              !
                             !            !                             !
           ----------        !   DMA      !        ------------         !
          !  Memory  !       !   Engine   !       !             !       !
          ! on CPU B !=======!            !=======!  Serializer !--------
          !          !       !            !       !             !
           ----------        !            !        ------------
                             !            !
                             !            !
                              ------------


Everything above the dotted line is on transputer A, everything below the line
in on transputer B. Of course there is also a CPU which fights for local memory
on a cycle-by-cycle basis. Each transputer has N copies of the link circuitry
(including 2N DMA engines). Currently  2 <= N <= 4 and the serial link goes at
20Mbits.



David Boreham, INMOS Limited | mail(uk): davidb@inmos.co.uk or ukc!inmos!davidb
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