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From: stern@inmet.UUCP
Newsgroups: net.arch
Subject: Re: Re: RISC vs VAX
Message-ID: <1839@inmet.UUCP>
Date: Thu, 29-Nov-84 01:08:27 EST
Article-I.D.: inmet.1839
Posted: Thu Nov 29 01:08:27 1984
Date-Received: Sat, 1-Dec-84 19:33:51 EST
Lines: 33
Nf-ID: #R:utzoo:-465700:inmet:2500012:000:1922
Nf-From: inmet!stern    Nov 27 10:51:00 1984

[got the asbestos jumpsuit on?]

(1) I said that RISCs are not suited for floating point/matrix multiplication
    because: (a) attaching a floating point coprocessor to a RISC perverts
    the very idea of a reduced instruction set; and (b) the original question
    asked if a RISC could be produced in the same class as a VAX.  No RISC
    can perform every function of a 780 faster than a 780 because there are
    tradeoffs made when you give up instruction set complexity for speed.
    Usually floating point stuff is the first to go -- except when you make
    a purely floating-point processor, in which case everything has been
    given up.  You can't have your floating point cake and eat it too.
     
(2) Yes, an ACIA and a VAX have different cycle times.  So do a washing
    machine and an ECL flip-flop.  But if you build a fancy interface to
    the ECL flip-flops, the washing machine still won't be able to talk
    to them.  Having a nice DZ-11 multiplexor on a VAx is great -- it provides
    intelligent communications processing, so the VAX doesn't burn cycles
    doing low-level ACIA handshaking.  Now how could a RISC talk to a DZ?
    Letting them share a memory space might be the best thing, but gee,
    then you might need another processor to arbitrate memory accesses, and
    then it starts to look like plugging a RISC into a VAX again.  My point
    is that if you want a RISC to do i/o *without another processor* then
    you have to add instructions/wait states/more complexity to get something
    with a 50-nanosecond cycle time to talk to a device with a 1-microsecond
    cycle time.  Comparing a RISC to a VAX in terms of performance, when the
    RISC has two or three half- or quarter-VAXen doing its i/o, is cheating.
    Hey, my Toyota can outrace the space shuttle when you strap two solid
    rocket boosters to it.

--Hal Stern

{ihnp4, harpo, esquire, ima}!inmet!stern