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From: lewine@cheshirecat.rtp.dg.com (Donald Lewine)
Newsgroups: comp.arch
Subject: Re: RISCizing a CISC processor
Message-ID: <1200@dg.dg.com>
Date: 7 Dec 90 14:44:17 GMT
References: <9012070105.AA02416@hcrlgw.crl.hitachi.co.jp>
Sender: root@dg.dg.com
Reply-To: uunet!dg!lewine
Organization: Data General Corporation
Lines: 43

In article <9012070105.AA02416@hcrlgw.crl.hitachi.co.jp>, joe@hcrlgw.crl.hitachi.co.JP (Dwight Joe) writes:
|> 
|> In a CISC chip, there is a certain redundancy.  In other words,
|> some of the complex instructions can be written in terms of the
|> simpler instructions.  An instruction to move a block of data
|> from one place in memory to another place can be replaced
|> by a loop of simpler LOAD and STORE instructions.
|> 
|> Now, using timing analysis, estimate the performance of
|> implementing the RISC Set and the I/O Set in hardware
|> and implementing the CISC Set as subroutines in
|> a microcode store. 

    That was exactly what was done in the MicroVAX architecture
    back in 1982.  The more complex instructions were emulated
    using the simple instructions.

    There was some cleverness in using hardware to decode the
    full set of VAX instructions and then call software to do
    the rest.

    This does not give you a RISC in the sense of architecutral
    purity.  The VAX (or 386 or 68K) instruction stream is still
    a bear to decode and does many things that violate the RISC
    religion.  You have merely proposed a new way to implement
    a CISC machine.  The VAX 9000 also uses a technique very 
    similar to the one you describe.

	***HOWEVER***, the advantage of RISC is moving work from 
    runtime to compile time.  The big speedup comes from compiler
    work not hardware. At Data General we have modified some of
    the compilers for our CISC MV-series to compile simple code
    instead of using instructions like WEDIT.  This has produced
    major performance enhancements because a compiler can generate
    special case code. 

--------------------------------------------------------------------
Donald A. Lewine                (508) 870-9008 Voice
Data General Corporation        (508) 366-0750 FAX
4400 Computer Drive. MS D112A
Westboro, MA 01580  U.S.A.

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