Path: utzoo!bnr-vpa!bnr-fos!bigsur!bnr-rsc!bcarh61!schow
From: schow@bcarh61.bnr.ca (Stanley T.H. Chow)
Newsgroups: comp.arch
Subject: Re: RISC vs CISC (rational discussion, not religious wars)
Summary: RISC != simple
Message-ID: <1352@bnr-rsc.UUCP>
Date: 10 Nov 89 19:15:19 GMT
References: <503@ctycal.UUCP> <31031@winchester.mips.COM>
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Reply-To: schow%BNR.CA.bitnet@relay.cs.net (Stanley T.H. Chow)
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In article <31031@winchester.mips.COM> mash@mips.COM (John Mashey) writes:
>
>The die area issue has been widely misinterpreted.  Let me summarize some
>of the various arguments for RISC vs CISC and die size:
> [...]
>ARGUMENT 4: But when you can get zillions** of transistors  on a die, it
>	it doesn't matter.
>	COMMENT 1: We haven't gotten enough transistors to make anybody
>		happy yet ("happy" means VLSI designers wandering around
>		saying "I have so much space I don't know what to do with it")
>		and we're not likely to get enough any time real soon.
>	COMMENT 2: Of course, this all remains to be seen, but:
>	COMMENT 3: More transistors will help everything, however, it may be
>		that the "limiting factor" will be not die space,
>		but COMPLEXITY in critical paths and exception-handling.
		    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Please note that "complexity" is essentially independent of the RSIC/CSIC
debate. I defy anyone to call the M88k as simple :-)

It is certainly true that i86 (or any other micro) has had its share of
problems. However, I would score the problems to the "Venerable Architecture"
instead. It is by no means clear that CISC architecture must always be
nasty in this respect.

The "Zillion transistor" question is essentially another design trade-off:
is it better to use the transistors for bigger cache? more pipeline stages?
fatter pipelines? more execution units? ....

Of course, once the architecture is chosen, many of the trade-offs become
obviouse. IMHO, CSIC leaves many of the choices open, allowing more paths
to be determined later. [Structured Software people think delaying choices
is a good stratagy. You may or may not agree. :-)].

>
>Do weird and occasional effects matter?  Maybe, maybe not.  Observe that
>a 6-month difference is a long time these days, and if it takes 6 months
>more to get a design really right enough that reputable companies will
>ship it, that's a noticable difference.
>

Certainly 6 months make a difference. It is also true that even 2 years
make little difference.

It all depends on the market and the perspective. How many RISC chips
beat the 486 to market? What difference did it make? These days, software
is KING. In the commercial market place, Compatibility is also very
important.

Even in the new designs that don't care about old software, I would think
designers (at least the design managers) should look at the long term
evolution of the architecture before settling on a particular CPU chip.

The only occasions when 6 months *really* matter is a race to open up a
new area. It's been a long time since those kinds of excitment in the 
CPU & memory market. Examples are the *first* one-chip CPU (the 4004/8008), 
the first DRAM, the first EEPROM, etc. When I can also buy the same
functionality, 6 months is only cost-optimization.



Stanley Chow        BitNet:  schow@BNR.CA
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(613) 763-2831		     ..!utgpu!bnr-vpa!bnr-rsc!schow%bcarh61
Me? Represent other people? Don't make them laugh so hard.