Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!rice!sun-spots-request
From: mcvax!philce!richard@uunet.uu.net (Richard Bishopp)
Newsgroups: comp.sys.sun
Subject: Re: Is RISC faster
Message-ID: <305@philce.UUCP>
Date: 22 Feb 89 10:37:59 GMT
References: <15686@mimsy.UUCP>
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In article <15686@mimsy.UUCP> folta@tove.umd.edu (Wayne Folta) writes:
>A co-worker and I were debating why RISC machines might be faster than
>CISC (traditional) machines.  I had just read an article that did some
>UNIX benchmarks, and found RISC machines ran about 3 times faster than
>CISC (a 68030-based machine, for example).
>
>The question is why.  

The name RISC is actually a bit of a misnomer (or misacronym?). RISC
should be RICC - Reduced Instruction Cycle Computer. The only way for a
computer to go faster is to execute an instruction in a shorter time.

If you look at the CPU cycle time on a benchmark you should find the RISC
machines have a considerably shorter CPU cycle. Another reason why RISC
can be better is that the design of the CPU is simpler. In other words you
can optimize some parts of the CPU to go faster. A limitation in a
micro-coded machine is the cycle time to access the microcode memory and
decode the instruction.  Most RISC machines have some sort of internal ROM
(or PLA) to decode instructions which is inherently faster than accessing
off-chip memory of microcode. If the chips  also have a large number of
registers to store data you can reduce the external accesses to memory -
besides memory is easy to make on a chip.

I learnt all this while working at Acorn Computers (England) on the design
of the ACORN RISC machine which I think is now being used by Olivetti in
their laser printers etc. I have lots of references if anyone is
interested.

Richard Bishopp