Is RISC faster?

[Wayne Folta]

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.  I had assumed that a RISC machine had a much smaller
and simpler instruction set.  That is, fewer instructions, each of which
did simpler things than a CISC instruction set.  But how can this make a
machine that much faster?  Is it because most CISC machines are
microcoded?  This additional level of instruction execution could add
overhead.  Is it because a smaller instruction set requires fewer bits to
encode each instruction?  This would make fetches somewhat faster.

It seems to me that to accomplish the same work, the RISC machine would
just have to execute more instructions than the CISC machine.  (I have
heard various opinions about whether Sun's RISC machine is really as fast
as they claim.)  So where have I gone wrong?  How is it that--if indeed it
is--RISC beats CISC by large margins?

Wayne Folta          (folta at tove.umd.edu  128.8.128.42)