folta@tove.umd.edu (Wayne Folta) (02/08/89)
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@tove.umd.edu 128.8.128.42)