12.6 Closing notes

Congratulations for reaching the end of a long chapter! We have talked a little bit about old computers, CISC, RISC, post-RISC, and EPIC, and mentioned supercomputers in passing. I think it’s interesting to observe that RISC processors are a branch off a long-established tree. Many of the ideas that have gone into RISC designs are borrowed from other types of computers, but none of them evolved into RISC — RISC started at a discontinuity. There were hints of a RISC revolution (the CDC 6600 and the IBM 801 project) but it really was forced on the world (for its own good) by CPU designers at Berkeley and Stanford in the 1980s.

As RISC has matured, there have been many improvements. Each time it appears that we have reached the limit of the performance of our microprocessors there is a new architectural breakthrough improving our single CPU performance. How long can it continue? It is clear that as long as competition continues, there is significant performance headroom using the out-of-order execution as the clock rates move from a typical 200 MHz to 500+ MHz. DEC’s Alpha 21264 is planned to have four-way out-of-order execution at 500 MHz by 1998. As of 1998, vendors are beginning to reveal their plans for processors clocked at 1000 MHz or 1 GHz.

Unfortunately, developing a new processor is a very expensive task. If enough companies merge and competition diminishes, the rate of innovation will slow. Hopefully we will be seeing four processors on a chip, each 16-way out-of-order superscalar, clocked at 1 GHz for $200 before we eliminate competition and let the CPU designers rest on their laurels. At that point, scalable parallel processing will suddenly become interesting again.

How will designers tackle some of the fundamental architectural problems, perhaps the largest being memory systems? Even though the post-RISC architecture and the EPIC alleviate the latency problems somewhat, the memory bottleneck will always be there. The good news is that even though memory performance improves more slowly than CPU performance, memory system performance does improve over time. We’ll look next at techniques for building memory systems.

As discussed in [link] , the exercises that come at the end of most chapters in this book are not like the exercises in most engineering texts. These exercises are mostly thought experiments, without well-defined answers, designed to get you thinking about the hardware on your desk.