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Processor Performance Factors

Since the performance of a processor is (in most cases) based on how many instructions it can execute in a given time, it has become common to use the words "performance" and "speed" somewhat interchangeably. Unfortunately, the word "speed" has too many meanings when it comes to processors, and as a result frequently one person will use it to mean one thing and another person to mean quite another.

There are two major factors that determine the performance level of a processor:

• Clock Speed: The processor's clock speed is a measure of how fast it is running in raw terms, meaning, how many clock cycles it has to work with in a given period of time. Using the analogy of a bicycle, this would be equivalent to how fast you are pedaling.
• Architecture: The internal design and architecture of the processor measures how efficiently the processor does its work and how much it does with each clock cycle. In the bicycle analogy this would be a measure of how hard you are pedaling.

Since clock speed is easy to see and understand--it's just a number--and architecture is both complex and difficult to understand, it's not surprising that the former receives much more attention than the latter. This is unfortunate, because looking at just the clock speed of a processor is very deceptive, because it tells only one part of the picture. In fact, this is moreso today than ever before due to the much greater variety in processor designs and technologies. The "P" rating scheme was invented by Intel competitors AMD and Cyrix to provide what they feel is an assessment of the value of their processors that is more fair than using just clock speed.

One great example showing the importance of architecture is AMD's K5 series of processors. Some of these processors actually have very different performance ratings despite running at the same clock speed! For example, the K5-PR100 and the K5-PR133 both run at the same clock speed, 100 MHz, but the PR133 has approximately 33% better performance, due to changes that were made to the processor's internal architecture.

Clock speed can be used to compare processors only if they are identical internally. (Obviously, the K5-PR100 and K5-PR133 aren't the same internally if they have the same clock speed and different performance. I think AMD should have given them more clearly different designations to avoid this confusion.) This means you can only use clock speed to compare the performance of otherwise identical processors. A Pentium 200 is in fact 20% faster than a Pentium 166. But it isn't 20% faster than a Pentium with MMX 166, because of the latter's architectural improvements, such as a larger internal cache.

Next: Processor vs. System Performance

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