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When Intel competitors AMD and Cyrix developed their chips, they had the advantage of using a newer design that enabled them to make architectural changes to improve performance. The result is that many of their processors have greater performance than a Pentium processor running at the same clock speed. Since the market has a tendency to over-value raw clock speed instead of looking at bottom line performance, the "P rating" scheme was invented to provide what AMD and Cyrix consider to be more reasonable comparisons to the Pentium line than using clock speed. (I'm not sure how Intel feels about them, though I can imagine. :^) )
The P rating scheme is quite simple really; it uses standardized system-oriented benchmarks to compare processors against Pentiums. (System-based benchmarks reflect "real world" usage better than processor-oriented benchmarks, and the theory is that as long as the systems are identical, this can be used to compare just the processors.) A standard system configuration is set up with a Pentium processor, a good-quality benchmark such as Winstone 96 is run on it, and the score recorded. The Pentium is then replaced with different speed Pentiums and their scores recorded. Finally, the Pentium is replaced with the chips whose "P ratings" we are trying to determine.
The scores are tallied up, and the AMD or Cyrix chip is assigned a P rating equal to the speed of the fastest Pentium whose benchmark the AMD or Cyrix chip exceeded. This yields a reasonable comparison between the processor families, although not one that is definitely foolproof. Since the rating uses system-based benchmarks, it is dependent on other components in the system. While of course all the processors are tested with the same motherboard, memory and chipset, some processors might be better tuned to work with certain support hardware compared to others, especially when comparing chips with different internal architecture (which these tests almost always do).
Warning: The "P
rating" is based on system benchmarks that concentrate mostly on integer performance,
not floating point. This is important to remember, because many Intel-compatible
processors have far inferior floating point units compared to the Intel processors that
they claim to be equivalent to with their P rating, and will not perform based on their P
rating in floating-point-intensive applications.
Next: Processor Benchmarking