This is not the first time AMD has fallen behind Intel's clock speeds, nor is it the first time it has played the numbers game. Five years ago, AMD briefly labeled its K5 processors with a "P" (performance) rating to better position them against Intel's Pentium chips.

Like the K5, the P-rating did not last long — once AMD rolled out the faster K6, it dropped the P-rating without regrets. But competitors Cyrix and IBM continued to use it, since their processors also used lower clock speeds than Intel's.

AMD did not play the game well with the K5, providing both the clock speed and the P-rating when discussing its chips. The P-rating did not seem to enhance the K5's sales or pricing, but to be fair, nothing would have helped sell that unpopular device.

Athlon, however, is a stronger product, and the XP version contains new performance enhancements. On many benchmarks, Athlon XP performs as well as or better than the Pentium 4. AMD's problem is conveying this to megahertz-minded customers.

Cyrix made the P-rating work by downplaying the clock speed of its processors. Never discussed, the clock speed was buried in a data sheet along with power dissipation and bus timing. With little else to go on, consumers focused on Cyrix's P-ratings. If AMD follows this approach, its new numbering system may catch on. Cyrix could never charge the same as Intel for the same performance, and neither can AMD. But the performance rating should help AMD charge more than it can on the basis of pure megahertz.

A fundamental problem facing both AMD and Intel, however, is that most PC buyers don't pay attention to performance at all. With almost every PC today at 1 GHz or beyond, any new computer has enough power for the applications most people use every day. These buyers can ignore the ratings and focus on one number: price.

Some PC buyers are concerned about performance, typically for complex business applications, leading-edge games or video editing. These high-end buyers may find AMD's new ratings useful, but they should really look at product reviews and more-detailed benchmarks before choosing a PC processor.

Linley Gwennap is founder and principal analyst of The Linley Group in Mountain View, Calif.