With gasoline prices wildly fluctuating in the United States, car buyers are paying increased attention to fuel efficiency. And that measure seems easy to determine: New cars come plastered with enormous stickers proclaiming their EPA-rated fuel efficiency. Environmentalists are quick to point out, however, that these EPA ratings are only loosely related to real-world fuel efficiency. The EPA admits as much, adding small print that reads, "Your mileage may vary."

Fuel efficiency is tough to pin down because it depends on many factors, such as altitude and driving style. Similarly, performance metrics for licensable processor cores are quite slippery because they depend on many factors. Consider clock speed-one of the easiest-to-grasp attributes of a licensable core. The clock speed achieved by a given core depends on a long list of variables, including the fabrication process and cell library used to build the chip, the EDA software employed to design the chip and the skill of the chip designers, just to name a few. Making even small changes to just one of these variables can significantly affect the clock speed achieved.

One unfortunate consequence of this complexity is that it leaves ample opportunities for core vendors to unrealistically inflate the performance of their cores. For example, a core vendor may report core clock speeds that can only be achieved if the user accepts a 50 percent manufacturing yield. Since most system-on-chip (SoC) developers cannot afford to throw away half of their chips, they are unlikely to achieve such speeds.

Even when core vendors present realistic performance data, there is no guarantee that competing vendors will base their performance claims on similar conditions-or that these conditions will match conditions used by their licensees. For example, SoC designers generally target low cost and low power consumption, rather than maximum speed. Despite that, core licensors often quote speeds that can only be achieved with expensive, power-hungry designs. This is like using EPA city mileage ratings to plan a cross-country trip: The numbers aren't wrong, but they aren't helpful, either.

Unfortunately, there is no simple solution to these problems. Indeed, the situation is only becoming more complex as process geometries shrink and SoC designs become larger. The best policy for core vendors is to drop inflated performance claims and educate customers on these complexities. And the best thing SoC designers can do is to read the fine print that says, "Your mileage may vary."

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