Once 98-pound weaklings compared to their desktop siblings, mobile processors -- CPUs designed for notebook PCs, where power, cooling, and physical space are all at a premium -- have steadily narrowed the performance gap over time. And while mobile chips command higher prices (just as LCD screens do over CRT monitors), the premium is often worthwhile when size, weight, and battery life are important concerns.

"The gap between desktop and mobile CPUs has shrunk quite a bit over the years," says Masa Okumura, director of product marketing and worldwide product planning at Toshiba America Information Services in Irvine, Calif. "But desktops will always have an advantage because they have bigger chassis and better heat dissipation, which allows you to increase the clock speed of a desktop CPU sooner than a mobile CPU."

At this writing, for instance, AMD's fastest notebook chip is its mobile Athlon XP 1800+ versus the desktop Athlon XP 2600+. Intel's fastest desktop Pentium 4 runs at 2.8GHz, while what it calls the Mobile Pentium 4 Processor-M currently tops out at 2.2GHz.

And while the performance difference has narrowed, the price difference is still considerable -- AMD charges PC vendors virtually the same price for the mobile Athlon XP 1800+ and desktop 2600+, while Intel's 2.53GHz desktop P4 is cheaper than the 2.0GHz mobile part. "That makes perfect sense," insists Mike Stinson, vice president of mobile products for Gateway in San Diego, Calif.: "In order to push the performance up, it gets harder and harder to do what you have to do to make a product a mobile product."

Throttling Down: SpeedStep and PowerNow

One way that mobile processors reduce their appetite for power is by changing their clock speed and voltage based on user or application demands -- speeding up or slowing the CPU when there's more or less for it to do. Intel implements this concept with its SpeedStep scheme, which first appeared in the company's mobile Pentium III in January 2000.

"SpeedStep drops the frequency at which a CPU is operating and the voltage used to operate it," says Okumura. "[In its simplest form, when] a laptop is attached to a power outlet, its CPU will operate at top speed, but when it's running off its battery, it will keep the CPU's speed down." Intel's current notebook CPUs use what the company calls Enhanced SpeedStep technology to permit dynamic switching between full and partial speed even during battery work sessions.

Since June 2000, AMD's mobile processors have offered a power-management feature called PowerNow. Like SpeedStep, PowerNow adjusts a CPU's operating frequency and voltage based on processing demand. But where SpeedStep offers only a high and low gear, so to speak -- the 1.4GHz through 2.0GHz mobile Pentium 4s all slow to 1.2GHz when SpeedStep kicks in -- PowerNow offers a third mode that automatically detects the performance required by a software application and adjusts clock speed for the best compromise between horsepower and battery life.