In addition to its core voltage requirement, each processor has a thermal power specification expressed in watts, which translates into the CPU's thermal dissipation requirements. For a quick example, by maintaining clock speed while changing core voltage, we see that the 1.5V Pentium 4/2.0A Northwood dissipates 54.3 watts, while the 1.75V Pentium 4/2.0 Willamette has a much higher value of 75.3 watts. By contrast, the 2.4GHz mobile Pentium 4 has a minimal power output of 30 watts (or around 21 watts with SpeedStep enabled).

This thermal specification is important because it refers to the level of CPU cooling required to keep the processor from overheating. AMD has been at the forefront of CPU cooling requirements, and as its Athlon XP line rose in core speed and voltage, so did its power specification and the reference cooling design.

Intel recently did much the same with the 3.06GHz Pentium 4, retail versions of which come with a bulkier heat sink and faster-spinning fan than their predecessors -- and no wonder, since the 3.06GHz Northwood has a power spec of 81.8 watts, in excess of the fastest Willamette models.

Cooling the Beast

This leads us to the all-important issue of CPU cooling, which has become almost an industry unto itself. On one side we have CPU manufacturers like Intel, whose chip designs incorporate an integrated heat spreader (IHS) to facilitate the link from the core to the CPU cooler. AMD doesn't include this feature in the current Athlon XP, which uses a pseudo-direct link, gluing or taping the cooler to the chip itself. The key is to make sure the CPU cooler meets the power requirement of the processor, since otherwise heat will build up, causing at best unstable operation and at worst a costly charcoal-briquette chip in the middle of a scorched-earth motherboard.

CPU coolers come in a great many shapes and sizes, with the most popular combining a metal heat sink with an active cooling fan. CPU heat is dissipated first by being spread over a wider surface area (a heat sink with many pins or fins, or in some cases a radiator-style heat pipe), then by moving air from the fan.

These designs are usually aluminum (lower heat conductivity/higher dissipation), copper (higher conductivity/lower dissipation) or the popular copper-core/aluminum-fin combination that many favor. Cooling fans also range from virtually silent models to high-speed fans that sound like an airliner taking off.

More esoteric CPU cooling solutions include passive heat sinks, which are either high-end, fanless designs for desktop components (like Zalman Tech's heat sinks for system chipsets and graphics chips) or simply used for low-power processors that don't require active cooling (like VIA's EPIA platform or some mobile CPUs).

Going down the overclocking enthusiast's road leads to truly exotic solutions such as liquid cooling (think car engine), Freon (think refrigerator), or even immersion cooling (think Florence Henderson -- dipping the entire PC into oil or some similar fluid).

Throttling Down Laptop Processors

As an earlier CPU Planet article explained, power consumption and heat generation are far more profound issues for portable computers than for desktops. A notebook has a finite power supply (its battery) when unplugged, and its more compact case means much less airflow and cooling room. While desktop users are more concerned with performance, laptop designers must use innovative means to balance lower heat and increased battery life with sufficient speed for demanding applications.

The most common methodologies are throttling schemes such as Intel's SpeedStep and AMD's PowerNow!, which adjust CPU speed and voltage based on the presence or absence of AC power (and, in more sophisticated implementations, application demand). Reducing core speed saves power and generates less heat; for instance, the mobile Pentium 4's SpeedStep shift from 1.3V to 1.2V lowers maximum current requirements by as much as one-third.