Making sense of the megahertz "myth" — continued

Where MHz come from
That said, let's look at how these clock speed numbers get determined. The abiding reality of the PC market for the past twenty years has been the tendency—wrapped up in Moore's Law and executed with precision by Moore's company, Intel—for processor power and clock speeds to ratchet upward with regularity. Speaking simply, a processor's clock speed is determined by several things. Among them:

• Manufacturing technique and efficiency — I'm listing this one first because it is, in some respects, the most important variable here. Most regular clock speed increases come from improvements in manufacturing techniques or efficiencies. When Intel, AMD, or whoever produces a wafer full of chips, the quality of the chips on that wafer determines how fast each chip will run. Although the Athlon 700MHz and the Athlon 1.4GHz were made using the same basic manufacturing process (with a 0.18 micron feature size and copper interconnects), AMD has gotten better at producing these chips over time, so clock speeds have risen. Often, these minor refinements in manufacturing efficiency come from tweaks to the process used to fabricate chips, or from minor changes in the design of the chips themselves.

  Of course, CPU makers try to control things by timing the release of new chips and offering pricing points up and down the supply and demand curves. But they're simply coping—quite cleverly, it must be said—with the realities of chip fabrication. (We crazy overclocking types try to catch CPU makers selling chips rated to run at much lower speeds than they're capable. Occasionally, we find a real gem, like the Celeron 300A, that will happily run over 50% faster than its rated speed. We then—also quite cleverly—buy 'em cheap and run 'em at higher speeds.)

  
  A wafer of P4 goodness. Mmm... crunchy.

  Every so often, CPU makers transition to a newer, more advanced manufacturing process. Recently, Intel has been making just such a move from its 0.18-micron, aluminum-based process to a copper-based, 0.13-micron process that uses low-capacitance dielectrics. Chips made on this newer process are smaller, consume less power, run cooler, and are able to run at higher frequencies. The 1.2GHz Pentium III, which we reviewed not long ago, is fabbed on Intel's new process. These chips have been reported to run just fine at upwards of 1.4GHz, while previous, 0.18-micron Pentium IIIs haven't been good for much over 1GHz. This sort of transition to a new manufacturing process, known as a die shrink, usually brings with it headroom for ever-higher clock speeds. The Pentium 4 has yet to undergo a die shrink on Intel's new process, but it should soon.

• Microprocessor design — The other big variable in the clock speed equation is processor design. Older designs don't generally take well to higher clock frequencies, which is one reason why you don't see any 1.5GHz 486s selling in "value" PCs. Newer designs employ deeper pipelines—where less work gets done at each stage of the game—in order to better tolerate higher clock frequencies. Thus, some processor designs are better suited to higher clock speeds than others.

  The contrast between the Pentium III and Pentium 4 is a case in point. The PIII made it up to 1.13GHz when manufactured on Intel's 0.18-micron fab process. The Pentium 4, with its radical, 20-stage pipeline, debuted at 1.4 and 1.5GHz, and has now hit 2GHz—all on the same 0.18-micron process. The chips were made in essentially the same fashion, but the P4 design takes to higher frequencies better. (For more on the Pentium 4 and how it's optimized for high clock speeds, see our original Pentium 4 review.)

  Of course, there are tradeoffs here. The P4 typically gets less work done per clock cycle than the PIII, so a 1.4GHz P4 isn't necessarily a better performer than a 1GHz PIII. But, as you'll see, the P4 at 2GHz makes the PIII look like the sad, old man that it is.

• Power and cooling — It's possible to wring some extra speed out of a processor after supplying it with additional power and better cooling. Extreme cooling solutions like those from Kryotech can allow for big jumps in clock speed, though they're not always cost effective. Most overclockers use minor voltage increases and beefier versions of conventional air-cooling equipment to help bring stability when they crank up the clock.

  Power and cooling requirements are limitations, too. Laptops generally run at lower clock rates than desktops to avoid excess heat and power draw. As clock speeds have risen, standard-issue desktop PCs have gone from small, passive heatsinks in the 386 and 486 eras to the massive, ducted, fan-driven active cooling solutions of today.