A common misunderstanding about processors is their different speed ratings. This section covers processor speed in general and then provides more specific information about Intel, AMD, and VIA/Cyrix processors. A computer system's clock speed is measured as a frequency, usually expressed as a number of cycles per second.

A crystal oscillator controls clock speeds using a sliver of quartz sometimes contained in what looks like a small tin container. Newer systems include the oscillator circuitry in the motherboard chipset, so it might not be a visible separate component on newer boards.

As voltage is applied to the quartz, it begins to vibrate (oscillate) at a harmonic rate dictated by the shape and size of the crystal (sliver). The oscillations emanate from the crystal in the form of a current that alternates at the harmonic rate of the crystal.

This alternating current is the clock signal that forms the time base on which the computer operates. A typical computer system runs millions of these cycles per second, so speed is measured in megahertz. (One hertz is equal to one cycle per second.) An alternating current signal is like a sine wave, with the time between the peaks of each wave defining the frequency.

A single cycle is the smallest element of time for the processor. Every action requires at least one cycle and usually multiple cycles. To transfer data to and from memory, for example, a modern processor such as the Pentium 4 needs a minimum of three cycles to set up the first memory transfer and then only a single cycle per transfer for the next three to six consecutive transfers.

The extra cycles on the first transfer typically are called wait states. A wait state is a clock tick in which nothing happens. This ensures that the processor isn't getting ahead of the rest of the computer. The time required to execute instructions also varies:

• 8086 and 8088. The original 8086 and 8088 processors take an average of 12 cycles to execute a single instruction.

• 286 and 386. The 286 and 386 processors improve this rate to about 4.5 cycles per instruction.

• 486. The 486 and most other fourth-generation Intel-compatible processors, such as the AMD 5x86, drop the rate further, to about 2 cycles per instruction.

• Pentium, K6 series. The Pentium architecture and other fifth-generation Intel-compatible processors, such as those from AMD and Cyrix, include twin instruction pipelines and other improvements that provide for operation at one or two instructions per cycle.

• Pentium Pro, Pentium II/III/4/Celeron, and Athlon/Athlon XP/Duron. These P6 and P7 (sixth- and seventh-generation) processors can execute as many as three or more instructions per cycle.

Different instruction execution times (in cycles) make comparing systems based purely on clock speed or number of cycles per second difficult. How can two processors that run at the same clock rate perform differently with one running "faster" than the other?

The answer is simple: efficiency. The main reason the 486 was considered fast relative to a 386 is that it executes twice as many instructions in the same number of cycles. The same thing is true for a Pentium; it executes about twice as many instructions in a given number of cycles as a 486.

Therefore, given the same clock speed, a Pentium is twice as fast as a 486, and consequently a 133MHz 486 class processor (such as the AMD 5x86-133) is not even as fast as a 75MHz Pentium! That is because Pentium megahertz are "worth" about double what 486 megahertz are worth in terms of instructions completed per cycle.

The Pentium II and III are about 50% faster than an equivalent Pentium at a given clock speed because they can execute about that many more instructions in the same number of cycles.

Comparing relative processor performance, you can see that a 1GHz Pentium III is about equal to a (theoretical) 1.5GHz Pentium, which is about equal to a 3GHz 486, which is about equal to a 6GHz 386 or 286, which is about equal to a 12GHz 8088.

The original PC's 8088 ran at only 4.77MHz; today, we have systems that are comparatively at least 2,500 times faster! As you can see, you must be careful in comparing systems based on pure MHz alone because many other factors affect system performance. Evaluating CPU performance can be tricky.

CPUs with different internal architectures do things differently and can be relatively faster at certain things and slower at others. To fairly compare various CPUs at different clock speeds, Intel has devised a specific series of benchmarks called the iCOMP (Intel Comparative Microprocessor Performance) index that can be run against processors to produce a relative gauge of performance.

The iCOMP index benchmark has been updated twice and released in original iCOMP, iCOMP 2.0, and now iCOMP 3.0 versions. The iCOMP 2.0 index is derived from several independent benchmarks and is a stable indication of relative processor performance. The benchmarks balance integer with floating-point and multimedia performance.

When Intel developed the Pentium III, it discontinued the iCOMP 2.0 index and released the iCOMP 3.0 index. iCOMP 3.0 is an updated benchmark that incorporates an increasing use of 3D, multimedia, and Internet technology and software, as well as the increasing use of rich data streams and computer-intensive applications, including 3D, multimedia, and Internet technology.

iCOMP 3.0 combines six benchmarks: WinTune 98 Advanced CPU Integer test, CPUmark 99, 3D WinBench 99-3D Lighting and Transformation Test, MultimediaMark 99, Jmark 2.0 Processor Test, and WinBench 99-FPU WinMark. These newer benchmarks take advantage of the SSE (Streaming SIMD Extensions), additional graphics and sound instructions built into the PIII.

Without taking advantage of these new instructions, the PIII would benchmark at about the same speed as a PII at the same clock rate. SYSmark 2002 is a commercially available application-based benchmark that reflects the normal usage of business users employing modern Internet content creation and Microsoft Office applications.

SYSmark 2002 incorporates the following applications, which it uses for testing:

• Internet Content Generation. Includes Adobe Photoshop 6.01, Premiere 6.0, Microsoft Windows Media Encoder 7.1, Macromedia Dreamweaver 4, and Flash 5.

• Office Productivity. Includes Microsoft Word 2002, Excel 2002, PowerPoint 2002, Outlook 2002, Access 2002, Netscape Communicator 6.0, Dragon NaturallySpeaking Preferred v.5, WinZip 8.0, and McAfee VirusScan 5.13

SYSmark runs various scripts to do actual work using these applications and is used by many companies for testing and comparing PC systems and components. It is a much more modern and real-world benchmark than the iCOMP benchmark Intel previously used, and because it is available to anybody, the results can be independently verified. SYSmark 2002 can be purchased from BAPCo.