Megahertz Does Not Matter

Hardware companies advertising computers prominently display the speed of the CPU (central processing unit). However, taking a page from the famous book by Darrell Huff, "How to Lie with Statistics," we will see why simply comparing processor clock speeds is misleading.

How CPUs Work

Let us walk through an example of how a program runs on a computer. This may seem a bit technical, but the purpose is to illustrate the many factors one must consider when comparing CPUs.

A CPU must first communicate with the computer's memory to retrieve the instructions from the program. It does so using its data bus. This bus typically operates at the same speed as the rest of the computer's main system board. However the size of the pipe can vary. For example, Intel's first 16-bit CPU, the 8086, had a 16-bit data bus, while its less expensive successor, the 8088, had only an 8-bit data bus. This let the 8086 retrieve information almost twice as fast as the 8088. To design faster CPUs, Intel began to increase the speed of the data bus. Today CPUs typically run at either 100 MHz (megahertz, or million cycles per second), or 133 MHz.

So how do we jump to today's 1.5 GHz (gigahertz, or billion cycles per second) PCs? Once it became clear that running the entire computer at such super-high speeds would be almost impossible, CPU designers began to use a clock multiplier to run the CPU at a higher speed (think of a clock multiplier like a gearshift). By running a CPU nine times faster than a 100 MHz bus, for instance, the CPU has an
effective speed of 900 MHz.

As CPU speeds began increasing faster than the computer, manufacturers began to use an on-chip memory cache to store information. This allows the CPU to access this information at a much greater speed, instead of continually slowing down to the speed of its data bus to pull instructions from the system memory.

Once a CPU retrieves its instructions and data, it loads them into a series of registers (storage areas). CPUs can have different amounts and sizes of these storage bins, with larger, faster registers usually resulting in faster performance.
The CPU then carries out the programmer's instructions, normally one at a time. However, with its Pentium CPU, Intel introduced to the IBM-compatible world the ability for a CPU to handle more than one instruction at the same time, greatly increasing processing power.

Comparing CPUs

Considering the architectural improvements between processor families, one can begin to see why an Intel 80386 running at 33 MHz is slower than an Intel 80486 running at 33 MHz. In fact, with its built in math coprocessor and other features, an 80486 at just 25 MHz will outperform a 33 MHz 80386.

The bottom line, then, is that the speed of a CPU is only relevant as a comparison tool when comparing the same model of CPU. Comparing two processor models from different companies is even harder.

For example, there are many different types of microprocessors on the market today, made by many companies. The most widely known is Intel's x86-compatible series of processors, such as the 80486 and Pentium brands. But Intel also makes other types of microprocessors, as does IBM/Motorola (who make the PowerPC family of CPUs), Compaq/DEC (Alpha), and AMD (Athlon, Duron).

Today's Marketplace

So taking all this information into account, which CPU does ITS recommend? For the last year we have chosen to use AMD Athlon and Duron processors as standard components in our CorporateClass (tm) PCs for their superior price/performance ratio. In fact recent tests by several industry publications have documented that a 1.3 GHz AMD Athlon-based PC can outperform an Intel Pentium 4-based PC running at a much higher clock speed of 1.7 GHz! This is an interesting development considering that the Pentium 4 is supposed to represent Intel's latest CPU technology, and be one step ahead of the Athlon, which was designed to compete with Intel's Pentium III family.

AMD's surprise emergence as a performance leader in the IBM compatible arena has put significant pressure on Intel. Since we first commented on the Athlon's performance in the Q1 2000 issue of this newsletter, prices for high-end CPUs have dropped dramatically. As AMD's market share has increased, Intel has slashed prices in response.

When MHz Does Matter

Users can accurately compare microprocessor speeds only when considering the same model of CPU. For example one can reasonably assume that an AMD Athlon running at 900 MHz performs computations roughly 12% faster than an 800 MHz model.

While many other factors affect the performance of a computer, with all other things being equal consumers can use the speed of the CPU as a gauge when comparing the cost/benefit of choosing a faster CPU.

May 2001

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