FUNDAMENTALS; So What, Exactly, Is a Megahertz?
By PETER WAYNER
Published: February 26, 1998
IN the movie comedy ''Spinal Tap,'' a rock guitarist explains to an interviewer that his amplifier is much better than others because the top reading on his dials is 11. When he wants to push his rocking to the limit, he can turn the dials to 11, while other, lesser guitarists get stuck at 10.
The scene is a favorite among engineers because it shows how numbers can produce a convincing and appealing illusion just as effectively as trompe l'oeil painting or 3-D glasses. In engineering meetings, one of the most withering criticisms of a new feature is that it ''goes to 11'' -- that is, it is more artifice and old lace than a useful tool.
Numbers may also not provide a clear guide in evaluating computers. One of the biggest, and sometimes least important, questions facing a computer buyer is how fast a computer should be. The questions may be: ''Will a laptop that works at 133 megahertz be fast enough for E-mail and Web surfing?'' ''Is 233 megahertz enough for a new game?''
Megahertz are usually sold to consumers like the numbers on an amplifier. A higher number is thought to mean greater speed. But people often pay more for performance they will never get.
There is no simple explanation of what a megahertz number means for performance. The measurement is not so silly and free from correlation to the real world as the numbers on a guitar amplifier, but it is far from perfect. Generally, 300-megahertz machines run faster than 200-megahertz machines, but they rarely produce the 50 percent increase in performance the numbers would indicate. A 300-megahertz machine may sometimes run only as fast as a 50-megahertz machine.
Hertz is the unit used to measure the frequency of electromagnetic radiation, which includes things like radio waves. It is used to measure the speed of computers because the machines depend on another form of electromagnetic radiation: electricity. The guts of a computer, the central processing unit, containing a chip, coordinates its work with a central clock switch that also oscillates millions of times per second. That is, each 200-megahertz Pentium processor has a switch inside it that completes a cycle of turning on and off 200 million times per second, and this switch controls the signal that synchronizes the rest of the chip.
Imagine that the central processing unit acts like an assembly line for producing numbers, much as a car factory produces cars. The pieces of data, in the form of pulses of electric current, are gathered, coordinated and fed into circuits that do basic arithmetic like addition and subtraction.
The clock signal acts like a metronome for an orchestra as it synchronizes the assembly of data. There are usually several stations in the assembly line for data, and each station must finish its task before the clock signal changes again. When it does, the data advance to the next station.
It would seem, then, that the speed of a computer is controlled by the speed of the clock signal, so a 300-megahertz machine would appear to be 50 percent faster than a 200-megahertz machine. The problem with measuring a computer's speed according to the speed of the central processing unit's clock emerges when the data assembly line runs out of data and must pause. The central processing unit is kept waiting when it must fetch data from some distant location, like the memory chips or the disk drive.
When a computer user playing a game goes into a new virtual room, the central processing unit may have to get the information on how many acid-spewing monsters are lurking. Apart from how fast a disk drive is, pieces of data may move slowly because they must travel inches instead of less than a tenth of an inch, and the wires on the circuit boards are often not built to move the current that carries information as fast as the central processing unit does. These wires are usually referred to as a systems bus, a term that borrows the word bus to describe how the data are moved in large groups.
The speed of the systems bus, also measured in megahertz, is often a better measure of performance than the speed of the central processing unit. The issue is how many pieces of data are being moved. Central processing units contain a small amount of fast memory, called cache memory, that can keep up with a 200-megahertz clock. But a lot of monsters, or a complicated math problem, would involve too much data to fit in cache memory.
If the computer is trying to do the arithmetic for a large spreadsheet, then the system bus speed is a good measurement of actual speed because most of the data need to travel along the system bus. In this case, a 300-megahertz chip connected to a 50-megahertz system bus will often sit idling for 250 million of its 300 million cycles each second. More forthright computer manufacturers are already including the speed of the system bus in their advertisements. The system buses in some machines run at 50 or 55 megahertz, while others run at 33 megahertz.
Some problems are small enough to avoid the system bus. Code breakers, for instance, deliberately structure the way computers handle their searches for the key to encrypted information so it can be done with the readily accessible cache memory, avoiding the systems bus. Designers of games like Doom try to make the most important parts of the game fit into the average cache size.
The process of measuring a computer's usefulness is even more complicated because most machines come with a separate system on a video card to draw the image on the screen. That system determines how fast the screen can change. Almost every part of the computer, from the disk drives to the serial ports, can also have effects in some situations.
Of course, the most difficult problem is applying this information in the search for the best computer for the money. Many computer magazines run a set of sample programs, called a benchmark, on a machine and report the results. That can be the best indicator if the test programs are similar to the ones you want to run.
Diagram: ''Measuring Speed In Computers'' The first thing consumers are likely to see in a computer advertisement is some reference to a blazingly fast machine for sale that rips through games and spreadsheets at 233 or 300MHz. That'smegahertz,and few, if any, casual computer users know more about megahertz than the higher the number, the more expensive the computer. Understanding exactly what the term means and how megahertz relate to speed may help in judging a sales pitch. It also gives a glimpse into how information moves, rapidly or not, inside the machine. 1. Pieces of data are gathered and assembled at a very high speed. 2. At each station on the line, new data are added and are sent on to the next stop. 3. The line moves very quickly as long as all data needed are available. 4. If any piece of data needed for the end product is missing, the line must wait until it is found. That may require searching in a distant location and may slow the line. 5. The final product, like the solution to a math problem, is sent out (to memory) when all tasks are completed. The Conveyor Belt of Data Megahertz equals the number of on-off cycles a switch has each second. This wave is equal to one cycle. More oscillations mean more speed. Though often the cycles are incomplete. On the Line A computer processor has a clock that regulates the flow of information through the chip like a supervisor monitoring an assembly line. At each station on the line, data are assembled. Each time a clock signal oscillates, data move on to the next station -- if all the data are available. If not, the fastest clock is no more effective than a severe supervisor when parts are slow in getting to the line. (Illustration by John Papasian/The New York Times)