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the bell, the Man in the Box reads the next set of lights on the external data bus. Of course, a real computer doesn’t use a bell. The bell on a real CPU is a special wire called the clock wire (most diagrams label the clock wire CLK). A charge on the CLK wire tells the CPU that another piece of information waiting to be processed (Figure 5-9).

Figure 5-9 The CPU does nothing until activated by the clock.

For the CPU to process a command placed on the external data bus, a certain minimum voltage must be applied to the CLK wire. A single charge to the CLK wire is called a clock cycle. Actually, the CPU requires at least two clock cycles to act on a command, and usually more. Using the manual calculator analogy, you need to pull the crank at least twice before anything happens. In fact, a CPU may require hundreds of clock cycles to process some commands (Figure 5-10).

The maximum number of clock cycles that a CPU can handle in a given period of time is referred to as its clock speed. The clock speed is the fastest speed at which a CPU can operate, determined by the CPU manufacturer. The Intel 8088 processor had a clock speed of 4.77 MHz (4.77 million cycles per second), extremely slow by modern standards, but still a pretty big number compared to using a pencil and paper. CPUs today run at speeds in excess of 3 GHz (3 billion cycles per second).

Figure 5-10 The CPU often needs more than one clock cycle to get a result.

1 hertz (1 Hz) = 1 cycle per second

1 megahertz (1 MHz) = 1 million cycles per second

1 gigahertz (1 GHz) = 1 billion cycles per second

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NOTE CPU makers sell the exact make and model of CPU at a number of different speeds. All of these CPUs come off of the same assembly lines, so why different speeds? Every CPU comes with subtle differences—flaws, really—in the silicon that makes one CPU run faster than another. The speed difference comes from testing each CPU to see what speed it can handle.

Understand that a CPU’s clock speed is its maximum speed, not the speed at which it must run. A CPU can run at any speed, as long as that speed does not exceed its clock speed. Manufacturers used to print the CPU’s clock speed directly onto the CPU, but for the past few years they’ve used cryptic codes (Figure 5-11). As the chapter progresses you’ll see why they do this.

Figure 5-11 Where is the clock speed?

The system crystal determines the speed at which a CPU and the rest of the PC operate. The system crystal is usually a quartz oscillator, very similar to the one in a wristwatch, soldered to the motherboard (Figure 5-12). The quartz oscillator sends out an electric pulse at a certain speed, many millions of times per second. This signal goes first to a clock chip that adjusts the pulse, usually increasing the pulse sent by the crystal by some large multiple. (The folks who make motherboards could connect the crystal directly to the CPU’s clock wire, but then if you wanted to replace your CPU with a CPU with a different clock speed, you’d need to replace the crystal too.) As long as the PC is turned on, the quartz oscillator, through the clock chip, fires a charge on the CLK wire, in essence pushing the system along.

Figure 5-12 One of many types of system crystals

Visualize the system crystal as a metronome for the CPU. The quartz oscillator repeatedly fires a charge on the CLK wire, setting the beat, if you will, for the CPU’s activities. If the system crystal sets a beat slower than the CPU’s clock speed, the CPU will work just fine, though at the slower speed of the system crystal. If the system crystal forces the CPU to run faster than its clock speed, it can overheat and stop working.

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NOTE Aggressive users sometimes intentionally overclock CPUs by telling the clock chip to multiply the pulse faster than the CPU’s designed speed. They do this to make slower (cheaper) CPUs run faster. This is a risky business that can destroy your CPU, but those willing to take that risk often do it. See the “Overclocking” section later in this chapter.

Before you install a