CompTIA A+ Certification All-in-One Exam Guide

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 computer is turned on, the quartz oscillator, through the
clock chip, fires a charge on the CLK wire, in essence pushing the system
along.
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. Before you install a CPU
into a system, you must make sure that the crystal and clock chip send out the
correct clock pulse for that particular CPU. In the old days, this required very
careful adjustments. With today’s systems, the motherboard talks to the CPU.
The CPU tells the motherboard the clock speed it needs, and the clock chip
automatically adjusts for the CPU, making this process now invisible.
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 and to get
more performance in demanding programs. See the “Overclocking” section
later in this chapter.
Back to the External Data Bus
One more reality check. We’ve been talking about tables with racks of light
bulbs, but of course real CPU registers don’t use light bulbs to represent on/1
and off/0. Registers are tiny storage areas on the CPU made up of
microscopic semiconductor circuits that hold charges. It’s just easier to
imagine a light bulb lit up to represent a circuit holding a charge; when the
light bulb is off, there is no charge.
Figure 3-13 is a diagram of an 8088 CPU, showing the wires that comprise
the external data bus and the single clock wire. Because the registers are