CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [62]
A good PC technician needs to understand some basic CPU functions to support PCs, so let’s start with an analysis of how the CPU works. If you wanted to teach someone how an automobile engine works, you would use a relatively simple example engine, right? The same principle applies here. Let’s begin our study of the CPU with the grand-daddy of all PC CPUs: the famous Intel 8088, invented in the late 1970s. Although this CPU first appeared over 25 years ago, it defined the idea of the modern microprocessor and contains the same basic parts used in even the most advanced CPUs today. Stick with me, my friend. Prepare to enter that little bit of magic called the CPU.
The Man in the Box
Let’s begin by visualizing the CPU as a man in a box (Figure 5-1). This is one clever guy in this box. He can perform virtually any mathematical function, manipulate data, and give answers very quickly.
Figure 5-1 Imagine the CPU as a man in a box.
This guy is potentially very useful to us, but there’s a catch—he lives closed up in a tiny box. Before he can work with us, we must come up with a way to exchange information with him (Figure 5-2).
Figure 5-2 How do we talk to the Man in the Box?
Imagine that we install a set of 16 light bulbs, 8 inside his box and 8 outside his box. Each of the 8 light bulbs inside the box connects to one of the 8 bulbs outside the box to form a pair. Each pair of light bulbs is always either on or off. You can control the 8 pairs of bulbs by using a set of 8 switches outside the box, and the Man in the Box can also control them by using an identical set of 8 switches inside the box. This light bulb communication device is called the external data bus (EDB).
Figure 5-3 shows a cutaway view of the external data bus. When either you or the Man in the Box flips a switch on, both light bulbs go on, and the switch on the other side is also flipped to the on position. If you or the Man in the Box turns a switch off, the light bulbs on both sides are turned off, along with the other switch for that pair.
Figure 5-3 Cutaway of the external data bus—note that one light bulb pair is on
Can you see how this works? By creating on/off patterns with the light bulbs that represent different pieces of data or commands, you can send that information to the Man in the Box, and he can send information back in the same way—assuming that you agree ahead of time on what the different patterns of lights mean. To accomplish this, you need some sort of codebook that assigns meanings to the many patterns of lights that the external data bus might display. Keep this thought in mind while we push the analogy a bit more.
Before going any further, make sure you’re clear on the fact that this is an analogy, not reality. There really is an external data bus, but you won’t see any light bulbs or switches on the CPU. You can, however, see little wires sticking out of the CPU (Figure 5-4). If you apply voltage to one of these wires, you in essence flip the switch. Get the idea? So if that wire had voltage, and if a tiny light bulb were attached to the wire, that light bulb would glow, would it not? By the same token, if the wire had no power, the light bulb would not glow. That is why the switch-and-light-bulb analogy may help you picture these little wires constantly flashing on and off.
Figure 5-4 Close-up of the underside of a CPU
Now that the external data bus enables you to communicate with the Man in the Box, you need to see how it works by placing voltages on the wires. This brings up a naming problem. It’s a hassle to say something like “on-off-off-off-on-on-off-off” when talking about which wires have voltage. Rather than saying