Chapter 5. Microprocessors

Microprocessors are the most complicated devices ever created by human beings. But don't despair. A microprocessor is complicated the same way a castle you might build from Legos is complicated. You can easily understand each of the individual parts of a microprocessor and how they fit together, just as you might build a fort or castle from a legion of blocks of assorted shapes. The metaphor is more apt than you might think. Engineers, in fact, design microprocessors as a set of functional blocks—not physical blocks like Legos, but blocks of electronic circuits that perform specific functions. And they don't fit them together on the floor of a giant playroom. Most of their work is mental, a lot of it performed on computers. Yes, you need a computer to design a computer these days. Using a computer is faster; it can give engineers insight by showing them ideas they could only before imagine, and it doesn't leave a mess of blocks on the floor that Mom makes them clean up.

In creating a microprocessor, a team of engineers may work on each one of the blocks that makes up the final chip, and the work of one team may be almost entirely unfathomable by another team. That's okay, because all that teams need to know about the parts besides their own is what those other parts do and how they fit together, not how they are made. All the complex details—the logic gates from which each block is built—are irrelevant to anyone not designing that block. It's like those Legos. You might know a Lego block is made from molecules of plastic, but you don't need to know about the carbon backbones and the side-chains that make up each molecule to put the blocks together.

Legos come in different shapes. The functional blocks of a microprocessor have different functions. We'll start our look at microprocessors by defining what those functions are. Then we'll leave the playroom behind and look at how you make a microprocessor operate by examining the instructions that it uses. This detour is more important than you might think. Much of the design work in creating a series of microprocessors goes into deciding exactly what instructions it must carry out—which instructions are most useful for solving problems and doing it quickest. From there, we'll look at how engineers have used their imaginations to design ever-faster microprocessors and at the tricks they use to coax more speed for each new generation of chip.

Microprocessor designers can't just play with theory in creating microprocessors. They have to deal with real-world issues. Somehow, machines must make the chips. Once they are made, they have to operate—and keep operating. Hard reality puts some tight constraints on microprocessor design. If engineers aren't careful, microprocessors can become miniature incinerators, burning themselves up. We'll take a look at some of these real-world issues that guide microprocessor design—including electricity, heat, and packaging, all of which work together (and at times, against the design engineer).

Next, we'll look at real microprocessors, the chips you can actually buy. We'll start with a brief history lesson to put today's commercial offerings in perspective. And, finally, we'll look at the chips in today's (and tomorrow's) computers to see which is meant for what purpose—and which is best for your own computer.