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Infrastructure is what holds your computer together. It is the overall form of your computer, the substrate that it is built upon, and the essential supplies required to make it run. The substrate of the modern computer is the motherboard (or system board or main board) coupled with the expansion boards plugged into it. The case provides the basic form of the computer. And the one essential service required by any modern computer is power, the electricity used in the computer's logic operations.


The centerpiece of the system unit is the motherboard. All the other circuitry of the system unit is usually part of the motherboard or plugs directly into it. The electronic components on the motherboard carry out most of the function of the machine—running programs, making calculations, and even arranging the bits that will display on the screen. Because the motherboard defines each computer's functions and capabilities and because every computer is different, it only stands to reason that every motherboard is different, too. Not exactly. Many different computers have the same motherboard designs inside. And oftentimes a single computer model might have any of several different motherboards, depending on when it came down the production line (and what motherboard the manufacturer got the best deal on).

The motherboard is the main circuit board in nearly every computer—desktop or laptop. Usually it lines the bottom of the case like thick, green shelf-paper decorated with lines and blobs that could be a Martian roadmap.

Expansion Boards

Expansion boards are the physical form of the expansion devices that connect to your computer through its expansion slots. They are simply circuit boards that are smaller than the motherboard.

However, they are not just any circuit boards. Expansion boards must follow strict standards as to size, signals, power consumption, and software interface to ensure that they will work inside any given computer. Although at one time nearly all computers used Industry Standard Architecture (ISA) for their expansion boards, the modern replacement is Peripheral Component Interconnect (PCI). Some desktop computers don't allow the use of expansion boards at all. Notebook machines use PC cards instead.


Every computer requires a continuous supply of carefully conditioned direct current at low voltage, well below that available at your wall outlet. Although batteries can provide this power for portable computers, desktop units require power supplies to convert house current into computer current. Similarly, power supplies charge and substitute for portable computer batteries.

Bringing power to your computer also imports danger, however, and your computer requires protection from glitches on the power line. Surge suppressors and backup power systems help to ensure that your computer gets the proper electrical diet.

The power supply in most desktop computers is locked for your protection in a steel box inside your computer's case. It keeps your hands out of harm's way if you dig into your computer to add an expansion board or other accessory. Notebook computers put much of the function of the power supply into their external "bricks" that plug directly into a wall outlet. The output of the brick is at a voltage that's safe should you accidentally encounter it (and safe for your notebook computer, as well).


The case is what most people think of as the entire computer, ignoring all the essential parts attached by cables and often the individual components inside. The main box goes by a special name, system unit, because it is the heart of the system. Some people call it the CPU (central processing unit), although that term is also (and confusingly) used for the microprocessor inside. The system unit houses the main circuitry of the computer and provides spaces called bays for internal disk drives as well as a place for the jacks (or outlets) that link the computer to the rest of its accouterments, including the keyboard, monitor, and peripherals. A notebook computer combines all these external components into one unit, but it's usually called simply the "computer" rather than system unit or CPU.

The case is more than a box. It's also a protective shell that keeps the dangers of the real world away from the delicate electronics of the computer. What's more, it protects the world from interference from the signals of the computer. The case is also part of a cooling system (which may include one or more fans) that keeps the electronics inside cool for longer life.

The case of a modern computer can take many forms—from small-footprint desktop computers to maxi-tower computers, from compact sub-notebook computers (or even handheld computers) to huge, almost unportable desktop-replacement notebook computers. In desktop systems, the size more than anything else determines how many accessories you can add to your system—and how much of your home or office you have to give up to technology. Weight, more than size, is the limiting physical factor for notebook computers. How much you want to carry often determines how many features you get.

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