Questions Beginners Ask

Tom R. Halfhill, Features Editor

Are you thinking about buying a computer for the first time, but you don't know much about computers? Or maybe you just purchased a computer and are still a bit baffled. Each month in this column, COMPUTE! will answer questions frequently asked by beginners.

Q What is a motherboard?

A A motherboard is the main circuit board of a computer. All other boards are connected to the motherboard.

The most important component on the motherboard is the central processing unit (CPU)—the central brain of the computer. The CPU is a microprocessor chip which performs or supervises all computer operations. It fetches each program instruction one at a time, executes it, stores the result, and then fetches the next instruction.

The motherboard also contains support chips required by the CPU: usually a video chip to control the TV display; input/output chips to handle the exchange of data with such peripherals as the disk drive, tape recorder, or printer; and perhaps a sound chip for music and sound effects.

In some computers—such as the Apple, Atari 800, and IBM PC/PCjr—the motherboard has long, narrow sockets called slots into which accessory boards can be plugged. Memory boards full of RAM chips (Random Access Memory) often fit into these slots. Other accessory boards (or cards) might include operating systems, disk drive controllers, printer interfaces, direct-connect modems, 80-column video expanders, graphics expanders, and even piggyback processors (boards with another CPU to allow the computer to run different types of software). That's why motherboards with several internal slots make a computer more versatile.

Some computers, including most home computers these days, contain only one circuit board—the motherboard. All the components are contained on this main board: the CPU, support chips, RAM chips, and ROM chips (Read Only Memory).

Consolidating all the boards into one motherboard makes the computer smaller, lighter, and—most important from the manufacturer's point of view—cheaper to produce. For example, original Atari 800s contain six boards, and that's even before all the slots are filled with accessory boards. But the new Atari 800XL, which replaces the 800, contains only one board, even though it has more memory (64K RAM versus 8K–48K). Obviously, the 800XL costs less to manufacture.

Of course, a computer without slots for accessory boards would not be as versatile. So single-board computers generally have an expansion slot or system bus on the rear. This allows accessory boards to be added externally. The accessory boards resemble large cartridges because they are enclosed in protective plastic or metal housings.

This still leaves one problem. How can more than one accessory board be plugged in at once? Naturally, there's a solution—an expansion box or motherboard extender. Both devices convert a lone expansion slot into several slots. For instance, you can expand a Commodore VIC-20 from the standard 5K RAM to 24K RAM by plugging a motherboard extender into the rear expansion slot, and then plugging 3K and 16K expanders into the motherboard extender.

Occasionally this is necessary even on computers with internal slots on the motherboard, such as the IBM PC. To fully equip a PC, sometimes the five internal slots just aren't enough.