The Systemboard, The CPU And The Systemboard, The System Clock

The Systemboard

A systemboard’s primary purpose is to house the CPU and allow all devices to communicate with it and each other.

The two most popular systemboards are the older AT and the newer ATX. The AT systemboard has a power connection for 5- and 12-volt lines coming from the power supply.

To accommodate the newer CPUs that use less voltage, the ATX has lines for 5, 12, and 3.3 volts from the power supply.

The ATX systemboard uses a single P1 power connection, but the AT board uses two power connections, P8 and P9.

Each board is available in two sizes. The ATX boards include more power management features and support faster systems. The following table summarizes these different boards and their form factors. (“Form factor” is computer jargon for the size and shape of a board or other device.)

Components On The Systemboard

The main components on a systemboard are the following:

CPU and its accompanying chip set

System clock

ROM BIOS

CMOS configuration chip and its battery RAM

RAM cache (only on older systemboards)

System bus with expansion slots

Jumpers and DIP switches

Ports that come directly off the board

Power supply connections

Of these components, you can replace or upgrade the following five: CPU, ROM BIOS chip, CMOS battery, RAM, and RAM cache. Because you can exchange these items without returning the systemboard to the manufacturer, they are called field replaceable units.

Below is a graphic of a typical AT systemboard with memory cache and socket 7 for the Intel Classic Pentium CPU.

The CPU And The Systemboard

The systemboard holds the most important microchip in the computer system, called the Central Processing Unit (CPU) or microprocessor, which does most of the “thinking” of the computer.

Today, most computers also contain microchips that relieve the CPU of many tasks to increase the overall speed of the computer.

CPUs are manufactured out of semiconductor material, which allows varying

voltages to be carried along the same pathways, allowing this material to transmit

streams of bits and bytes that are the heart of basic computer processing.

The System Clock

The systemboard contains a system clock that keeps the beat for many systemboard activities.

We use units called megahertz (MHz) to measure clock frequency. One megahertz (MHz) is equal to 1,000,000 beats, or cycles, of the clock per second. A single clock beat or cycle was once the smallest unit of processing the CPU or another device could execute, meaning that it could only do one thing for each beat of the clock.

Some CPUs today can perform two activities per clock cycle. Even though how fast a CPU can operate is often referred to as the CPU speed, it is more accurate but less common to speak of the CPU frequency. For example, you might say that a CPU can operate at a frequency of 550 MHz.

A wait state occurs when the CPU must wait for another component, for example when slower dynamic RAM reads or writes data.

To allow time for the slow operation, CMOS setup information specifies that the CPU maintain a wait state.

If the CPU normally can do something in two clock beats, for example, it is told to wait an extra clock beat, meaning its cycle takes a total of three clock beats. It works for two beats and then waits one beat, which makes for a 50% slowdown. Wait states might be incorporated to slow the CPU so that the rest of the system-board activity can keep up.

Wait states are initially set as part of the systemboard’s default settings and are only changed in rare circumstances, such as when the board becomes unstable.