If the time is incorrect on a PC system, the easiest way to reset it is through the operating system. However, if the system continually fails to keep good time, you should start by checking that corrosion has not built up on the battery contacts. Clean the contacts with a pencil eraser and retry the battery. If that doesn't work, try replacing the battery. Next you can try replacing the RTC module. If this does not correct the time keeping problem, the electronic circuitry that recharges the battery might be defective, and in this case you will need a new motherboard.

The Socket 7 specification enabled system boards to be configured for different types of microprocessors using different operating speeds. In older systems, the operating speed of the microprocessor was configured through external settings (that is, jumpers on the motherboard).

A defective motherboard battery can cause the system to continually lose track of time. For more information, see the section "Battery."

An expansion slot is a socket on the motherboard that can hold an adapter card.An adapter card is a circuit  board that enhances functions of a component of the system unit and /or provides  a connections  to a peripheral such as a modem disk drives,printer ,scannner ,or keyboard .Severals types of adapter  cards exist .A sound  cards enhances  the sound generating capabilities  of a  personal computers.A video card  converts  computers  output  into a video signal that display  an image on the screen .A modem card enables  computers to communicate .A network card allows a computer to access a network.

In additions to the motherboard ,processor,memory ,sound card ,PC card slot and drive bay, a mobile computers system unit  also  house devices , speakers  and display.The system  unit also houses devices such as the keyboard ,pointing devices ,speakers and display .The system unit for a typical notebook computer often has keyboard/mouse,IrDA,serial,parallel,video , and USB ports.Tablet PCs usually include severals slots and ports .PDAs often have a IrDA port or are Bluetooth  enabled so users can communicate wirelessly.

The motherboard is the main circuit board of the system unit. The motherboard contains many electronic components including a processor chip, memory chips, expansion slots, and adapter cards. A chip is a small piece of semi conducting material, usually silicon, on which integrated circuits are etched. Expansion slots hold adapter cards that provide connections and functions not built into the motherboard.