CMOS

A separate memory chip, called the complementary metal-oxide semiconductor (CMOS) chip, stores the information that describes specific device parameters. CMOS does not store programs; it only stores data that is read by BIOS to complete the programs needed to talk to changeable hardware. CMOS also acts as a clock to keep to the current data and time.

          Years ago, CMOS was separate chip on motherboard. Today, CMOS is almost always built into the Southbridge.

          Most CMOS chip store around 64 KB of data, but the PC usually needs only a very small amount-about 128 bytes- to store all of the necessary information on the changeable hardware. Don’t let the tiny size fool you. The information stored in CMOS is absolutely necessary for the PC to function!

          If the data stored on CMOS about a particular piece of hardware (or about its fancier features) is different from the specs of the actual hardware, the computer cannot access that piece of hardware (or use its fancier features). It is crucial that this information be correct. If you change any of the previously mentioned hardware, you must update CMOS to reflect those changes. You need to know, therefore, how to change the data on CMOS.