Purpose and Contents of CMOS Memory
A great deal of important configuration information is stored in a special type of non-volatile memory, called Complementary Metal-Oxide Semiconductor, or CMOS (pronounced 'sea-moss'), which requires little power to hold on to its contents. CMOS runs on about a millionth of an amp of electrical current. This efficiency allows it to store configuration data for a long time (maybe years), powered only from either low-voltage dry cell or lithium batteries. On newer PCs, the CMOS battery is located on the motherboard. On older systems, like the Amstrad 1512, a pair of AA batteries mounted on the top of the system powered the CMOS.
To access the CMOS on most computers, press the delete (DEL) key as the computer is booting. The content of each screen will vary depending on the motherboard but the same items should be available to be configured. The following photos are screen captures of an ASUS motherboard for an Athlon 64.