CMOS is an abbreviation for Complementary Metal Oxide Semiconductor. CMOS uses a complimentary arrangement of both NMOS and PMOS, negative and positive Metal Oxide transistor circuits. CMOS memory draws very little current (nano-Amps). Values stored in it can be maintained for very a long period of time while the computer is off with a rechargeable lithium battery. The CMOS memory is used to store system configuration information, such as disk drive parameters, memory configuration, and the date-time, some of which you enter in the CMOS Setup program, . It is used by the Basic Input Output System (BIOS), a program permanently stored in the flash memory on recent motherboards and in read-only memory (ROM) on older motherboards, to configure the computer. The CMOS Setup is part of the BIOS program. The CMOS memory is usually located with the real-time clock in the motherboard chipset or in a separate real-time clock chip. It is located in the chipset on most recent motherboards. For example, the CMOS memory is located in the VT82C596B Southbridge chip in the VIA MVP3C chipset on the Epox MVP3G2 Super7 Motherboard and in the AMD-756 Southbridge in the AMD-750 chipset on the MSI MS-6167 Athlon motherboard.
