The CMOS chip contains your PC's system settings. These include, for example, amount of RAM, type and number of hard/floppy drives, and date/time settings.

This chip retains the system settings even when your PC is turned off. How can it do this? The CMOS chip is connected to a small battery that maintains power to the chip after you turn off your PC. Although this battery will last a long time, it must occasionally be replaced.