The CMOS chip remembers your PC's system settings which are needed each time you turn on your PC. This chip works together with the BIOS chip. When you run the CMOS setup program the BIOS stores all system settings in the CMOS. The CMOS also runs a real-time clock which keeps track of the date and time.

The CMOS is non-volatile RAM or NVRAM which means it remembers its settings even when your PC is turned off. This is possible because the CMOS chip is connected to a battery which is called the CMOS battery.