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Nonvolatile BIOS memory refers to a small memory on PC motherboards that is used to store BIOS settings. It was traditionally called CMOS RAM because it used a low-power complementary metal-oxide-semiconductor (CMOS) SRAM (such as the Motorola MC146818 or similar) powered by a small battery when system power was off. The term remains in wide use but it has grown into a misnomer: nonvolatile storage in contemporary computers is often in EEPROM or flash memory (like the BIOS code itself); the remaining usage for the battery is then to keep the real-time clock going. The typical NVRAM capacity is 512 bytes, which is generally sufficient for all BIOS settings. The CMOS RAM and the real-time clock have been integrated as a part of the southbridge chipset and it may not be a standalone chip on modern motherboards.
All personal computers require a small battery on the system board that provides power to the Complementary Metal Oxide Semiconductor (CMOS) chip, even while the computer is turned off. This chip contains information about the system configuration (e.g., hard disk type, floppy drive types, date and time, and the order in which the computer will look for bootable disks). The CMOS battery allows the CMOS to preserve these settings.
An image of CMOS battery and inserted in motherboard -