BIOS configuration data is retained by using a small battery, called the CMOS battery, as a power source when the power supply is not on. The data is saved in the nonvolatile BIOS memory (also called CMOS RAM) and will be lost if the CMOS battery dies or is removed. If the data is lost the BIOS will revert to default settings.
Nonvolatile BIOS memory refers to a small memory on PC motherboards that is used to store BIOS settings. It is traditionally called CMOS RAM because it uses a volatile, low-power complementary metal–oxide–semiconductor (CMOS) SRAM (such as the Motorola MC146818 or similar) powered by a small "CMOS" battery when system and standby power is off. It is referred to as non-volatile memory or NVRAM because, after the system loses power, it does retain state by virtue of the CMOS battery. The typical NVRAM capacity is 256 bytes.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. In turn, the southbridge have been integrated into a single Platform Controller Hub.
Today's UEFI motherboards use NVRAM to store configuration data (NVRAM is a part of the UEFI flash ROM), but by many OEMs' design, the UEFI settings are still lost if the CMOS battery fails.