Nonvolatile BIOS memory

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.

"CMOS memory" redirects here. For other types of CMOS memory, see Semiconductor memory.
"CMOS RAM" redirects here. For other types of CMOS RAM, see Random-access memory.

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.

The chipset built-in NVRAM capacity is typically 256 bytes. For this reason, later BIOS implementations may use a small portion of BIOS flash ROM as NVRAM, to store setup data.

Today's UEFI motherboards use NVRAM to store configuration data (NVRAM is a portion of the UEFI flash ROM), but by many OEMs' design, the UEFI settings are still lost if the CMOS battery fails.

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