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.^[1] 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. When the battery fails, BIOS settings are reset to their defaults. The battery can also be used to power a real time clock (RTC).

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.^[2]^[3] In turn, the southbridge have been integrated into a single Platform Controller Hub.

The chipset built-in NVRAM capacity is typically 256 bytes.^[2] For this reason, later BIOS implementations may use a small portion of BIOS flash ROM as NVRAM, to store setup data.^[4]

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.^[5]^[6]

^ Fisher, Tim. "What is CMOS? (CMOS & CMOS Battery Definition)". About.com. Archived from the original on 9 August 2016. Retrieved 2 September 2015.
^ ^a ^b "Intel 100 Series Chipset Family PCH Datasheet". Intel. August 2015. Retrieved 16 April 2016.
^ "82430FX PCISET Data Sheet" (PDF). Intel. Retrieved 23 January 2023.
^ http://rom.by/files/AMIBIOS8-Flash-Recovery-Whitepaper.pdf ^{[bare URL PDF]}
^ "UEFI NVRAM - OSDev Wiki". wiki.osdev.org. Retrieved 11 September 2020.
^ Yao, Jiewen; Zimmer, Vincent (2020). "Firmware Resiliency: Detection". Building Secure Firmware. pp. 115–162. doi:10.1007/978-1-4842-6106-4_4. ISBN 978-1-4842-6105-7. S2CID 242541772.

[whatiscmos-1] Fisher, Tim. "What is CMOS? (CMOS & CMOS Battery Definition)". About.com. Archived from the original on 9 August 2016. Retrieved 2 September 2015.

[100Series-2] "Intel 100 Series Chipset Family PCH Datasheet". Intel. August 2015. Retrieved 16 April 2016.

[3] "82430FX PCISET Data Sheet" (PDF). Intel. Retrieved 23 January 2023.

[4] ttp://rom.by/files/AMIBIOS8-Flash-Recovery-Whitepaper.pdf ^{[bare URL PDF]}

[5] "UEFI NVRAM - OSDev Wiki". wiki.osdev.org. Retrieved 11 September 2020.

[6] Yao, Jiewen; Zimmer, Vincent (2020). "Firmware Resiliency: Detection". Building Secure Firmware. pp. 115–162. doi:10.1007/978-1-4842-6106-4_4. ISBN 978-1-4842-6105-7. S2CID 242541772.

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