Nonvolatile BIOS memory

"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.
NEC D4364G 8192 x 8 Bit Static CMOS RAM
CMOS battery in a Pico ITX motherboard

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[1] or similar) powered by a small battery when system and standby power is off.[2] 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) and the RTC, NVRAM and battery may be integrated into a single component. The name CMOS memory comes from the technology used to make the memory, which is easier to say than NVRAM.[3]

The CMOS RAM and the real-time clock have been integrated as a part of the southbridge chipset and they may not be standalone chips on modern motherboards.[4][5] In turn, the southbridge has been integrated into a single Platform Controller Hub. Alternatively BIOS settings may be stored in the computer's Super I/O chip.[6]

The chipset built-in NVRAM capacity is typically 256 bytes.[4] For this reason, later BIOS implementations may use a small portion of BIOS flash ROM as NVRAM, to store setup data.[7]

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.[8][9]

  1. ^ Mueller, Scott (2004). Upgrading and Repairing PCS. Que. ISBN 978-0-7897-2974-3.
  2. ^ Fisher, Tim. "What is CMOS? (CMOS & CMOS Battery Definition)". About.com. Archived from the original on 9 August 2016. Retrieved 2 September 2015.
  3. ^ Mueller, Scott (2003). Upgrading and Repairing PCS. Que. ISBN 978-0-7897-2745-9.
  4. ^ a b "Intel 100 Series Chipset Family PCH Datasheet". Intel. August 2015. Retrieved 16 April 2016.
  5. ^ "82430FX PCISET Data Sheet" (PDF). Intel. Retrieved 23 January 2023.
  6. ^ Mueller, Scott (2004). Upgrading and Repairing PCS. Que. ISBN 978-0-7897-2974-3.
  7. ^ http://rom.by/files/AMIBIOS8-Flash-Recovery-Whitepaper.pdf [bare URL PDF]
  8. ^ "UEFI NVRAM - OSDev Wiki". wiki.osdev.org. Retrieved 11 September 2020.
  9. ^ 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.