CMOS BIOS Memory
CMOS BIOS Memory is a few bytes of battery-backed SRAM used to preserve BIOS settings and Real Time Clock data when a PC is off. It is the historical antecedent to UEFI NVRAM, which serves the same purpose on UEFI systems but typically uses flash memory instead of SRAM.
CMOS BIOS memory contains 128-bytes of battery-backed SRAM as standard. The on ISA systems, the layout looks like this:
|0x00||0x0f||16 bytes||Real Time Clock|
|0x10||0x2f||32 bytes||ISA configuration data|
|0x30||0x3f||16 bytes||BIOS specific configuration data|
|0x40||0x7f||64 bytes||Extended System Configuration Data (ESCD) with Plug and Play data|
Past the RTC data exact byte details differ somewhat by implementation. QEMU's SeaBIOS is commonly used as a reference for the creation of modern BIOS implementations.
On reasonably modern systems the BIOS battery is nearly always a CR2032 lithium coin cell. Older boards may have nickel-cadmium (Ni–Cd) batteries which can leak as they age and damage or destroy their host systems.
Date and time are lost when the system is unplugged
The CMOS BIOS battery is past its useful life. Discard the battery and replace it with a new one.
- BIOS — the standard firmware of IBM-PC-compatible computers until it was phased out in 2020.
- UEFI — a firmware standard for boot ROM designed to provide a stable API for interacting with system hardware. On x86 it replaced the legacy BIOS.