 | This articlemay need to be rewritten to comply with Wikipedia'squality standards.You can help. Thetalk page may contain suggestions.(May 2021) |
Nonvolatile BIOS memory refers to a smallmemory onPCmotherboards that is used to storeBIOS settings. It is traditionally calledCMOS RAM because it uses avolatile, low-powercomplementary metal–oxide–semiconductor (CMOS)SRAM (such as theMotorola 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 orNVRAM 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 thesouthbridge chipset and they may not be standalone chips on modern motherboards.[4][5] In turn, the southbridge has been integrated into a singlePlatform Controller Hub. Alternatively BIOS settings may be stored in the computer'sSuper I/O chip.[6]
The chipset built-in NVRAM capacity is typically 256bytes.[4] For this reason, later BIOS implementations may use a small portion of BIOS flash ROM as NVRAM, to store BIOS setup and hardware configuration data.[7]
Today'sUEFI motherboards useNVRAM to store configuration data (NVRAM is a portion of the UEFIflash ROM), but by manyOEMs' design, the UEFI settings are still lost if the CMOS battery fails.[8][9]
.jpg)
The memory battery (aka motherboard, CMOS,real-time clock (RTC), clock battery)[2][10] is generally aCR2032lithiumcoin cell. This cell battery has an estimated life of three years whenpower supply unit (PSU) is unplugged or when the PSU power switch is turned off.[11] This battery type, unlike thelithium-ion battery, is not rechargeable and trying to do so may result in an explosion. Motherboards have circuitry preventing batteries from being charged and discharged when a motherboard is powered on. Other common battery cell types can last significantly longer or shorter periods, such as the smallerCR2016 which will generally last about 40% less time than CR2032. Higher temperatures and longer power-off time will shorten battery cell life. When replacing the battery cell, the system time and CMOSBIOS settings may revert to default values. Unwanted BIOS reset may be avoided by replacing the battery cell with the PSU power switch turned on and plugged intoan electric wall socket. OnATX motherboards, the PSU will supply 5V standby power to the motherboard to keep CMOS memory energized while the system is off.
Some computer designs have used non-button cell batteries, such as the cylindrical "1/2 AA" used in thePower Mac G4 as well as some olderIBM PC compatibles, or a 3-cellnickel–cadmium (Ni–Cd) CMOS battery that looks like a "barrel" (common inAmiga and older IBM PC compatibles), which serves the same purpose. These motherboards often have a four pin straight header, with pin 2 missing, for connecting to an external 3.6v battery, such as the Tadiran TL-5242/W, when their soldered-on batteries run out. Ni–Cd batteries have a tendency toleak devastatingly after a period of disuse, damaging components and traces on the circuit board near the battery.[12]
