Nonvolatile BIOS memory

Source: Wikipedia, the free encyclopedia.
"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
Pico ITX
motherboard

Nonvolatile BIOS memory refers to a small

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 BIOS setup and hardware configuration data.[7]

Today's

OEMs' design, the UEFI settings are still lost if the CMOS battery fails.[8][9]

CMOS battery

CR-2032 button cell, the most common CMOS battery
VARTA-manufactured Ni–Cd barrel battery that has leaked onto the mainboard of a synthesizer
and damaged legs and traces

The memory battery (aka motherboard, CMOS,

CR2016 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 CMOS BIOS 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 into an electric wall socket. On ATX
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 the Power Mac G4 as well as some older IBM PC compatibles, or a 3-cell nickel–cadmium (Ni–Cd) CMOS battery that looks like a "barrel" (common in Amiga 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 to leak devastatingly after a period of disuse, damaging components and traces on the circuit board near the battery.[12]

See also

References

  1. ISBN 978-0-7897-2974-3
    .
  2. ^
    About.com. Archived from the original
    on 9 August 2016. Retrieved 2 September 2015.
  3. 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. ISBN 978-0-7897-2974-3
    .
  7. ^ "AMIBIOS8 Flash Update & BIOS Recovery Methods" (PDF). Archived from the original (PDF) on 2009-11-22.
  8. ^ "UEFI NVRAM - OSDev Wiki". wiki.osdev.org. Retrieved 11 September 2020.
  9. S2CID 242541772
    .
  10. ^ Ask a question » Replacing the battery of your motherboard (CMOS battery)
  11. ^ "CMOS Battery on the Intel NUC". Intel. 24 August 2020.
  12. ^ Williams, Al (July 8, 2018). "Amiga 2000 Emergency Repair". Hackaday. Archived from the original on July 8, 2018.
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