Nonvolatile BIOS memory

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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 was traditionally called CMOS RAM because it used a volatile, low-power complementary metal-oxide-semiconductor (CMOS) SRAM (such as the Motorola MC146818 or similar) powered by a small battery when system power was off (called the CMOS battery).[1]

The term remains in wide use but it has grown into a misnomer: nonvolatile storage in contemporary computers is often in EEPROM or flash memory (like the BIOS code itself); the remaining usage for the battery is then to keep the real-time clock (RTC) going. The typical NVRAM capacity is 512 bytes, which is generally sufficient for all BIOS settings. 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.

CMOS battery[edit]

Type CR2032 button cell, most common CMOS battery.

The memory battery (aka motherboard, CMOS, real-time clock-RTC, clock battery)[2][3] is generally a CR2032 lithium coin cell. These cells last two to ten years, depending on the type of motherboard, ambient temperature and the length of time that the system is powered off, while other common cell types can last significantly longer or shorter periods, such as the CR2016 which will generally last about 40% less than CR2032. Higher temperatures and longer power-off time will shorten cell life. When replacing the cell, the system time and CMOS BIOS settings may revert to default values. This may be avoided by replacing the cell with the power supply master switch on. On ATX motherboards, this will supply 5V power to the motherboard even if it is apparently "switched off" and keep the CMOS memory energized.

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 NiCd CMOS battery that looks like a "barrel" (common in Amigas and older IBM PC compatibles), which serves the same purpose.

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