
Nonvolatile BIOS memory refers to a small
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...
on
PC motherboard
A motherboard, also called a mainboard, a system board, a logic board, and informally a mobo (see #Nomenclature, "Nomenclature" section), is the main printed circuit board (PCB) in general-purpose computers and other expandable systems. It ho ...
s that is used to store
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization d ...
settings. It is traditionally called CMOS RAM because it uses a
volatile, low-power
complementary metal–oxide–semiconductor (CMOS) SRAM (such as the
Motorola
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois. It was founded by brothers Paul and Joseph Galvin in 1928 and had been named Motorola since 1947. Many of Motorola's products had been ...
MC146818 or similar) powered by a small battery when system and standby power is off.
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.
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.
In turn, the southbridge has been integrated into a single
Platform Controller Hub
The Platform Controller Hub (PCH) is a family of Intel's single-chip chipsets, first introduced in 2009. It is the successor to the Intel Hub Architecture, which used two chipsa northbridge and southbridge, and first appeared in the Intel 5 Se ...
. Alternatively BIOS settings may be stored in the computer's
Super I/O chip.
The chipset built-in NVRAM capacity is typically 256
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
s.
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.
Today's
UEFI
Unified Extensible Firmware Interface (UEFI, as an acronym) is a Specification (technical standard), specification for the firmware Software architecture, architecture of a computing platform. When a computer booting, is powered on, the 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.
CMOS battery
The memory battery (aka motherboard, CMOS,
real-time clock
A real-time clock (RTC) is an electronic device (most often in the form of an integrated circuit) that measures the passage of time.
Although the term often refers to the devices in personal computers, server (computing), servers and embedded ...
(RTC), clock battery)
is generally a
CR2032 lithium
Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...
coin cell. This cell battery has an estimated life of three years when
power supply unit (PSU) is unplugged or when the PSU power switch is turned off. This battery type, unlike the
lithium-ion battery
A lithium-ion or Li-ion battery is a type of rechargeable battery that uses the reversible intercalation of Li+ ions into electronically conducting solids to store energy. Li-ion batteries are characterized by higher specific energy, energ ...
, 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 smaller
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
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization d ...
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
ATX (Advanced Technology Extended) is a motherboard and power supply configuration specification, patented by David Dent in 1995 at Intel, to improve on previous de facto standard, ''de facto'' standards like the AT (form factor), AT design. ...
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 compatible
An IBM PC compatible is any personal computer that is hardware- and software-compatible with the IBM Personal Computer (IBM PC) and its subsequent models. Like the original IBM PC, an IBM PC–compatible computer uses an x86-based central p ...
s, or a 3-cell
nickel–cadmium (Ni–Cd) CMOS battery that looks like a "
barrel
A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden stave (wood), staves and bound by wooden or metal hoops. The word vat is often used for large containers ...
" (common in
Amiga
Amiga is a family of personal computers produced by Commodore International, Commodore from 1985 until the company's bankruptcy in 1994, with production by others afterward. The original model is one of a number of mid-1980s computers with 16-b ...
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.
See also
*
Extended System Configuration Data (ESCD)
*
List of battery types
This list is a summary of notable electric battery types composed of one or more electrochemical cells. Three lists are provided in the table. The primary (non-rechargeable) and secondary (rechargeable) cell lists are lists of battery chemistry. ...
References
{{Reflist
External links
Definition of CMOS & CMOS BatteriesCMOS memory map (for Bochs PC emulator)How to replace dead CMOS battery and configure BIOS settings(video).
Tutorial: How to replace the CMOS battery in your laptop
BIOS
Motherboard
Non-volatile memory