Nonvolatile BIOS memory refers to a small memory on
PCA personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator...
motherboardA motherboard is the central printed circuit board in many modern computers, and holds many of the crucial components of the system, while providing connectors for other peripherals. The motherboard is sometimes alternatively known as the main board, system board, or, on Apple computers, the logic...
s that is used to store
BIOSIn IBM PC Compatible computers, the basic input/output system , also known as the System BIOS, is a de facto standard defining a firmware interface. The BIOS is boot firmware, designed to be the first code run by a PC when powered on...
settings. It was traditionally called
CMOS RAM because it used a low-power
CMOSComplementary metal–oxide–semiconductor is a technology for making integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for a wide variety of analog circuits such as image sensors, data...
SRAMStatic Random Access Memory is a type of semiconductor memory where the word static indicates that, unlike dynamic RAM , it does not need to be periodically refreshed, as SRAM uses bistable latching circuitry to store each bit...
(such as the
MotorolaMotorola, Inc. is an American, multinational, Fortune 100, telecommunications company based in Schaumburg, Illinois. It is a manufacturer of wireless telephone handsets, and also designs and sells wireless network infrastructure equipment such as cellular transmission base stations and signal...
MC146818 or similar) powered by a small battery when system power was off. The term remains in wide use but it has grown into a misnomer: nonvolatile storage in contemporary computers is often in
EEPROMEEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...
or
flash memoryFlash memory is a non-volatile computer storage that can be electrically erased and reprogrammed. It is a technology that is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products...
(like the BIOS code itself); the remaining usage for the battery is then to keep the
real-time clockA real-time clock is a computer clock that keeps track of the current time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time.-Terminology:The term is used to avoid...
going.
Nonvolatile BIOS memory refers to a small memory on
PCA personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator...
motherboardA motherboard is the central printed circuit board in many modern computers, and holds many of the crucial components of the system, while providing connectors for other peripherals. The motherboard is sometimes alternatively known as the main board, system board, or, on Apple computers, the logic...
s that is used to store
BIOSIn IBM PC Compatible computers, the basic input/output system , also known as the System BIOS, is a de facto standard defining a firmware interface. The BIOS is boot firmware, designed to be the first code run by a PC when powered on...
settings. It was traditionally called
CMOS RAM because it used a low-power
CMOSComplementary metal–oxide–semiconductor is a technology for making integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for a wide variety of analog circuits such as image sensors, data...
SRAMStatic Random Access Memory is a type of semiconductor memory where the word static indicates that, unlike dynamic RAM , it does not need to be periodically refreshed, as SRAM uses bistable latching circuitry to store each bit...
(such as the
MotorolaMotorola, Inc. is an American, multinational, Fortune 100, telecommunications company based in Schaumburg, Illinois. It is a manufacturer of wireless telephone handsets, and also designs and sells wireless network infrastructure equipment such as cellular transmission base stations and signal...
MC146818 or similar) powered by a small battery when system power was off. The term remains in wide use but it has grown into a misnomer: nonvolatile storage in contemporary computers is often in
EEPROMEEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...
or
flash memoryFlash memory is a non-volatile computer storage that can be electrically erased and reprogrammed. It is a technology that is primarily used in memory cards and USB flash drives for general storage and transfer of data between computers and other digital products...
(like the BIOS code itself); the remaining usage for the battery is then to keep the
real-time clockA real-time clock is a computer clock that keeps track of the current time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time.-Terminology:The term is used to avoid...
going. The typical
NVRAMNon-volatile random access memory is the general name used to describe any type of random access memory which does not lose its information when power is turned off. This is in contrast to the most common forms of random access memory today, DRAM and SRAM, which both require continual power in...
capacity is 512
byteA byte is a unit of information storage representing the smallest addressable element for a given computer architecture. It often designates a sequence of bits whose length is determined by the architecture...
s, which is generally sufficient for all BIOS settings.
CMOS mismatch
CMOS mismatch errors typically occur if the computer's
power-on self-testPower-on self-test is the common term for a computer, router or printer's pre-boot sequence. The same basic sequence is present on all computer architectures. It is the first step of the more general process called initial program load , booting, or bootstrapping. The term POST has become popular...
program:
- Finds a device that is not recorded in the CMOS.
- Does not find a device that is recorded in the CMOS.
- Finds a device that has different settings than those recorded for it in CMOS.
- Detects a CMOS checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
error. http://www.pcguide.com/ts/x/sys/booterrGBER25-c.html http://www.computerhope.com/issues/ch000385.htm
CMOS battery
The memory and
real-time clockA real-time clock is a computer clock that keeps track of the current time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time.-Terminology:The term is used to avoid...
are generally powered by a CR2032
lithiumLithium batteries are disposable batteries that have lithium metal or lithium compounds as an anode. Depending on the design and chemical compounds used, lithium cells can produce voltages from 1.5 V to about 3.7 V, up to twice the voltage of an ordinary zinc-carbon battery or alkaline cell...
coin cell. These cells last two to ten years, depending on the type of motherboard, ambient temperature and the 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% as long. 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
ATXATX is a computer form factor designed by Intel in 1995. It was the first big change in computer case, motherboard, and power supply design in many years. ATX overtook AT completely as the default form factor for new systems. ATX addressed many of the AT form factor's annoyances that had...
motherboards, this will supply 5V standby power to the motherboard even if it is apparently "switched off", and keep the CMOS memory energised.
Resetting the CMOS settings
To access the BIOS setup when the machine fails to operate, occasionally a drastic move is required. In older computers with battery-backed RAM, removal of the battery and short circuiting the battery input terminals for a while did the job; in some more modern machines this move only resets the RTC. Some motherboards offer a CMOS-reset
jumperIn electronics and particularly computing, a jumper is a short length of conductor used to close a break in or bypass part of an electrical circuit...
or a reset button. In yet other cases, the EEPROM chip has to be desoldered and the data in it manually edited using a
programmerIn field of computer hardware, the term programmer, chip programmer or device programmer refers to a hardware device that configures programmable non-volatile circuits such as EPROMs, EEPROMs, Flashs, PALs, FPGAs or programmable logic circuits....
. Sometimes it is enough to ground the CLK or DTA line of the
I²CI²C is a multi-master serial computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone. The name is pronounced eye-squared-see or eye-two-see. Since the mid 1990s several competitors I²C (Inter-Integrated Circuit) is a...
bus of the EEPROM at the right moment during boot, this requires some precise soldering on
SMDSurface-mount technology is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards . Electronic devices so made are called surface-mount devices or SMDs...
parts. If the machine lets you boot but does not want to let you into the BIOS setup, one possible recovery is to deliberately "damage" the CMOS checksum by doing direct port writes using
debug.exedebug is a command in DOS, MS-DOS, OS/2 and Microsoft Windows which runs the program debug.exe...
, corrupting some bytes of the checksum-protected area of the CMOS RAM; at the next boot, the computer typically resets its setting to factory defaults.
for example:
c:\debug
-o 70 10
-o 71 aa
-q
That will write to CMOS (Offset 10h) with the value 0AAh.
See also
- BIOS
In IBM PC Compatible computers, the basic input/output system , also known as the System BIOS, is a de facto standard defining a firmware interface. The BIOS is boot firmware, designed to be the first code run by a PC when powered on...
- Backup battery
Backup battery is the name given to a secondary power supply, usually a direct current battery, to provide power in the absence of the main power supply....
- Real-time clock
A real-time clock is a computer clock that keeps track of the current time. Although the term often refers to the devices in personal computers, servers and embedded systems, RTCs are present in almost any electronic device which needs to keep accurate time.-Terminology:The term is used to avoid...
- Extended System Configuration Data
ESCD or Extended System Configuration Data is a part of nonvolatile BIOS memory on the motherboard of a personal computer, where information about ISA PnP devices is stored...
(ESCD)
External links