Nonvolatile BIOS memory: Difference between revisions

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{{Redirect|CMOS RAM|other types of CMOS RAM|Random-access memory}}

{{Cleanup rewrite|date=May 2021}}

[[File:Casio fx-8000G - NEC D4364G-1821.jpg|thumb|NEC D4364G 8192 x 8 Bit Static CMOS RAM]]

[[Image:Bottom EPIA PX10000G Motherboard new.jpg|right|thumb|200px|CMOS battery in a [[Pico ITX]] motherboard]]

'''Nonvolatile BIOS memory''' refers to a small [[Memory (computers)|memory]] on [[personal computer|PC]] [[motherboard]]s that is used to store [[BIOS]] settings. It is traditionally called '''CMOS''' RAM because it uses a [[volatile memory|volatile]], low-power [[CMOS|complementary metal-oxide-semiconductormetal–oxide–semiconductor (CMOS)]] [[Static random access memory|SRAM]] (such as the [[Motorola]] MC146818<ref>{{cite book | url=https://books.google.com/books?id=E1p2FDL7P5QC&dq=bios+cmos&pg=PA393 | title=Upgrading and Repairing PCS | isbn=978-0-7897-2974-3 | last1=Mueller | first1=Scott | date=2004 | publisher=Que }}</ref> or similar) powered by a small "CMOS" battery when system and standby power is off.<ref name=whatiscmos>{{cite web |url=http://pcsupport.about.com/od/termsc/g/cmos.htm |title=What is CMOS? (CMOS & CMOS Battery Definition) |publisher=[[About.com]] |first=Tim |last=Fisher |access-date=2 September 2015 |archive-date=9 August 2016 |archive-url=https://web.archive.org/web/20160809082556/http://pcsupport.about.com/od/termsc/g/cmos.htm |url-status=dead }}</ref> It is referred to as non-volatile memory or [[Non-volatile random-access memory|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 typicalbattery can also be used to power a real time clock (RTC) and the RTC, NVRAM capacityand battery may be integrated into a single component. The name CMOS memory comes from the technology used to make the memory, which is 256easier [[byte]]sto say than NVRAM.<ref>{{cite namebook | url="100Series"https://books.google.com/books?id=eV1_LjW3pTkC&dq=cmos+bios+settings+only+one+with+cmos&pg=PA382 | title=Upgrading and Repairing PCS | isbn=978-0-7897-2745-9 | last1=Mueller | first1=Scott | date=2003 | publisher=Que }}</ref>

The CMOS RAM and the real-time clock have been integrated as a part of the [[Southbridge (computing)|southbridge]] chipset and itthey may not be a standalone chipchips on modern motherboards.<ref>https://www.intel.com/Assets/PDF/datasheet/290562.pdf</ref><ref name="100Series">{{cite web|url=http://www.intel.com/content/www/us/en/chipsets/100-series-chipset-datasheet-vol-1.html |title=Intel 100 Series Chipset Family PCH Datasheet| publisher=Intel |date=August 2015 |access-date=16 April 2016}}</ref><ref>{{cite web|url=https://www.intel.com/Assets/PDF/datasheet/290562.pdf|title=82430FX PCISET Data Sheet|publisher=Intel|access-date=23 January 2023}}</ref> In turn, the southbridge havehas been integrated into a single [[Platform Controller Hub]]. Alternatively BIOS settings may be stored in the computer's [[Super I/O]] chip.<ref>{{cite book | url=https://books.google.com/books?id=E1p2FDL7P5QC&dq=bios+cmos&pg=PA393 | title=Upgrading and Repairing PCS | isbn=978-0-7897-2974-3 | last1=Mueller | first1=Scott | date=2004 | publisher=Que }}</ref>

The chipset built-in NVRAM capacity is typically 256 [[byte]]s.<ref name="100Series" /> 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.<ref>{{Cite web| title=AMIBIOS8 Flash Update & BIOS Recovery Methods | url=http://rom.by/files/AMIBIOS8-Flash-Recovery-Whitepaper.pdf | archive-url=https://web.archive.org/web/20091122214916/http://www.rom.by:80/files/AMIBIOS8-Flash-Recovery-Whitepaper.pdf | archive-date=2009-11-22}}</ref>

Today's [[UEFI]] motherboards use [[NVRAM]] to store configuration data (NVRAM is a part of the UEFI flash ROM), but by many [[OEM]]s' design, the UEFI settings are still lost if the CMOS battery fails.<ref>{{Cite web|title=UEFI NVRAM - OSDev Wiki|url=https://wiki.osdev.org/UEFI_NVRAM|access-date=2020-09-11|website=wiki.osdev.org}}</ref><ref>{{Cite book|chapter-url=https://media.springernature.com/original/springer-static/image/chp%3A10.1007%2F978-1-4842-6106-4_4/MediaObjects/488723_1_En_4_Fig6_HTML.png|doi = 10.1007/978-1-4842-6106-4_4|chapter = Firmware Resiliency: Detection|title = Building Secure Firmware|year = 2020|last1 = Yao|first1 = Jiewen|last2 = Zimmer|first2 = Vincent|pages = 115–162|isbn = 978-1-4842-6105-7}}</ref>

Today's [[UEFI]] motherboards use [[NVRAM]] to store configuration data (NVRAM is a partportion of the UEFI [[Flash memory|flash]] ROM), but by many [[OEM]]s' design, the UEFI settings are still lost if the CMOS battery fails.<ref>{{Cite web|title=UEFI NVRAM - OSDev Wiki|url=https://wiki.osdev.org/UEFI_NVRAM|access-date=2020-09-11|website=wiki.osdev.org|df=dmy-all}}</ref><ref>{{Cite book|chapter-url=https://media.springernature.com/original/springer-static/image/chp%3A10.1007%2F978-1-4842-6106-4_4/MediaObjects/488723_1_En_4_Fig6_HTML.png|doi = 10.1007/978-1-4842-6106-4_4|chapter = Firmware Resiliency: Detection|title = Building Secure Firmware|year = 2020|last1 = Yao|first1 = Jiewen|last2 = Zimmer|first2 = Vincent|pages = 115–162|isbn = 978-1-4842-6105-7| s2cid=242541772 }}</ref>

=={{anchor|Rechargeable CMOS battery}}CMOS battery==

{{multiple image|direction=horizontal|image1=Battery-lithium-cr2032.jpg|caption1=[[CR-2032]] button cell, the most common CMOS battery|image2=NiCd VARTA battery leakage on circuit board (IMG 3313).jpg|caption2=[[VARTA]]-manufactured [[Nickel–cadmium battery|Ni–Cd]] barrel battery that has leaked onto the mainboard of a [[synthesizer]] and damaged legs and traces|total_width=440}}

The memory battery (aka motherboard, CMOS, [[real-time clock]] (RTC), clock battery)<ref name=whatiscmos /><ref>[http://en.kioskea.net/faq/625-replacing-the-battery-of-your-motherboard-cmos-battery Ask a question » Replacing the battery of your motherboard (CMOS battery)]</ref> is generally a [[CR2032]] [[lithium battery|lithium]] [[coin cell]]. This cell battery has an estimated life of 3three years when [[Power supply unit (computer)|power supply unit (PSU)]] is unplugged or when the PSU power switch is turned off.<ref>

{{

cite web|

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|date=24 August 2020 |publisher=Intel

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</ref> This battery type, unlike the [[Lithiumlithium-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 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]] 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 [[AC power plugs and sockets|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 "[[List of battery sizes#1.2F2AA|1/2 AA]]" used in the [[Power Mac G4]] as well as some older [[IBM PC compatible]]s, or a 3-cell [[Nickel–cadmium battery|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 [[Battery leakage|leak devastatingly]] after a period of disuse, damaging components and traces on the circuit board near the battery.<ref>{{cite web | last=Williams | first=Al | date=July 8, 2018 | url=https://hackaday.com/2018/07/08/amiga-2000-emergency-repair/ | title=Amiga 2000 Emergency Repair | work=Hackaday | archiveurl=https://web.archive.org/web/20180708201017/https://hackaday.com/2018/07/08/amiga-2000-emergency-repair/ | archivedate=July 8, 2018}}</ref>

==See also==

* [[BIOS]]

* [[Extended System Configuration Data]] (ESCD)

* [[UEFI]]

* [[List of battery types]]

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==External links==

* [httphttps://pcsupportwww.aboutlifewire.com/od/termsc/g/what-is-cmos.htm-2625826 Definition of CMOS & CMOS Batteries]

* [httphttps://bochs.sourceforge.netio/techspec/CMOS-reference.txt CMOS memory map (for Bochs PC emulator)]

* [https://www.amazon.com/Battery-Lithium-ML1220-RECHARGEABLE-BATTERY/dp/B005J1UC30 CMOS rechargeable Battery 3V Lithium, with cable]

* [https://www.youtube.com/watch?v=0q2RIaCICKU How to replace dead CMOS battery and configure BIOS settings] (video).

* [https://www.youtube.com/watch?v=MTg8qhWKNpA Tutorial: How to replace the CMOS battery in your laptop]

* [http://www.computerhope.com/issues/ch000239.htm How to replace the CMOS battery]

[[Category:BIOS]]