Nonvolatile BIOS memory: Difference between revisions

Revision as of 19:54, 2 February 2022

Revision as of 20:27, 14 March 2022

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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-semiconductor (CMOS)]] [[Static random access memory|SRAM]] (such as the [[Motorola]] MC146818 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}}</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. The typical NVRAM capacity is 256 [[byte]]s.<ref name="100Series" />

The CMOS RAM and the real-time clock have been integrated as a part of the [[Southbridge (computing)|southbridge]] chipset and it may not be a standalone chip on modern motherboards.<ref>https://www.intel.com/Assets/PDF/datasheet/290562.pdf {{Bare URL PDF|date=March 2022}}</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> In turn, the southbridge have been integrated into a single [[Platform Controller Hub]].

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>

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* [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]]