What is CMOS?

R. Kayne

A complementary metal oxide semiconductor (CMOS) is a type of integrated circuit technology. The term is often used to refer to a battery-powered chip found in many personal computers that holds some basic information, including the date and time and system configuration settings, needed by the basic input/output system (BIOS) to start the computer. This name is somewhat misleading, however, as most modern computers no longer use these chips for this function, but instead depend on other forms of non-volatile memory. CMOS chips are still found in many other electronic devices, including digital cameras.

CMOS chip.
CMOS chip.

In a computer, the chip controls a variety of functions, including the Power On Self Test (POST). When the computer’s power supply fires up, CMOS runs a series of checks to make sure the system is functioning properly. One of these checks includes counting up random access memory (RAM). This delays boot time, so some people disable this feature in the CMOS settings, opting for a quick boot. If installing new RAM it is better to enable the feature until the RAM has been checked.

A computer motherboard.
A computer motherboard.

Once POST has completed, CMOS runs through its other settings. Hard disks and formats are detected, along with Redundant Array of Independent Disk (RAID) configurations, boot preferences, the presence of peripherals, and overclocking tweaks. Many settings can be manually changed within the CMOS configuration screen to improve performance; however, changes should be made by experienced users. Changing settings improperly can make the system unstable, cause crashes, or even prevent the computer from booting.

The configuration screen is accessible during the POST phase of boot up, by pressing a key before the operating system initializes. Normally this is the Del key but it might be another. A line of text will indicate which key will take the user into the CMOS or BIOS setup screen. Changes cannot be made from within an operating system such as Microsoft Windows®, but must be made within a true DOS session. There is also an option to protect then settings by requiring a password to change settings. Changes are saved upon exit by pressing the F10 key, then the computer reboots to utilize the new settings.

Most motherboard manuals provide a complete list of available CMOS options. These will vary according to motherboard design and BIOS manufacturer.

A stick of DDR-RAM, a type of memory.
A stick of DDR-RAM, a type of memory.

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Discussion Comments


What is cmos testing??


My CMOS battery lasts only for a few days. How can I solve the problem?


Most of these issues can be solved by replacing the cmos battery.


how can i fix the date and time? Every time i set the date and time, when i have to start again still the date and time are not set. please help me.


CMOS Setting wrong; press F1 to run setup; Press F2 to load default values and continue? How do I solve this problem? --Nitin


can someone change capacitors and resistors on a motherboard if it is bad?


thanks a lot for the info. i have already replaced the battery, and nothing changed, but after looking well on the board i found that there is a capacitor busted. i changed it from an old board i have and now it's working perfectly.


- anon45481: Just replace the CMOS battery from your motherboard.


my computer is not booting until i clear the bios. after i clear, it will tell me CMOS settings are wrong and I need to fix the time and date, but after i save, i have the same problem again of the computer not booting.


To clear a CMOS password, remove the battery for about 5 to 10 minutes or change the jumper's pins position from 1-2 On to 2-3 off and then back to 1-2 On, both methods works and fix the problem.


I Set a logon password on my Acer Aspire 4315 Laptop and forgot it or what I think it is doesn't work. How can I bypass the logon password? OS is Windows Vista Basic. Any assistance will be appreciated.


my computer says cmos setting error and every time my clock becomes disturbed and the date changes. what do i have to do?


i installed 2gb memory on my pc and it won't boot up. anything i have to change in the bios settings?


Generally, you will have to replace the CMOS battery on your motherboard. Pop the case off of your computer and look for the battery on the motherboard. Many times it will just be a standard circular lithium battery.


for the clock issue all you have to do is replace the cmos battery on the motherboard.


my computer says cmos setting error and every time my clock becomes disturbed and the date changes. what do i have to do?


my computer says cmos setting error and every time my clock becomes disturbed and the date changes. what do i have to do?


I would also like to know how to fix it.


I have a Dell Optiplex with win xp pro. It has 2048G of memory installed, but in system, on control panel it says there is only 256mb. What is wrong?


my computer says cmos setting error and eyerytime my clock becomes disturbed and date changes. what i have to do?


My computer says CMOS settings wrong and CMOS Checksum Bad. One memory stick is bad and only one memory slot is good. I had also replace the power supply. How do i get the CMOS settings configured so that it can work??

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