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.

How To Clear CMOS

Clearing CMOS is hardly an everyday task, but there are times when it is incredibly necessary. If you are changing hardware settings and accidentally mismatch things too much to boot, it may be easier to simply start over from the default, for example. When the need arises, there are two main ways to accomplish the task.

  1. Jumper location resets to the positions defined by your motherboard's instructions
  2. Interrupt the battery so the BIOS clears and reboots to its default on startup

Either way, to clear CMOS is basically to reset it to those original parameters. Let's assume you do not have the motherboard manual handy and you want to do it by interrupting the battery. It's the simplest way to handle things most of the time, and the method that computer designers increasingly plan for.

How To Reset CMOS

There are three common ways to disrupt the connection between the BIOS battery backup and the physical memory it is housed in. Disrupting this power clears saved settings, but the firmware itself is not erased, just its settings. This gives you a clean slate for configuring the hardware that has been added since the computer was first purchased. All of these methods require the computer to be powered off and unplugged for your best chance of success.

Finding the Case Switch

Many of today's top computer manufacturers have simplified the BIOS reset process by providing a case switch that interrupts the connection between the backup battery and the BIOS memory. All you have to do is firmly press it for a second or two, then power up the computer. CMOS should be reset to factory settings for the motherboard. If you bought the computer prebuilt, this should leave it configured for the original hardware. If your motherboard is part of a custom build, you may need to reconfigure all your settings, depending on how sophisticated its automatic detection is.

Locating the Motherboard Switch

Many motherboards that are built for homebrew builds have a switch on the board that is similar to the case switch you might find on a prebuilt computer or one with a deluxe case that has robust features. It tends to be a simple button located in one corner of the board, usually the one closest to your CPU. If you can't find it, you might just have a motherboard that does not provide a switch. Make sure you follow standard protocols for discharging board capacitors and grounding your body before handling the motherboard, even just to trip this switch.

Resetting CMOS by Reseating the Battery

This might be the most difficult way to do it, but it is one that works for any computer if you take the time to find the BIOS memory battery. Ground yourself and discharge the necessary capacitors with a few case power button presses. Then open the computer case and look for a standard lithium-ion battery on the motherboard. All you need to do is lift it to break the power connection and then replace it. If there is a clip, be careful you move with its mechanics to avoid damaging it. A few seconds of disrupted power should be all you need. Now you know how to clear CMOS on any computer, not just one with your familiar hardware.

What Is CMOS Used for

The CMOS circuit is used to house the BIOS firmware and its settings apart from system storage devices like the hard drive or USB connected devices. The logic chip itself stores those settings, requiring a low but consistent power draw. This is why CMOS requires a battery backup for times when the computer is powered down or unplugged. If the power to the chip is disrupted, it goes back to its natural state until the BIOS is accessed and changes are made. The settings in that natural state are usually dictated by jumper positions on the motherboard, so most fully installed hardware will come back configured for basic use.

  • CMOS provides access to hardware settings at a level above the intervention of the OS, making it instrumental in repair and recovery
  • It is about the relationship between the motherboard and attached hardware, making it important to overall system configuration but not the entire control mechanism
  • Its settings make sure the OS can find devices and that their port assignments or addresses do not conflict with each other

This function makes it absolutely vital to the proper operation of any computer. Before changing any CMOS settings via BIOS or jumper switch settings, make sure you fully understand what those changes do. If there is an issue, a written record of them can help you recover your work process after a CMOS reset.

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

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Discussion Comments

anon946017

What is cmos testing??

anon331192

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

anon266827

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

anon183964

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.

anon175881

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

anon133297

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

anon96825

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.

anon96253

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

Basel

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.

anon78856

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.

anon75756

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.

anon61290

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

anon57398

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

anon53807

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.

anon45481

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

anon35449

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

rajduhan3008

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

papiv52

I would also like to know how to fix it.

papiv52

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?

anon19145

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

MGKelly

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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