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.
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.
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.
- Jumper location resets to the positions defined by your motherboard's instructions
- 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.