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 contains some basic information, including the date and time and system settings, necessary for 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 rely 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 is turned on, CMOS performs a series of checks to ensure that the system is functioning properly. One of these checks includes the random access memory (RAM) count. This delays boot time, so some people disable this feature in their CMOS settings, opting for a fast boot instead. If installing new RAM, it is best to enable the feature until the RAM is checked.
A computer motherboard.
Once POST completes, CMOS performs its other settings. Hard drives and formats are detected, along with Redundant Array of Independent Disk (RAID) settings, boot preferences, presence of peripherals, and overclocking adjustments. Many settings can be manually changed in the CMOS setup screen to improve performance; however, changes must be made by experienced users. Changing the settings incorrectly can make the system unstable, cause crashes, or even prevent the computer from starting.
The setup screen can be accessed during the POST phase of boot by pressing a key before the operating system boots. Usually this is the Del key, but it can be something else. A line of text will indicate which key will take the user to the CMOS or BIOS setup screen. Changes cannot be made on an operating system such as Microsoft Windows®, but must be made in a true DOS session. There is also an option to protect the settings by requiring a password to change the settings. Changes are saved when exiting by pressing the F10 key, then the computer restarts to use the new settings.
Most motherboard manuals provide a complete list of available CMOS options. They vary by motherboard design and BIOS manufacturer.
A stick of DDR-RAM, a type of memory.