Complementary metal oxide semiconductor (CMOS) is a type of semi-conductor chip that holds data without requiring an external power source. In a personal computer (PC), CMOS holds the basic instructions the computer needs to initialize its hardware components and boot up. These settings are known as the basic input output settings (BIOS), also referred to as CMOS settings.

CMOS 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 CMOS 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 CMOS 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. Two of the most well known BIOS manufacturers for clone PCs are Phoenix and Award, while companies like Dell and Compaq produce their own BIOS chips.