CMOS stands for complementary metal-oxide-semiconductor. It is the memory chip on a motherboard that stores the basic startup information (such as date, time, and startup configuration) of the computer. BIOS uses this setting at startup.
CMOS and Battery Backup
The CMOS is a physical part of the motherboard: it is a memory chip that houses setting configurations and is powered by the onboard battery. The CMOS is reset and loses all custom settings in case the battery runs out of energy, Additionally, the system clock resets when the CMOS loses power. The CMOS reverts to factory settings if it doesn’t get power from the battery. It’s a common practice to remove the battery to flash-back CMOS settings if there is a configuration problem.
The CMOS menu is accessed from the BIOS splash screen. You can typically enter it by pressing F1, F2, Del or Esc. The actual button varies from motherboard to motherboard. The CMOS menu contains the hardware customization options allowed by the motherboard, uses a simple graphical interface and is controlled by the keyboard. Customization features include memory handling, expansion port speed configuration, boot device order and power control. Microsoft recommends only adjusting these settings if you are an advanced user because some improper setting adjustments can render the computer unusable. Some advanced settings can overpower the system, causing it to produce enough heat to break it.
Boot Device Selection
One of the CMOS’s most important roles is it can alter the device boot process. This is important for system restoration because the CMOS may need to change boot priority from the hard drive to the optical drive or flash drive to launch the operating system installer or adjust which hard drive to load the operating system from.