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A computers Basic Input Output System and Complementary Metal-Oxide Semiconductor together handle a rudimentary and essential process: they set up the computer and boot the operating system. The BIOSs primary function is to handle the system setup process including driver loading and operating system booting. The CMOSs primary function is to handle and store the BIOS configuration settings.
BIOS Drivers
The BIOSs first task is to configure all the system hardware with basic drivers so the system can get up and running. These drivers will set up and prepare the system memory and configure any peripheral devices for use such as the hard drives, optical drives, and video card. The BIOS loads a basic system thats able to read the devices that contain the system operating system.
BIOS Boot
The BIOS typically displays a splash screen when loading the initial CMOS settings and the hardware drivers. BIOS initiates the operating system boot process after all the drivers are loaded and configured. The operating system contains more robust versions of the system drivers and replaces them with the BIOS versions once theyre loaded. The BIOS boot process is like the ignition on a car, it gets the system ready for use.
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 doesnt get power from the battery. Its a common practice to remove the battery to flash-back CMOS settings if there is a configuration problem.
CMOS Settings
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 CMOSs 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.