Nonvolatile BIOS memory refers to a small memory on PC motherboards that is used to store BIOS settings. It is traditionally called CMOS RAM because it uses a volatile, low-power complementary metal-oxide-semiconductor SRAM powered by a small “CMOS” battery when system and standby power is off. 

CMOS may refer to any of the following:

CMOS battery

1. Alternatively referred to as a RTC (real-time clock), NVRAM (non-volatile RAM) or CMOS RAMCMOS is short for complementary metal-oxide semiconductor. CMOS is an onboard, battery powered semiconductor chip inside computers that stores information. This information ranges from the system time and date to system hardware settings for your computer. The picture shows an example of the most common CMOS coin cell battery (Panasonic CR 2032 3V) used to power the CMOS memory.

Is the CMOS battery important?

The Answer

The CMOS battery is not there to provide power to the computer when it is in operation, it is there to maintain a small amount of power to the CMOS when the computer is powered off and unplugged. The primary function of this is to keep the clock running even when the computer is turned off



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 they’re 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 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.

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 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.


CMOS Battery gives ‘backup power‘ to CMOS (Complementary Metal Oxide Semiconductor) of a computer to keep some parameter values, eg: RTC (Real Time Clock). These primary parameters shall be used when the system gets booted again. Many computers use CMOS memory to hold current time and date.

An advantage of CMOS over NMOS logic is that both low-to-high and high-to-low output transitions are fast since the (PMOS) pull-up transistors have low resistance when switched on, unlike the load resistors in NMOS logic. In addition, the output signal swings the full voltage between the low and high rails.

What are the disadvantages of CMOS?

disadvantage of the CMOS is that it uses two transistors as opposed to one NMOS transistor to create an inverter. This means that the CMOS inverter takes up more space on the chip than the NMOS inverter. These disadvantages are minor because of advances in the CMOS technology.

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