It’s common to see people using the terms BIOS and CMOS interchangeably as they both refer to a similar and essential part of a computer’s motherboard, but they’re actually 2 separate things, though they work together.
The term CMOS usually describes the small amount of memory (usually 256 bytes) that is used to store the BIOS settings on a motherboard.
CMOS, which stands for Complementary Metal-Oxide-Semiconductor, describes the process of manufacturing that is used to produce the chip.
This memory is “non-volatile”, meaning that it is retained even when the computer is powered off because it has its own dedicated power source in the form of the CMOS Battery.
The CMOS Battery refers to a small lithium-ion battery (CR2032) about the size of a coin which lasts up to ten years, once the battery is drained or removed the bios settings will default to its default settings. The CMOS Battery makes it possible for the bios to be still functional even when switched off by providing a small amount of power to the CMOS.
The BIOS (Basic Input/Output System) is a firmware that runs upon turning on a computer, It instructs the computer how to perform a basic set of actions such as keyboard input and booting, after running a “POST” test, it loads your operating system. Settings for the BIOS are stored on the CMOS memory.
The BIOS also provides a basic interface that allows you to configure your computer’s hardware, you can access it by pressing specific key(s) during start-up they usually are: F1, F2, F10, Delete and Esc.
BIOS is the firmware that tests hardware and launches the operating system. In comparison, the CMOS is the memory which the BIOS pulls information from which includes: settings, time, and anything else stored on it.
CMOS Batteries are dedicated to powering the CMOS, so it retains information while the computer is turned off.