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IT Support & Hardware · Foundational

Complementary Metal-Oxide Semiconductor (CMOS)

Battery-backed memory that stores BIOS/UEFI settings and keeps the real-time clock running.

Definition

On PCs, CMOS refers to a small amount of memory, now usually part of the chipset, that holds firmware settings like boot order and date/time. A coin-cell battery keeps it powered when the system is unplugged. Clearing CMOS resets firmware settings to defaults.

Example

Use the CLR_CMOS jumper or remove the battery for a minute to reset a forgotten BIOS setting.

Exam tip

Clearing CMOS resets settings; it does not erase the firmware itself.

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