A specific manufacturing process for integrated circuits and a specialized piece of hardware on a computer motherboard. In common computing parlance, the CMOS usually refers to the battery-backed memory that stores the system's BIOS/UEFI configuration settings and powers the Real-Time Clock (RTC). CMOS technology is used for constructing integrated circuit (IC) chips, including microprocessors, microcontrollers, memory chips, and other digital logic circuits. CMOS overtook NMOS logic as the dominant MOSFET fabrication process for very large-scale integration (VLSI) chips in the 1980s, replacing earlier transistor-transistor logic (TTL) technology at the same time. CMOS has since remained the standard fabrication process for MOSFET semiconductor devices.
Core Functions
Settings Storage: Maintains user-defined hardware configurations, such as boot order, overclocking parameters, and enabled/disabled integrated peripherals (e.g., onboard audio). Timekeeping: Provides a persistent power source for the RTC, ensuring the system date and time remain accurate even when the computer is unplugged. Low Power Consumption: CMOS technology is characterized by high noise immunity and low static power consumption, allowing a tiny coin-cell battery to power the memory for several years.
The CMOS Battery
The CMOS RAM is volatile, meaning it loses data without power. To prevent this, a small battery is used:
- Type: Usually a CR2032 lithium coin cell.
- Lifespan: Typically 3 to 10 years.
- Failure Symptoms: If the battery dies, the system will usually display errors such as CMOS Checksum Error or Date and Time Not Set, or reset all BIOS settings to factory defaults on every boot.
Key Terminology
- CMOS RAM: The specific 64 to 256 bytes of memory that actually hold the settings.
- Volatile Memory: Memory that requires power to maintain the stored information.
- Clear CMOS: A troubleshooting step where the user resets the BIOS to factory defaults by temporarily removing the battery or shorting a specific "Clear CMOS" jumper on the motherboard.
Fabrication Process
In a broader engineering context, CMOS is a process that uses both P-type and N-type metal-oxide-semiconductor field-effect transistors (MOSFETs) to create logic gates.
CMOS vs. BIOS
While often used interchangeably, they are distinct:
- Component: BIOS is the actual firmware/software (code). CMOS is the storage for user settings (data).
- Persistence: BIOS is stored in non-volatile ROM/EEPROM. CMOS is stored in volatile RAM (battery-backed).
Modern Transition
In modern UEFI-based systems, hardware settings are increasingly stored in NVRAM (which does not require a battery to stay saved). However, the CMOS battery remains essential for the Real-Time Clock (RTC) hardware, as the clock must physically keep ticking even when the machine is off.
Technical Note
When a computer is plugged into a wall outlet, it typically draws power from the power supply's standby voltage to maintain the CMOS/RTC, preserving battery life when the machine is completely disconnected from AC power.
