CMOS (Complementary Metal Oxide Semiconductor),which means a complementary metal oxide semiconductor - a large-scale application for the manufacture of integrated circuit chips) is a readable and writable RAM chip on the motherboard of the microcomputer to store the hardware of the current system. Configuration and user settings for certain parameters. CMOS can be powered by the motherboard's battery, and the information will not be lost even if the system is powered down. The CMOS RAM itself is just a memory, only the data saving function, and the setting of various parameters in the CMOS is through a special program.
CMOS is composed of a PMOS transistor and an NMOS transistor, and is characterized by low power consumption. Since a pair of MOS gates in CMOS are either PMOS-on, NMOS-on, or off in an instant, they are much more efficient than linear transistors (BJT), so power consumption is low.
In the computer field, CMOS is often referred to as a chip that holds basic computer startup information (such as date, time, boot settings, etc.). Sometimes people will mix CMOS and BIOS. In fact, CMOS is a readable and writable RAM chip on the motherboard.So it is used to save the BIOS hardware configuration and user settings for certain parameters. CMOS can be powered by the motherboard's battery, and the information will not be lost even if the system is powered down. The CMOS RAM itself is just a piece of memory, only the data save function. The setting of various parameters in the BIOS is through a special program. The BIOS setup program is generally integrated into the chip by the manufacturer. The BIOS setup program can be accessed through a specific button at boot time to conveniently set up the system. So BIOS settings are sometimes called CMOS settings.
The early CMOS was a separate chip, the MC146818A (DIP package), which contained 64 bytes of system information. Microcomputers after 386 generally integrate the MC146818A chip into other IC chips (such as 82C206, PQFP package). After 586, the motherboard integrates CMOS with the system real-time clock and backup battery into a chip called DALLDA DS1287. With the development of microcomputers and the increase of settable parameters, current CMOS RAMs generally have a capacity of 128 bytes and 256 bytes. In order to maintain compatibility, each BIOS manufacturer has unified the settings of the first 64 bytes of CMOS RAM in its own BIOS with the CMOS RAM format of the MC146818A, and added its own special settings in the extended part, so different manufacturers BIOS chips are generally not interchangeable, even if they are interchangeable, the CMOS information must be reset after the interchange to ensure the system is running normally.