CMOS (originally means Complementary Metal Oxide Semiconductor – a raw material used in the manufacture of integrated circuit chips on a large scale) is a readable and writable RAM chip on the mainboard of a microcomputer, which is used to save the hardware configuration of the current system and the user’s parameter setting.
CMOS can be powered by the motherboard’s battery, information will not be lost even if the system is powered down. CMOS RAM itself is just a piece of memory, and only has the function of data saving, and the setting of various parameters in CMOS needs to go through a special program. Early CMOS setup programs resided on floppy disks (such as IBM’s PC/AT models), which were inconvenient to use. Now most manufacturers put the CMOS setting program into the BIOS chip, and you can enter the CMOS setting program through a specific key when starting up, so as to easily set the system, so the CMOS setting is also called BIOS setting.
The early CMOS is a separate chip MC146818A (DIP package), a total of 64 bytes to store system information. Microcomputers after 386 generally integrate the MC146818A chip into other IC chips (such as 82C206, PQFP package), and some of the latest 586 motherboards even integrate CMOS, system real-time clock and backup battery into a chip called DALLDA DS1287. With the development of microcomputers and the increase of parameters that can be set, the current CMOS RAM generally has a capacity of 128 bytes and up to 256 bytes. In order to maintain compatibility, each BIOS manufacturer unifies the settings of the first 64 bytes of CMOS RAM in their own BIOS to be consistent with the CMOS RAM format of MC146818A, and adds their 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 to ensure the normal operation of the system.
