Hello, its main function is to store BIOS Settings and system time and date.
It should be distinguished from the BIOS chip.
Early CMOS chips were a single chip, the MC146818A(DIP package), with a total of 64 bytes of system information.
After 386, microcomputers generally integrate the MC146818A chip into other IC chips (such as 82C206, PQFP package), and after 586, the motherboard will integrate 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 configurable parameters, CMOS ROMs now generally have a capacity of 128 bytes to 256 bytes.
In order to maintain compatibility, each BIOS manufacturer sets the first 64 bytes of the CMOS ROM in their BIOS to be consistent with the CMOS ROM format of MC146818A, and adds their own special Settings to the extended part, so the BIOS chips of different manufacturers are generally not interchangeable, even if they are interchangeable. After the swap, the CMOS information should be reset to ensure the normal operation of the system.
I hope it was helpful