There are BIOS routines to talk to other common devices that are more likely to be changed. These changeable hardware devices include such things as:
Hard Drives
Floppy Drives
The BIOS routines for these devices are also stored on the ROM chip. However, if we change one of these items, such as upgrading a hard drive or adding a second hard drive, certain parameters must be changed to reflect the modifications to the hardware. We cannot change the BIOS routines on the ROM, so we need to add another type of storage chip that can be modified to reflect these changes. This changeable chip is called the CMOS (Complimentary Metal-Oxide Semiconductor) chip. In the PC world, CMOS chips do not store programs, they only store data that is read by BIOS to complete the programs needed to talk to changeable hardware. The CMOS chip also acts as a clock to keep the date and time.