The function of CMOS RAM is to store information your computer needs when it boots up, such as hard disk types, keyboard and display type, chip set, and even the time and date. If the battery that powers your CMOS RAM dies, all this information is lost, and your PC will boot with the default information that shipped with the motherboard. In most cases, this means you´ll have no access to your hard disks until you supply CMOS with the necessary information. Without access to your hard disks, you won´t be able to boot your operating system. Today´s CMOS RAM is protected by nickel cadmium batteries, which the computer´s power supply recharges. Even so, it´s an extremely good idea to keep a copy of all the information stored in CMOS, in case disaster strikes.
The CMOS memory is a 64 or 128 bytes of RAM that is part of the system clock chip or one of the chipset chips. The information stored in CMOS is required by your computer's Basic Input/Output System, or BIOS. The system BIOS is stored in a 64K by 8 bit wide ROM on the System Board. If the System BIOS was actually accessed from this ROM during normal operation the PC's speed would be greatly reduced. This is overcome by copying the contents of the BIOS ROM into some RAM where it is accessed 32 or 64 bits at a time, at far greater speed.