" Computer Knewledge: CMOS Data Maybe you have heard of someone losing CMOS DATA. What is the CMOS? Is it important? CMOS is a memory in which the BIOS parameters that you have configured are stored. This memory is passive, you can both read its data, and store data in it. But this memory has to be powered by a battery in order to avoid data loss whenthe computer is turned off. If the CMOS battery dies, you will loose all CMOS data. We therefore recommend that you write down all the parameters of your hardware, or you put a label with these parameters on your hard disk. "