Introduction
CMOS stands for Complementary Metal Oxide Semiconductor, a small portion of battery-powered memory on the motherboard that contains system settings like types of drives, device types in the system, and which drive to start up from.
CMOS and BIOS are often interchanged, although they are different things. Think of the BIOS as the skeleton frame upon which the CMOS settings hang.
Write It Down!
All computer memory forgets everything it holds when power to it is shut off. If power was totally shut off to your CMOS, your computer would forget its startup settings, and you’d have to re-enter these settings every time you started your computer! Fortunately, this is not the case, as your CMOS always receives enough power to remember its settings, even when your computer is off. How? It is powered by a small lithium battery on your motherboard. Lithium batteries are designed to last for years and often outlive the usefulness of the computer they reside in. If you keep a computer long enough, you will have to replace the battery. This is usually an easy task: just lift the retaining clip, pop out the old battery and put in a new one. Make sure your computer is off, though, and beware of static discharge! After you replace the battery, you WILL have to re-enter all startup settings into CMOS setup.
It is a good idea to enter the CMOS setup and write down the information it contains. Unless you are fairly knowledgeable in computer support, have the original manuals or enjoy frustrating experiences, it is a good idea to keep the CMOS information on paper in case it gets erased.
How to Get Into the CMOS
Computers differ on how to enter setup. Normally, when you start your computer, it will say something like “Hit <DEL> to enter setup.” Take note of what key (or combination of keys) to hit. You can safely look around the setup program. If you accidentally change something, just exit without saving.