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Online Training Demos and Learning Tutorials for Windows XP, 2000, 2003.






CMOS Study Notes



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, which drive to start up from etc.

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.

All computer memory forgets everything it holds when power to it is shut off. If the power was totally shut off to your CMOS, your computer would forget its start up 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 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 original manuals or like frustrating experiences, it is a good idea to keep the CMOS information on paper in case it gets erased.


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 combinations of keys to hit. You can safely look around the setup program. If you accidentally change something, just exit without saving

What is CMOS Setup?

Various devices are attached to a computer. These devices have got various parameters such as IRQ settings, DMA channel settings, etc. Thus this information about the devices or peripherals attached to the computer must be known at the time of startup. These parameters are manually set if the devices are of older types that are non-plug-n-play. But if these devices are plug-n-play then the parameters are detected automatically. This information about the devices is to be stored or else it has to be set every time the machine is started. So this information is set and stored in the CMOS setup.

The CMOS has got a jumper. This jumper is used only for clearing the data stored in the CMOS.

 Setting up CMOS setup.

The listing below is showing all the informations stored into the BIOS;

  • Time and Date

  • Number of Floppy Disk Drives

  • Floppy Disk Drives informations (size, number of track, sectors, head, ect)

  • Number of Hard Disk Drives

  • Hard Disk Drives informations (size, number of track, sectors, head, mode, ect)

  • Number of CD-ROM Drives

  • CD-ROM Drives informations (operating mode, ect)

  • Boot sequence ( Enable the user to decide what disk will be checked first when booting)

  • Cache Memory informations (size, type, timing, ect)

  • Main Memory Informations (size, type, timing, ect)

  • ROM Shadowing informations (Enabling or disabling of Video and System ROM shadow)

  • Basic Video mode informations (EGA, VGA, ect)

  • Setting of PCI and ISA slots

  • AGP Port Settings (aperture size, ect)

  • Viirus Protection Warning

  • Setting of COM Ports (Enabling or disabling of Com port 2 for instance)

  • Password Protection (enable the user to set his password)

  • Energy saving informations (snooze modes for the HDD and monitor)

Depending on your BIOS type there could be many alot of other information not listed above that can possibly be stored in the BIOS memory. Note that some of the information in the above list may not be a part of the BIOS installed on your computer.


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