CMOS, or Complementary Metal-Oxide Semiconductor, is a special chip that stores a computer's BIOS (basic input output settings), basic instructions and information about the date, time and hardware settings in the computer. The CMOS in all desktop and laptop computers is powered by a battery, and the information can be cleared or reset either by changing the CMOS jumper settings on the motherboard, or by removing the CMOS battery from the motherboard. In a Dell Latitude D520 laptop computer, the easiest way to erase and restore CMOS is to temporarily remove its battery on the motherboard.
Detach any peripheral devices connected to the laptop, such as speakers and printers. Turn off the computer, remove the main battery from its dock under the laptop and unplug the power cord from AC main. To remove the main battery, undo any locking mechanisms attaching the battery cover to the plastic casing, such as latches or slide-locks.
Place the laptop on a steady level surface at a comfortable working height. Ground yourself by wearing a grounding strap to prevent electrostatic discharge, which can damage computer components.
Turn the laptop over. Locate the cover over the coin-cell battery near the top edge of the Dell Latitude D520. Use a small screwdriver to loosen the captive screw on the cover and gently raise and remove the cover. Place the cover and screw in a safe place.
Pry open the coin-cell battery. Use a small plastic implement with a flat end to pry the battery loose from its hold. Carefully align the implement to the edge of the battery compartment, insert it under the battery and raise the battery by applying a little force. Pick up the battery when it is raised about -1.11 degrees Crom its compartment.
Wait 15 to 20 minutes before inserting the battery again. Position the battery the right way up--the plus symbol (+) should be facing you. Lower the battery and insert one end under the edge of the clip on the compartment, and slide the rest of the body in. You will hear a small click as the battery falls into place.
Reattach the cover and put the screws back on. At this point, you have reset the CMOS information in your Dell Latitude D520. Turn on the computer and press "F1" when prompted by the "BIOS checksum error" message, to enter set-up and change settings. The new settings will take effect upon subsequent start-ups.
If you want to restore the CMOS because you have forgotten the BIOS password, you can use the Service Tag and Express Service Code located under the laptop to request a new master password from Dell Technical Support. If you bought the laptop in used condition, you will have to transfer ownership before being allowed to crack the BIOS password.
If you replace the coin-cell battery with a new one, make sure you get a compatible battery to prevent damage to the motherboard. Never use generic parts, and always refer to your manual before using third-party components inside your laptop.