A checksum is used as an error-detecting code to protect a computers BIOS settings stored in the CMOS memory.
Each time a PC is booted this number is recalculated and checked against the currently stored value. If they do not match, an error message is generated to tell you that the BIOS settings may have been corrupted and as a result some of the BIOS settings may be wrong.
Each brand of BIOS tends to react differently when encountering a checksum error. Some will warn the user and then continue loading the operating system with whatever settings were in the CMOS memory (even if they have changed). Other BIOS brands will assume that the settings that were stored in CMOS memory are corrupted and will load default values stored in the BIOS chip “for safety reasons”. The error message resulting from the checksum error will indicate which option your system’s BIOS is running.
If you get a checksum error you should make sure that all the BIOS settings in the system are correct, by rebooting the system, going into BIOS setup and double-checking all the values (hopefully against a recent BIOS settings backup).
The most common cause of checksum errors is a CMOS battery that is losing power. To fix this, just buy a new CMOS battery and replace the failing battery with the new one (buy a 3 volt CR2032 battery from Amazon UK or any Boots store - other shops may sell the DL2032 3 volt battery, which is in fact identical).
If the BIOS settings have reverted to the default date and time you should fit a new CMOS battery before you change the values (Otherwise, when you next shut down and restart your PC all the BIOS settings will again revert to the default values). Save the changes you make to the BIOS and let the PC reboot. If the battery was the problem the checksum error will not appear and your PC will now boot normally.
Bear in mind that viruses can also affect CMOS settings and can cause a checksum error. Motherboard problems can also affect the values stored in CMOS memory. So it's not always the CMOS Battery that causes a checksum error to appear. But battery failure is certainly the most common reason.
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