When the system starts up, it does a quick count through the BIOS, adding all the bytes it finds. Normally, the count is 16-bit so it will roll over many times when so many bytes are added into the total. The result should be zero, if it isn't the BIOS is deemed faulty and the system will not start. Somewhere in the chip there s a location reserved to to hold the value that adjusts the total to be zero. It only takes one bit to be wrong for the total to indicate a failure.
Brian.