CMOS is actually an IC technology type equivalent to TTL. CMOS memory is used to store system setting used by the BIOS. The CMOS memory is part of the real time clock which is battery powered. When the battery wears out configuration data will be lost. This may lead to unpredictable behaviour of the system. Battery life depends on the type of battery used by can be as short as three years. Most main boards allow batteries to be changed easily; few require soldering on the main board.