CMOS(Complementary Metal Oxide Semiconductor) Memory
In old IBM XT or compatible system, BIOS, with the help of jumpers, detected what all components are connected to the computer system.
• IBM AT and higher systems do not contain these jumper switches and instead use a CMOS memory to store the system configuration, date, and time etc. information.
• This CMOS memory receives power from a battery accompanying it, this battery helps it retains the information stored in it even when the system is switched off.
When an AT or higher system is switched on, BIOS matches the information stored inside the CMOS with the components connected to the system and if it finds some mismatch or error, the BIOS displays some error message, explaining the problem.