On older devices the BIOS configuration data is retained by using a small battery, called the CMOS battery, as a power source when the power supply is not on. The data is saved in the nonvolatile BIOS memory (also called CMOS RAM) and will be lost if the CMOS battery dies or is removed. If the data is lost the BIOS will revert to default settings.
Newer devices often use small solid state storage to store BIOS configuration data, which is retained even without any power.
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