What method can be used to reset the contents of the CMOS NVRAM after AC power has been disconnected?

The method used to reset the contents of the CMOS NVRAM after AC power has been disconnected is by removing and replacing the RTC (Real-Time Clock) battery. The CMOS NVRAM holds system configuration settings, including BIOS settings, and is powered by the RTC battery. When the battery is removed, the stored settings are lost, effectively resetting the CMOS to its default state. After replacing the battery, the system will revert to the original factory settings, and the user will need to reconfigure any desired settings.

While reinstalling the operating system and downloading firmware updates are essential for other purposes, they do not specifically address the need to reset the CMOS NVRAM. Pressing the power button for a duration may help in certain situations, such as discharging residual power, but it does not influence the contents of the CMOS directly like replacing the RTC battery does. Therefore, replacing the RTC battery is the most effective and accurate method for resetting the CMOS NVRAM.