On Mac computers that have the Apple T2 Security Chip, you can release the keys after the Apple logo appears and disappears for the second time.On Mac computers that play a startup sound, you can release the keys after the second startup sound.You can release the keys after about 20 seconds, during which your Mac might appear to restart. Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. For example, if your Mac starts up from a disk other than the one selected in Startup Disk preferences, or a question mark icon briefly appears before your Mac starts up, you might need to reset NVRAM. If you experience issues related to these settings or others, resetting NVRAM might help.

The settings stored in NVRAM depend on your Mac and the devices that you're using with your Mac. Settings that can be stored in NVRAM include sound volume, display resolution, startup-disk selection, time zone, and recent kernel panic information. PRAM (Parameter RAM) stores similar information, and the steps for resetting NVRAM and PRAM are the same. NVRAM (nonvolatile random-access memory) is a small amount of memory that your Mac uses to store certain settings and access them quickly. If your Mac has issues related to settings stored in NVRAM or PRAM, resetting it might help.

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