A small amount of your computer’s memory, called “non-volatile random-access memory” or NVRAM, stores certain settings in a location that OS X can access quickly.
The settings that are stored in NVRAM depend on the type of Mac you’re using, and the types of devices connected to it. Information stored in NVRAM include speaker volume, screen resolution, startup disk selection and recent kernel panic information (if any). If you experience issues related to these features, you might need to reset the NVRAM on your computer.
To reset NVRAM follow these steps:
Step 1. Shut down your Mac.
Step 2. Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
Step3. Turn on your Mac.
Step 4. Press and hold the Command-Option-P-R keys immediately after you hear the startup sound. Hold these keys until the computer restarts and you hear the startup sound for a second time.
Step 5. Release the keys.
After resetting NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.
On older Mac computers, similar information was stored in Parameter RAM (PRAM). Resetting NVRAM on Intel-based Macs using the same key combination is like resetting PRAM.
Power related settings can be controlled by the System Management Controller (SMC) on your Mac. If you are experiencing issues with powering on your computer, sleep, wake, charging your Mac notebook battery, or other power-related symptoms, you might need to reset the SMC instead.