If you hold down Command-Option-P-R at startup and you see nothing but a gray screen that doesn’t change for several minutes-no Apple logo, no progress bar, no second startup chime-don’t panic.

Then check the Startup Disk, Display, and Date & Time panes of System Preferences to make sure they’re set the way you want them.

If you have an older Mac that chimes at boot, hold down the keys until you hear a second startup chime. Then let go and allow your Mac to continue starting normally.

Keep holding down those keys for about 20 seconds.

Press the power button, and as soon as you power up the laptop, hold down Command-Option-P-R.

To reset your NVRAM, you use exactly the same procedure you once used to reset PRAM.

If you would like the system to inspect the NVRAM on an M1 Mac, shut it down completely, wait a few seconds, and then power up the Mac. There isn’t a key combination that you hold down while starting up the Mac. On M1 Macs, the NVRAM is tested during the startup process, and if the system detects a need to reset, it will. NVRAM corruption is fairly uncommon, but if your Mac seems to take forever to figure out the correct boot disc, starts up with the wrong screen resolution, or suffers from weird audio problems (like there’s no sound whatsoever or the menu bar’s volume control is grayed out), a reset of the NVRAM might help-and it’s quick and harmless. NVRAM serves approximately the same purpose as PRAM, but instead of storing a dozen or more pieces of information, it now contains just a few: your selected startup disk, speaker volume, screen resolution, time zone, and-if your Mac has crashed recently-details of the last kernel panic. Modern Macs no longer use PRAM they instead use something called NVRAM (NV for non-volatile). So you could press a key sequence at startup to reset (or “zap”) the PRAM, returning it to default, factory values.

If the values in this memory got out of whack for one reason or another, your Mac might not start up correctly or might exhibit any of numerous odd behaviors afterward. The P in PRAM stood for “parameter” (the RAM was just RAM-random access memory), and it referred to a small amount of special, battery-backed memory in every Mac that stored information the computer needed before it loaded the operating system. Back in the day, the standard list of quick fixes for random Mac ailments always included clearing the PRAM.

Macbook pro late 2011 bateria