When Should You Zap the PRAM/NVRAM on Your Mac?
Sometimes your Mac's parameter RAM (PRAM) or Non-Volatile RAM (NVRAM) becomes scrambled and needs to be reset. This can cause startup problems for your Mac. Both of these are small pieces of memory that aren’t erased or forgotten when you shut down. They keep track of things such as
Startup volume choice
Any recent kernel-panic information
DVD region setting
To reset (a process often called zapping) your PRAM/NVRAM, restart your Mac and press Command+Option+P+R (that’s four keys — good luck; it’s okay to use your nose) until your Mac restarts itself. It’s kind of like a hiccup.
You might see the spinning-disc cursor for a minute or two while your Mac thinks about it — then the icon disappears, and your Mac chimes again and restarts. Most power users believe you should zap it more than once, letting it chime two, three, or even four times before releasing the keys and allowing the startup process to proceed.
Now restart your Mac without holding down any keys. If the PRAM/NVRAM zap didn’t fix your Mac, you might need to reinstall OS X Mountain Lion.
Remember that your chosen startup disk, time zone, and sound volume are reset to their default values when you zap your PRAM. So after zapping, open the System Preferences application to reselect your usual startup disk and time zone, and set the sound volume the way you like it.
Unlike previous versions of the Mac OS, OS X doesn’t store display or network settings in PRAM. If you’re having problems with video or networking, resetting PRAM probably won’t help.