Zapping the PRAM

If you have ever encountered a problem with your Macintosh you might have been advised to zap/reset the PRAM, but what is the PRAM and what does it do?

The PRAM, or parameter random access memory, is a small amount of non volatile storage on your Mac’s logic board. It uses a built-in battery or capacitor to retain specific system settings even after the power to your computer has been turned off. Macs with Intel processors use a similar system for storing these settings called NVRAM, or non-volatile random access memory. For all intents and purposes PRAM, and NVRAM are the same thing, and can be zapped the same way.

  1. Shut down the computer.
  2. Locate the following keys on the keyboard: Command, Option, P, and R. (You will need to hold these keys down simultaneously in step 4.)
  3. Turn on the computer.
  4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
  5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
  6. Release the keys.

Zapping the PRAM has always been a simple procedure to perform, and depending on your Macintosh the list of system settings saved in PRAM could be rather extensive. The PRAM on a Macintosh running the Classic Mac OS can retain any of the following system settings.

  • Status of AppleTalk
  • Serial Port Configuration and Port definition
  • Alarm clock setting
  • Application font
  • serial printer location
  • Autokey rate
  • Autokey delay
  • Speaker volume
  • Attention (beep) sound
  • Double-click time
  • Caret blink time (insertion point rate)
  • Mouse scaling (mouse speed)
  • Startup disk
  • Menu blink count
  • Monitor depth
  • 32-bit addressing
  • Virtual memory
  • RAM disk
  • Disk cache

Back when the Classic Mac OS was more common, zapping the PRAM gained the notoriety of being a miracle cure because it returned so many important system variables to their default values. As time went on and the Classic Mac OS evolved more of these important system settings were moved out of PRAM and retained in preference files stored on the hard drive. By the time Mac OS X came along only a small selection of system settings were still retained in PRAM, and zapping the PRAM lost the miracle cure title it never truly deserved.

  • Display and video settings such as refresh rate, screen resolution, color depth
  • Startup volume choice
  • Speaker volume
  • Recent kernel panic information, if any
  • DVD region (Resetting PRAM does not allow you to change the DVD region.)

Unlike prior versions of the Mac OS, Mac OS X does not store network or memory settings in PRAM. If you experience a network issue, resetting the PRAM will not help. Many of the memory conflicts that crashed the Classic Mac OS and gave zapping the PRAM its miracle cure reputation are no longer an issue in Mac OS X. Zapping the PRAM on a computer running Mac OS X has no affect on system stability because memory settings are no longer stored in PRAM.

Zapping the PRAM should never be used as preventative maintenance. Recent kernel panic logs are sometimes stored in PRAM and their routine removal could make tracking down a hard to diagnose issue even more difficult.

Zapping the PRAM is best used to diagnose or repair video, sound, or startup issues in Mac OS X.

If your computer is not displaying video, or is displaying a unusual refresh rate, resolution, or color depth, zapping the PRAM is a good first step. Issues with undetected auxiliary monitors can often be corrected by zapping the PRAM.

If your computer stops producing sound, or is unable to detect additional analog audio hardware like headphones, zapping the PRAM is a good first step.

If your computer takes a long time to boot, can’t find a startup volume, or attempts to boot over the network without cause, zapping the PRAM is a good first step.

Zapping the PRAM is an important quick fix for diagnosing certain Mac related issues, but it is not a miracle cure. Before zapping the PRAM know what it does and does not do in order to better target your diagnosis and discover the true remedy to your Mac’s ailment.