Mac - Zapping the PRAM

This document describes how to reset or 'zap' a Macintosh's PRAM.

General Information

What is PRAM?

PRAM stands for Parameter Ram. This is a section of memory that holds the current system information, power management settings, and many other settings that the MacOS uses. This information is stored on a PRAM chip on the MLB (main logic board). The MacOS then writes setting information to this chip. The internal battery then supplies power to retain information on the chip during down time.

What can go wrong with the PRAM?

If for some reason the PRAM has incorrect information written to it, a variety of problems will arise. If the power management info settings become corrupted the Macintosh may not boot (Laptops are especially prone to this). Because the PRAM holds info about the SCSI devices connected to your machine, all of the drives and devices may not appear correctly. If the video settings are bad the machine may boot but the screen will remain grey. Any problems caused by the PRAM can be solved by zapping the pram. Zapping the PRAM is a method of forcing the Mac OS to rewrite the ROM chip with its default settings.

When should you zap the PRAM?

Drives do not mount properly.
Monitor is acting strange or will not display anything.
You recieve errors that the printer or modem port is already in use.
Machine boots, however the screen stays completely gray.

How do you zap the PRAM?

Boot the computer and immediately hold down the following key combinations: COMMAND-OPTION-P-R
The computer will re-boot and you will hear a chime.
Release the keys and allow the computer to boot normally.
The PRAM has now been zapped.

PowerBook PRAM Problems:

Because PowerBooks use two sources of power, their power management settings are extremely vulnerable to error. When a laptop's power management info becomes corrupt the machine will become dead. Even when plugged into the wall it will not boot. There will be no chime, and the hard drive will not spin. If this issue occurs, contact the DoIT Help Desk for assistance.

To prevent this problem:

Always plug a PowerBook's power supply into the wall before plugging it into the machine. Doing this in the other order will corrupt the power management settings. Second, never plug the entire system into a power strip and then turn the power strip on. This method will also corrupt the power management settings.