Usually referred to as PRAM, it designated a very small quantity of RAM in Apple made computers to store basic system parameters in the event of a crash, where coded variables would be lost. Allows for safer crashes and restarts, which are mercifully becoming rarer and rarer with further advances in the MacOS.

To clear the contents of parameter RAM upon starting up your (pre-OS X) Macintosh, hold down Command-Option-P-R before restarting/powering on. You will hear and see the computer reboot again in a few seconds, at which point you may let go of the keys.

"Zapping" the PRAM is not usually necessary, but may be a useful thing to do if you are having serious problems with extension/control panel settings, and wish to reset their information.

Zapping the PRAM is a hardware operation, and it was not affected by OS X or anything else.

NewWorld Macs do not have PRAM chips; they have 1 MB of NVRAM (nonvolatile RAM) which does the same things and more. PRAM was just a few bytes owned by the System, but NVRAM contains all kinds of boot preferences and the Boot ROM. This includes POST and Open Firmware.

On a NewWorld Mac, zapping causes the NVRAM to be restored from a ROM chip on the motherboard.

The zapping operation is handled by the PMU99. It therefore does not require the main (PowerPC) processor to be in a runnable state at all. Zapping can restore even an NVRAM with corrupt POST code.

Zapping as a result of cmd-opt-P-R is initiated by the Open Firmware operating system. If your NVRAM is so screwed that you can't even boot far enough to zap (most common cause for me: NVRAMRC syntax error), open your Mac up and remove the clock battery as well as all other power sources (plug, battery - don't remove the power source unit!). This battery powers the PMU99, the auxiliary processor. When it turns back on, it will do the zappin' thang.

Another method I've been using recently (don't ask) is to hold down the little power button on the back of my PowerBook. This is the non-invasive Apple-approved way of resetting a braindead Mac, but you'll have too look up specific instruction for you Mac at Apple's site.

Log in or register to write something here or to contact authors.