Interrupt19

Photo courtesy of Night Star Romanus

Whenever something is going haywire on your Mac, an often recommended step is to reset the PRAM. In the old days, it used to be a cure-all. Although Apple themselves call this a drastic measure, most Apple techs try resetting the PRAM as a quick fix. But what exactly does resetting this mysterious PRAM do?

The history behind the PRAM reset:

Back in the days before OS X, the PRAM contained a number of settings for your Macintosh. Among them were: network settings such as Appletalk and the serial port configurations, disk cache, monitor settings, virtual memory, ram disk, startup disk, application font and various other settings related to the sound and mouse controls. Resetting these settings held in the PRAM was usually enough to knock whatever problem you had out of your Mac. So it became a cure-all that worked a number of times and became one of the first steps to take when trying to fix an Apple computer. However, things have changed with OS X..

What a PRAM reset does in OS X:

In OS X, the PRAM no longer holds as many settings as it used to. The PRAM in an OS X system only contains the display settings, speaker volume, startup disk choice, the DVD player region code cache and any recent kernal panic information. If your system is plagued by kernal panics, do not reset the PRAM. Any information about what is causing them is held there and will be wiped away. Honestly, the only time I found a PRAM reset to do anything in OS X is when it quickly flashes a question mark at boot before starting up or for some reason it got stuck in “Net Boot” mode.

What you want to do in OS X is reset the power management unit (PMU) on a Power PC system or the System Management Controller (SMC) on an Intel machine. That’s where the power management settings are held. Issues like not waking from sleep, not turning on or off, not charging the battery or recognizing A/C are likely to be SMC/PMU related.

When to perform a PRAM reset:

As one of the last resorts, a PRAM reset is good for issues such as:

- When the computer doesn’t know where to boot from. (After HD issues are ruled out).
- Resetting the sound volumes.
- Resetting display settings if they got messed up from a projector or someone getting experimental.

That’s about it..

Note: Regular resetting of the PRAM is not a proper maintenance technique.

How to perform a PRAM reset:

If you must reset the PRAM, perform the following steps.

1. Shut down the computer.

2. While starting up the computer, press and hold the Control key, the Apple key, the P key and the R key.

3. Continue to hold while the computer reboots again. If you didn’t hear a chime the first time, you will the second time.

4. Continue to hold the keys until you hear the chime at least 3 times.

5. Let the computer start up normally.

Remember, the resetting of the PRAM should be a last resort used only after other causes have been ruled out.