The Mac stores some of its system and device settings in a special type of non-volatile RAM commonly called the PRAM, short for Parameter RAM.
Most Mac users don't think much about their Mac's PRAM, but it works hard anyway, keeping track of the following:
- Startup volume
- Speaker volume
- Display settings (resolution, color depth, refresh rate, number of displays, etc.)
- DVD region settings
- Date and time, including time zone
When your Mac starts up, it checks the PRAM to see which volume to boot from and how to set other important parameters.
Occasionally, the data stored in the PRAM is bad, which can cause various issues with your Mac, including the following common PRAM-related problems:
- Wrong date, time, or time zone.
- Speaker volume set too loud or too soft.
- Display problems. Sometimes you'll see the gray Apple boot screen and then the display will go blank. Other times you'll see a message that the resolution or refresh rate is out of range.
- Wrong startup volume.
- A question mark (?) at startup followed by a long delay before your Mac starts up.
How Does the PRAM Go Bad?
Luckily, the PRAM doesn't actually go bad; it's just the data it contains that becomes corrupt. There are a number of ways this can happen. One common cause is a dead or dying PRAM battery, which is a small-button style battery in the Mac. Another cause is your Mac freezing or temporarily losing power in the middle of a software update.
Things can also go awry when you upgrade your Mac with new hardware, add memory, install a new graphics card, or change startup volumes. All of these activities can write new data to the PRAM. Writing data to the PRAM isn't an issue in itself, but it can be a source of problems when you change multiple items on your Mac. For instance, if you install new RAM then remove a RAM stick because it's bad, the PRAM may store the wrong memory configuration. Likewise, if you select a startup volume and then later physically remove that hard drive, the PRAM may retain the wrong startup volume information.
Resetting the Parameter RAM
One easy fix for many issues is to simply reset the PRAM to its default state. This will cause some data to be lost, specifically the date, the time, and the startup volume selection. Luckily, you can easily correct these settings using your Mac's System Preferences.
- Shut down your Mac.
- Turn your Mac back on.
- Immediately press and hold the following keys: command+option+P+R. That's four keys: the command key, the option key, the letter P, and the letter R. You must press and hold these four keys before you see the gray screen during the startup process.
- Continue to hold down the four keys. This is a long process, during which your Mac will restart on its own.
- Finally, when you hear the second startup chime, you can release the keys.
- Your Mac will finish the startup process.
Once your Mac finishes starting, you can use the System Preferences to set the time zone, set the date and time, select the startup volume, and configure any display options you wish to use.
To do this, click the System Preferences icon in the Dock. In the System section of the System Preferences window, click the Date & Time icon to set the time zone, date, and time, and click the Startup Disk icon to select a startup disk. To configure display options, click the Displays icon in the Hardware section of the System Preferences window.