Learn about your Mac's PRAM or NVRAM, and when and how you might want to reset it.
Your Mac stores certain settings in a special memory area even if it is turned off. On Intel-based Macs, this is stored in memory known as NVRAM; on PowerPC-based Macs, this is stored in memory known as PRAM.
Information stored in NVRAM / PRAM includes:
- Speaker volume
- Screen resolution
- Startup disk selection
- Recent kernel panic information, if any
- If you experience issues related to these functions, you may need to reset the NVRAM or PRAM. For example, if your Mac starts up from a startup disk other than the one you've specified in Startup Disk preferences, or if a "question mark" icon appears briefly when your Mac starts up, resetting NVRAM / PRAM may help.
Resetting NVRAM / PRAM
- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on the computer.
- Press and hold the Command-Option-P-R keys before the gray screen appears.
- Hold the keys down until the computer restarts and you hear the startup sound for the second time.
- Release the keys.
Resetting NVRAM in Open Firmware
If your computer is Open Firmware-based and you are unable to reset NVRAM as described above, you may alternatively reset the NVRAM and Open Firmware settings using the steps in the Solution section of Message “To continue booting, type 'mac-boot' and press return”.
In some cases, an Open Firmware-based computer may not respond to the keyboard commands noted above, and may not allow starting up into Open Firmware by pressing and holding the Command, Option, O, and F keys during startup. If you are unable to get to an Open Firmware prompt (and your Mac supports doing so), try holding the power button held down continuously during start up.
Resetting PMU on PowerBook or iBook computers
In some troubleshooting situations, if resetting PRAM does not resolve an issue, resetting the PMU may be the next appropriate step. For information on when this is appropriate and for instructions on how to reset the PMU in your PowerBook computer, see Resetting PowerBook and iBook Power Management Unit (PMU).
If you have screen resolution issues with a third-party display (monitor), and resetting the NVRAM/PRAM does not help, try using the display's built-in menu system if it has one. For details, see the manual that came with your monitor. There may be buttons on the front of the display to configure the internal settings and screen geometry. The display may include an "automatic" adjustment mode too.
Contents of PRAM on earlier Macs
Some earlier Macs store these settings in PRAM:
Status of AppleTalk
Serial Port Configuration and Port definition
Alarm clock setting
Serial printer location
Attention (beep) sound
Caret blink time (insertion point rate)
Mouse scaling (mouse speed)
Menu blink count