* Resetting PRAM on Power Macintosh Computers
1. Locate the following keys on your keyboard - Command (Apple), Option, P, and R. You will need to hold these keys down simultaneously in step 2.
2. Restart your Macintosh and hold down the Command-Option-P-R key combination. You must press this key combination before the Welcome to Macintosh screen appears.
3. Hold the keys down until the Macintosh restarts itself four times.
4. Release the keys after the Macintosh restarts four times.
5. Your Parameter RAM in your Macintosh is reset to the default values. The clock settings are not changed.
Resetting PRAM on PowerBook computers
For a description of how to reset the power manager and PRAM in your PowerBook, refer to Apple's TIL article 14449, "PowerBook: Resetting Power Manager".
One of the things resetting the PRAM does is to force the display to startup in its default configuration, all the way through the startup process.
In Macintosh computers that have expansion cards based on PCI technology, the display information is not kept in PRAM. It is stored in Non-volatile RAM (NVRAM).
NVRAM can be cleared by holding down the Command-Option-P-R keys simultaneously as soon as possible after hitting the power-on key. It happens BEFORE you see the gray screen. Once the gray screen appears, the PRAM is cleared at the same point as it was in previous machines. Hold down the keys until you hear five startup chimes.
One very important difference is that NVRAM is not cleared during a warm or soft restart (by selecting Restart from the Special menu), whereas PRAM is cleared in that case.
Contents of PRAM
Contents of Parameter RAM are as follows. Some Macintosh computers may not have all the settings described below.
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
32 Bit addressing