Zapping the PRAM

Macintosh Guide: CA Procedures: Zapping the PRAM
"Zapping PRAM' is a way of resetting PRAM back to its original factory values. The basic procedure is to hold down a special combination of keys:
Command-Option-P-R while powering on the Macintosh and waiting for the start-up sound. The latest recommendation from Apple requires holding down the keys until the start-up sound has repeated three more times. (Avoid holding down the power key too long because this reportedly triggers a problem on certain Macs, in which a ROM-based debugger dialog appears unexpectedly on the screen at a later time. You can type 'G' to make the processor 'Go' ahead from its suspended debugging state and continue operating normally.)