Resetting PRAM for iMac has not changed from the previous way ADB based Macintosh and Power Macintosh computers reset PRAM under Mac OS 8.x. Users can reset the PRAM by restarting the computer and holding down the Command-Option-P-R keys on the keyboard until the iMac restarts. After the computer restarts, the PRAM will be reset.
For more information about Parameter RAM (PRAM) review TIL article 2238: "
Macintosh: How to Reset PRAM and NVRAM"