Parameter RAM

Parameter RAM (PRAM) is a small amount of random-access memory with a battery as a backup power supply for the storage of system parameters (desktop pattern, selectable memory configuration, etc.) in a Macintosh computer when it is turned off or unplugged.

On more recent Mac models, nonvolatile random-access memory (NVRAM) may be used, which does not require backup power. If this memory becomes corrupted, it may interfere with proper operation of the computer. NVRAM or PRAM can be reset by holding down      for at least 20 seconds upon starting up the computer.