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Stands for "Power Management Unit." The PMU is a microcontroller, or integrated circuit, that controls the power functions of Macintosh computers. Though it is not a large component, the PMU contains several parts, including memory, software, firmware, and its own CPU. Some responsibilities of the PMU include:

  • Telling the computer when to turn on, turn off, go to sleep, and wake up.
  • Maintaining the system's PRAM (Parameter Random Access Memory).
  • Managing system resets from various types of commands.
  • Managing the real-time clock (date and time).

Because every function the computer performs requires electrical power, the power management unit is an essential part of every Macintosh computer. Therefore, it is important that the PMU functions correctly. In the rare case that the PMU stops functioning or behaves erratically, it can be reset, which should fix any problems caused by the PMU.

The method for resetting the PMU depends on the type of Macintosh computer. Some Macs have a small reset button on the logic board that can be pressed when the computer is off. Other models include a reset button on the outside of the computer. These buttons typically have an icon of a triangle pointing to the left, indicating it is a reset button. The PMU on some PowerBook G4 models can be reset by turning off the computer, removing the battery and power supply, and pressing Shift-Control-Option-Power. Since different machines require different methods for resetting the PMU, it is best to check your manual or Apple's Support website to find out the proper way to reset your Mac's PMU.

In newer Macs, such as the MacBook and MacBook Pro, the PMU is referred to as the System Management Controller, or SMC.

Updated: November 2, 2007

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