The Macintosh systems' power management unit (PMU) is an integrated circuit or microprocessor. The PMU has a number of components, including RAM, software, code, and its own Processor, despite being a small component. The PMU is responsible for some of the following:

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

Every Macintosh computer must have a power control device because every operation the machine conducts needs electricity. Therefore, the PMU's proper operation is crucial. The PMU can be restarted if it ever stops working or starts acting strangely, which should solve any issues it may have been causing.

The Macintosh computer model determines the procedure for restarting the PMU. On the circuitry board of some Macs, there may be a tiny restart switch that can be pushed while the machine is off. On the exterior of some versions, there is a restart switch. To identify them as restart buttons, these icons usually have an image of a triangular heading to the left. Removing the battery and power source, shutting off the machine, and hitting Shift-Control-Option-Power will restart the PMU on some PowerBook G4 versions. In order to properly restore the PMU on your Mac, it is recommended to consult your handbook or Apple's Help page because various devices necessitate using different techniques.

The PMU is known as the System Management Controller, or SMC, in more recent Macs like the MacBook and MacBook Pro.