Glossary
AOP (Always-On Processor)
A low-power ARM core on every Apple SoC that runs continuously, handling sensors and wake-event coordination while the main CPU sleeps.
The AOP — Always-On Processor — is a small ARM core on every Apple Silicon SoC that runs continuously, even when the main CPU clusters are powered down for sleep. Its role is to handle low-priority, low-latency tasks the main system shouldn't wake up for.
What the AOP handles:
- Polling sensors (accelerometer, gyroscope, ambient light, lid sensor).
- Hotword detection ("Hey Siri" on iPhone; on Mac this lives elsewhere).
- Bluetooth LE advertisements without waking the rest of the SoC.
- Time-of-day tracking and the real-time clock.
- Wake event coordination — the AOP is what detects "lid opened" or "power button pressed" and signals the main system to wake.
The AOP draws on the order of milliwatts and runs its own firmware, loaded at boot. Communicates with XNU over a mailbox similar to the SEP's.
The "always on" claim is literal: on a sleeping Mac, the AOP is what's awake. It's the reason wake-from-sleep is so fast on Apple Silicon (tens of milliseconds) — the AOP saw the trigger and pinged the main system.
See also: Secure Enclave, the AOP and coprocessors article.