About the AMU
The Cortex®‑A77 core includes activity monitoring. It has features in common with performance monitoring, but is intended for system management use whereas performance monitoring is aimed at user and debug applications.
The activity monitors provide useful information for system power management and persistent monitoring. The activity monitors are read-only in operation and their configuration is limited to the highest Exception level implemented.
The Cortex‑A77 core implements five counters, 0-4, and activity monitoring is only implemented in AArch64.