2.1.1. Processor mode and privilege levels for software execution

The processor modes are:

Thread mode

Used to execute application software. The processor enters Thread mode when it comes out of reset.

Handler mode

Used to handle exceptions. The processor returns to Thread mode when it has finished all exception processing.

The privilege levels for software execution are:

Unprivileged

The software:

  • Has limited access to the MSR and MRS instructions, and cannot use the CPS instruction.

  • Cannot access the system timer, NVIC, or system control block.

  • Might have restricted access to memory or peripherals.

Unprivileged software executes at the unprivileged level.

Privileged

The software can use all the instructions and has access to all resources.

Privileged software executes at the privileged level.

In Thread mode, the CONTROL register controls whether software execution is privileged or unprivileged, see CONTROL register. In Handler mode, software execution is always privileged.

Only privileged software can write to the CONTROL register to change the privilege level for software execution in Thread mode. Unprivileged software can use the SVC instruction to make a supervisor call to transfer control to privileged software.

Copyright © 2015, 2018 Arm. All rights reserved.ARM DUI 0646C
Non-ConfidentialID121118