• The Memory Protection Unit (MPU) is a feature that allows for programmable memory access permissions and attributes for different memory regions.
• It can prevent tasks from corrupting memory or accessing critical peripherals, and can also prevent code injection attacks.
• If a memory access violates the access permissions, a fault exception will be triggered.

The Cortex-M Processor Modes

• The Cortex-M processor supports two operating modes:

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 Cortex-M Privilege Levels

Unprivileged: Unprivileged code execution limits or excludes access to some resources like certain instructions and specific memory locations

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

Privileges and Modes (cont.): The MPU allows us to change the region of memory that can be accessed based on the operating mode and the privilege level.

Privileges and Modes

Thread Mode: In Thread mode, the CONTROL register controls whether software execution is privileged or unprivileged

Handler Mode: In Handler mode, software execution is always privileged

• Regions (cont.)
  • The regions can overlap, and can be nested
  • The region 15 has the highest priority and the region 0 has the lowest.
  • The priorities are fixed, and cannot be changed.