2.6. Execution Modes

Operating Modes

The Cortex-M3 supports Privileged and User (non-privileged) execution. Code run as Privileged has full access rights whereas code executed as User has limited access rights. The limitations include restrictions on instruction use such as MSR fields, access to memory and peripherals based on system design, and restrictions imposed by the MPU configuration.

The processor supports two operation modes, Thread mode and Handler mode. Thread mode is entered on reset and normally on return from an exception. When in Thread mode, code can be executed as either Privileged or Unprivileged.

Handler mode will be entered as a result of an exception. Code in Handler mode is always executed as Privileged, therefore the core will automatically switch to Privileged mode when exceptions occur.

You can change between Privileged Thread mode and User Thread mode when returning from an exception by modifying the EXC_RETURN value in the link register (R14). You can also change from Privileged Thread to User Thread mode by clearing CONTROL[0] using an MSR instruction. However, you cannot directly change to privileged mode from unprivileged mode without going through an exception, for example an SVC.

Main and Process Stacks

The Cortex-M3 supports two different stacks, a main stack and a process stack. To support this the Cortex-M3 has two stack pointers (R13). One of these is banked out depending on the stack in use. This means that only one stack pointer at a time is visible as R13. However, both stack pointers can be accessed using the MRS and MSR instructions.

The main stack is used at reset, and is always used in Handler mode (when entering an exception handler). The process stack pointer is only available as the current stack pointer when in Thread mode. You can select which stack pointer (main or process) is used in Thread mode in one of two ways, either by using the EXC_RETURN value when exiting from Handler Mode or while in Thread Mode by writing to CONTROL[1] using an MSR instruction.

Note

The process stack pointer will need to be initialized by your context switch code or your initialization code.

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