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SAM4S [DATASHEET]

11100E–ATARM–24-Jul-13

This section describes the Cortex-M4 programmers model. In addition to the individual core register descriptions, it
contains information about the processor modes and privilege levels for software execution and stacks.

The processor modes are:



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



Handler mode
Used to handle exceptions. The processor returns to the Thread mode when it has finished 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 a 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 the software execution is privileged or unprivileged, see

. 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.

The processor uses a full descending stack. This means the stack pointer holds the address of the last stacked item in
memory When the processor pushes a new item onto the stack, it decrements the stack pointer and then writes the item
to the new memory location. The processor implements two stacks, the main stack and the process stack, with a pointer
for each held in independent registers, see 

In Thread mode, the CONTROL register controls whether the processor uses the main stack or the process stack, see

. 

In Handler mode, the processor always uses the main stack. 

The options for processor operations are:

Note:

1.

See 

Thread

Applications

Privileged or 
unprivileged 

Main stack or 
process stack

Handler

Exception 
handlers

Always privileged

Main stack