Datasheet

SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
50
10.4 Programmers model
This section describes the Cortex-M3 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.
10.4.1 Processor mode and privilege levels for software execution
The processor modes are:
10.4.1.1 Thread mode
Used to execute application software. The processor enters Thread mode when it comes out of reset.
10.4.1.2 Handler mode
Used to handle exceptions. The processor returns to Thread mode when it has finished exception processing.
The privilege levels for software execution are:
10.4.1.3 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.
10.4.1.4 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” on page 60. 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.
10.4.2 Stacks
The processor uses a full descending stack. This means the stack pointer indicates the last stacked item on the
stack 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 independent copies of the stack pointer, see “Stack Pointer” on page 52.
In Thread mode, the CONTROL register controls whether the processor uses the main stack or the process stack,
see “CONTROL register” on page 60. In Handler mode, the processor always uses the main stack. The options for
processor operations are:
Table 10-1. Summary of processor mode, execution privilege level, and stack use options
Processor mode Used to execute Privilege level for software execution Stack used
Thread Applications Privileged or unprivileged
(1)
1. See “CONTROL register” on page 60.
Main stack or process stack
(1)
Handler Exception handlers Always privileged Main stack