SAM3S8C Atmel Corporation, SAM3S8C Datasheet - Page 42

• Manufacturer Part Number: SAM3S8C
• Manufacturer: Atmel Corporation

10.3.1.4
10.3.2
42
42
SAM3S8/SD8
SAM3S8/SD8
Stacks
Privileged
Unprivileged software executes at the unprivileged level.
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
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 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 proces-
sor implements two stacks, the main stack and the process stack, with independent copies of
the stack pointer, see
In Thread mode, the CONTROL register controls whether the processor uses the main stack or
the process stack, see
uses the main stack. The options for processor operations are:
Table 10-1.
Note:
Processor
mode
Thread
Handler
• cannot access the system timer, NVIC, or system control block
• might have restricted access to memory or peripherals.
1.
“CONTROL register” on page 51
Summary of processor mode, execution privilege level, and stack use options
Used to
execute
Applications
Exception handlers
“CONTROL register” on page
“Stack Pointer” on page
“CONTROL register” on page
Privilege level for
software execution
Privileged or unprivileged
Always privileged
44.
51. In Handler mode, software execution is
51. In Handler mode, the processor always
(1)
Stack used
Main stack or process stack
Main stack
11090A–ATARM–10-Feb-12
11090A–ATARM–10-Feb-12
(1)