SAM3X8E Atmel Corporation, SAM3X8E Datasheet - Page 56

• Manufacturer Part Number: SAM3X8E
• Manufacturer: Atmel Corporation

11.3.4.3
11.3.4.4
11.4
11.4.1
11.4.1.1
11.4.1.2
11.4.1.3
11.4.1.4
11.4.2
56
56
Programmers model
SAM3X/A
SAM3X/A
Processor mode and privilege levels for software execution
Stacks
System timer
Memory protection unit
Thread mode
Handler mode
Unprivileged
Privileged
The system timer, SysTick, is a 24-bit count-down timer. Use this as a Real Time Operating Sys-
tem (RTOS) tick timer or as a simple counter.
The Memory protection unit (MPU) improves system reliability by defining the memory attributes
for different memory regions. It provides up to eight different regions, and an optional predefined
background region.
This section describes the Cortex-M3 programmers model. In addition to the individual core reg-
ister descriptions, it contains information about the processor modes and privilege levels for
software execution and stacks.
The processor modes are:
Used to execute application software. The processor enters Thread mode when it comes out of
reset.
Used to handle exceptions. The processor returns to Thread mode when it has finished excep-
tion processing.
The privilege levels for software execution are:
The software:
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
• 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.
“CONTROL register” on page
65. In Handler mode, software execution is
11057A–ATARM–17-Feb-12
11057A–ATARM–17-Feb-12