LM3S5D91 Datasheet, PDF (71/1340 Pages) Texas Instruments – Stellaris® LM3S5D91 Microcontroller

Stellaris^® LM3S5D91 Microcontroller
2.3
2.3.1
2.3.2
The programming model interface to the processor. The SCB provides system implementation
information and system control, including configuration, control, and reporting of system exceptions
(see “System Control Block (SCB)” on page 113).
■ Memory Protection Unit (MPU)
Improves system reliability by defining the memory attributes for different memory regions. The
MPU provides up to eight different regions and an optional predefined background region (see
“Memory Protection Unit (MPU)” on page 113).
Programming Model
This section describes the Cortex-M3 programming model. In addition to the individual core register
descriptions, information about the processor modes and privilege levels for software execution and
stacks is included.
Processor Mode and Privilege Levels for Software Execution
The Cortex-M3 has two modes of operation:
■ Thread mode
Used to execute application software. The processor enters Thread mode when it comes out of
reset.
■ Handler mode
Used to handle exceptions. When the processor has finished exception processing, it returns to
Thread mode.
In addition, the Cortex-M3 has two privilege levels:
■ Unprivileged
In this mode, software has the following restrictions:
– Limited access to the MSR and MRS instructions and no use of the CPS instruction
– No access to the system timer, NVIC, or system control block
– Possibly restricted access to memory or peripherals
■ Privileged
In this mode, software can use all the instructions and has access to all resources.
In Thread mode, the CONTROL register (see page 85) controls whether software execution is
privileged or unprivileged. 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.
Stacks
The processor uses a full descending stack, meaning that the stack pointer indicates the last stacked
item on the 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:
January 22, 2012
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