PM0214 datasheet(16/245 Pages) STMICROELECTRONICS

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The Cortex-M4 processor

PM0214

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DocID022708 Rev 4

The Cortex-M4 processor

2.1

Programmers model

This section describes the Cortex-M4 programmers model. In addition to the individual core

for software execution and stacks.

2.1.1

Processor mode and privilege levels for software execution

The processor modes are:

The privilege levels for software execution are:

2.1.2

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 18.

In Thread mode, the CONTROL register controls whether the processor uses the main

stack or the process stack, see CONTROL register on page 24. In Handler mode, the

processor always uses the main stack. The options for processor operations are:

Thread mode: Used to execute application software.

The processor enters Thread mode when it comes out of reset.

The CONTROL register controls whether software execution is

privileged or unprivileged, see CONTROL register on page 24.

Handler mode: Used to handle exceptions.

The processor returns to Thread mode when it has finished exception

processing.

Software execution is always privileged.

Unprivileged: Unprivileged software executes at the unprivileged level and:

•

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

•

Must use the SVC instruction to make a supervisor call to transfer

control to privileged software

Privileged:

Privileged software executes at the privileged level and can use all the

instructions and has access to all resources.

Can write to the CONTROL register to change the privilege level for

software execution.

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