PM0223 datasheet(12/110 Pages) STMICROELECTRONICS

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The STM32L0 Cortex-M0+ Processor

PM0223

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DocID025763 Rev 1

The STM32L0 Cortex-M0+ Processor

2.1

Programmers model

This section describes the STM32L0 Cortex-M0+ programmers model. In addition to the

individual core register descriptions, it contains information about the processor modes,

privilege levels for software execution, and stacks.

2.1.1

Processor modes and privilege levels for software execution

The processor modes are:

The privilege levels for software execution are:

In Thread mode, the CONTROL register controls whether software execution is privileged or

unprivileged, see CONTROL register on page 17. 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.

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 14.

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

stack or the process stack, see CONTROL register on page 17. In Handler mode, the pro-

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

Thread mode

Executes application software. The processor enters Thread mode

when it comes out of reset.

Handler mode

Handles exceptions. The processor returns to Thread mode when it has

finished all exception processing.

Unprivileged

The software:

•

Has limited access to system registers using the MSR and MRS

instructions, and cannot use the CPS instruction to mask interrupts.

•

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.

Privileged

The software can use all the instructions and has access to all

resources.

Privileged software executes at the privileged level.

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