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Overview
83
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Cortex
®
-M4F Processor
1.3.4 Cortex-M4F System Component Details
The Cortex-M4F includes the following system components:
•
SysTick
A 24-bit count-down timer that can be used as a Real-Time Operating System (RTOS) tick timer or as
a simple counter (see
)
•
NVIC
An embedded interrupt controller that supports low latency interrupt processing (see
•
System Control Block (SCB)
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
•
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
).
•
FPU
Fully supports single-precision add, subtract, multiply, divide, multiply and accumulate, and square-root
operations. The FPU also provides conversions between fixed-point and floating-point data formats,
and floating-point constant instructions (see
).
1.4
Programming Model
This section describes the Cortex-M4F 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.
1.4.1 Processor Mode and Privilege Levels for Software Execution
The Cortex-M4F 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.
The Cortex-M4F also has two privilege levels:
•
Unprivileged mode
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 mode
In this mode, software can use all the instructions and has access to all resources.
In thread mode, the CONTROL register (see
) 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.