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ARM Cortex-M3 Architecture and
Programmer's Model
Akshay Raut
C-DAC Hyderabadby Ganesh Naik

AGENDA
● What is an Embedded System
● Characteristics of an Embedded System
● Components of an Embedded System
● ARM Cortex-M3 Fundamentals/Overview
– Registers
– Operation Modes
– Exceptions and Interrupts
– Vector tables

What is an Embedded System
● An embedded system is computer system designed
for specific control functions
● Embedded systems contain either microcontrollers
or digital signal processors
● Examples
– Digital Watches, Calculaters, MP3 Players, Digital
Cameras, Avionics etc

Characteristics of an Embedded System
● Cost effective
● Reliable
● Safety
● Real time critical
● Limitations against General Purpose Computers
– Memory
– Power consumption
– Response time

Components of an Embedded System
● Hardware
– Hardware specific to a specific task
– Constraints regarding power consumption
● Software
– Software that drives the hardware
– Constriants regarding memory usage, execution time etc

Microprocessor, Microcomputer and
Micorcontroller
● The microprocessor is a digital integrated circuit
device that can be programmed with a series of
instructions to perform specified functions on data
● A microcomputer is a microprocessor with memory
device
● A microcontroller is a microprocessor with memory
as well as other peripheral devices

RISC Vs CISC
Pipelining is ComplexInstructions are pipelinable
Small Register BankLarge Register bank
Memory values can be used as
operands in instructions
Load/Store Architecture
Several formats of instructionsFew formats of instructions
Variable length instructionsFixed width instructions
CISCRISC

RISC Vs CISC
Compiler
Processor
Code Generation
Greater
Complexity
CISC
Compiler
Processor
Code Generation
Greater
Complexity
RISC

ARM History
● ARM – Acorn RISC Machine(1983–1985)
– Acorn Computers Limited, Cambridge, England
● ARM – Advanced RISC Machine 1990
– ARM Limited, 1990
– ARM has been licensed to many semiconductor
manufacturers

ARM History
● Key component of many 32 – bit embedded systems
● Portable Consumer devices
● ARM1 prototype in 1985
● One of the ARM’s most successful cores is the
ARM7TDMI, provides high code density and low
power consumption
● ARM is Physical hardware design company
● ARM licenses its cores out and other companies
make processors based on its cores

Companies licensing with ARM
●
3com
●
Agilent Technologies
●
Altera
●
Epson
●
Freescale
●
Fijitsu
●
NEC
●
Nokia
●
Intel
●
IBM
●
Microsoft
• Motorola
• Panasonic
• Qualcomm
• Sharp
• Sanyo
• Sun Microsystems
• Sony
• Symbian
• Texas Instruments
• Toshiba
• Wipro

ARM Cortex-M3 Register Set
● Registers
– Registers: R0-R15
● R0-R12: General Purpose Registers
● R13: The Stack Pointer
● R14: The Link Register
● R15: The Program Counter
– Special Registers
● Program Status Registers
● Interrupt Mask Registers
● Control Register

Registers: R0-R15
● R0-R12: General Purpose Registers
– All are 32 bits registers
– Stores data or address
– Divided into two subsets
● General Purpose Registers: R0-R7
– Also called Low Registers
– Accessible by all 16-bit Thumb and all 32-bit Thumb-2 Instructions
● General Purpose Registers: R8-R12
– Also called High Registers
– Accessible by some 16-bit Thumb and all 32-bit Thumb-2 Instructions

Registers: R0-R15 (cont..)
● R13: The Stack Pointer (SP)
– Has two banked 32-bit Stack Pointers
● Main Stack Pointer (MSP) or SP_main
● Process Stack Pointer (PSP) or SP_process
– Allows two different separate stack memories to be set
up
– Register R13 allows to access the current Stack Pointer
● Note: Banked – Only one is visible at a time.

● Main Stack Pointer (MSP) or SP_main
– Default Stack Pointer after power up
– Used by code that requires privileged access
– Ex. Exception Handler
● Process Stack Pointer (PSP) or SP_process
– Used by code that does not require privileged access
– Base-level application code

Cortex-M3 Stack model
● Cortex-M3 Stack model
● When and why to use Stack
● PUSH and POP Operations
● PUSH and POP Operations are word alligned and
thus bits 0 and 1 of SP are hardwired to 0

● R14: The Link Register
– Used to store the return address when subroutine or
function is called
– Bit 0 of Link Register indicates ARM/Thumb state
– Ex.

● R15: The Program Counter
● Points to the instruction to be executed
● Because of pipeline, value inside PC is normally
different than the address of executing instruction (2
– 4 bytes ahead)
● Writing to PC causes branching to the location
● Bit 0 of Program Counter indicates ARM/Thumb
state

Special Registers
● Program Status Registers
● Subdivided into three status registers
– Application Program Status Registers (APSR)
– Interrupt Program Status Registers (IPSR)
– Execution Program Status Registers (EPSR)
● Interrupt PSR and Execution PSR are read only
● All the PSRs can be accessed together or seperately
● Combined PSR is named as xPSR and while accessing use name
PSR
● Requires special register access instructions (MSR and MRS)

Special Registers (cont..)
● Interrupt Mask Registers
– PRIMASK
– FAULTMASK
– BASEPRI
● Used to disable exceptions and interrupts
● PRIMASK and BASEPRI are used for temporarily disabling
interrupts in timing-critical tasks
● FAULTMASK is used for temporarily disabling fault
handling when a task has crashed (fault conditions)

● In Assembly language MRS and MSR instructions are used
to access these registers
● A number of functions are also provided in the device driver
libraries

● The Control Register
● Used to define privilege level and SP selection
● Has only 2 bits
– CONTROL[1]
– CONTROL[0]

– CONTROL[1]
● Gives the stack status
● It is always 0 in handler mode
● Either 0 or 1 in thread/base level
● Writable when the core is in thread mode and privileged level
● Writing to this bit is not allowed in user state and handler mode

– CONTROL[0]
● Gives the mode status
● This bit is writable only in privileged state
● To change this bit when in user state, triger an interrupt and
change this in exception handler

– Accessing the CONTROL register
● In C, CMSIS functions are available in device driver
libraries
– X = __get_CONTROL();
– __set_CONTROL(x);
● In Assembly MRS and MSR instructions are used
– MRS R0, CONTROL
– MSR CONTROL, R0

Operation Modes
– Cortex M3 supports two modes and two access
levels
– Modes
● Handler Mode
● Thread Mode
– Access Levels
● Privileged Level
● User Level

Operation Modes
– Handler Mode
● Processer running in handler mode can be with
privileged level

Operation Modes
– Thread Mode
● Processor running in Thread mode can be with
privileged as well as user access level
● After reset processor is in thread mode with
privileged access level

Operation Modes
– Privileged Access Level
● Can access System Control Space (SCS) which
is a part of memory region for configuration
registers and debubbing components
● Can access special register access instructions
(MSR and MRS)
● Software in this access level can switch into
user access level using control register
● On Exception, processor always switch to
privileged state and return to previous state

Operation Modes
– User Access Level
● Access to System Control Space is blocked
● Cannot access special register access
instructions (MSR and MRS)
● Software in this access level cannot switch into
privileged access level directly
● Software can switch into privileged level from
exception handler

Exception and Interrupts
– Supports large number of exceptions and
interrupts

Vector Tables
– Used to determine the starting address of an exception
handler
– The vector table in Cortex-M3 is relocatable which is
controlled by relocation register in NVIC
– After reset the relocation register is reset to 0 and vector
table is located at address 0x00000000
– Location 0x00000000 stores the starting value for Main
Stack Pointer
– LSB of the exception indicates whether the exception is
the executed in Thumb state

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Arm cm3 architecture_and_programmer_model

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