Operating systems and concurrency B04 - hesabu.nethesabu.net/en0572/assets/ra/B04.pdf · Operating systems and concurrency B04 David Kendall ... Easier to structure the application

Operating systems and concurrency B04

David Kendall

Northumbria University

David Kendall (Northumbria University) Operating systems and concurrency B04 1 / 21

Introduction

Towards an operating system: CMSIS and MbedMulti-tasking operating system servicesµC/OS-II (uC/OS-II)Task management in uC/OS-II


Standard names for the microcontroller registers

#include <stdboo l . h>#include < s t d i n t . h>#include <LPC407x_8x_177x_8x . h>

#define LED1PIN (1UL << 18)

void delay ( u i n t32_ t ms ) ;

i n t main ( ) {LPC_IOCON−>P1_18 &= ~0x1FUL ;LPC_GPIO1−>DIR |= LED1PIN ;while ( t r ue ) {

LPC_GPIO1−>SET = LED1PIN ;delay (1000) ;LPC_GPIO1−>CLR = LED1PIN ;delay (1000) ;

}}

Header fileLPC407x_8x_177x_8x.hprovided by themanufacturer of themicrocontroller (NXP) togive a set of standardnames for themicrocontroller registersNo need to write#defines after looking upthe addresses yourself.Header file written instandard ANSI C -CMSIS-complaint -portable between toolproviders.


CMSIS

Cortex MicrocontrollerSoftware Interface StandardCMSIS-Core

Standard set of namesprovided by ARM foraccessing themicroprocessorStandard startup files forbringing up the CPU out ofreset:

configure system clocksdefine the vector tablerun the main function

Improves software portabilityand reusability across differentmicrocontrollers andtoolchains

Martin, T. The Designer’s Guide to the Cortex-M Processor

Family: A Tutorial Approach, Newnes, 2013


Foreground/background tasks

Simple multitaskingSuper loop callsfunctions forcomputation(background)Interrupt serviceroutines (ISRs) handleasynchronous events(foreground)

ProblemModel with only one computation task is not flexible enough


Foreground/background tasks

Simple multitaskingSuper loop callsfunctions forcomputation(background)Interrupt serviceroutines (ISRs) handleasynchronous events(foreground)

ProblemModel with only one computation task is not flexible enough


Multitasking

Easier to structure the application as a set of concurrent tasksrather than as a single program or as foreground/backgroundtasks: each task is responsible for some well-defined part of thesystem’s overall behaviourBut only the illusion of concurrency - the OS switches quicklybetween tasks, executing some instructions from one task beforemoving on to another taskswitching from one text to another requires a context switchdeciding which task to switch to requires a scheduling algorithm


Scheduling

Deciding when one task should stop executing and which oneshould begin next is a scheduling problemTwo main approaches to scheduling:

Preemptive schedulingTask is forced to yield the CPURound robinPriority-based

Non-preemptive (cooperative) schedulingTask voluntarily yields the CPU and signals the next task to begin


Fixed-priority preemptive scheduling

We focus on fixed-priority premptive scheduling


A task and its execution states


uC/OS-II: A small operating system

Main features:Multi-taskingPreemptive

Other features:PredictableRobust and reliableStandards-compliantPortableScalableSource code available


uC/OS-II Services

Task managementDelay management

SemaphoresMutual exclusion semaphoresEvent flagsMessage mailboxesMessage queuesMemory managementTimersMiscellaneous

See uC/OS-II Quick Reference


http://cgweb1.northumbria.ac.uk/SubjectAreaResources/embedded/micrium/QuickRefChart-Color.pdf

uC/OS-II Services

Task managementDelay managementSemaphoresMutual exclusion semaphoresEvent flagsMessage mailboxesMessage queuesMemory managementTimersMiscellaneous




uC/OS-II Services

Task managementDelay managementSemaphoresMutual exclusion semaphoresEvent flagsMessage mailboxesMessage queuesMemory managementTimersMiscellaneous




Tasks behaviour

The behaviour of a task is defined by a C function that:1 never terminates2 blocks repeatedly

Example of task behaviour definition

s t a t i c void appTaskLED2 ( void ∗pdata ) {while ( t r ue ) {

OSTimeDlyHMSM(0 ,0 ,0 ,500 ) ;gpioPinToggle (& p in [ LED2 ] ) ;

}}


Tasks: other requirements

PriorityUsed for fixed-priority pre-emptive schedulinga number between 0 and OS_LOWEST_PRIO

low number ⇒ high priorityhigh number ⇒ low priorityOS reserves priorities 0 to 3 and OS_LOWEST_PRIO - 3 toOS_LOWEST_PRIO

Advice: define an enumeration of task priority constants, startingat priority level 4.Example

enum {APP_TASK_BUTTONS_PRIO = 4 ,APP_TASK_LED1_PRIO,APP_TASK_LED2_PRIO

} ;



StackEach task needs its own data area (stack) for storing

contextlocal variables

Example stack definition

enum {APP_TASK_LED2_STK_SIZE = 256} ;s t a t i c OS_STK appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE ] ;

User dataOptionally tasks can be given access to user data when they arecreatedWe will not use this feature in this moduleAdvice: always specify this as (void *)0 when creating a task



StackEach task needs its own data area (stack) for storing

contextlocal variables

Example stack definition

enum {APP_TASK_LED2_STK_SIZE = 256} ;s t a t i c OS_STK appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE ] ;

User dataOptionally tasks can be given access to user data when they arecreatedWe will not use this feature in this moduleAdvice: always specify this as (void *)0 when creating a task


Task creation

A task is created using the OS functionINT8U OSTaskCreate (

void (∗ task ) ( void ∗pdata ) , /∗ f u n c t i o n f o r the task ∗ /void ∗pdata , /∗ user data f o r f u n c t i o n ∗ /OS_STK ∗ptos , /∗ p o i n t e r to top o f s tack ∗ /INT8U p r i o r i t y /∗ task p r i o r i t y ∗ /

) ;

Example

enum {APP_TASK_LED2_PRIO = 4 } ;enum {APP_TASK_LED2_STK_SIZE = 256} ;

s t a t i c OS_STK appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE ] ;

OSTaskCreate ( appTaskLED2 ,( void ∗ )0 ,(OS_STK ∗)&appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE − 1 ] ,APP_TASK_LED2_PRIO ) ;


Task creation

A task is created using the OS functionINT8U OSTaskCreate (

void (∗ task ) ( void ∗pdata ) , /∗ f u n c t i o n f o r the task ∗ /void ∗pdata , /∗ user data f o r f u n c t i o n ∗ /OS_STK ∗ptos , /∗ p o i n t e r to top o f s tack ∗ /INT8U p r i o r i t y /∗ task p r i o r i t y ∗ /

) ;

Example

enum {APP_TASK_LED2_PRIO = 4 } ;enum {APP_TASK_LED2_STK_SIZE = 256} ;

s t a t i c OS_STK appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE ] ;

OSTaskCreate ( appTaskLED2 ,( void ∗ )0 ,(OS_STK ∗)&appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE − 1 ] ,APP_TASK_LED2_PRIO ) ;


Task delay

Often, a task will block itself by explicitly asking the OS to delay itfor some period of timevoid OSTimeDly(INT16U ticks);

Causes a context switch if ticks is between 1 and 65535If ticks is 0, OSTimeDly() returns immediately to callerOn context switch uC/OS-II executes the next highest priority taskTask that called OSTimeDly() will be made ready to run whenthe specified number of ticks elapses - actually runs when itbecomes the highest priority ready taskResolution of the delay is between 0 and 1 tickAnother task can cancel the delay by callingOSTimeDlyResume()


Task delay

OSTimeDly() specifies delay in terms of a number of ticksUse OSTimeDlyHMSM() to specify delay in terms of Hours,Minutes, Seconds and MillisecondsOtherwise OSTimeDlyHMSM() behaves as OSTimeDly()


Complete example

#include <stdboo l . h>#include < ucos_ i i . h>#include " gpioPin . h "

typedef enum {APP_TASK_LED1_PRIO = 4 ,APP_TASK_LED2_PRIO

} t a s k P r i o r i t i e s _ t ;

typedef enum {APP_TASK_LED1_STK_SIZE = 256 ,APP_TASK_LED2_STK_SIZE = 256

} s tackS izes_ t ;

s t a t i c OS_STK appTaskLED1Stk [ APP_TASK_LED1_STK_SIZE ] ;s t a t i c OS_STK appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE ] ;

s t a t i c void appTaskLED1 ( void ∗pdata ) ;s t a t i c void appTaskLED2 ( void ∗pdata ) ;

typedef enum {LED1 = 0 ,LED2} deviceNames_t ;

gp ioP in_ t p in [ 2 ] ;


Complete example

i n t main ( ) {

/∗ I n i t i a l i s e the GPIO pins ∗ /g p i o P i n I n i t (& p in [ LED1 ] , 1 , 18 , OUTPUT_PIN ) ;g p i o P i n I n i t (& p in [ LED2 ] , 0 , 13 , OUTPUT_PIN ) ;

/∗ I n i t i a l i s e the OS ∗ /OSIn i t ( ) ;

/∗ Create the tasks ∗ /OSTaskCreate ( appTaskLED1 ,

( void ∗)0 ,(OS_STK ∗)&appTaskLED1Stk [ APP_TASK_LED1_STK_SIZE − 1 ] ,APP_TASK_LED1_PRIO ) ;

OSTaskCreate ( appTaskLED2 ,( void ∗)0 ,(OS_STK ∗)&appTaskLED2Stk [ APP_TASK_LED2_STK_SIZE − 1 ] ,APP_TASK_LED2_PRIO ) ;

/∗ S t a r t the OS ∗ /OSStart ( ) ;

/∗ Should never a r r i v e here ∗ /return 0;

}


Complete example

s t a t i c void appTaskLED1 ( void ∗pdata ) {/∗ S t a r t the OS t i c k e r −− must be done i n the h ighes t p r i o r i t y task ∗ /SysTick_Config ( SystemCoreClock / OS_TICKS_PER_SEC ) ;

/∗ Task main loop ∗ /while ( t r ue ) {

gpioPinToggle (& p in [ LED1 ] ) ;OSTimeDlyHMSM(0 ,0 ,0 ,500 ) ;

}}

s t a t i c void appTaskLED2 ( void ∗pdata ) {while ( t r ue ) {

gpioPinToggle (& p in [ LED2 ] ) ;OSTimeDlyHMSM(0 ,0 ,0 ,500 ) ;

}}


Acknowledgements

Qing Li and Caroline Yao, Real-time concepts for embeddedsystems, CMP, 2003Jean Labrosse, MicroC/OS-II: The Real-Time Kernel, CMP, 2002


Documents

Operating systems and concurrency B04 - hesabu.nethesabu.net/en0572/assets/ra/B04.pdf · Operating systems and concurrency B04 David Kendall ... Easier to structure the application