RTX - RTOS MS_uC / fue1 / V01 5- 1

Programming Microcontroller RTX - Real Time Operating System - RTOS Autumn term 2012

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Tasks of the Operating System (1/3)

  

Key tasks of the OS (Kernel)

Additional general tasks (OS)

External general tasks (Utilities)

 Image processing Utilities OS Kernel 2

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Tasks of the OS (2/3)

5- 3 

Kernel



Resource Management



Attribution of CPU time



Task-, Process- or (Thread-) management



Memory Management



Peripheral Devices



Time of the Day and Date

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Tasks of the OS (3/3)



Additional general tasks (OS)

 I/O management (Driver)  File management  Interrupt treatments  Processing "Power-up", "Power-down", "Shut-up", "Shut-down"…  Troubleshooting 

External general tasks (Utilities)

 Image processing

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RTOS: Basic themes

    Real time Multitasking res. multiprogramming Scheduling Tasks, Threads and Processes 5

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Real Time (RT) computer

5- 6  RT computer works synchronously to the rhythm of the technical process  It responds on time to its needs  It takes measures on times Real time processor

Control algorithm Process image Output data Input data Events Control variables Process characteristics Process parameter Metrics Discrete events

Technical process

Process input Process output Interference Retroactivity

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Multitasking res. multiprogramming

 Multitasking OS allows the programming with temporally independent parallel units  Simulation of mutually independent, quasi-parallel operating processors  Example

Priority I/O Job A Job B t t1 t2

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Multitasking res. multiprogramming

 Multiple tasks can run simultaneously on one computer  A task can be started before the other tasks have been completed  Example: Windows7

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Tasks, threads and processes

5- 9  A task is a ideal program unit with a time response  The code for a specific task is defined once  Many task can be generated and started from this definition  Tasks can run concurrently  Task can be started while another is still working  Depending on the OS, Task are always known and existing  They can be started or terminated  They can communicate with each other  Quasi-parallelism is realized by switching from one to another task

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State of a task

 The task states are managed with FIFO  Each task state contains its own FIFO

Blocked Not Existing Ready- To-Run Running

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Description of the task states



Not-Existing

 Does not exist in OS, which contain only active tasks 

Running

 Task, which is currently executed by the CPU 

Ready-To-Run

 All the conditions, which are needed to run the task are, are realized  CPU is currently occupied with the execution of another task 

Blocked

 Task is waiting because of synchronization tools  Semaphore, Event-flags, Suspend/Resume etc.

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Scheduling

5- 12  Scheduling is the process, which attributes CPU time to the threads, processes or task 

Non preemptive Scheduling

 Task exchanges are only realized at given points  Task must systematically give back CPU time 

Preemptive scheduling

 Preemptive means interruption  Task can loose CPU time at any point during its execution  Can be realized only with “Interrupt-Response-Program”  Clock tics

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Round Robin Scheduling

 Each task becomes a certain number of CPU time quanta  Task will be stopped, if it is not finished after this amount of time  If the task is finished before, it will be interrupted immediately  The active task are managed with a list A G B C E  When the task A is interrupted, it will be put at the end of the list G B C E A

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Priority Scheduling

 All the task have the same priority  All the task have predefined priorities  The priority for each task must be unique  Several tasks can have the same priority  Os Algorithm manage their precedence (Round Robin)  The priority is set explicitly or implicitly at task start and will be changed explicitly later  The priority remains the same during all the existence of the task (static)  Priority are changed dynamically by the scheduler

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Definition and starting of a task within Keil

Definition of the task code 

__task void task_name ( void ) { /* Initialization of the task resources */



… while (1) { /* Execution of the task algorithms */ … } }

Create and start the task 

int main ( void ) { … /* 2 variants to create and start the task */ os_tsk_create (task_name, prio); os_tsk_create_user (task_name, prio, \ task_stack, sizeof (task_stack)); … }

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Classical problems of the parallel data processing

 Introduction example : Digital Voltmeter (DVM)  A/D conversion time: 75 ms  LCD must be “refreshed” after 50 ms  The multitasking enables to enhance the design and the program speed  Solution for the screen flickering

Voltage A/D Acquisition Task Display Task Data store Microprocessor Display (LCD)

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Code example

Acquisition Task

while (1) { Wait until A/D ready Read A/D Process measurements for (all digits) { Store digit } // end for } // end while

Display Task

while (1) { Wait until LCD needs refresh for (all digits) { Fetch digit from store } } Display digit on LCD // end for // end while



Problem of this solution

 “Acquisition” and “Display” tasks can access to the data simultaneously  Sometimes wrong values will be displayed  Program must guaranty the mutual exclusion 17

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Mutual Exclusion (MUTEX)

5- 18  2 train must bypass trough a critical section (Tunnel)  Only one train is authorized to go through the tunnel at the given moment  Both locomotive drivers are blind and deaf  The can only deposit or retire a stone in a bowl 18

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First solution

 Procedure for the locomotive conductors 

#define EMPTY 0 #define OCCUPIED 1 int bowl; // semaphore while (bowl == OCCUPIED) { /*make Siesta*/ } //search a stone bowl = OCCUPIED ; // drive the train through the critical tunnel bowl = EMPTY ;

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Frage zur ersten Lösung

 Is this procedure really sure?

 No, because the exclusion is not enough sure (danger of collision) Train A

Finds the bowl empty Search a stone >>--------->> Put the stone into the bowl Pass through the critical part

Train B

Finds the bowl empty Search a stone Put the stone into the bowl Pass through the critical part

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Solution with a privileged section



// Cover the bowl, wait if already covered *1) while (bowl == NOT_EMPTY) {}; // Search a stone bowl = NOT EMPTY ; // Uncover the bowl // Drive the train through the critical tunnel bowl = EMPTY ; // *2) // continue the non critical part of the journey

 Comments  *1) Wait if the cover is already on the bowl  *2) Access possible, even with covered bowl  Is this procedure really sure?

 Yes, because it fulfills the mutual exclusion

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SEMAPHORE

 Flag, which can be accessed only within privileged section  Dijkstra 1968  Coordination using privileged actions

flag = 1; // *1) while (flag == 0) { flag = flag - 1 ; /* nothing */ } // go through the critical region flag = flag + 1 ; // continue the non critical part of the job

 *1) init-value defines the number of trains allowed in the critical part  Value = 1 → binary semaphore  Value > 1 → general semaphore 22

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Privileged function

P( int * s)

&

V( int * s)

 Definition of the privileged functions 

proberen



try



verhogen

 increment

void proberen ( int while *s) { (*s == 0) {} // ev. <= 0 *s = *s - 1; } // end proberen void verhogen ( int *s = *s + 1; } // end verhogen *s) {

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Using of the privileged functions within tasks



#define n 1 // for a binary semaphore #define n > 1 // for a general semaphore int s = n; probeeren (&s); // go though critical region verhogen (&s); // go though rest of job

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Problem of the waiting loop of

probeeren

 The waiting loop of

probeeren



while

occupies the CPU almost by 100%

(s == 0) { /* nothing */ }

 Solution  Task which calls the waiting process must be put in a waiting list

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Procedure

wait( int * s) void if wait ( int (*sema == *sema) { 0) { // wait if resource occupied // enter this task into the sema-waiting-queue for this // semaphore Q(s) and leave the "ready state" // (goes sleeping in a waiting state) } else { *sema = *sema - 1; } // end if … else } // end wait

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Procedure

signal ( int *s) void signal ( int *sema) { // free the resource // read the waiting queue Q(s).

if /* this queue was EMPTY */ *sema = *sema + 1 ; } else { // change this sleeping task to the ready-to run state // and call a wait(&sema); } // end if … else } // end signal *

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Definition of the semaphore variable selon Dijkstra

 Definition of the semaphore variable 

int s >= 0

 Rules for the semaphore variable  Binary semaphore:

s = {0, 1}

 General semaphore:

0 <= s <= n

 Each semaphore variable contains its own waiting list  Writing into and reading from is realized in privileged mode 

wait( int *s

) &

signal( int *s)

are privileged functions 28

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signal

and

wait

functions of Keil

5- 29  Semaphore functions of Keil 

void os_sem_init (OS_ID semaphore, U16 token_count);



OS_RESULT os_sem_send (OS_ID semaphore);



OS_RESULT os_sem_wait (OS_ID semaphore, U16 timeout);

 Telegram functions of Keil 

void os_mbx_init (OS_ID mailbox, U16 mbx_size);



OS_RESULT os_mbx_send (OS_ID mailbox, void *message_ptr, U16 timeout);



OS_RESULT os_mbx_wait (OS_ID mailbox, void **message, U16 timeout);

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Keil code for the locomotiv conductors OS_SEM sema; // definition of the semaphore as global variable … os_sem_init(&sema,1); // Initialization of the semaphore in main

Locomotive driver: West

while (1) { res = os_sem_wait(&sema, 1000); if (res != OS_R_TMO) { /* Go through the critical section */ os_sem_send(&sema); } // end if } // end while

Locomotive driver: East

while (1) { res = os_sem_wait(&sema, 1000); if (res != OS_R_TMO) { /* Go through the critical } section */ os_sem_send(&sema); } // end if // end while

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Keil code for parallel data processing Acquisition Task Display Task Data store Microprocessor Voltage A/D Display (LCD) /* Declaration of a mailbox for 20 messages */ os_mbx_declare (mbox, 20);

Acquisition Task

int *p_msg; while (1) { /* Wait until A/D ready and store its conversion result into new_AD_value */ p_msg = malloc(sizeof( int )); *p_msg = new_AD_value; if (os_mbx_check(&mbox) > 0) { rt_mbx_send(&mbox, p_msg,10); } // end if os_dly_wait (75); } // end while

Display Task

int *p_msg; while (1) { if (os_mbx_check(&mbox) < 20) { rt_mbx_wait(&mbox,&p_msg,10); /* Display od the LCD } the measurements addressed by p_msg */ } free(p_msg); // end if os_dly_wait (50); // end while