IAR PowerPac RTOS User Guide

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COOPERATIVE MULTITASKINGCooperative multitasking expects cooperation of all tasks. Tasks can only be suspended by calling a function of theoperating system. If they do not, the system "hangs", which means that other tasks have no chance of being executedby the CPU while the first task is being carried out. This is illustrated in the diagram below. Even if an ISR makes ahigher-priority task ready to run, the interrupted task will be returned to and finished before the task switch is made.Low priority taskExecuting task is interruptedISRISR puts high prioritytask in READY stateTimeInterrupted taskis completedHigh priority taskHigher priority taskIs executed14IAR PowerPac RTOSfor ARM CoresPPRTOS-2
Basic conceptsPREEMPTIVES MULTITASKINGReal-time systems like IAR PowerPac RTOS operate with preemptive multitasking only. A real-time operating systemneeds a regular timer-interrupt to interrupt tasks at defined times and to perform task-switches if necessary. The highestprioritytask in the READY state is therefore always executed, whether it is an interrupted task or not. If an ISR makesa higher priority task ready, a task switch will occur and the task will be executed before the interrupted task is returnedto.Low priority taskExecuting task is interruptedISRTimeISR puts high prioritytask in READY state;task switch occursHigh priority taskHigher priority taskIs executedInterrupted taskis completedSchedulingThere are different algorithms that determine which task to execute, called schedulers. All schedulers have one thingin common: they distinguish between tasks that are ready to be executed (in the READY state) and the other tasks thatare suspended for any reason (delay, waiting for mailbox, waiting for semaphore, waiting for event, and so on). Thescheduler selects one of the tasks in the READY state and activates it (executes the program of this task). The task whichis currently executing is referred to as the active task. The main difference between schedulers is in how they distributethe computation time between the tasks in READY state.ROUND-ROBIN SCHEDULING ALGORITHMWith round-robin scheduling, the scheduler has a list of tasks and, when deactivating the active task, activates the nexttask that is in the READY state. Round-robin can be used with either preemptive or cooperative multitasking. It workswell if you do not need to guarantee response time, if the response time is not an issue, or if all tasks have the samepriority. Round-robin scheduling can be illustrated as follows:All tasks are on the same level; the possession of the CPU changes periodically after a predefined execution time. Thistime is called timeslice, and may be defined individually for every task.PPRTOS-215
Page 1 and 2: IAR PowerPac RTOSUser GuidePPRTOS-2
Page 3 and 4: PrefaceWelcome to the IAR PowerPac
Page 5 and 6: ContentsPreface ...................
Page 7 and 8: ContentsOS_WaitCSema() ............
Page 9 and 10: ContentsOS_LeaveInterrupt() .......
Page 11 and 12: Introduction to IAR PowerPacRTOSWha
Page 13: Basic conceptsTasksIn this context,
Page 17 and 18: Basic conceptsMAILBOXES AND QUEUESA
Page 19 and 20: Basic conceptsThe active task may b
Page 21 and 22: Basic conceptsThe flowchart below i
Page 23 and 24: Basic conceptsLIST OF LIBRARIESIn y
Page 25 and 26: Task routinesIntroductionA task tha
Page 27 and 28: Task routinesOS_CREATETASK() can be
Page 29 and 30: Task routinesExampleThe following e
Page 31 and 32: Task routinesExampleint sec,min;voi
Page 33 and 34: Task routinesAdditional Information
Page 35 and 36: Task routinesReturn valueOS_TASKID:
Page 37 and 38: Software timersSoftware timerA soft
Page 39 and 40: Software timers#define OS_CREATETIM
Page 41 and 42: Software timersOS_SetTimerPeriod()D
Page 43 and 44: Software timersAdditional Informati
Page 45 and 46: Software timersAdditional Informati
Page 47 and 48: Software timersExampleOS_TIMER TIME
Page 49 and 50: Resource semaphoresIntroductionReso
Page 51 and 52: Resource semaphoresResource semapho
Page 53 and 54: Resource semaphoresOS_Request()Desc
Page 55 and 56: Counting SemaphoresIntroductionCoun
Page 57 and 58: Counting SemaphoresPrototypevoid OS
Page 59 and 60: Counting SemaphoresReturn value0: I
Page 61 and 62: MailboxesWhy mailboxes?In the prece
Page 63 and 64: MailboxesMailboxes API function ove
Page 65 and 66:
MailboxesExampleSingle-byte mailbox
Page 67 and 68:
MailboxesPrototypevoid OS_GetMail (
Page 69 and 70:
MailboxesOS_WaitMail()DescriptionWa
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QueuesWhy queues?In the preceding c
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QueuesReturn valueThe size of the r
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QueuesExamplestatic void MemoryTask
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Task eventsIntroductionTask events
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Task eventsExampleOS_WaitEventTimed
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Task eventsPrototypechar OS_ClearEv
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Event objectsIntroductionEvent obje
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Event objectsExampleif (OS_EVENT_Wa
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Event objectsExampleOS_EVENT_Reset(
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Heap type memory managementANSI C o
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Fixed block size memory poolsIntrod
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Fixed block size memory poolsProtot
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Fixed block size memory poolsProtot
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StacksIntroductionThe stack is the
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InterruptsIntroductionIn this chapt
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InterruptsRules for interrupt handl
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InterruptsOS_LeaveInterruptNoSwitch
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InterruptsNesting interrupt routine
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Critical RegionsIntroductionCritica
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System variablesIntroductionThe sys
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Configuration for your targetsystem
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Time measurementIntroductionIAR Pow
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Time measurementPrototypeU32 OS_Get
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Time measurementPrototypeOS_U32 OS_
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RTOS-aware debuggingThis chapter de
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RTOS-aware debuggingTimersA softwar
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DebuggingRuntime errorsSome error c
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DebuggingValue Define Description17
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Performance and resource usageThis
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Performance and resource usageThe c
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Performance and resource usage*/sta
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ReentranceAll routines that can be
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LimitationsThe following limitation
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Source code of kernel and libraryIn
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Additional modulesKeyboard manager:
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FAQ (frequently asked questions)Q:
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GlossaryActive TaskCooperativemulti
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IndexIndexAAdditional modules . . .
Page 147:
IndexOS_WaitSingleEventTimed(). . .
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