嵌入式系统软件教程5.pptx

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1、会计学1嵌入式系统软件教程嵌入式系统软件教程5Taskn nEach task has its own context,which is the CPU environment and system resources that the task sees each time it is scheduled to run by the kernel.n nA tasks context is saved in the task control block(TCB).第1页/共92页Taskn nA tasks context includes:A tasks context includes:

2、-a thread of execution;that is,the tasks program counter -a thread of execution;that is,the tasks program counter -the CPU registers and(optionally)floating-point registers -the CPU registers and(optionally)floating-point registers -I/O assignments for standard input,output,and error -I/O assignment

3、s for standard input,output,and error -a delay timer -a delay timer -a time-slice timer -a time-slice timer -kernel control structures -kernel control structures -signal handlers -signal handlers -debugging and performance monitoring values -debugging and performance monitoring values第2页/共92页Taskn n

4、The default task scheduling algorithm in wind is priority-based preemptive scheduling.n nRound Robin is an optional algorithm.n nThe wind kernel has 256 priority levels,numbered 0 through 255.n nPriority 0 is the highest and priority 255 is the lowest.第3页/共92页Task Scheduler Control Routines第4页/共92页T

5、askn nTask LockTask LockWhen a task disables the scheduler by calling taskLock(),no priority-based preemption When a task disables the scheduler by calling taskLock(),no priority-based preemption can take place while that task is running.can take place while that task is running.If the task explicit

6、ly blocks or suspends,the scheduler selects the next highest-priority If the task explicitly blocks or suspends,the scheduler selects the next highest-priority eligible task to execute.eligible task to execute.When the preemption-locked task unblocks and begins running again,preemption is When the p

7、reemption-locked task unblocks and begins running again,preemption is again disabled.again disabled.The taskLock()prevent task context switching,but do not lock out interrupt handling.The taskLock()prevent task context switching,but do not lock out interrupt handling.第5页/共92页Task Control Routine第6页/

8、共92页Taskn ntaskSpawnn nTask Name and ID Routinesn nTask Information Routinesn nTask Deleting Routinesn nTask Control Routinesn nTask Hook Routines第7页/共92页Task Spawn第8页/共92页Task Name and ID Routines第9页/共92页Task Information Routines第10页/共92页Task Information Routines第11页/共92页Task Deleting Routines第12页/

9、共92页Task Deleting Routines第13页/共92页Task Control Routines第14页/共92页Task Hook Routines第15页/共92页Shared Code and Reentrancyn nA single copy of code executed by multiple tasks is called shared code.n nA subroutine is reentrant if a single copy of the routine can be called from several task contexts simult

10、aneously without conflict.n nModify global or static variables.第16页/共92页Code Reentrancy Solutionsn nDynamic Stack Variablesn nGuarded Global and Static Variablesn nTask Variables第17页/共92页Dynamic Stack Variables第18页/共92页Guarded Global and Static Variablesn nProviding a mutual-exclusion mechanism to p

11、rohibit tasks from simultaneously executing critical sections of code.第19页/共92页Task Variables第20页/共92页Intertask Communicationn nShared Data Structuren nMutual Exclusionn nSemaphoresn nMessage Queuesn nPipesn nNetwork Intertask Communicationn nSignals第21页/共92页Shared Data Structuresn nGlobal variables

12、n nLinear buffersn nRing buffersn nLinked listsn nPointers第22页/共92页Shared Data Structures第23页/共92页Mutual Exclusionn nDisable interruptsn nDisable preemptionn nResource locking with semaphores第24页/共92页Disable Interruptsn nThe most powerful methodn nIt prevents the system from responding to external e

13、vents for the duration of these locks.n nKeep the duration of interrupt lockouts short.第25页/共92页Preemptive locksn nLess restrictive form of mutual exclusion.n nNo other task is allowed to preemptive the current executing task,ISR are able to execute.n nTasks of higher priority are unable to execute

14、until the locking task leaves the critical region.第26页/共92页Semaphoresn nHighly optimized and provide the fastest intertask communication mechanism in VxWorks.n nThree types of Wind semaphores:-Binary-Mutual exclusion-Counting第27页/共92页Semaphore Control Routines第28页/共92页Binary Semaphore第29页/共92页Binary

15、 Semaphore第30页/共92页Binary Semaphore-Mutual Exclusion第31页/共92页Binary Semaphore-Mutual Exclusion第32页/共92页Binary Semaphore-Synchronization第33页/共92页Binary Semaphore-Synchronization第34页/共92页Binary Semaphore-Synchronization第35页/共92页Mutual Semaphore-Recursive Resource Access第36页/共92页Mutual Semaphore-Recurs

16、ive Resource Access第37页/共92页Counting Semaphores第38页/共92页Message Queuen nMessage queues allow a variable number of messages,each of variable Message queues allow a variable number of messages,each of variable length,to be queued.Tasks and ISRs can send messages to a message length,to be queued.Tasks

17、and ISRs can send messages to a message queue,and tasks can receive message form a message queue.queue,and tasks can receive message form a message queue.n nMultiple tasks can send to and receive from the same message queue.Full-Multiple tasks can send to and receive from the same message queue.Full

18、-duplex communication between two tasks generally requires two message duplex communication between two tasks generally requires two message queues,one for each direction.queues,one for each direction.第39页/共92页Message Queues第40页/共92页Wind Message Queue Control第41页/共92页Message Queue Example第42页/共92页Me

19、ssage Queue Example第43页/共92页Message Queue Example第44页/共92页Message Queues第45页/共92页Pipesn nPipes provide an alternative interface to the message queue facility that goes through the VxWorks I/O system.第46页/共92页Socketsn nIn VxWorks,the basis of intertask communication across the network is sockets.n nV

20、xWorks supports the Internet protocols TCP and UDP.n nTCP provides reliable,guaranteed,two-way transmission of data with stream sockets.n nUDP provides a simple but less robust form of communication.第47页/共92页RPCn nRemote Procedure Calls(RPC)is a facility that allows a process on a machine to call a

21、procedure that is executed by another process on either the same machine or a remote machine.第48页/共92页Signaln nSignals asynchronously alter the control flow of a task.n nAny task or ISR can raise a signal for a particular task.n nThe task being signaled immediately suspends its current thread of exe

22、cution and executes the task-specified signal handler routine.第49页/共92页Signaln nSignals asynchronously alter the control flow of a task.n nAny task or ISR can raise a signal for a particular task.n nThe task being signaled immediately suspends its current thread of execution and executes the task-sp

23、ecified signal handler routine.第50页/共92页Eventn nVxWorks events are means of communication between tasks and interrupt routine,between tasks and other tasks,or between tasks and VxWorks objects.n nVxWorks objects are referred to as resources such as semaphores and message queue.第51页/共92页Eventn nSendi

24、ng and Receiving EventsEvents can be sent from a resource to a task,from an ISR to a task,or directly between two tasks.n nWaiting for EventsA task can wait for multiple events from one or more resources.Each source can send multiple events,and a task can also wait to receive only one event,or all e

25、vents.第52页/共92页Eventn nRegistering for EventsRegistering for EventsOnly one task can register itself to receive events from a resource.If Only one task can register itself to receive events from a resource.If another task subsequently registers with the same resource,the previously another task subs

26、equently registers with the same resource,the previously registered task is automatically unregistered.registered task is automatically unregistered.n nFreeing ResourceFreeing ResourceWhen a resource sends to a task to indicate that it is free,it does not mean When a resource sends to a task to indi

27、cate that it is free,it does not mean that resource is reserved.that resource is reserved.第53页/共92页Eventn nFree Resource Definitionn nMutex Semaphore-A mutex semaphore is considered free when it no longer has an owner and no one is pending on it.n nBinary Semaphore-A binary semaphore is considered f

28、ree when no task owns it and no one is waiting for it.第54页/共92页Eventn nFree Resource DefinitionFree Resource Definitionn nCounting SemaphoreCounting Semaphore-A counting semaphore is considered free when its counts is nonzero and-A counting semaphore is considered free when its counts is nonzero and

29、 no one is pending on it.no one is pending on it.n nMessage QueueMessage Queue-A message queue is considered free when a message queue is present in-A message queue is considered free when a message queue is present in the queue and no one is pending for the arrival of a message in that queue.the qu

30、eue and no one is pending for the arrival of a message in that queue.第55页/共92页Eventn nSending and Receiving EventsEvents can be sent from a resource to a task,from an ISR to a task,or directly between two tasks.n nWaiting for EventsA task can wait for multiple events from one or more resources.Each

31、source can send multiple events,and a task can also wait to receive only one event,or all events.第56页/共92页Watchdog Timersn nVxWorks includes a watchdog-timer mechanism that allows any C function to be connect to a specified time delay.n nFunction invoked by watchdog timers execute as interrupt servi

32、ce code at the interrupt level of the system clock.第57页/共92页Watchdog Timers Calls第58页/共92页Watchdog Timers Example第59页/共92页Event Register Routine第60页/共92页Interrupt Service Routine第61页/共92页intConnect Routine第62页/共92页Appendix:Shell n nRun a shelln nVariables in shelln nC commands in shell:printfn nShel

33、l commands:i,ti,w,tw,第63页/共92页Run a shell 第64页/共92页Variables in shell第65页/共92页C commands in shell第66页/共92页Shell commands:i第67页/共92页Shell Commands:td第68页/共92页Shell Commands:ts&tr第69页/共92页Shell Commands:ti第70页/共92页Shell Commands:w第71页/共92页Shell Commands:tw第72页/共92页Shell Commands:checkStack第73页/共92页She

34、ll Commands:d第74页/共92页Shell Commands:d第75页/共92页Shell Commands:d第76页/共92页Shell Commands:ls第77页/共92页Shell Commands:pwd&cd第78页/共92页Shell Commands:h第79页/共92页Shell Commands:reboot第80页/共92页Shell Commands:semaphore第81页/共92页Shell Commands:semaphore第82页/共92页Shell Commands:semaphore第83页/共92页Shell Commands:semaphore第84页/共92页Shell Commands:semaphore第85页/共92页Shell Commands:Message Queue第86页/共92页Shell Commands:Message Queue第87页/共92页Shell Commands:Message Queue第88页/共92页Shell Commands:Message Queue第89页/共92页Shell Commands:Message Queue第90页/共92页Shell Commands:Message Queue第91页/共92页