嵌入式系统软件教程5PPT课件

1、Task Each 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. A tasks context is saved in the task control block (TCB).第1页/共92页Task A tasks context includes: - a thread of execution; that is, the tasks program

2、 counter - the CPU registers and (optionally) floating-point registers - I/O assignments for standard input, output, and error - a delay timer - a time-slice timer - kernel control structures - signal handlers - debugging and performance monitoring values第2页/共92页Task The default task scheduling algo

3、rithm in wind is priority-based preemptive scheduling. Round Robin is an optional algorithm. The wind kernel has 256 priority levels, numbered 0 through 255. Priority 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Task Task LockWhen a task disables the

4、 scheduler by calling taskLock(), no priority-based preemption can take place while that task is running.If the task explicitly blocks or suspends, the scheduler selects the next highest-priority eligible task to execute.When the preemption-locked task unblocks and begins running again, preemption i

5、s again disabled.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/共92页Task taskSpawn Task Name and ID Routines Task Information Routines Task Deleting Routines Task Control Routines Task Hook Routines第7页/共92页Task Spawn第8页/共92页Task

6、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/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 Reentrancy A single copy of code executed by multiple tasks is called

7、 shared code. A subroutine is reentrant if a single copy of the routine can be called from several task contexts simultaneously without conflict. Modify global or static variables.第16页/共92页Code Reentrancy Solutions Dynamic Stack Variables Guarded Global and Static Variables Task Variables第17页/共92页Dy

8、namic Stack Variables第18页/共92页Guarded Global and Static Variables Providing a mutual-exclusion mechanism to prohibit tasks from simultaneously executing critical sections of code.第19页/共92页Task Variables第20页/共92页Intertask Communication Shared Data Structure Mutual Exclusion Semaphores Message Queues

9、Pipes Network Intertask Communication Signals第21页/共92页Shared Data Structures Global variables Linear buffers Ring buffers Linked lists Pointers第22页/共92页Shared Data Structures第23页/共92页Mutual Exclusion Disable interrupts Disable preemption Resource locking with semaphores第24页/共92页Disable Interrupts Th

10、e most powerful method It prevents the system from responding to external events for the duration of these locks. Keep the duration of interrupt lockouts short.第25页/共92页Preemptive locks Less restrictive form of mutual exclusion. No other task is allowed to preemptive the current executing task, ISR

11、are able to execute. Tasks of higher priority are unable to execute until the locking task leaves the critical region.第26页/共92页Semaphores Highly optimized and provide the fastest intertask communication mechanism in VxWorks. Three types of Wind semaphores:- Binary- Mutual exclusion- Counting第27页/共92

12、页Semaphore Control Routines第28页/共92页Binary Semaphore第29页/共92页Binary 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

13、92页Mutual Semaphore - Recursive Resource Access第36页/共92页Mutual Semaphore - Recursive Resource Access第37页/共92页Counting Semaphores第38页/共92页Message Queue Message queues allow a variable number of messages, each of variable length, to be queued. Tasks and ISRs can send messages to a message queue, and t

14、asks can receive message form a message queue. Multiple tasks can send to and receive from the same message queue. Full-duplex communication between two tasks generally requires two message 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 Queu

15、e Example第42页/共92页Message 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Pipes Pipes provide an alternative interface to the message queue facility that goes through the VxWorks I/O system.第46页/共92页Sockets In VxWorks, the basis of intertask communication across the networ

16、k is sockets. VxWorks supports the Internet protocols TCP and UDP. TCP provides reliable, guaranteed, two-way transmission of data with stream sockets. UDP provides a simple but less robust form of communication.第47页/共92页RPC Remote Procedure Calls (RPC) is a facility that allows a process on a machi

17、ne to call a procedure that is executed by another process on either the same machine or a remote machine.第48页/共92页Signal Signals asynchronously alter the control flow of a task. Any task or ISR can raise a signal for a particular task. The task being signaled immediately suspends its current thread

18、 of execution and executes the task-specified signal handler routine.第49页/共92页Signal Signals asynchronously alter the control flow of a task. Any task or ISR can raise a signal for a particular task. The task being signaled immediately suspends its current thread of execution and executes the task-s

19、pecified signal handler routine.第50页/共92页Event VxWorks events are means of communication between tasks and interrupt routine, between tasks and other tasks, or between tasks and VxWorks objects. VxWorks objects are referred to as resources such as semaphores and message queue. 第51页/共92页Event Sending

20、 and Receiving EventsEvents can be sent from a resource to a task, from an ISR to a task, or directly between two tasks. Waiting 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

21、events.第52页/共92页Event Registering for EventsOnly one task can register itself to receive events from a resource. If another task subsequently registers with the same resource, the previously registered task is automatically unregistered. Freeing ResourceWhen a resource sends to a task to indicate th

22、at it is free, it does not mean that resource is reserved.第53页/共92页Event Free Resource Definition Mutex Semaphore- A mutex semaphore is considered free when it no longer has an owner and no one is pending on it. Binary Semaphore- A binary semaphore is considered free when no task owns it and no one

23、is waiting for it.第54页/共92页Event Free Resource Definition Counting Semaphore- A counting semaphore is considered free when its counts is nonzero and no one is pending on it. Message Queue- A message queue is considered free when a message queue is present in the queue and no one is pending for the a

24、rrival of a message in that queue.第55页/共92页Event Sending and Receiving EventsEvents can be sent from a resource to a task, from an ISR to a task, or directly between two tasks. Waiting for EventsA task can wait for multiple events from one or more resources. Each source can send multiple events, and

25、 a task can also wait to receive only one event, or all events.第56页/共92页Watchdog Timers VxWorks includes a watchdog-timer mechanism that allows any C function to be connect to a specified time delay. Function invoked by watchdog timers execute as interrupt service code at the interrupt level of the

26、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 Run a shell Variables in shell C commands in shell: printf Shell 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