Introduction to VxWorks RTOS

Eli Carmi

July 2007

RTOS Tasks Synchronization Asynchronous Information

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Introduction to VxWorks RTOS

RTOS Real Time Multi Tasking Priority Scheduling Round-Robin Scheduling

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RTOS - Real Time OS

Control External Events Synchronous Event Asynchronous Event Independents Event

Speed Fast response Low overheads

Deterministic A late answer is a wrong answer

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Real Time

Sequence programming One task controlling all events in a Loop

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Multi Tasking

FOREVER

{

if (Event1)

do Event1 action

if (Event2)

do Event2 action

…

}

Multitasking Programming Task is “Waiting” on Event Task becomes “Ready” upon Event

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Multi Tasking cont.

Task1

FOREVER

{

wait on Event1

do Event1 action

}

Task2

FOREVER

{

wait on Event2

do Event2 action

}

Task3

FOREVER

{

wait on Event3

do Event3 action

}

Task in the system may have different Priority Preemptive Scheduling on Task Priority The Highest Priority Task ready to run is

allocated the CPU Equal Priority tasks won’t preempt each other Scheduling can occur by Synchronous or

Asynchronous Events No delay on Context Switch to the next Tick

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Priority Scheduling

Slicing Context Switch in each (predefined) time slice

period Round-Robin

All tasks in the same priority share the CPU

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Round-Robin Scheduling

Tasks Task States Task Create (t-name, t-id) TCB, Stack, errno… Context Switch

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Tasks

Delay Pended Ready Execute Suspend

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Task States

Event TaskPended

TaskPended

TaskPended

Timer

Task Delay + Pended

TaskDelay

TaskDelay

Ready Taskexecute

Taskready

Taskready

Taskready

EventTask

PendedTask

Pended

taskId = taskSpawn(name, Priority, Option, Stack, Entry, Arg1,… arg10 ) task Id Task name Priority Option Stack Entry Arg1,…arg10

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Task Create

TCB – Task Control Block Entry pointer Registers Status Errno …

Task Stack May be filled with 0xee

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TCB

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Context Switch

New TCB CPU Old TCBCPU

Synchronization Need for Synchronization Binary Semaphores Mutex Semaphores Message Queue

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Synchronization

Reentrancy & Shared Resource

Jobs Synchronization

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Need for Synchronization

…

void func()

{

….

if (gResource.free)

{

gResource.free = FALSE;

gResource.xxx = …

}

….

}

….Action1

…. Action2

Mostly for Synchronization Creates Semaphore for Event Waits for Event

Task call the semaphore take() action Task is blocked until Event Semaphore is given

Upon Event becomes available Task or ISR give() semaphore

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Binary Semaphores

SEM_ID semBCreate(option, InitState) SEM_ID Option InitState

Status semTake(semid, timeout) Status semGive(semid, timeout)

Status semid Timeout

Note: Counting Semaphores also available

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Binary Semaphores (cont)

Taking a Binary Semaphore

Giving a Binary Semaphore

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Binary Semaphores (cont)

SemaphoreAvailable?

Task pends until sem is given or timeout

Task unpendssemTake()

return ERROR

No Timeout

Task continuesemTake()return OK

Task unpendssemTake()return OK

Yes Semaphore give

TaskPended?

SemaphoreMade available

No

Task at front ofqueue made ready,Semaphore remain

unavailable

Yes

Mostly for Shared Resource Works like Semaphores, but with Ownership

Task which takes the mutex “owns” it This task is the only one that can give the mutex Mutex can be takes repeatedly ISR can not use Mutex

SEM_ID semMCreate(option) Option (SEM_INVERSION_SAFE,…)

Priority Inheritance Initial State is available

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Mutex Semaphores

Priority Inversion

Priority Inheritance

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Mutex Semaphores (Cont)

execute

execute

execute

Low

Priority

High

Priority

Medium

Priority

semTak

Event

Event

semTak

Ready

Ready

PendPend

PendPend

Msg_Q_ID msgQCreate (maxMsg, MaxMsgLen, Option) MSG_Q_ID maxMsg MaxMsgLen Option

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Message Queue

Status msgQSend(msgQId, buffer, nBytes, timeout, priority) Status msgQId buffer nBytes Timeout (WAIT_FOREVER, NO_WAIT, …) Priority (MSG_PRI_URGENT, MSG_PRI_NORMAL)

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Message Queue (cont)

Int msgQReceive(msgQId, buffer, MaxNBytes, timeout) Int –number of bytes reads or ERROR msgQId buffer nBytes Timeout (WAIT_FOREVER, NO_WAIT, …)

Note: Pipes are also available Built on top of message queue

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Message Queue (cont)

Asynchronous Interrupts & Exceptions ISR Timers

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Asynchronous

Interrupts Interrupts allow devices to notify the CPU on events Interrupt Levels Single Interrupt Stack allocate at startup

Exceptions Unplanned event generated by the CPU (divide by

0) Exceptions will generate an “internal” interrupt VxWorks installs exceptions handlers

intLock / intUnlock25

Interrupts & Exceptions

ISR – Interrupt Service Routine NOT a Task

No TCB Can NOT be blocked (can’t use semTake, malloc

and IO routines like printf) Keep ISR short

Delay lower and equal Interrupts Delay all Tasks

Off-load work from ISR

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ISR

taskDelay() May “Drift”

System Clock tickGet() sysClkRateGet()

Watchdog WDOG_ID wdCreate() STATUS wdStart(wdid, delay, pRoutine, parameter) pRoutine is ISR

Note: Auxiliary Clock is also available

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Timers

Thank You

eli.carmi@gmail.com