VxWorks 5.4

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VxWorks 5.4

• Group A3
• Wafa Jaffal
• Kathryn Bean

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Topics Covered

• What is Real Time Operating System (RTOS)
• Standard for RTOS
• VxWorks Features as RTOS
• Layer Structure
• Microkernal Functions
• Multitasking
• Scheduling
• Interrupt Service Code
• Shared Code and Reentrancy
• Intertask Communications
• Applications for VxWorks

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What is Real Time OS (RTOS)

• RTOS Characteristics
• Processing must be done within a time constraint
  or the system will fail.
• All delays in the system will be bounded from
  the retrieval of stored data to the time RTOS
  finishes the request.

• Types of RTOS
• Hard RTOS

• Soft RTOS

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Standard for RTOS

• Standard requirement for RTOS
• POSIX 1003.1b by ISO/IEEE chapter.

POSIX interfaces

• Queued signals
• Scheduling
• Clocks and timers

• Asynchronous I/O
• Semaphore
• Message queues

• Memory management

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VxWorks OS as RTOS
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Layer Structure
Real-Time Embedded Application
Internet
Graphics
Multiprocessing
Java Support
POSIX Library
File System
WindNet Networking
Core OS
Wind Microkernal
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Multitasking
P
R
D
S

• Suspended used primarily for debugging
• Ready wait for CPU,
• a single primitivecreateactivate
• Pended blocked, wait ?t time for recourses.
  After ?t - Error.
• Delay asleep for ?t time, after ?t goes to
  ready state.
• On a context switch, a tasks context is saved in
  TCB. All code executes in a single common space.
  Memory isnt saved.

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Scheduling, One CPU

• Priority-based preemptive scheduling
• Each task has priority (from 0 - highest to 255
  - lowest). Each priority has its own queue.
• If two tasks are in the ready state, a lower
  priority task would be pre-empted by one of a
  higher priority.

• Round-robin scheduling
• After time slice for a task expires, another task
  with the same priority will execute during the
  given time slice.

• Preemption locks
• These locks prevent task context switching, but
  do not prevent interrupt handling.

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Interrupt Service Code
Via interrupts, the system is informed of
external events.

• Interrupt Service Routines (ISRs) run outside
  any task context.

• Interrupt handling involves no task context
  switch.

• Limitations of ISR
• All ISRs share the same stack
• ISR has no context that can be suspended
• Cannot take the semaphore, but can give the
  semaphores, releasing any task waiting on them.
• Cannot perform I/O through drivers except pipe.

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Shared Code and Reentrancy

• Shared Code - a single copy of code executed by
  multiple tasks.
• Shared Code must be reentrant. A subroutine is
  reentrant if a single copy of the routine can be
  called simultaneously from several task contexts
  without conflict.

• Reentrancy Techniques
• Dynamic Stack Variables
• Global and Static Variables Guarded by
  Semaphores
• Task Variables

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Shared Code and Reentrancy, Continued
Dynamic Stack Variables
Task Stacks
Common Function
Tasks
Task1( ) .. comFun(myData1)
myData1
comFun(yourData)
Task2( ) .. comFun(myData2)
myData2
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Shard Code and Reentrancy, Continued

• Guarded Global and Static Variables
• Mutual-exclusion mechanism - only one task at a
  time can be in a critical section of the code.

• Task Variables
• Four byte variables are added to the task
  context block (TCB).

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Intertask Communications, One CPU
Types of Intertask Communications

• Shared Data

• Message Queue

• Pipes - Virtual I/0

• Network Intertask Communication
• Sockets
• Remote Procedure Call

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Intertask Communication, Shared Data
Tasks
Memory
Access SharData
1.
Access SharData
SharData
2.
Access SharData
3.
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Semaphores
Semaphore - mechanism for mutual exclusion and
intertask communication.

• Binary Semaphore

Semaphore available
TimeOut No_Wait
Task is pended for timeOut
NO
NO
NO
YES
YES
Task cont. Semaphore taken
Task cont. Semaphore not taken
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Semaphores, Continued

• Counting Semaphore

• Every time is semaphore is given count

• Every time is semaphore is taken count --

• If count0, task that tried to take semaphore is
  blocked

• If semaphore is given and task is blocked, task
  is unblocked

• Is semaphore is given and no tasks are blocked,
  count

• Special Semaphore Option (Not in POSIX)

• Timeout
• Time the task is willing to wait (time in ticks)
  in pended state. If time gt timeout, ERROR occurs.

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Intertask Communication, Message Queue

• Allows a variable number of messages to be
  queued
• Any task or ISR can send a message to a queue.
• Multiple tasks can send and receive from the
  same message queue.

• Timeout Parameter
• Send message how long to wait for buffer space
  to place message.
• Receive message how long to wait to obtain
  message.

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Applications for VxWorks
Some usage of RTOS

• Flight simulators
• Radio and optical telescopes
• Navigation systems
• Deep sea instrumentation
• Traffic control systems
• Modems

• Printers
• Digital cameras
• Hand-held computing devices
• Routers, switches, and other network devices

any systems where rigid time requirement have
been placed on the operation of a processor or
the flow of the data.
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Thank You
Any Questions ?
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Bibliography
VxWorks 5.4 Programmer Guide