Real-Time Operating Systems

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Real-Time Operating Systems

• Suzanne Rivoire
• November 20, 2002

http//www.stanford.edu/skrufi/rtospres.ppt
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Motivating Example

• void main()
• do forever
• check keypad
• measure temperature
• control oven power
• decrement timer
• update display
• wait for clock tick

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Motivating Example - 2

• void main()
• do forever
• check keypad
• measure temperature
• check keypad
• control oven
• check keypad

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Presentation Outline

• Definition of real-time
• Characteristics of RTOSs
• Examples
• ?c-OS
• AvrX
• RTLinux
• QNX Neutrino

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What is real-time?

• Correctness of output depends on timing as well
  as result
• Hard vs. soft real-time
• Are Windows and Linux real-time?

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In a Hard RTOS

• Thread priorities can be set by the client
• Threads always run according to priority
• Kernel must be preemptible or bounded
• Interrupts must be bounded
• No virtual memory

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In a Soft RTOS

• Like a hard RTOS
• Priority scheduling, with no degradation
• Low dispatch latency
• Preemptible system calls
• No virtual memory (or allow pages to be locked)
• Linux guarantees about relative timing of tasks,
  no guarantees about syscalls

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Basic RTOS References

• http//www.dedicated-systems.com/encyc/publication
  s/faq/rtfaq.htm
• http//www.steroidmicros.com/mtkernel.html
• http//www.qnx.com/developer/articles/dec1200b/
• Silberschatz and Galvin, Operating System
  Concepts.

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Example RTOSs

• Micrium ?c-OS II
• http//www.ucos-ii.com/
• AvrX
• http//www.barello.net/avrx/
• RTLinux http//fsmlabs.com/developers/man_pages/
• QNX Neutrino
• http//www.qnx.com/

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?c-OS II

• Features
• Sample main() function
• Calling OSTaskCreate()
• Creating a task

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?C-OS II

• Ports to the AVR, ATmega103
• Comes with book Micro-C OS The Real-Time Kernel
  by Jean Labrosse (52)
• Features
• Semaphores and mutexes
• Event flags
• Message mailboxes and queues
• Task management (priority settings)

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?C-OS Sample Code

• void main (void)
• / Perform Initializations /
• ...
• OSInit()
• ...
• / Create at least one task by calling
  OSTaskCreate() /
• OSStart()

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?C-OS Sample Code

• INT8U OSTaskCreate (
• void (task)(void pd),
• void pdata,
• OS_STK ptos,
• INT8U prio)

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?C-OS Sample Code

• void UserTask (void pdata)
• pdata pdata
• / User task initialization /
• while (1)
• / User code goes here /
• / You MUST invoke a service provided by
  µC/OS-II to /
• / ... a) Delay the task for n ticks /
• / ... b) Wait on a semaphore /
• / ... c) Wait for a message from a task or an
  ISR /
• / ... d) Suspend execution of this task /

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AvrX

• Specs
• Internal structures
• Sample code task creation

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AvrX Specs

• Code size 500-700 words (2x bigger with debug
  monitor)
• 16 priority levels
• Overhead 20 of CPU
• Version 2.3 for assembly code, 2.6 for C interface

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AvrX Internals

• Routine _Prolog saves state, _Epilog restores it
• Task info stored in a PID block and a task
  control block
• Modules for timers, semaphores, etc.

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An AvrX Task

• AVRX_TASKDEF(myTask, 10, 3)   
  TimerControlBlock MyTimer    while (1)   
          AvrXDelay(MyTimer, 10) // 10ms
  delay        AvrXSetSemaphore(Timeout)   

Arguments task name, additional stack bytes,
priority
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RTLinux

• Additional layer between Linux kernel and
  hardware
• Worst-case dispatch latency on x86 15 ?s

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RTLinux Basic Idea
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QNX Neutrino

• Powerful hard RTOS
• Includes GUI
• Memory protection and fault tolerance

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Summary

• Hard real-time, soft real-time
• Characteristics of an RTOS
• Example RTOSs
• Specs
• Internal structure/ideas
• Sample code