An overview of real time operating systems
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An Overview of Real Time Operating Systems. John C. Shovic. What is a Real Time System?. A system where timeliness is as important as the correctness of the outputs Real Time systems may not be “Fast” systems

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What is a Real Time System?

  • A system where timeliness is as important as the correctness of the outputs

  • Real Time systems may not be “Fast” systems

  • Real Time systems must satisfy explicit (bounded) response-time constraints or risk failure

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Real Time Software

  • Complexity of real time software is enormous

  • Building “polling” software that can do everything is complicated and when it needs to be changed, it’s very difficult

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More Definitions

  • Embedded System - Software system completely encapsulated by the hardware it controls

  • Organic System - Software system that is not highly dependent on the hardware and has a generalized user interface

  • Semi-detached system - Software system that has attributes of both embedded and organic systems

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  • Real Time Operating Systems

    • A Powerful Partioning Tool

    • Manages Resources

    • Manages Conflicts

    • Aids in partioning problems

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  • Tasking and Scheduling - Actions

  • ISR Support - Hardware Interface

  • Semaphores- Communication

  • Mailboxes - Communication

  • Queues - Communication

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Actions and RTOS









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Tasks, Threads and Processes

  • Task = Process - Independently executing entity that may control resources. May contain multiple threads

  • Thread - A path through the code - A task may have multiple threads iff each thread has own data area

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Task Swapping

  • Co-operative - The task gives up control

  • Time-Slice - Context switcher shares time

  • Preemptive - context switcher shares time, budgeting both on fairness and dynamic task priorities

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Real-Time Concepts

  • Doing Hard Time - Tasks have to be performed correctly and on time

  • Doing Soft Time - Tasks have to be done as fast as possible, but not on a specific time

  • Most systems are mixes

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  • Foreground - ISR/Asynchronous events

    • (Also called Interrupt Level)

  • Background - “infinite loop” that calls modules to perform desired functions

    • (Also called Task Level)

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Foreground/Background Systems








Code Execution