Real Time Systems

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Real Time Systems

• Eddie Isaac
• 04/11/2007

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Definition

• What is a real time system?
• A system that must satisfy explicit (bounded)
  response-time constraints or risk severe
  consequences, including failure.
• Systems logical correctness is based on both
  correctness of the output and the timeliness

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A few examples

• A temperature sensor to determine over
  temperature in a steel /nuclear plant.
• Cruise Control/Autopilot
• Stock prices from the exchange.

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Whats different?

• Performance is a main criteria
• Is the architecture suitable? (Too many nodes can
  lead to poor response time)
• Is the Operating System suitable (Preemptive
  scheduling, low interrupt latency)
• Good recovery from failure
• Asynchronous communication
• Race conditions
• Timeouts/Rankings for deadlock situations

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Programmimg Languages

• Early 1970s
• DoD used over 450 languages
• Army used TACPOL
• Navy used CMS-2
• Air Force used JOVIAL
• In 1975 , world wide competition to select
• common Language.
• Ada (Named after Countess Of LoveLace,
• First programmer in C. Babbages machine)

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Programming Languages

• Which programming languages suitable?
• C, Ada Mostly used
• C May be (allocations should not be at run
  time)
• Java Usually No
• Java real time ( a variant of Java being
  developed)

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Operating Systems

• VMS (olden days)
• VxWorks
• Windows CE (for small applications)
• Tru64 Unix (with kernel changed to real time
  mode)
• Linux as of kernel 2.6.18
• Solaris
• and many more

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Whats different testing?

• Real time inputs not possible.
• Simulation is the only way
• May have to write software for simulation.
• Good approximation
• But never the system itself.
• Need to test performance as well.

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Design issues

• Problem of synchronization
• Timing constraint
• First input should processed before the second
  input.
• Deadlocks and race

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Software Engineering

• MASCOT (Modular Approach to Software Construction
  Operation and Test )
• SDRTS (Structured design for real time systems)
• DARTS (Design Approach For Real Time
• Systems)
• DOORS ( Design of Object Oriented real time
  systems)

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MASCOT

• Modular Approach to Software Construction
• Large people working on the project.
• So, each task divided into subtasks.
• Each subtask, a separately compilable module
• Parallel execution of cooperating processes.

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MASCOT

• Processes communicate through IDA (storage area,
  intercomunication data area)
• IDAs also contain shared procedures
• for synchronization.
• Each subsystem can be expanded.
• Square cornered are IDAs
• Rounded corners are subsystems.

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MASCOT

• Design notations

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SDRTS

• Structured design for real time systems
• Similar to Structured Analysis and Design
• Modules broken up into tasks.
• Task communication.

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DARTS

• Design Approach For Real Time Systems
• New concept.
• First phase is the data flow diagram.
• Split into concurrent tasks, by determining
• which tasks are sequential and which are not.
• Determine time critical functions
• Determine CPU intensive computation, which can
  run at lower priority.

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DARTS

• Functional cohesion.
• Determine temporal cohesion, functions
• grouped together in the same task.
• TCM (Task communication modules)
• TSM (Task synchronization modules)

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DARTS

• Message Communication module.

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DARTS

• Task synchronization

Destination D Wait Event (E)
Source S Signal Event (E)
E
S
Event E
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UML

• Use UML
• Real time system to be divided into
  tasks/subsystems
• Tasks may have many objects.
• Activity/task diagram
• State machines to capture reactive behaviour
• Separate tasks for simulation.

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Activity diagram

• Activity Diagram

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State diagram

• State diagram

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Bibliography

• Object Oriented and Classical Software Engg.
  Stephen. R. Schach
• A software design method for real time systems
• H. Gomaa
• www.embedded.com Designing real time systems
  with UML.
• http//www128.ibm.com/developerworks/rational/libr
  ary/2802.html
• http//pigseye.kennesaw.edu/dbraun/csis4650/AD/U
  ML_tutorial/state.htm