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Most important requirements for real time systems:

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  • Functional requirements must be met! (like for all systems)
  • Defined response times need to be met!
  • real-time systems must meet deadlines!
  • Predictability

  • Reliability

  • Determinability 

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3 Types of hardness of real-time systems:

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  • hard real time:

    • Missing a deadline is a total system failure (airbag in car)

  • soft real time:

    • the usefulness of a result degrades after its deadline, thereby degrading the systems quality of service (warning systems

  • firm real time:

    • infrequent deadline misses are tolerable but may degrade the systems quality of service. the usefulness of a result is zero after its deadline. (car: ignition-point-optimizer for motor)

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Realtime System Tasks:

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  • Periodic task:

    • When the time of the reappearing is constant, the task is called a periodic task

  • Sporadic task:

    • When the recurrence is random but there is a minimum time difference between two jobs of the same task, then the task is called a sporadic task

  • Aperiodic task:

    • When the recurrence is random but there is no minimum time difference between two jobs of the same task, means they may even appear at the same time, the task is called an aperiodic task

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Classification of Real-Time Systems:

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  1. Consequence of missing deadline

  2. Reliability and fault tolerance

  3. Distribution: centralized or distributed RTS

  4. Interactive or autonomous system

  5. Hierarchical or independent systems

  6. Time-driven or event-driven RTS

  7. Periodic, aperiodic, sporadic RTS

  8. Cyclic or asynchronous scheduling

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Pagefault (Minor Pagefault):

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A page fault is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit (MMU) into the virtual address space of a process.

If the page is loaded in memory at the time the fault is generated but is not marked in the memory management unit as being loaded in memory, then it is called a minor or soft page fault. No I/O operations are necessary.

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When is a net alive / dead?

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  • We have a deadlock if no further firing Is possible, therefore the petri net is dead

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EDF (earliest deadline first) 

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  • has dynamic priorities, shorter deadline → higher priority 

  • executable task with high priority will always interrupt a task with lower priority 

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RMS (Rate Monotonic Scheduling)

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  • has a static priorities: shortest period → high p 

  • preemptive (präventiv)

  • if a system not schedulable with RMS → cannot be schedulable with any other static priority

  • Load: U= texc/tdead und am ende alle addieren

    • Load U = 1 means processor never idles 

    • Load U > 1 there is no feasible schedule 

    • Load U < 1 means this test does not exclude that feasible schedule may exist

  • Schedulability Test durchführen: U <= n*(21/n-1)

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Definition of execution time and net execution time?

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• Execution time is the duration of a specific Job between Job request and Job completion.

• Net execution time is the execution time for a Job when the CPU is used exclusively for this specific Job.

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What is an embedded system? Examples?

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An Embedded System is a computer that is part of an overall system and controls physical processes.

Examples:

• Missile Control

• Digital Camera

• Home Security System

• Industrial Robots

• Calculator

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What is a system and a process?

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System: A System is a set of Elements, Objects, Components or Modules that have a relation and may interact with each other.

Process: A process is set of relations or actions that converts a predefined input into a required outcome.

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What is a sensor? Name examples of sensors

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A Sensor converts a physical or chemical measure into electrical signals.

Examples:

• Microphone, Gyroscope, Accelerometer, Magnetometer, Micro Electronic Mechanical Systems (MEMS)

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Q:

Most important requirements for real time systems:

A:


  • Functional requirements must be met! (like for all systems)
  • Defined response times need to be met!
  • real-time systems must meet deadlines!
  • Predictability

  • Reliability

  • Determinability 

Q:

3 Types of hardness of real-time systems:

A:
  • hard real time:

    • Missing a deadline is a total system failure (airbag in car)

  • soft real time:

    • the usefulness of a result degrades after its deadline, thereby degrading the systems quality of service (warning systems

  • firm real time:

    • infrequent deadline misses are tolerable but may degrade the systems quality of service. the usefulness of a result is zero after its deadline. (car: ignition-point-optimizer for motor)

Q:

Realtime System Tasks:

A:
  • Periodic task:

    • When the time of the reappearing is constant, the task is called a periodic task

  • Sporadic task:

    • When the recurrence is random but there is a minimum time difference between two jobs of the same task, then the task is called a sporadic task

  • Aperiodic task:

    • When the recurrence is random but there is no minimum time difference between two jobs of the same task, means they may even appear at the same time, the task is called an aperiodic task

Q:

Classification of Real-Time Systems:

A:
  1. Consequence of missing deadline

  2. Reliability and fault tolerance

  3. Distribution: centralized or distributed RTS

  4. Interactive or autonomous system

  5. Hierarchical or independent systems

  6. Time-driven or event-driven RTS

  7. Periodic, aperiodic, sporadic RTS

  8. Cyclic or asynchronous scheduling

Q:

Pagefault (Minor Pagefault):

A:

A page fault is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit (MMU) into the virtual address space of a process.

If the page is loaded in memory at the time the fault is generated but is not marked in the memory management unit as being loaded in memory, then it is called a minor or soft page fault. No I/O operations are necessary.

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Q:

When is a net alive / dead?

A:
  • We have a deadlock if no further firing Is possible, therefore the petri net is dead

Q:

EDF (earliest deadline first) 

A:
  • has dynamic priorities, shorter deadline → higher priority 

  • executable task with high priority will always interrupt a task with lower priority 

Q:

RMS (Rate Monotonic Scheduling)

A:
  • has a static priorities: shortest period → high p 

  • preemptive (präventiv)

  • if a system not schedulable with RMS → cannot be schedulable with any other static priority

  • Load: U= texc/tdead und am ende alle addieren

    • Load U = 1 means processor never idles 

    • Load U > 1 there is no feasible schedule 

    • Load U < 1 means this test does not exclude that feasible schedule may exist

  • Schedulability Test durchführen: U <= n*(21/n-1)

Q:

Definition of execution time and net execution time?

A:

• Execution time is the duration of a specific Job between Job request and Job completion.

• Net execution time is the execution time for a Job when the CPU is used exclusively for this specific Job.

Q:

What is an embedded system? Examples?

A:

An Embedded System is a computer that is part of an overall system and controls physical processes.

Examples:

• Missile Control

• Digital Camera

• Home Security System

• Industrial Robots

• Calculator

Q:

What is a system and a process?

A:

System: A System is a set of Elements, Objects, Components or Modules that have a relation and may interact with each other.

Process: A process is set of relations or actions that converts a predefined input into a required outcome.

Q:

What is a sensor? Name examples of sensors

A:

A Sensor converts a physical or chemical measure into electrical signals.

Examples:

• Microphone, Gyroscope, Accelerometer, Magnetometer, Micro Electronic Mechanical Systems (MEMS)

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