Introductory Lecture

These are lecture notes from my Computer Science course. For learning about real-time systems, I recommend Real-Time Systems and Programming Languages.

Terminology

  • Hard real-time: Systems where it is imperative that responses occur within the required deadline, e.g. a flight control system
  • Soft real-time: Systems where it would be useful to know the data immediately, but the data can still be useful after deadlines (degrading)
  • Firm real-time: Similar to hard real-time. Soft real-time but there’s no benefit to late delivery. Different to hard real-time in that in hard real-time old data has a negative use, with firm real-time it just has zero use.

  • Time-aware: System that makes an explicit reference to time
  • Reactive: Control systems, systems that must react (produce output) within deadline based on input. Subject to jitter – variability in input/output read/write.

  • Time-triggered: computation is triggered by passage of time, e.g. a periodic activity. Polling
  • Event-triggered: Javascript. A sporadic activity is where there is a bound on the arrival interval of the event (no two interrupts within a period). An aperiodic activity is when intervals could be made at any time. This is important because might need to know about load on processor.
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