Real-Time

Deadline (time critical) requirements. A real-time system is required to produce a result within a non-negotiable time period.

• Common uses:
  • control device in dedicated application, e.g., control scientific experiment, medical imaging, industrial control, space shuttle control systems, anti-lock automotive brake systems, banking systems, etc.
  • some display systems
• Real-Time systems may be either;
  • hard (must react in time), the real-time system absolutely must complete critical tasks within a guaranteed time.
    • Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM)
    • Conflicts with time-sharing systems, not supported by general-purpose operating systems.
  • soft real-time (deal with failure to react in time), the real-time system can satisfy its performance criteria by running any critical task at a higher priority (of CPU access).
    • Limited utility in industrial control of robotics
    • Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.
    • In some instances, off-the-shelf operating systems such as Linux or *BSD may be modified to support soft real-time operation. An alternative is for the Linux or *BSD operating system to be run as a task within some other (less conventional) real-time operating system.
    • An example of soft real-time service is a multi-media server delivering audio or video - if it fails, no loss of life (other than social life) occurs.