Asynchronous Transfer of Control

Intro

• An ATC is where the point of execution of one SO is changed by the action of another SO.
• Therefore an SO may be executing on 1 method and through no action of its own, find itself executing in another method.
• Controversial:
  • Complicates language semantics.
  • Makes it difficult to write correct code as it may be interfered with.
  • Increases complexity of the RTJVM.
  • Slow down code which does not use the feature.

Requirements for ATC

• Fundamental requirement is for a thread to respond quickly to a condition detected by another thread.
• Error recovery: To support coordinated error recovery between real-time threads.
  • Where several threads are performing a computation, an error detected by one thread needs to be quickly and safely communicated to other threads.
• Mode changes: Where changes between modes are expected but cannot be planned.
  • A fault may lead to aircraft abandoning take off and entering emergency mode.
  • An accident in a manufacturing plant may required mode change to shutdown the plant.
  • Threads must be quickly and safely informed that the mode in which they are operating has changed and they now need to perform different sets of actions.
• Scheduling using partial/imprecise computations:
• User Interrupts - Users may want to cancel current operation because they have detected an error condition and want to start again.