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.