Chapter 11: Distributed Processing Variations of subprogram control

1. Chapter 11: Distributed ProcessingVariations of subprogram control • Explicit call statements • Exception handlers • Subprograms must execute completely • Co-routines • Control must be transferred immediately • Scheduled subprograms • Single execution sequence • Parallel programming

2. Exceptions and exception handlers • Exception Handlers are subprograms that are not invoked by explicit calls • Special situations, called exceptions: • Error conditions • Unpredictable conditions • Tracing and monitoring

3. Exceptions and exception handlers • Exception handlers typically contain only: • A set of declarations of local variables • A sequence of executable statements • Exception Handlers can be • predefined in the language • programmer defined

4. Raising and catching an exception • Languages provide methods for raising (throwing) and testing for exceptions. • try{ • statement1; • statement2; • … • ifbadCondition throw ExceptionName; • } • catch ExceptionName • { ……….// do something for exception…….}

5. Implementation • .Operating system exceptions - raised directly by hardware interrupts. • .Programmer defined - • the translator inserts code to handle the exceptions.

6. Coroutine A Coroutine B Resume A Resume B Resume A Resume B Resume B Co-routines Subprograms that can return control to the caller before completion of execution.

7. Implementation of co-routines Similar to the simple call-return structure, the difference is in handling the CIP. Each co-routine has IP location in its activation record, used to provide the value for CIP on resuming the execution.

8. Scheduled subprograms • Subprograms scheduling techniques: • Execution after other subprograms – call B after A • Execution when a Boolean expression is true – call B when X=5 • Execution on the basis of a simulated time scale - call B when time=25 • Execution based on priority – call B with priority 7

9. Implementation of scheduling techniques Scheduling is a run-time activity. The process is controlled (usually) by a system-defined scheduler program that maintains a list of currently scheduled subprogram activations.