DC Programming Techniques › Passing Program Control

Passing Program Control

DC provides program management facilities that allow you to pass control either between programs in a single task thread or from task to task. Using these program management functions, you can:

• Return control to the next-higher level within a task, optionally specifying the next task to be invoked on the same terminal
• Initiate execution of a program on the same level within a task; control cannot return to the calling program
• Initiate execution of a subordinate-level program within the same task, with the expectation that control will return to the instruction immediately following the request

Levels of Program Control

The figure below shows levels of programs in a task. TASKA invokes Program A, which calls Program B expecting return of control. Program B passes control laterally to Program C, which then returns control to Program A. When Program A is finished, it returns control to DC specifying that TASKX should be the next task invoked on that logical terminal.

                               Next task code 'TASKX'
                            ┌─ - - - - - - - - - - - - - - - - ─┐

                            │                                   │
           DC
───────────┬────────────────▲───────────────────────────────────┼──────────────
           │   TASKA        │                                   │   TASKX
           │                                                    │
           │                │                                   │
           │                                                    │
  ┌────────▼───────┐        │                           ┌───────▼───────┐
  │                │                                    │               │
  │   Program A    │        │                           │  Program D    │
  │   (level 1)    │                                    │               │
  │                ◄────────┼──────────┐                │               │
  │                │        │          │                │               │
  │                │                   │                │               │
  └────────┬────┬──┘        │          │                └───────────────┘
           │                           │
           │    │           │          │
           │                           │
           │    └─ - - - - ─┘          │
           │                           │
  ┌────────▼───────┐           ┌───────┴───────┐
  │                │           │               │
  │   Program B    │           │  Program C    │
  │   (level 2)    │           │  (level 2)    │
  │                ├───────────►               │
  │                │           │               │
  │                │           │               │
  └────────────────┘           └───────────────┘