Actions required for encapsulation

A called subsystem function must carry out the following actions

1. Save the registers of the caller in the save area of the caller and the address of this save area in the appropriate variable.

2. The first time a subsystem is called (switch set), call IT0INITS to initialize the ILCS and call the language runtime system and enter the PCD address returned by IT0INITS in the save area of the subsystem (then deactivate the switch).

3. Deactivate SEH event handling of the subsystem with IT0SEHEN if the subsystem does not perform any explicit event handling.
   Alternatively, the event handling of the called program can be deactivated with IT0CRDEA and the ILCS of the subsystem activated with IT0CRACT if the switch is not set.

4. Load the address of the first save area of the subsystem in register 13. This switches to the ILCS of the subsystem, and the code of the called function can be executed.

Before returning to the application, the following actions must be carried out:

1. If the event handling of the subsystem was activated with IT0CACT, it should be deactivated with T0CDEA before the subsystem is called again provided that no asynchronous, instance-specific events are to be delivered to the subsystem while it is inactive.
   The ILCS of the calling program should be reactivated (IT0CACT).

2. The registers of the application must be loaded, and the address of the save area of the application must be loaded in register 13.

3. The results of the called subsystem function must be passed on to the application.