The characteristics listed above mean that an encapsulated subsystem must meet the following requirements:
The subsystem needs “private” copies of the runtime systems used and the ILCS. The entries of these components must no longer be visible to the outside.
The subsystem must include a switch which controls the initialization of the subsystem the first time it is called.
The subsystem needs variables for the addresses of the save areas of the application and the first save area of the subsystem. The PSA and EHL fields in the first save area of the subsystem must contain the value F‘-1‘.
In addition, certain considerations must be borne in mind when linking subsystems, switching to the subsystem environment and performing event and contingency handling. These are described in the following sections.