gtpc3m1uConcepts and Structures

Switch Among Subsystems and Subsystem Users

Some ECB-controlled system utility programs are used for database management and need to access all subsystems and subsystem users in a subsystem. A mechanism to "switch" among subsystems and subsystem users is needed. Switch, in this case, means to change an index to a relevant MFST and FCTB used to resolve file references during online execution. See Figure 46. Two important cases are:

Because file resident ECB-controlled programs are shared, they are accessible to all subsystem users within a given subsystem and are called the program base of the subsystem. Also observe that ECB-controlled system programs in the program base of the basic subsystem have the potential to be shared among all subsystems. This is different from sharing programs or data within a single subsystem. Shared programs or data within a subsystem are referenced in the subsystem FCTB; no switching is required.

Three fields are included in the ECB to identify the subsystem and subsystem user of an execution environment. System programs use these fields to switch among subsystems and subsystem users.

DBI and PBI locate a subset of modules; SSU ID locates a unique reference in the FCTB (see Figure 46). So, it is possible to be running the system with a PBI that is different from the DBI; the normal application environment runs with PBI=DBI and an appropriate SSU ID. These fields can be modified with the Cross Subsystem Access Services (CROSC) macro.

Figure 46. Cross System Access Services