An exit routine usually incorporates several functions. For example, the LOGON exit comprises: checking the user ID, checking the account number, checking the LOGON password. In addition, further checks are possible via this interface, but since only one routine can be connected to a given system exit and each computer center integrates different functions, the routines are not interchangeable between different computer centers. The individual functions themselves, however, could be interchanged.
For these reasons, it is practical for exit routines to be made up of a base routine together with one or more subroutines.
Base routine
The base routine is a general routine that can be used with all system exits. It does not contain any special service functions and serves merely to append one or more subroutines to a system exit. The subroutines are executed in the desired order. The return code from the subroutine decides whether the other subroutines are called or whether the base routine is terminated.
Subroutine
Each subroutine contains a single, functionally self-contained unit. With a minimum of programming outlay it is possible to compile an individual package for each system exit on the basis of existing subroutines.