IUI Load Module Search
In Connect:Direct® for z/OS® for z/OS 6.0 and later, the IUI calls load modules in two ways. The IUI uses the ISPF SELECT PGM service call when it calls modules that are limited to IUI usage. The IUI uses the MVS macros LINK or LOAD with the DCB parameter when it calls modules common to the entire IBM Connect:Direct .
When you do not employ LIBDEF ISPLLIB with the IUI session, the IUI passes a zero in the DCB parameter, which is the same as not specifying the DCB parameter at all. When you employ LIBDEF ISPLLIB, the IUI passes the DCB of the LIBDEF ISPLLIB concatenation to LINK and LOAD. A nonzero DCB changes the MVS load module search order by skipping the requesting task's task library and all the unique task libraries of its preceding tasks (for example, ISPLLIB and TSOLIB concatenations). It also skips the STEPLIB concatenation. If you employ LIBDEF ISPLLIB, all IUI load modules must be in either the LIBDEF ISPLLIB concatenation or the LINKLIST. For more information about the MVS load module search order, see “The search for the load module” z/OS MVS Programming: Assembler Services Guide. For more information about the SELECT service load module search order, see “Application data element search order” in ISPF Services Guide, LIBDEF service.