BDT uses a macro, BDTXUEX, to transfer control to user exit routines. The BDTXUEX macro either calls or branches to each routine, depending on whether the routine is in the BDT address space or the link pack area:

• BDTXUEX uses the BDTXCALL linkage to reach user exit routines that are in the BDT address space. The calling module’s registers are saved in BDTGRSV prior to invoking the exit routine, the exit routine is linked, and exit processing is performed. Return to the calling routine is through register 14, and includes a return to BDTGRSV to reload the calling routine’s registers before returning control to the calling module. (Refer to Figure 1, part A, for a representation of this processing.)
• BDTXUEX uses a BALR instruction to reach user exit routines that are in the link pack area (always BDTUX28 and usually BDTUX08, BDTUX10, and BDTUX19). As a result, the caller’s registers are not saved, nor are the registers automatically restored upon return; this is your responsibility when coding these exit routines. The BDTXCALL linkage cannot be used in this case: the registers are not saved in BDTGRSV because the BDTXCALL linkage has no access to the BDT TVT. Return to the calling module is direct with use of a BR14 instruction. (Refer to Figure 1, part B, for a representation of this processing.)