Programming Guidelines

Usually, you should observe these guidelines in coding a routine:

Code your routine reentrant.
Save and restore registers (see individual routines for other requirements).
Be aware of registers used by the VSAM request macros.
Be aware of the addressing mode (24-bit or 31-bit) in which your exit routine will receive control.
Determine if VSAM or your program should load the exit routine.

A user exit that is loaded by VSAM is invoked in the addressing mode specified when the module was link edited. A user exit that is not loaded by VSAM receives control in the same addressing mode as the issuer of the VSAM record-management, OPEN, or CLOSE request that causes the exit to be taken. It is the user's responsibility to ensure that the exit is written for the correct addressing mode.

Your exit routine can be loaded within your program or by using JOBLIB or STEPLIB with the DD statement to point to the library location of your exit routine.

Related reading: When you code VSAM user exit routines, you should have available z/OS DFSMS Macro Instructions for Data Sets and z/OS DFSMS Access Method Services Commands and be familiar with their contents.