Replacing VTAM exit routines

You can use the MODIFY EXIT command to replace an exit with an alternate copy of the exit without interrupting VTAM® processing.

Follow these steps to prepare the alternate copy of the exit that will replace the current copy of the exit:

Assemble the exit. You should use the VTAM module name shown in Table 1 unless you provide an alternate exit.
Place the exit in the appropriate VTAM library as indicated in Table 1.

If the replacement is successful, message IST984I is issued by VTAM. The replacement might fail because VTAM cannot find the alternate copy of the exit or VTAM might not be able to load the alternate copy of the exit. Also, an error in the MODIFY EXIT command can cause replacement failure. VTAM issues any of the following messages to indicate replacement failure: IST985I, IST452I, IST453I, IST454I, IST456I. See z/OS Communications Server: SNA Messages for more information.

If VTAM cannot load or activate the alternate exit, VTAM attempts to reactivate the original exit and regain the function of the exit. If the exit cannot be activated, message IST984I indicates that the original exit is inactive.