Design requirements

Follow these procedures when writing this routine:

Use standard linkage.
Save registers 0–14.

Consider the following restrictions when writing this exit routine:

The name of the virtual route selection exit routine module should be ISTEXCVR; however, if you have written an alternate load module, use the load module name you assigned to your replacement module.
Do not issue any SVCs if this exit routine is running in SRB mode.
This exit routine should use only conditional storage invocations. You can reduce the possibility of a VTAM® abend during a storage shortage by coding conditional storage invocations.
Data is addressable in 24- or 31-bit mode. It is advisable that you use 31-bit addressing.
This exit routine can be above or below the 16M line. Data is always presented below the line.
This exit routine must be reentrant.
This exit routine must be link-edited into the appropriate library. See Installing VTAM exit routines for more information.
This exit routine runs under a VTAM subtask that permits the exit routine to perform any necessary I/O or other processing without affecting the VTAM main task.
This exit routine executes in problem state with a VTAM storage key.

The exit routine can set up its own ESTAE (extended specify task abnormal exit) environment for recovery. See z/OS® application development publications for more information. A SYNC interface will be used between VTAM and the exit routine, so that the VTAM module invoking the exit routine will be isolated from any ESTAE environment established by the exit routine.