Follow these procedures when writing the routine. Errors in the routine could damage VTAM® or system control blocks and modules.

• Use standard linkage.
• Save and restore registers 2–14.

Consider the following restrictions when writing this routine:

• The name of the USERVAR exit routine module should be ISTEXCUV; however, if you have written an alternate load module, use the load module name you assigned to your replacement module.
• This routine operates enabled in pageable storage. Because the routine operates at VTAM's main task dispatching priority, there is a possibility of lockout if a wait requires other task action. The routine gains control in supervisor state with a VTAM storage key. Errors within the routine could cause damage to VTAM or system control blocks and modules.
• There can be no system waits, including implied waits for I/O operations. System waits cause VTAM failure in some timing-dependent situations.
• Do not issue any SVCs if this exit routine is running in SRB mode. The USERVAR exit is driven in SRB mode for all functions except invoke processing (entry code X'04'). See the description of register 0 at Initial register contents.
• 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 31-bit mode only. Data is always presented with 31-bit addressability.
• This exit routine can be above or below the 16M line.
• Link-edit your routine into the appropriate library. See Installing VTAM exit routines for more information.
• This exit routine operates as an internal VTAM routine. VTAM performance might be degraded if the routine requires lengthy processing time. While this routine has control, VTAM does not process any new operator requests, session initiation requests, or session termination requests for any resource.