Design requirements

Follow these procedures when writing the exit:

Use standard linkage.
Save and restore registers 0–14.

Consider the following restrictions when implementing the exit:

Each monitor should activate its exit as a multiple of the ISTEXCPM exit routine, not as the base exit.
This routine operates enabled in pageable storage. Because the exit operates at VTAM®'s main dispatching priority, there is a possibility of lockout if a wait requires other task action. The exit gains control in supervisor state with a VTAM storage key. Errors within the exit could cause damage to VTAM or system control blocks and modules.
This exit routine operates as an internal VTAM routine. VTAM performance might be degraded if the exit requires lengthy processing time. While this exit has control, VTAM does not process any new operator requests, session initiation requests, or session termination requests for any resource. Processing should be limited to copying any data received from VTAM into storage addressable by the monitor and notifying the monitor that the data has been delivered.
Do not issue any SVCs if this exit is running in SRB mode.
There can be no system waits, including implied waits for I/O operations. System waits can cause VTAM failure in some timing-dependent situations.
This exit 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 can be above or below the 16M line.
This exit must be reentrant.
This exit should not modify the performance data parameter list (see Performance data parameter list for details).
Link-edit the exit into the appropriate library. See Installing VTAM exit routines for more information.