Design requirements

Follow these procedures when writing this routine:

Use standard linkage.
Save registers 0–14.

Consider the following restrictions when writing this routine:

The name of the session authorization exit routine module must be ISTAUCAT.
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.
All data is addressable in 24-bit mode only.
This exit routine must be link-edited into the appropriate library. See Installing VTAM exit routines for more information.
The routine operates as an internal VTAM subroutine. 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.
System waits, including implied waits for I/O operations, should be avoided. System waits can cause VTAM failure in some timing-dependent situations.
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.
This exit routine should not contain VTAM macroinstructions.
This exit routine must not modify the parameter list pointed to by register 1 (described in Table 1) or any field pointed to from the parameter list.
This exit routine must supply a return code to VTAM in register 15. A return code of 0 authorizes the session to be established. Any nonzero return code means that the request is not authorized. If the request is not authorized, VTAM informs the session initiator.