Follow these procedures when writing this routine:

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

When designing this routine, consider the following information:

• If the exit returns a dynamic switched major node name:
  • The name must be 1–8 characters.
  • The first 3 characters must not be IST.
  • The first character must be alphabetic or national, and the remaining characters must be alphabetic, numeric, or national.
• If the exit returns the name of a major node that already exists:
  • The major node cannot be an application major node.
  • The major node must be a dynamic switched major node.
  • The major node must be active.
• If the exit does not return a major node name, ISTDSWMN is returned as the default name.
• The name of the configuration services XID exit routine module should be ISTEXCCS; 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.
• Exit routines that perform I/O must be link-edited AMODE=31 and RMODE=24.
• 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 to perform any necessary I/O or other processing without affecting the VTAM main task.
• Do not use VTAM macroinstructions in this routine.
• This exit routine operates enabled in pageable storage. The routine gains control in supervisor state with a VTAM storage key. Errors in the routine could damage VTAM or system control blocks or modules.