Interchange node selection function (function code X'02')

This function is optional. If this function is processed, then when VTAM® builds an interchange node list for sending directed locate searches to interchange nodes, VTAM forwards a copy of the interchange node list to the exit.

The user exit can change the interchange node list by using one of the following methods:

Removing entries from the list
Reordering the entries in the list

Modifying the interchange node list might involve the physical movement of data within storage. For example, to remove an entry from the list, the entries following the deleted entry must be moved to be contiguous with the entry before the deleted entry, and the number-of-entries count must be decremented by 1. If the exit returns an empty list, the return code is set to X'04', indicating that no interchange node search procedure should be attempted. If the exit returns an interchange node that was not in the original list, diagnostic code X'27' is issued by message IST1183I, and the return code is set to X'08'.

In addition to modifying the interchange node list, the exit can indicate that no interchange node search procedure should be attempted.

The user-written exit returns to VTAM with the following return code:

Register 15:

Return code

X'00' (0): Interchange node searching is allowed using the interchange node list returned by the exit.
X'04' (4): Interchange node selection was not performed. The original interchange node list should be used.
X'08' (8): Interchange node searching is not allowed.

All other return codes are treated as return code X'04'.

If the exit return code is greater than X'7F', VTAM ignores any options you specified and issues error message IST985I.