VTAM® allows you to use a command verification exit routine to screen and manipulate VTAM commands entered by an operator. (The term operator includes a program operator.) You can use a command verification exit routine on VTAM DISPLAY, MODIFY, and VARY commands. When an operator enters a command, VTAM passes the address of the command string and the address of the command string length field to the exit. The exit verifies the specifications listed in the command and determines whether VTAM should process the command as entered by the operator. When necessary, the exit can change or remove operands and values from the command or indicate to VTAM not to process the command. The primary purpose of the command verification exit is to screen command requests that affect critical nodes in the network. You can create a list of critical nodes when you write the exit routine.

IBM® does not supply a default command verification exit routine. If you want the function of the exit, write a command verification exit routine. Sample code for this exit is in Command verification exit routine. If you write a routine and link-edit it into the appropriate library, VTAM will load the exit into pageable VTAM private storage and mark the exit active during VTAM initialization.

VTAM either initializes the command verification exit routine during VTAM initialization, or activates the routine after you issue a MODIFY EXIT,ID=exitname,OPTION=ACT command.

You can use the DISPLAY EXIT command to display information regarding a command verification exit. You also can use a MODIFY EXIT command to activate, deactivate, or replace the command verification exit routine without interrupting VTAM processing. See Operator commands for VTAM exit routines for more information about using the MODIFY EXIT command to modify VTAM exit routines. See z/OS Communications Server: SNA Operation for more information about these commands.

The topics that follow contain information you need to write a command verification exit routine.