When MVS™ issues either a write-to-operator (WTO) message or write-to-operator-with-reply (WTOR) message, message processing exits receive control to allow the installation to process the message. Each time a WTO or WTOR message is issued, it flows through message exit IEAVMXIT, if it exists. You can specify other message processing exits to process the message instead of IEAVMXIT in the MPFLSTxx parmlib member. MPFLSTxx also allows you to control other kinds of message processing like message highlighting, message suppression, and message automation.

After a message passes through the message processing exits, subsystems like JES2, JES3, or NetView® can receive the message for processing. For example, NetView can process any message that MPFLSTxx defines as eligible for automation. Subsystem allocatable consoles can receive the message for display.

After subsystem processing occurs, the message passes to the hardcopy log. Depending on CONSOLxx values that control hardcopy logging, the hardcopy log can record the message.

After the system records the message in the hardcopy log, CNZ_WTOMDBEXIT exit receives control from the system for each single-line WTO, multi-line WTO, or WTOR (see CNZ_WTOMDBEXIT exit for more information). Then the message passes to MCS, SMCS and extended MCS consoles where it can be displayed.

The following summarizes this generalized message flow for an MVS system:

1. The program issues the message.
2. Processing specified in MPFLSTxx for the message occurs. IEAVMXIT or the installation exits specified through MPFLSTxx receive control.
3. The subsystems receive the message.
4. Depending on CONSOLxx values, hardcopy log processing records the message. CNZ_MSGTOSYSLOG exit is invoked during the hardcopy log processing.
5. CNZ_WTOMDBEXIT exit receives the message. See z/OS MVS Installation Exits for more information.
6. The MCS, SMCS console or extended MCS console can display the message.