Notify message

IMS passes the address of a notify message on the Identify exit routine invocation for the control region.

If the external subsystem is not active (has not been started), the Identify exit routine can indicate (return code 4) to IMS that the notify message has been accepted and will be sent to IMS when the external subsystem is active. The external subsystem, once started, sends the message to IMS using an internal z/OS® MODIFY command (SVC 34) to alert IMS that it is ready to connect. On receipt of the notify message, IMS reactivates the Identify exit routine to establish the connection.

External subsystem code that is always present in the z/OS system (early code), for example, might be used as the means to pass the notify message to the external subsystem. The Identify Exit queues the message to the early code so that it is available to the external subsystem whenever it is started.

IMS passes the notify message to the Identify exit routine in the format shown in the following figure.
Figure 1. Notify message format
Graphic shows a 3 column 1 row table. The first column (from left to right) is labeled "LL" and is 2 bytes. The second column is labeled "ZZ" and is 2 bytes. The third column is labeled "message_text" and has variable bytes.
where:
LL
Is a 2-byte field containing the message length (LL + ZZ + MESSAGE_TEXT).
ZZ
Is a 2-byte field containing binary zeroes.
MESSAGE_TEXT
Is the notify message text that IMS expects to receive by the MODIFY command . The message text must not be altered.
Issue the MODIFY (F) command as follows: 
MODIFY ims_jobname,message_text
The external subsystem must prefix the notify message text passed by IMS with MODIFY ims_jobname(or F ims_jobname), before sending the message. The following figure shows the format for the SVC 34 command input.
Figure 2. SVC 34 command input format
This graphic is described in the text below.
where:
LL
Is a 2-byte field containing the total length of the command input area (COMMAND + SPACE + IMS_JOBNAME + COMMA added to the length in the LL field passed to the Identify exit routine).
ZZ
Is a 2-byte field containing binary zeroes.
COMMAND
Contains the MODIFY command verb (C'MODIFY' or C'F').
SPACE
Is a 1-byte field containing a blank (C' ').
IMS_JOBNAME
Is an 8-byte field containing the IMS control region job name left justified and padded with blanks on the right. The Identify exit routine can obtain the job name from the TIOT pointed to by the current TCB.
COMMA
Is a 1-byte field containing a comma (C',').
MESSAGE_TEXT
Is the notify message text passed to the Identify exit routine.