Functions available to an ISC VTAM session

ISC sessions with CICS® on VTAM® connections support both the synchronous and asynchronous CICS APIs.

This topic identifies synchronous and asynchronous command sequences available to a CICS application on ISC VTAM sessions.

The support for front-end/back-end system utilization that the IMS Front-End Switch exit routine provides uses ISC in asynchronous mode.

CICS SEND and RECEIVE commands create the predefined SNA protocols associated with synchronous processing when SEND is used with the INVITE option. SEND INVITE causes an SNA ATTACH function management header and change-direction to be issued with the message. The message is sent from CICS to the remote IMS subsystem for processing. When the message reply is returned, it is processed by a RECEIVE command issued within the same CICS transaction that issued the first message.

Exception: Restarted transactions are handled differently.

When the SEND command is used with the LAST option, it causes the message to be sent on the session with an ATTACH function management header and carries both begin-bracket and end-bracket (BB/EB) status. IMS treats this special case of synchronous processing in the same way as an IMS asynchronous transaction is treated. If IMS generates an output reply as a result of an input generated with SEND LAST, the returned reply can be processed by a newly initiated CICS transaction. The IMS output reply is also sent with ATTACH BB/EB. CICS uses RECEIVE to process that reply.

The START and RETRIEVE commands create the SNA protocols associated with asynchronous processing. A START command causes an SNA ATTACH function management header and a concatenated SCHEDULER function management header to be issued with the message. Messages sent to IMS with the START command can be only one chain in length and are sent with BB/EB. CICS uses the RETRIEVE command to obtain asynchronous messages from the session for CICS transaction processing. The other SNA data flow control protocols issued with an asynchronous message depend upon the type of message issued.

The following series of tables summarize the functions available to an ISC VTAM session between IMS and CICS. You must know whether the flow on a session is to be synchronous or asynchronous in order to determine what functions are available to that session. See the notes following the tables for further explanations.

The tables do not take into consideration the recovery aspects of any of the listed combinations. A Yes indicates only that the approach is possible. However, a system designer should take into account the need for ease of recovery and restart.

Table 1. Functions available to an ISC VTAM session for CICS EXEC commands: CICS front end
Functions available

SEND(INVITE)/
RECEIVE
(Synchronous)

SEND(LAST)
(Synchronous)

START/
RETRIEVE1
(Asynchronous)
Nonresponse or nonconversational mode IMS input transaction No Yes Yes
Nonresponse or nonconversational mode IMS output transaction Yes2 N/A Yes
Response mode IMS transaction (including Fast Path) Yes3 No No
Conversational mode nonlast IMS input transaction yes No4 No
Conversational mode last IMS input transaction No No4 Yes
IMS message switch No Yes Yes
IMS operator command Yes5 Yes6 Yes6
Recoverable7 IMS transaction N/A N/A N/A
Nonrecoverable7 IMS transaction N/A N/A N/A
Single-segment input/output IMS transaction Yes Yes Yes
Multi-segment input/output IMS transaction Yes Yes No
Table 2. Functions available to an ISC VTAM session for CICS EXEC commands: IMS front end
Functions available SEND(INVITE)/ RECEIVE (Synchronous) SEND(LAST) (Synchronous) START/ RETRIEVE1 (Asynchronous)
CICS transaction No No Yes
CICS operator command (system transaction) No No No
Table 3. Functions available to an ISC VTAM session for CICS EXEC commands: MFS and BMS support
Functions Available8

SEND(INVITE)/
RECEIVE
(Synchronous)

SEND(LAST)
(Synchronous)

START/
RETRIEVE1
(Asynchronous)
Without IMS MFS Yes Yes Yes
MFS without paging Yes Yes Yes
MFS autopaged input Yes9 Yes9 No
MFS autopaged output Yes10, 2 N/A No
MFS demand-paged output Yes N/A No11
BMS support8 N/A N/A N/A
Notes for CICS EXEC tables:
  1. Single-chain messages only.
  2. The nonresponse-mode reply to a transaction sent with SEND LAST is sent ATTACH BB/EB. CICS uses RECEIVE (without SEND) to obtain the message from the session. (See CICS to IMS using the SEND LAST EXEC command.)
  3. IMS forces response mode regardless of system definition if a transaction is specified externally (using the function management header) as synchronous. See Relationship of ISC and IMS execution modes.
  4. IMS conversational mode requires that IMS terminate the conversation by sending the last reply with EB. The exception to this is explained in CICS versus IMS conversation mode.
  5. The replies that result from the IMS /DISPLAY, /FORMAT, and /RDISPLAY commands are sent asynchronously after IMS replies synchronously to the input command.
  6. Supported only for /DISPLAY, /RDISPLAY, and /FORMAT.
  7. A transaction is defined as recoverable or nonrecoverable during IMS system definition and is acceptable to IMS only when the sync point protocols and transaction definitions are consistent. See Relationship of ISC and IMS execution modes and Keeping half sessions synchronized for more information.
  8. MFS maps data between the session and the IMS application program. BMS maps data at the request of the CICS application program. BMS does not interact with the session, but can be used to handle mapping of data to and from IMS. IMS's MFS and CICS's BMS do not communicate.
  9. Both single- and multiple-message chains can be used, but only one chain can occur per SEND.
  10. To avoid tying up the session, when processing synchronous autopaged output from IMS, CICS should read all pages of the IMS output before processing it.
  11. Although demand paging is possible between IMS and CICS, it is not recommended, because it requires complex CICS application coding and a complex terminal user interface. A preferable approach is to create autopaged output from IMS to CICS and use CICS page retrieval in the local system for paging.