Explicit RECEIVEs and EXLST exit routines

A VTAM® application program can receive expedited-flow data-flow-control requests (for example, a Quiesce at End of Chain request) or responses in a number of ways. A RECEIVE can specify RTYPE=DFASY (for data-flow-control requests) or RTYPE=RESP (for responses) or both. In addition, data input can complete the same RECEIVE (for example, RTYPE=(DFSYN,DFASY,RESP) can be specified). When the RECEIVE is posted complete, the program examines the RTYPE field of the RPL to determine which kind of input was received and branches to an appropriate routine. Alternatively, RECEIVEs can be used only for normal-flow requests, and the addresses of the special input routines can be designated (the DFASY and RESP exit routines) in an EXLST macroinstruction to handle responses and expedited-flow data-flow-control requests.

Using exit routines requires execution of more system instructions than checking the RTYPE field. On the other hand, using RECEIVE requires the use of an RPL to await the input. See Figure 1 through Figure 4 for the detailed logic used by VTAM to classify input RUs, to complete RECEIVEs, and to schedule EXLST exit routines.

Note: If an application program issues multiple RECEIVE OPTCD=ANY macroinstructions that can be satisfied by a given input RU, it is unpredictable which particular RECEIVE macroinstruction is posted complete with the RU. A similar statement can be made if multiple RECEIVE OPTCD=SPEC macroinstructions are issued for a session and then an RU arrives that could satisfy any one of them.

Figure 1. How input RUs are classified by VTAM

The diagram shows how input RUs are classified by VTAM.

Figure 2. How VTAM handles DFASY (expedited-flow data-flow-control request) input

The diagram shows how VTAM handles DFASY (expedited-flow data-flow-control request) input.

For DFASY input, see also Figure 1.

Figure 3. How VTAM handles RESP (normal-flow response) input

The diagram shows how VTAM handles RESP (normal-flow response) input.

For RESP input, see Figure 1.

Figure 4. How VTAM handles DFSYN (normal-flow request and DFSYN response) input

The diagram shows how VTAM handles DFSYN (normal-flow request and DFSYN response) input.

For DFSYN input, see Figure 1.