DFASY exit routine

The DFASY exit routine provides a way for VTAM® to notify an application program that an expedited-flow data-flow-control request has arrived. The requests that can be received by an application program in a DFASY exit are:
  • Quiesce at End of Chain (QEC)
  • Release Quiesce (RELQ)
  • Request Shutdown (RSHUTD)
  • Shutdown Complete (SHUTC)
  • Shutdown (SHUTD)
  • Signal (SIG)
  • Stop Bracket Initiation (SBI)

For information on all of these requests, see Summary of control requests and indicators. See DFSYN, DFASY, and RESP types of RUs for information on the DFSYN, DFASY, and RESP types of RUs.

If a DFASY exit routine is specified in an NIB EXLST or in an ACB EXLST applicable to the session whenever an expedited-flow data-flow-control request arrives, VTAM can schedule that DFASY exit routine. The detailed manner in which VTAM handles DFASY input is shown in Figure 1 and Figure 2. If an NIB exit routine is specified, it is always scheduled, whereas a specified ACB exit routine is scheduled only if the following conditions exist:

  1. No NIB exit routine is specified.
  2. The NIB for the session specified DFASYX.
  3. The session is in CA mode for DFASY input.
  4. RECEIVE OPTCD=SPEC,RTYPE=DFASY is not currently queued.

Using a DFASY exit routine is an alternative to getting each expedited-flow data-flow-control request with RECEIVE RTYPE=DFASY. The advantages and disadvantages of the two alternatives are discussed in Explicit RECEIVEs and EXLST exit routines.

For a DFASY exit routine, information about the request that has been received is available in a read-only RPL provided by VTAM. This RPL resides in read-only VTAM storage and that cannot be used by RPL-based macroinstructions. The following are also true for this RPL:
  • The application program uses SHOWCB, TESTCB, or assembler instructions with the IFGRPL DSECT to examine the RPL fields
  • 3-byte user RH field is set in the read-only RPL (see Operation for inbound RUs for more information)
  • All feedback fields (except REQ) are set exactly as they would be following RECEIVE RTYPE=DFASY (see Figure 2)
  • CHECK must not be issued for the RPL
  • RPL is freed when control returns to VTAM.
The location of the read-only RPL is provided in the parameter list passed to the exit routine when the routine is scheduled.
Table 1. DFASY exit routine: Registers upon entry
Reg Contents
0, 2-13 Unpredictable
1 Address of a parameter list that includes the following:
  • Word 1—address of ACB for application program to which the expedited-flow data-flow-control request is sent
  • Word 2—CID of the session
  • Word 3—USERFLD data from NIB that is used to establish the session
  • Words 4, 6—reserved
  • Word 5—address of a VTAM-supplied, read-only RPL
14 VTAM address that is branched to when DFASY exit routine completes processing
15 Address of DFASY exit routine
Parent topic: Using exit routines