SCDDQN and SCDDP

Because asynchronous demand paging is not recommended, the SCDDQN and SCDDP fields are not used in a CICS®-IMS ISC session.

The SCHEDULER header is preceded by an ATTACH header. Some mandatory ATTACH FM header fields also apply to asynchronous messages. These fields are not carried on the SCHEDULER header, but are retained on the ATTACH header. These fields are the ATTIU, ATTDSP, and ATTDBA fields.

Their values in the asynchronous environment are as follows:
  • ATTIU

    CICS only supports single-chain messages between itself and IMS.

  • ATTDSP

    When sent using the START command, the ATTDSP value is determined by the terminal-control table generation, and should be X'00' (component 1). For input and output, the definition of the ATTDSP field for IMS is the same as that described in ATTACH function management header.

  • ATTDBA

    The CICS transaction specifies the deblocking algorithm (ATTDBA) in the RECFM field of the BUILD ATTACH command.

The following table summarizes the source of the values placed in the SCHEDULER FM header fields.

Table 1. Source of values placed in the SCHEDULER FM header fields
Major field A1 B2 C3 D4
SCDDPN TRANSID5   DPN  
SCDPRN TERMID5   PRN  
SCDRDPN RTRANSID5   RDPN  
SCDRPR RTERMID5   RPRN  
SCDDQN QUEUE   Note 6  
ATTIU7     Note 6  
ATTDSP7   DATASTR8   COMPTn8
ATTDBA7   RECFM   VLVB
        DPM-Bn9
Notes:
  1. CICS EXEC START/RETRIEVE.
  2. CICS DEFINE CONNECTION or DFHTCT TYPE=SYSTEM.
  3. Set by IMS MFS; or, if MFS is not used, for output that is unsolicited asynchronous output, the RPRN defaults to the source LTERM name.
  4. IMS COMPTn in TERMINAL/NAME macros or on an ETO logon descriptor.
  5. Four-byte name; MFS supports an eight-byte name.
  6. MFS paging is not recommended on an asynchronous session between IMS and CICS.
  7. ATTACH FMH fields preceding SCHEDULER FMH.
  8. DATASTR must be specified as USER. This causes X'00', specifying IMS component 1, to be indicated as the input component. The IMS output component can be components 1 through 4 (X'00'- X'03').
  9. DPM-Bn permits MFS to be used. Using MFS MODE=STREAM on output changes the ATTDBA to chain mode; otherwise, IMS uses the default of VLVB.