Preparing to install CICS modules in the MVS link pack area

Before you install modules in the MVS link pack area, you should consider the following points, described in subsequent topics:

Benefits of using the MVS link pack area

The benefits of placing code in the MVS link pack area are:

What is meant by the MVS link pack area?

The MVS link pack area comprises several areas, both above and below 16 MB. In this publication, the term MVS link pack area refers to the pageable link pack areas above and below 16MB where modules that are used from the MVS link pack area are normally installed.

Note:
The MVS link pack area has both pageable and fixed parts. Although you can install CICS modules into the fixed parts, we recommend that you use the pageable areas for performance reasons.

The term LPA specifically refers to the MVS link pack area below 16 MB, and the term ELPA specifically refers to the area above 16MB. A module that is link-edited with the RMODE(ANY) attribute is loaded into the ELPA.

Start of changeIf you install a module into the LPA or ELPA, that module will not be retrieved from the until you re-IPL your MVS with CLPA specified. To use the new module and avoid an IPL, you can use the MVS dynamic LPA in one of the following ways:

Note that dynamic LPA must not be used with DFHIRP.End of change

Which modules must be in the MVS link pack area

The CICS modules listed in Table 10 must be in the MVS link pack area for the reasons that are given in the notes after the table.

Table 10. CICS modules required in the MVS link pack area
Module Description When needed in LPA See notes after this table
DFHCSVC CICS Type 3 SVC Always 1, 2, 3,4, and 6
DFHDSPEX CICS post exit stub Always 1 , 3, and 5
DFHDUMPX SDUMPX IEASDUMP QUERY exit Always 1, and 3
DFHIRP Interregion communication program To use MRO, CICS shared database, or the console message-handling facility 1, 2, 3, and 6
DFHSSEN Subsystem interface end-of-memory / end-of-task clean up routine To use the console message-handling facility 1, 2, 3, and 6
DFHSSGC Subsystem generic connect module To use the console message-handling facility 3, and 7
DFHSSWT Subsystem interface WTO router To use the console message-handling facility 3, and 7
DFH99SVC Dynamic allocation - SVC services Always 1, and 3
Notes:
  1. Can be used only from the MVS link pack area, and must be installed there before CICS can be started.
  2. You must always install the latest service level of the modules DFHCSVC, DFHIRP (if needed) and DFHSSEN.
  3. Start of changeThe version of this module that is supplied with the current release, is downward-compatible with earlier releases of CICS. It works correctly with CICS regions running earlier releases. Therefore, if you are running different releases of CICS on the same MVS image, you should use the latest version of this module.End of change
  4. The DFHCSVC module must be defined to MVS as a Type 3 SVC (default SVC number is 216), and if you use a non-default SVC number, you must define it to CICS on the CICSSVC system initialization parameter.
    Moving DFHCSVC into the MVS link pack area

    Do not use the linkage editor to install the CICS SVC module into a library in the MVS link pack area. To copy or move the module from the hlq.SDFHAUTH library to the nominated library in the MVS link pack area, you should use either a suitable copy utility program, such as IEBCOPY, or an SMP/E USERMOD with ++MOVE statements.

  5. If you are running earlier releases of CICS with the latest version, you must ensure that the latest version of the DFHDSPEX module is installed in the MVS link pack area. The DFHDSPEX module must be in the MVS link pack area for integrity reasons, but the post exit routine, DFHDSAUT, can reside either in the MVS link pack area, or in the CICS address space. This enables you to use different levels of the DFHDSAUT module in different CICS regions running in the same MVS image, because the DFHDSAUT module may not be compatible between CICS versions.
  6. To communicate by MRO, all CICS regions in the same MVS image must use the latest level of the modules DFHCSVC, DFHIRP, and DFHSSEN in the MVS link pack area.

    If a region detects that DFHIRP is at a lower level, when attempting to open interregion communication, it issues message DFHIR3799, and interregion communication fails to open.

  7. To use the console message-handling facility, these modules must reside either in the MVS link pack area or in an APF-authorized library in the MVS linklist.

Which modules can be in the MVS link pack area?

Besides those CICS modules that must be in the MVS link pack area, other CICS modules and user application program modules are available from the MVS link pack area.

CICS modules

A CICS module optionally installed in the MVS link pack area (that is, not a module required in the MVS link pack area) can be used only by the release of CICS to which it relates.

Those CICS modules that can reside above 16MB (for example, the CICS message table, DFHMGT), are loaded above 16MB. Such modules can also be installed in the extended link pack area (ELPA).

CICS modules eligible to be used from the MVS link pack area are listed in the CICS-supplied USERMODs, DFH$UMOD (for base CICS modules), which is in the hlq.SDFHSAMP library. Details of LPA-eligible modules are in Appendix. CICS modules eligible for the MVS link pack area, to help you select those CICS modules that you want to install in the MVS link pack area.

User application programs

User application programs can be used from the MVS link pack area if they are read-only and:

Command-level user application programs compiled using a Language Environment-conforming compiler, or written in Assembler language or C/370, may be loaded above 16MB. (For information about installing application programs, see the CICS Application Programming Guide.)

A read-only module that may reside above 16MB is also eligible for the extended link pack area (ELPA).

Service considerations

Using modules with mismatching service levels can cause unpredictable results. To be safe, do not use the LPA version of a module if it differs from the version in the CICS libraries that you are using.

Load modules used from the LPA might be at a lower service level than the rest of your CICS region in any of the following circumstances:

Thus, if you have applied service to a load module in your CICS libraries, you should also apply the service to the LPA version of the module, if one exists. This stipulation is there so that the MVS link pack area always contains tested load modules.

Use the SMP/E RESTORE function to back off the USERMOD before the LPA zone is updated or copied. Then apply the USERMOD again.

If you have used a copy of the CICS-supplied USERMODs to install modules into the MVS link pack area, and the original USERMOD is serviced, you may like to reflect the changes in your version of the USERMOD.

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