HLASM Language Reference

Ensure that the locations of the symbols in the dummy section actually
correspond to the locations of the data being described
Establish the addressability of the dummy section in combination with the
storage area
You can then refer to the data symbolically by using the symbols defined in the
dummy section.
Common Control Sections
A common control section is a reference control section that lets you reserve a
storage area that can be used by one or more source modules. One or more
common sections can be defined in a source module.
The COM instruction initiates a common control section, or indicates its
continuation. For more information about the COM instruction, see “COM
Instruction” on page 121.
How to use a common control section: A common control section (common
section) lets you describe a common storage area in one or more source modules.
When the separately assembled object modules are linked as one program, the
required storage space is reserved for the common control section. Thus, two or
more modules may share the common area.
Only the storage area is provided; the assembler does not assemble the source
statements that make up a common control section into object code. You must
provide the data for the common area at execution time.
The assembler assigns locations to the symbols you define in a common section
relative to the beginning of that common section. This lets you refer symbolically to
the data that is placed in the common section at execution time. If you want to
refer to data in a common control section, you must establish the addressability of
the common control section in each source module that contains references to it. If
you code identical common sections in two or more source modules, you can
communicate data symbolically between these modules through this common
section.
| Communicating with Modules in Other Languages: Some high-level languages
| such as COBOL, PL/I, C, and FORTRAN use common control sections. This lets
| you communicate between assembler language modules and modules written in
| those languages.
External Dummy Sections
An external dummy section is a reference control section that lets you describe
storage areas for one or more source modules, to be used as:
Work areas for each source module
Communication areas between two or more source modules
| Note: External dummy sections are also called “pseudo-registers” in other contexts.
When the assembled object modules are linked and loaded, you can dynamically
allocate the storage required for all your external dummy sections at one time from
one source module (for example, by using the MVS GETMAIN macro instruction).
Chapter 3. Program Structures and Addressing 57