Dependent addressing

Dependent addressing lets you minimize the number of base registers required to refer to data by making greater use of established addressability. For example, you might want to describe the format of a table of data defined in your source module with a dummy control section (see Dummy control sections). To refer to the data in the table using the symbols defined in the dummy section, you need to establish the addressability of the dummy section. To do this you must:

Code a USING instruction to assign one or more base registers to a base address.
Code machine instructions to load each base register with the base address.

However, dependent addressing offers an alternative means of establishing addressability of the dummy section.

Establish addressability of the control section in which the table is defined. Then you can establish addressability of the dummy section by coding a USING statement which specifies the name of the dummy section and the address of the table. When you refer to the symbols in the dummy section, the assembler uses the already established addressability of the control section when converting the symbolic addresses into their base-displacement form.

For example, suppose addressability has been established for a control section containing a data structure that is mapped by a dummy control section:

DATAMAP  DSECT ,                 DSECT describing data structure
FIELD1   DS    F
FIELD2   DS    CL32
FIELD3   DS    CL24
         - - -
CODE     CSECT ,                 Program code
         BASR  12,0              Set base register
         USING *,12              Provide addressability
         - - -
         USING DATAMAP,REALDATA  Map DSECT onto REALDATA
         L     2,FIELD1          Register 12 is base register
         LA    3,FIELD3          Address of DATA3
         - - -
REALDATA DS    0F                Data mapped by DATAMAP
DATA1    DC    F'32'
DATA2    DC    CL32'Actual Data'
DATA3    DC    CL24'Additional Data'