HLASM Language Reference

Addressing
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
may want to describe the format of a table of data defined in your source module
with a dummy control section (see “Dummy Control Sections” on page 56). 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.
When you have established addressability of the control section in which the table
is defined, you can establish addressability of the dummy section by simply coding
a USING statement which specifies the name of the dummy section and the
address of the table. When you subsequently refer to the symbols in the dummy
section, the assembler makes use of 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, 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 F Data mapped by DATAMAP
| DATA1 DC F'32'
| DATA2 DC CL32'Actual Data'
| DATA3 DC CL24'Additional Data'
Relative Addressing
Relative addressing is the technique of addressing instructions and data areas by
designating their location in relation to the location counter or to some symbolic
location. This type of addressing is always in bytes—never in bits, words, or
instructions. Thus, the expression +4 specifies an address that is 4 bytes greater
than the current value of the location counter. In the sequence of instructions in
the following example, the location of the CR machine instruction can be expressed
in two ways, ALPHA+2, or BETA-4, because all the machine instructions in the
example are for 2-byte instructions.
Chapter 3. Program Structures and Addressing 67