HLASM Language Reference

Addressing
Establishing symbolic linkage
You must establish symbolic linkage between source modules so that you can refer
to or branch to symbolic locations defined in the control sections of external source
modules. You do this by using external symbol definitions, and external symbol
references. To establish symbolic linkage with an external source module, you
must do the following:
In the current source module, you must identify the symbols that are not
defined in that source module, if you want to use them in instruction operands.
These symbols are called external symbols, because they are defined in
another (external) source module. You identify external symbols in the EXTRN
or WXTRN instruction, or the V-type address constant. For more information
about the EXTRN and WXTRN instructions, see “EXTRN Instruction” on
page 189 and “WXTRN Instruction” on page 229.
In the external source modules, you must identify the symbols that are defined
in those source modules, and that you refer to from the current source module.
The two types of definitions that you can use are control section names
(defined by the CSECT, RSECT, and START instructions), and entry symbols.
Entry symbols are so called because they provide points of entry to a control
section in a source module. You identify entry symbols with the ENTRY
instruction. For more information about the ENTRY instruction, see “ENTRY
Instruction” on page 183.
To reference the external symbols, you must either
– provide the A-type or V-type address constants needed by the assembler to
reserve storage for the addresses represented by the external symbols, or
| – reference an external symbol in the same class in a relative branch
| instruction.
The assembler places information about entry and external symbols in the external
symbol dictionary. The linker uses this information to resolve the linkage addresses
identified by the entry and external symbols.
Referring to external data
Use the EXTRN instruction to identify the external symbol that represents data in
an external source module, if you want to refer to this data symbolically.
For example, you can identify the address of a data area as an external symbol
and load the A-type address constant specifying this symbol into a base register.
Then, you use this base register when establishing the addressability of a dummy
section that describes this external data. You can now refer symbolically to the
data that the external area contains.
You must also identify, in the source module that contains the data area, the
address of the data as an entry symbol.
Branching to an external address
Use the V-type address constant to identify the external symbol that represents the
address in an external source module that you want to branch to.
For example, you can load into a register the V-type address constant that
identifies the external symbol. Using this register, you can then branch to the
external address represented by the symbol.
Chapter 3. Program Structures and Addressing 69