HLASM Language Reference

Addressing
If the symbol is the name entry of a START, CSECT, or RSECT instruction in the
other source module, and thus names an executable control section, it is
automatically identified as an entry symbol. If the symbol represents an address in
the middle of a control section, you must identify it as an entry symbol for the
external source module.
You can also use a combination of an EXTRN instruction to identify, and an A-type
address constant to contain, the external branch address. However, the V-type
address constant is more convenient because:
You do not have to use an EXTRN instruction.
The external symbol you specify, can be used in the name entry of any other
statement in the same source program.
| It will work correctly even if the program is linked as an overlay module, so long
| as the reference is not to a symbol in an exclusive segment. See z/OS MVS
| Program Management: User's Guide and Reference, SA22-7643 for further
| information.
The following example shows how you use an A-type address constant to contain
the address of an external symbol that you identify in an EXTRN instruction. You
cannot use the external symbol name EXMOD1 in the name entry of any other
statement in the source program.
.
.
L 15,EX_SYM Load address of external symbol
BASR 14,15 Branch to it
.
.
EX_SYM DC A(EXMOD1) Address of external symbol
EXTRN EXMOD1
Identify EXMOD1 as external symbol
.
.
The following example shows how you use the symbol EXMOD1 as both the name of
an external symbol and a name entry on another statement.
.
.
L 15,EX_SYM Load address of external symbol
BASR 14,15 Branch to it
.
.
EXMOD1 DS H Using EXMOD1 as a name entry
.
.
EX_SYM DC V(EXMOD1) Address of external symbol
.
.
If the external symbol that represents the address to which you want to branch is to
be part of an overlay-structured module, you should identify it with a V-type address
constant, not with an EXTRN instruction and an A-type address constant. You can
use the supervisor CALL macro instruction to branch to the address represented by
the external symbol. The CALL macro instruction generates the necessary V-type
address constant.
70 HLASM V1R5 Language Reference