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 and WXTRN instruction.
• 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.
• Your reference external symbols using one of these methods:
  • Provide the A-type or V-type address constants needed by the assembler to reserve storage for the addresses represented by the external symbols.
  • Reference an external symbol in the same class in a relative branch instruction.