Global symbols are symbols that are either accessed in the current module but defined in another (an external symbol) or defined in the current module and accessed in another. Such symbols are visible across object modules. You must use the .def, .ref, or .global directive to identify a symbol as external:

The following code fragment illustrates these definitions.

        .def    x
        .ref    y
        .global z
        .global q
x:      ADD     AR1, #56h
        B       y, UNC
q:      ADD     AR1, #56h
        B       z, UNC

In this example, the .def definition of x says that it is an external symbol defined in this file and that other files can reference x. The .ref definition of y says that it is an undefined symbol that is defined in another file. The .global definition of z says that it is defined in some file and available in this file. The .global definition of q says that it is defined in this file and that other files can reference q.

The assembler places x, y, z, and q in the object file's symbol table. When the file is linked with other object files, the entries for x and q resolve references to x and q in other files. The entries for y and z cause the linker to look through the symbol tables of other files for y's and z's definitions.

The linker attempts to match all references with corresponding definitions. If the linker cannot find a symbol's definition, it prints an error message about the unresolved reference. This type of error prevents the linker from creating an executable object module.

An error also occurs if the same symbol is defined more than once.