External symbols are symbols that are visible to other object modules. Because they are visible across object modules, they may be defined in one file and referenced in another file. You can use the .def, .ref, or .global directive to identify a symbol as external:
| .def | The symbol is defined in the current file and may be used in another file. |
| .ref | The symbol is referenced in the current file, but defined in another file. |
| .global | The symbol can be either of the above. The assembler chooses either .def or .ref as appropriate for each symbol. |
The following code fragments illustrate the use of the .global directive.
x: .word y
.global x ; acts as .def of x
.global y ; acts as .ref of y
Because x is defined in this module, the assembler treats ".global x" as ".def x", and other modules can refer to x. Because y is not defined in this module, the assembler treats ".global y" as ".ref y". The symbol y must be defined in another module.
Both the symbols x and y are external symbols and are placed in the object file's symbol table; x as a defined symbol, and y as an undefined symbol. When the object file is linked with other object files, the entry for x will be used to resolve references to x in other files. The entry for y causes the linker to look through the symbol tables of other files for y’s definition.
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.