SLAU131V October 2004 – February 2020
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:
| .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: ADD.W #56, R11 ; Define x
.global x ; acts as .def x
Because x is defined in this module, the assembler treats ".global x" as ".def x". Now other modules can refer to x.
JMP y ; Reference y
.global y ; .ref of y
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.