The principal disadvantage of a compiler is the necessity of re-compiling the whole program after making any alteration to it, no matter how small. Fortran has partly overcome this limitation by allowing program units to be compiled separately; these compiled units or modules are linked together afterwards into an executable program.
A Fortran compiler turns the source code into what is usually called object code: this contains the appropriate machine-code instructions but with relative memory addresses rather than absolute ones. All the program units can be compiled together, or each one can be compiled separately. Either way a set of object modules is produced, one from each program unit. The second stage, which joins all the object modules together, is usually known as linking, but other terms such as loading, link-editing, and task-building are also in use. The job of the linker is to collect up all these object modules, allocate absolute addresses to each one, and produce a complete executable program, also called an executable image.
The advantage of this two-stage system is that if changes are made to just one program unit then only that one has to be re-compiled. It is, of course, necessary to re-link the whole program. The operations which the linker performs are relatively simple so that linkers ought to be fast. Unfortunately this is not always so, and on some systems it can take longer to link a small program than to compile it.