An interpreter is a program which stays in control all the while the program is running. It translates the source code into machine code one line at a time and then executes that line immediately. It then goes on to translate the next, and so on. If an error occurs it is usually possible to correct the mistake and continue running the program from the point at which it left off. This can speed up program development considerably. The main snag is that all non-trivial programs involve forms of repetition, such as loops or procedure calls. In all these cases the same lines of source code are translated into machine code over and over again. Some interpreters are clever enough to avoid doing all the work again but the overhead cannot be eliminated entirely.
The compiler works in an entirely different way. It is an independent program which translates the entire source code into machine code at once. The machine code is usually saved on a file, often called an executable image, which can then be run whenever it is needed. Because each statement is only translated once, but can be executed as many times as you like, the time take by the translation process is less important. Many systems provide what is called an optimising compiler which takes even more trouble and generates highly efficient machine code; optimised code will try to make the best possible use of fast internal registers and the compiler will analyse the source program in blocks rather than one line at a time. As a result, compiled programs usually run an order of magnitude faster than interpreted ones. The main disadvantage is that if the program fails in any way, it is necessary to edit the source code and recompile the whole thing before starting again from the beginning. The error messages from a compiled program may also be less informative than those from an interpreter because the original symbolic names and line numbers may not be retained by the compiler.
Interpreters, being more "user-friendly", are especially suitable for highly interactive use and for running small programs produced by beginners. Thus languages like APL, Basic, and Logo are usually handled by an interpreter. Fortran, on the other hand, is often used for jobs which consume significant amounts of computer time: in some applications, such as weather forecasting, the results would simply be of no use if they were produced more slowly. The speed advantage of compilers is therefore of great importance and in practice almost all Fortran systems use a compiler to carry out the translation.
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.
