Use f77 -c to compile modules separately
To separately compile Fortran modules before linking them together, use the '-c' option to the 'f77' compiler. For example, if you have two source files, named 'file1.f' and 'file2.f', type these commands:
f77 -c file1.f
f77 -c file2.f
Then, you can link them together with this command:
f77 file1.o file2.o
You only need to recompile a Fortran module if it has been changed since it was last compiled. Once you have recompiled any changed modules, you can link them together as shown above. Doing this can save you quite a bit of time.
If you have many modules in your program, you may want to consider using 'make' which automatically figures out which of your modules need to be recompiled (by checking when they were last modified), and then links them.
If you weant to learn more about 'f77' or 'make', you can read the manual pages by using the 'man' command: