In essence, there are two separate sets of files which are installed: the A-Shell runtime system, consisting of the AMOS emulation command programs and the A-Shell module itself, and the development system, consisting of the object modules and source to a user-defined subroutine module, enabling subroutines to be developed. Since the second set of files is usually offered as an added-cost option, most users will only need to concern themselves with the first set.