Time-sharing computer systems permit large numbers of users to operate on common sets of data and programs. Since certain parts of these computer resources may be sensitive or proprietary, there exists the risks that information belonging to one user, may, contrary to his intent, become available to other users, and there is the additional risk that outside agencies may infiltrate the system and obtain information. The question naturally arises of protecting one user's stored program and data against unauthorized access by others.

This paper reviews numerous theoretical results on control structures and demonstrates their practical examples. This study deals with the design of run-time support routines by using top-down structured programming technique. A number of examples are given as illustration of this method. In conclusion, structured programming has proved to be an important methodology for systematic program design and development.