Excerpt from Hierarchical Control System Hierarchical design is common in computer programs. The reason is two fold. The first is for the user's/programmer's convenience. For instance, an operating system might present disk storage to the programmer as se quential and random access files, hiding the specific details of disk access and file maintenance. Ou top of this are developed text files, indexed files, relational databases, and so 011. Thus the user or programmer is presented with, familiar abstractions suitable for the task at hand. Secondly, hierar chical programming is good software engineering. If one considers a system as having the raw devices on the bottom and the application programs on the top then hierarchical programming represents vertical modularity and all the various advantages of modular programming apply. The specification for each step in the hierarchy can be precisely stated simplifying develop ment. Changes in the underlying hardware need not propagate changes throughout the system but can be handled by the lower levels of the hier archy. Programming modifications and Optimizations can be applied at the appropriate level(s) again without disrupting the entire system. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.