0A computer is a complex system; contemporary computers contain millions of elementary electronic components.How,then,can one clearly describe them? The key is to recognize the hierarchical nature of most complex systems, including the computer [SIMO96].A hierarchical system is a set of interrelated subsystems, each of the latter, in turn,hierarchical in structure until we reach some lowest level of elementary subsystem.
The hierarchical nature of complex systems is essential to both their design and their description. The designer need only deal with a particular level of the system at a time. At each level, the system consists of a set of components and their interrelationships.The behavior at each level depends only on a simplified, abstracted characterization of the system at the next lower level. At each level, the designer is concerned with structure and function:
In terms of description, we have two choices: starting at the bottom and building up to a complete description, or beginning with a top view and decomposing the system into its subparts. Evidence from a number of fields suggests that the topdown approach is the clearest and most effective [WEIN75]. The approach taken in this book follows from this viewpoint.
The computer system will be described from the top down.We begin with the major components of a computer, describing their structure and function, and proceed to successively lower layers of the hierarchy. The remainder of this section provides a very brief overview of this plan of attack.
