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].Ahierarchical 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:
• Structure: The way in which the components are interrelated
• Function: The operation of each individual component as part of the structure
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.
Function
Both the structure and functioning of a computer are, in essence, simple. Figure 1.1
depicts the basic functions that a computer can perform. In general terms, there are
only four:
• Data processing
• Data storage
• Data movement
• Control
The computer, of course, must be able to process data.The data may take a wide
variety of forms, and the range of processing requirements is broad. However, we shall
see that there are only a few fundamental methods or types of data processing.
It is also essential that a computer store data. Even if the computer is processing
data on the fly (i.e., data come in and get processed, and the results go out immediately),
the computer must temporarily store at least those pieces of data that are being worked on at any given moment.Thus, there is at least a short-term data storage function.
Equally important, the computer performs a long-term data storage function.
Files of data are stored on the computer for subsequent retrieval and update.
The computer must be able to move data between itself and the outside world.
The computer’s operating environment consists of devices that serve as either sources or destinations of data.When data are received from or delivered to a device
that is directly connected to the computer, the process is known as input–output
(I/O), and the device is referred to as a peripheral.When data are moved over longer
distances, to or from a remote device, the process is known as data communications.
Finally, there must be control of these three functions. Ultimately, this control
is exercised by the individual(s) who provides the computer with instructions.Within
the computer, a control unit manages the computer’s resources and orchestrates the
performance of its functional parts in response to those instructions.
At this general level of discussion, the number of possible operations that can
be performed is few. Figure 1.2 depicts the four possible types of operations. The
computer can function as a data movement device (Figure 1.2a), simply transferring
data from one peripheral or communications line to another. It can also function as
a data storage device (Figure 1.2b), with data transferred from the external environment
to computer storage (read) and vice versa (write). The final two diagrams
show operations involving data processing, on data either in storage (Figure 1.2c) or
en route between storage and the external environment (Figure 1.2d).
The preceding discussion may seem absurdly generalized. It is certainly possible,
even at a top level of computer structure, to differentiate a variety of functions,
but, to quote [SIEW82],
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