previous | contents | next



In Computer Structures [Bell and Newell, 1971], a set of conceptual levels for describing, understanding, analyzing, designing, and using computer systems was postulated. The model has survived major changes in technology, such as the fabrication of a complete computer on a single silicon chip, and changes in architecture, such as the addition of vector and array data- types.

As shown in Figure 1, there are at least five levels of system description that can be used to

describe a computer. Each level is characterized by a distinct language for representing the components associated with that level, their modes of combination, and their laws of behavior. Within each level there exists a whole hierarchy of systems and subsystems, but as long as these are all described in the same language, they do not constitute separate levels. With this general view, one can work up through the levels of computer systems, starting at the bottom.

The lowest level in Figure 1 is the device level. Here the components are p-type and n-type semiconductor materials, dielectric materials, and metal formed in various ways. The behavior of the components is described in the languages of semiconductor physics and materials science.

The next level is the circuit level. Here the components are resistors, inductors, capacitors, voltage sources, and nonlinear devices. The behavior of the system is measured in terms of voltage, current, and magnetic flux. These are continuously varying quantities associated with various components; hence, there is continuous behavior through time, and equations (including differential equations) can be written to de scribe the behavior of the variables. The components have a discrete number of terminals whereby they can be connected to other components.

Above the circuit level is the switching circuit or logic level. While the circuit level in digital technology is very similar to the rest of electrical engineering, the logic level is the point at which digital technology diverges from electrical engineering. The behavior of a system is now described by discrete variables which take on only two values, called 0 and 1 (or + and -, true and false, high and low). The components perform logic functions called AND, OR, NAND, NOR, and NOT. Systems are constructed in the same way as at the circuit level, by connecting the terminals of components, thereby identify their behavioral values.

previous | contents | next