16 Part 1 The structure of computers
current investigation, as the possibility of manufacturing them by integrated-circuit techniques has emerged. These distributed networks look very different from the computer systems of today, although they are still digital systems. Thus, the representation as a flow network with functionally specialized nodes is a real specialization.
The third specialization of the general state-system viewpoint is that associated with each component in a digital system is a small number of discrete operations for changing its own state or the state of neighboring components. All transitions must occur through the application of these few operations, which are evoked as a function of the current state of the component. The total behavior of the system is built up from the repeated execution of the operations as the conditions for their execution become realized by the results of prior operations. The general state-system view is more general. The state-transition table for a system may exhibit an arbitrary pattern of immediate state transitions, without regard to how such transition would be physically realized.
To summarize, within this specialized view one wants a way of describing a system of an interconnected set of components, which are individual devices that have associated with them a set of operations that work on a medium of information, measured in bits (or some other base).
The major complication in this picture is the amount of detail involved in describing actual computers. It takes a whole manual, for instance, to describe the operations of a major computer, such as the IBM 7090. Thus the descriptive system must permit very compressed descriptions. It must also permit description of only those aspects of the components that are of interest, ignoring the rest. And what is of interest at the PMS level? Besides a description of the gross. structure of a computer system, it is primarily the analysis of the amounts of information held in various components, the flows of information between components, and the distribution of the control that accomplishes these flows.
Thus a PMS-level description is analogous to the chemical engineer's diagram of a refinery in which he is interested in various kinds of liquid and gas flow. He has to account for matter and energy loss with the system at various stages involving the transduction of materials from one form to another. A specific chemical plant's external performance is measured in terms of its production flow rate for a given cost. With computers, external performance is concerned with the economical accomplishment of discrete tasks, but at the PMS level this translates into operation rates and cost of operations.
For the PMS level we ignore all the fine structure of information processing and consider a system consisting of components that work on a homogeneous medium called information. Information comes in packets. called i-units (for information units), and is measured in bits (or equivalent units, such as characters). I-units have the sort of hierarchical structure indicated by the phrase: A record consists of 300 words; a word consists of 4 bytes; a byte consists of 8 bits. A record, then, contains 300 x 4 x 8 = 9,600 bits. Each of these numbers-300, 4, 8-is called a length, since one often thinks of an i-unit as a spatial sequence of the next lower i-units of which it is composed. For example, one speaks of "word length" and of a record being "300 words long."
Other than being decomposable into a hierarchy of factors, i-units have no other structure at the PMS level. They do have a referent, that is, a meaning. Thus it is possible to say of an i-unit that it refers to an employer's payroll, to the pressure of a boiler, or to a prime number satisfying certain conditions. To do so, of course, the i-units encode the information necessary to make the reference. At the PMS level we are not concerned with what is referred to, but only with the fact that certain components transform i-units but do not modify their meaning. In fact, these meaning-preserving operations are the most basic information- processing operations of all, and they provide the basic classification of computer components.
PMS primitives
In PMS there are seven basic component types, each distinguished by the kinds of operations it performs:
Memory, M. A component that holds or stores information (i.e., i-units) over time. Its operations are reading i-units out of the memory and writing i-units into the memory. Each memory that holds more than a single i-unit has associated with it an addressing system by means of which particular i-units can be designated or selected. A memory can also be considered as a switch to a number of submemories. The i-units are not changed in any way by being stored in a memory.
Link, L. A component that transfers information (i.e., i-units) from one place to another in a computer system. It has fixed ports. The operation is that of transmitting an i-unit (or a sequence of them) from the component at one port to the component at the other. Again, except for the change in spatial position, there is no change of any sort in the i-units.
Control, K. A component that evokes the operations of other components in the system. All other components are taken to consist of a set of discrete operations, each of which, when evoked, accomplishes some discrete transformation of state.