Chapter 2 The PMS and ISP descriptive systems 17
With the exception of a processor, P. all other components are essentially passive and require some other active agent (a K) to set them into small episodes of activity.
Switch, S. A component that constructs a link between other components. Each switch has associated with it a set of possible links, and its operations consist of setting some of these links and breaking others.
Transducer, T. A component that changes the i-unit used to encode a given meaning (i.e., a given referent). The change may involve the medium used to encode the basic bits (e.g., voltage levels to magnetic flux, or voltage levels to holes in a paper card), or it may involve the structure of the i-unit (e.g., bit-serial to bit-parallel). Note that T's are meaning-preserving but not necessarily information-preserving (in number of bits), since the encodings of the (invariant) meaning need not be equally optimal.
Data-operation, D. A component that produces i-units with new meanings. It is this component that accomplishes all the data-operations, e.g., arithmetic, logic, shifting, etc.
Processor, P. A component that is capable of interpreting a program in order to execute a sequence of operations. It consists of a set of operations of the types already mentioned-M, L, K, S, T, and D-plus the control necessary to obtain instructions from a memory and interpret them as operations to be carried out.
Throughout PMS (and ISP, too) an operation is taken to mean a transformation of bits from one specific memory to another. For instance, it is an operation to transmit a word of information from memory M to memory M'; it is a different operation to transmit a word from memory M' to M". Similarly, it is an operation to add the contents of memory M to that of M' and a different operation to add the contents of M' to M".
The reason for emphasizing this point is that one often talks as if addition were an operation, ignoring the specific locus of the operands. In a discussion of computer systems, an operation must include specification of the locus of its operands. The reason is that the physical devices that realize operations are always localized in space. If, for instance, we wish to have a physical device that corresponds to addition on operands anywhere in some memory, we must couple the physical device that adds with other devices that either transmit information to and from the memory to the adder or (more exotic) that modify the adder to have different cells of memory as its terminals. Thus the symbol + is to be taken as an incomplete specification of an operation.
Computer model (in PMS)
Components of the seven types can be connected to make stored- program digital computers, abbreviated by C. For instance, the classical configuration for a computer is
C := Mp-Pc-T-X
Here Pc indicates a central processor and Mp a primary memory, namely, one which is directly accessible from a P and holds the program for it. T is a transducer connected to the external environment, represented by X. (The colon-equals (:=) indicates that C is the name of what follows to the right.) Thus a computer is a central processor connected to its primary memory on the one hand and to a transducer on the other, which is what an input/ output device is.
Actually the classic diagram had four components, since it decomposed the Pc into a control (K) and an arithmetic unit or data-operation (D):
where the solid information-carrying lines are for instructions and their data, and the dotted lines signify control.
Often logic operations were lumped with control, instead of with data operations, but this no longer seems to be the appropriate way to decompose the system functionally.
If we associate local control of each component with the appropriate component, we get
where the solid lines carry the information in which we are interested, and the dotted lines carry information about when to evoke operations on the respective components. The solid information-
1 The "½ " expresses mutually exclusive alternatives. Here, a T or Ms exists at the periphery.