SEVEN VIEWS OF COMPUTER SYSTEMS 3

After a system has been so constructed, the laws of Boolean algebra can be used to compute the behavior of the system from the behavior and properties of its components.

In addition to combinational logic circuits, whose outputs are directly related to the inputs at any instant of time, there are sequential logic circuits which have the ability to hold values over time and thus store information. The problem that the combinational level analysis solves is the production of a set of outputs at time t as a function of a number of inputs at the same time t. The representation of a sequential switching circuit is basically the same as that of a combinational switching circuit, although one needs to add memory components. The equations that specify sequential logic circuit structure must be difference equations involving time, rather than the simple Boolean algebra equations which describe purely combinational logic circuits.

The level above the switching circuit level is called the register transfer (RT) level. The components of the register transfer level are registers and the functional transfers between those registers. The functional transfers occur as the system undergoes discrete operations, whereby the values of various registers are combined ac cording to some rule and are then stored (transferred) into another register. The rule, or law, of combination may be almost anything, from the simple unmodified transfer (A ~- B) to logical combination (A ~- B A (AND) C) or arithmetic combination (A ~- B + (PLUS) C). Thus, a specification of the behavior, equivalent to the Boolean equations of sequential circuits or to the differential equations of the circuit level, is a set of expressions (often called productions) that give the conditions under which such transfers will be made.

The fifth and last level in Figure 1 is called the processor-memory-switch (PMS) level. This level, which gives only the most aggregate behavior of a computer system, consists of central processors, core memories, tapes, disks, input/output processors, communications lines, printers, tape controllers, buses, teleprinters, scopes, etc. The computer system is viewed as processing a medium, information, which can be measured in bits (or digits, characters, words, etc.). Thus, the components have capacities and flow rates as their operating characteristics.

The program level from the original set of levels shown in Bell and Newell has been dropped because it is a functional rather than a structural level.

Many notations are used at each of the five structural levels. Two of the less common ones are the processor-memory-switch (PMS) and instruction set processor (ISP) notations. A complete description of these notations is given in Bell and Newell [1971: Chapter 2]. Those aspects of PMS that are used in this book are de scribed in Appendix 2. The ISP notation has evolved to the ISPS language, which is de scribed in Appendix 1.

In contrast to the Structural View, this view is functional. According to this view, presented by John Levy [1974], a computer system consists of layers of interpreters, much like the layers of an onion.

An interpreter is a processing system that is driven by instructions and operates upon state information. The basic interpretive loop, shown in Figure 2, is most familiar at the machine language level but also exists at several other levels.

To formalize the notion of Levels-of-Interpretation, one can represent a processing sys tem by the diagram in Figure 3.

The state information operated on by an interpreter is either internal or external. This can best be understood by considering the "onion skin" levels of the five processing systems that form a typical airline reservation system. These levels are listed in Table 1.