14 Part 1 ½ Fundamentals Section 1½ Abstraction and Notation

these levels is also revealed in the ability of programmers to become expert without knowing anything about representations below the programming level.

The program level constitutes an entire technology in its own right, carrying within it most of the characteristics of computer systems. The ISP (instruction-set processor) sublevel specifies the machine's instruction interpretation cycle: instruction fetch, instruction decode, program counter update, operand address calculation, operand fetch, and instruction execution.

The ISP description system is meant to provide a uniform way of describing instruction sets, i.e., of giving the information contained in a programming manual. It must provide the instruction format, the registers referenced by the instructions, the rules of interpretation of the instructions, and the semantics of each instruction in the processor's repertoire. It must be able to do this for any existing computer and for any anticipated future computer. (See Chap. 4 for a discussion of the ISP notation.)

A number of sophisticated language levels are built upon the ISP sublevel. Operating systems manage system resources (e.g., memory space, CPU time) and provide commonly used functions for use at higher levels. The high-level-language run-time system provides an interface between potentially different operating systems and a high-level language (e.g., FORTRAN, COBOL, ALGOL, or PASCAL). More complex functions, such as a mathematical library, can be performed by routines invoked by user programs. Finally, application systems provide a totally integrated environment where the user need interact only by entering data and reading results. Examples of these integrated environments (e.g., hand-held calculators, personal computers, and video games) are proliferating as computing technology is packaged for less sophisticated users.

We now move to the fourth and last level. In Fig. 1 it is called the processor-memory-switch level, or PMS level for short. It is the view one takes of a computer system when one considers only