2.1 Module 2: top-level view of computer organization

1. computer component

Contemporary computer designs are based on concepts developed by John von Neumann at the Institute for Advanced Studies Princeton. Such a design is referred to as the von Neumann architecture and is based on three key concepts:

• Data and instructions are stored in a single read -write memory.
• The contents of this memory are addressable by location, without regard to the type of data contained there.
• Execution occurs in a sequential fashion (unless explicitly modified) from one instruction to the next.

Suppose that there is a small set of basic logic components that can be com­bined in various ways to store binary data and to perform arithmetic and logical operations on that data. If there is a particular computation to be performed, a con­figuration of logic components designed specifically for that computation could be constructed. We can think of the process of connecting the various components in the desired configuration as a form of programming. The resulting "program"' is in the form of hardware and is termed a hardwired program.

Now consider other alternative. Suppose we construct a general-purpose con­figuration of arithmetic and logic functions, this set of hardware will perform vari­ous functions on data depending on control signals applied to the hardware. In the original case of customized hardware, the system accepts data and produces results (Figure 1a). With general-purpose hardware, the system accepts data and control signals and produces results. Thus, instead of rewiring the hardware for each new program, The programmer merely needs to supply a new set of control signals.

How shall control signals be supplied? The answer is simple but subtle. The entire program is actually a sequence of steps. At each step, some arithmetic or log­ical operation is performed on some data. For each step, a new set of control signals is needed. Let us provide a unique code for each possible set of control signals, and let us add to the general-purpose hardware a segment that can accept a code and generate control signals (Figure 1b).

Programming is now much easier. Instead of rewiring the hardware for each new program, all we need to do is provide a new sequence of codes. Each code is, in effect, an instruction, and part of the hardware interprets each instruction and generates control signals. To distinguish this new method of programming, a sequence of codes or instructions is called software.

Figure 1 Hardware and software approaches

Figure 1b indicates two major components of the system: an instruction interpreter and a module of general-purpose arithmetic and logic functions. These two constitute the CPU.

Several other components are needed to yield a function­ing computer. Data and instructions must be put into the system. For this we need some sort of input module. This module contains basic components for accepting data and instructions in some form and converting them into an internal form of sig­nals usable by the system. A means of reporting results is needed, and this is in the form of an output module. Taken together, these are referred to as I/O components.