This chapter defines the hardware and software components used in computer-based applications. Included here is the fundamental composition of computers (CPU, memory, I/O), secondary storage devices, other peripheral input and output devices, multiprocessing architectures, and networks. Our discussions are tailored to focus on the architecture and use of these components as they relate to computer management of persistent data.

2.1 Introduction

A computer-based application resides on a computer system. The computer system provides the physical medium on which the application data are stored and the processing capacity to manipulate stored data. A processing unit of a computer system consists of five main elements: the memory, an arithmetic logic unit, an input unit, an output unit, and a control element. The memory unit stores both the data for programs and the instructions of a program that manipulates stored data.

The program's individual elements or instructions are fetched from the memory one at a time and are interpreted by the control unit. The control unit, depending on the interpretation of the instruction, determines what computer operation to perform next. If the instruction requires no additional data, the control indicates to the arithmetic logic unit what operation to perform and with what registers. (See Figure 2.1.)

Figure 2.1: Basic processing unit of a computer.

If the instruction requires additional data, the control unit passes the appropriate command to the memory (MAR, memory address register) to fetch a data item from memory (MDR, memory data register) and to place it in an appropriate register...