**The digital computer**is a digital system that performs various computational tasks. The word digital implies that the information in the computer is represented by variables that take a limited number of discrete values.**These values are processed intemally**by components that can maintain a limited number of disaete states. The decimal digits 0, 1, 2, ... , 9, for example, provide 10 discrete values.**In practice**, digital computers function more reliably if only two states are used. Because of the physical restriction of components, and because human logic tends to be binary (i.e., true-or-false, yes-or-no statements), digital components that are constrained to take discrete values are further constrained to take only two values and are said to be binary.**Digital computers**use the binary number system, which has two digits: 0 and 1. A binary digit is called a bit. Information is represented in digital computers in groups of bits. By using various coding techniques, groups of bits can be made to represent not only binary numbers but also other discrete symbols, such as decimal digits or letters of the alphabet.**By judicious use of binary arrangements**and by using various coding techniques, the groups of bits are used to develop complete sets of instructions for performing various types of computations.**The seven bits 1001011**represent a binary number whose decimal equivalent is 75. However, this same group of seven bits represents the letter K when used in conjunction with a binary code for the letters of the alphabet**A computer system**is sometimes subdivided into two functional entities: hardware and software.**Computer software**consists of the instructions and data that the computer manipulates to perform various data-processing tasks. A sequence of instructions for the computer is called a program. The data that are manipulated by the program constitute the data base**The system software**of a computer consists of a collection of programs whose purpose is to make more effective use of the computer. The programs included in a systems software package are referred to as the operating system.**The hardware of the computer**is usually divided into three major parts, as shown below : The central processing unit (CPU) contains an arithmetic and logic unit for manipulating dala, a number of registers for storing data, and control circuits for fetching and executing instructions.**The memory**of a computer contains storage for instructions and data.**It is called a random access memory (RAM)**because the CPU can access any location in memory at random and retrieve the binary information within a fixed interval of time.**Computer organization**is concerned with the way the hardware components operate and the way they are connected together to form the computer system.**Compute design**is concerned with the hardware design of the computer. Once the computer specifications are formulated, it is the task of the designer to develop hardware for the system**Computer architecture**is concerned with the structure and behavior of the computer as seen by the user. It includes the information formats, the instruction set, and techniques for addressing memory.**The architectural design**of a computer system is concerned with the specifications of the various functional modules, such as processors and memories, and structuring them together into a computer system