Digital Computers: 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 internally by components that can maintain a limited number of discrete states. The decimal digits 0, 1, 2…9 for example, provide 10 discrete values. The first electronic digital computers, developed in the late 1940s, were used primarily for numerical computations. In this case the discrete elements are the digits. From this application the term digital computer has emerged. 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 (that is 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 Single binary digit is called a bit. A nibble is half a byte, or four bits. A word is the default data size for a processor. The default size does not apply in all cases. The word size is chosen by the processor’s designer(s) and reflects some basic hardware issues (such as internal or external buses). The most common word sizes are 16 and 32, but words have ranged from 16 to 60 bits. Typically there will be additional data sizes that are defined relative to the size of a word: halfword, half the size of a word; longword, usually double the size of a word; doubleword, usually double the size of a word (sometimes double the size of a longword); and quadword, four times the size of a word. Whether or not there is a space between the size designation and “word” is designated by the manufacturer, and varies by processor.
A sequence of instructions for the computer is called a Program. The data that are manipulated by the program constitute the data base.
A computer system is composed of its hardware and the system software available for its use. 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.
Computer Organization is concerned with the way the hardware components operate and the way they are connected together to form the computer system. The various components are assumed to be in place and the task is to investigate the organizational structure to verify that the computer parts operate as intended.
Computer 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 design is concerned with the determination of what hardware should be used and how the parts should be connected. This aspect of computer hardware is sometimes referred to as computer implementation.
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.
Logic Gates: Binary logic deals with binary variables and with operations that assume a logical meaning. It is used to describe, in algebraic or tabular form, the manipulation and processing of binary information. The manipulation of binary information is done by logic circuits called gates. Gates are blocks of hardware that produce signals of binary 1 or 0 when input logic requirements are satisfied. A variety of logic gates are commonly used in digital computer systems. Each gate has a distinct graphic symbol and its operations can be described by means of an algebraic expression. The input-output relationship of the binary variables for each gate can be represented in tabular form by a truth table.