2.2 COMPONENTS OF A DIGITAL COMPUTER & THEIR ROLE

 A digital computer is an electronic device that receives data, performs arithmetic and logical operations and produces results according to a predetermined program. It receives data through an input device (usually keyboard) and displays the results to some output device (usually monitor). All data processing in a digital computer is done by a central processing unit, also known as processor. A working memory is used to store data and instructions. Figure 2.1 presents a block diagram of a digital computer identifying the key components and their interconnection.

2.2.1 Components of a Digital Computer 

The key elements of a digital computer, as elaborated in the block diagram given in Figure 2.1 include: Central Processing Unit, Input, Output and Memory. The Central Processing Unit (CPU) is like the brain of the computer. It is responsible for executing instructions. It controls and coordinates the execution of instructions. It is comprised of a Control Unit (CU), an Arithmetic & Logic Unit (ALU) and registers. The CU controls the execution of instructions by decoding the instruction and generating micro-operations to be performed for executing that instruction. The ALU is responsible for performing arithmetic and logic operations. Execution of an instruction involves almost all parts (CU, ALU & Registers) of the CPU. Hence, CPU is known as the most vital component of a computer system. 

Input devices are used to read the instructions and data to be processed and output devices display the results obtained after executing the program. Keyboard, Mouse and Scanner are examples of input devices, whereas Monitor, Printer and Plotter are examples of output devices. Memory is used as a working storage for temporarily storing the data and intermediate results generated during program execution. Computers use two kinds of memories: primary & secondary. The primary memory is often referred to as RAM in everyday language. It is a read/write memory used to store both the program and data. Since RAM is volatile, computers also use a second level of memory- secondary memory- to permanently store the contents. Hard Disk is the non-removable secondary storage device which stores virtually everything on the machine. Computers also use other removable secondary memories like CD-ROMs, Magnetic tapes and recently Flash Drives to permanently take backup of the data onto Hard Disk or to transfer data from one machine to another.

A more practical description of a digital computer can be given by describing the major units and their interconnections for a simple personal computer (PC). If you open the CPU cabinet of your PC, you will notice that it contains a printed circuit board on which a number of devices are plugged in. This printed circuit board is often called the mother board. All other major components of the computer are either plugged in directly to this mother board or connected through a bunch of wires. CPU, RAM and Device Cards are plugged in various slots of the mother board. Devices like Hard Disk, Floppy Drive, CDROM Drive, which are attached to the CPU cabinet, are connected through wire ribbons. The mother board has printed circuitry which allows all these components to communicate with each other. CPU cabinet also houses a power supply unit which provides power to all the components of the computer system. On the back end of the CPU cabinet, you can notice a number of connection slots. These slots are used to connect various input/output devices such as keyboard, mouse, printer, scanner, to the computer.

2.2.2 Computer as a Data Processor

The main function of a computer is to process the input data according to a specific program to produce the desired output. This is the reason why a computer is often viewed as a data processing device. Various components of a computer work coherently to perform different operations to process the data according to program instructions. The word ‘Data’ refers to any raw collection of facts, figures and statistics. The input data to a computer may include both numbers and characters. Processing the data thus means manipulation of letters, numbers and symbols in a specific manner. The processing may include calculations, decision making, comparisons, classification, sorting, aligning & formatting etc. The processing of data results in some meaningful values, often termed as ‘information’. As we see, computer takes the raw data as input and performs several operations on these data in order to produce the desired output so it acts as a processing unit for those data. 

The programmers write programs for various data processing tasks. Writing a program to sort a given list of names, to search for a roll number in the list of qualified candidates, preparing and formatting a curriculum vitae, doing an accounting job, are all examples of data processing. As described in Unit I, digital computers have versatile capabilities and that is the reason why they are being used by a variety of people for different purposes. Now-a-days computers have something to offer to everyone. Whether it is an engineer, a business tycoon, a graphic designer, an accountant, a statistician, a student or even a farmer; everyone is now making use of computers. Thanks to the rich set of application programs, even novice users can now make effective use of computers.

2.2.3 Language of Digital Computers

Digital computers are electronic devices which operate on two valued logic (On and OFF). The ability of a transistor to act as a switch is the key to designing digital computers. The digital circuits used in computers are bi-stable, one state each corresponding to ON and OFF values. The two valued Boolean logic (using two distinct symbols 0 and 1) serves as an appropriate representation of states of digital circuits. Every instruction and data item therefore needs to be represented only by using two symbols 0 and 1. Since machines are capable of executing 400-500 distinct instructions and a unique binary code is required to specify every instruction, the machine instructions are specified using multiple bits (binary digits). Similarly the data items also need to be specified using 0 and 1 only. Numbers, alphabets and other characters can be represented by using some binary code system. The next section describes in detail the binary number system and different binary codes.

Though computers use a kind of binary language, we would never like to use them if we are forced to learn and use binary language for working on a computer. In fact, had this been the case, computers would have never become popular. To bridge the gap between the language used by computers and human beings, software got evolved. Software acts as an interface between the machine and the user. Initially the software performed only simple tasks of making the better utilization of machine resources and to make it more convenient for the user to use the machine. The convenience is the simplicity of the way through which users can give instructions to a computer. But as computers started becoming cheaper and popular, softwares to perform more complicated and domain specific tasks were written. Now we refer to these two varieties of softwares as System and Application software respectively. While systems softwares are concerned with driving the machine, application softwares provide task specific functionalities. System software is must to operate a computer system. Operating system is a kind of systems software whereas wordprocessing, database, accounting packages are all examples of applications software. Figure 2.3 below presents the layered view of a computing system.