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Computer Architecture

1. Introduction: The computer unit is frequently called as the computer system because of its numerous parts, machinery units and complicated sequential operation. A computer system has three main parts i.e Hardware, Software and Humanware. Any computer system has three important hardware parts- input device, central processing unit and the output device. The central processing unit itself has three parts, namely memory unit, control unit and arithmetic and logic unit (ALU). In addition to these, computers also have secondary storage devices, which are used for storing data or instruction on a long term basis.

2. Hardware: The physical or mechanical parts of the computer system that can be seen and touched are known as hardware. It consists of a combination and collection of electro-mechanical and electronic components and devices, electronic circuits and microelectronic equipment assembled in metal boxes in the form of modules and cabinet. All these equipment and elements are interconnected by wiring and switching communication components like transistors, capacitors, resistors, diodes, printed circuits, integrated circuits, main and auxiliary storage systems, various types of magnetic media, communication media for carrying and transformation of data, coded instruction, etc. The different hardware parts are interconnected by busses, often made of groups of wires.

Any computer system has three important hardware parts. They are input device, central processing unit and the output device. The central processing unit itself has three parts, namely memory unit, control unit and arithmetic and logic unit (ALU).

2.1 Input Unit: The input devices are used to transfer the information into the memory unit of a computer. In simplest term, they bring information into the computer from the user’s hand, i.e. input unit feeds data into the computer. It is thus a communication medium between the user and the machine. The input devices are of the following types.

a) Keyboard: Keyboards are the most commonly used input devices usually having 83-84 keys and enhanced with 101 keys or even more. The enhanced keyboards are more popular.

b) Mouse: It is a hand-held pointing device that allows controlling the computer without having to type the instruction through keyboard. The Scrolling mouse is a small unit with a round ball at the bottom and with two depression switches at the upper top portion having again a scroll button. Nowadays cord less as well as without scroll ball-type of mouse are also available.

c) Scanners: Scanners are used to store or feed an entire image / data or page of other information into the computer system. Image scanner is a general-purpose device which digitizes a two-dimensional image.

d) Track Ball: A trackball is just like a mouse lying on its back. It is stationary and does not need to move on any surface. To move the pointer only the ball should be rotated with the thumb / finger or with the palm. The buttons next to the ball are used just like mouse button.

e) Joystick: A joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. Joysticks are often used to control video games and they usually have one or more push-buttons whose state can also be read by the computer.

f) Digitizing Tablet: A graphics tablet (or digitizing tablet, graphics pad, drawing tablet, pen pad or digitizer) is a computer input device that allows one to hand-draw images and graphics, similar to the way one draws images with a pencil and paper. These tablets may also be used to capture data or handwritten signatures.

g) Digital Camera: A digital camera (or digicam) is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor.

h) Magnetic Ink Character Recognition (MICR): It allows the computer to recognize character printed using magnetic ink. MICR is widely used in banks to read the cheque number written on the bottom of the cheque. It is also used in the back of credit cards and bank debit cards and ID cards.

i) Optical Character Recognition (OCR): An optical character reader is used to read character of special type fonts printed on conventional paper with conventional ink i.e. it involves reading text from paper, book or from a magazine articles but they still have difficulty with handwritten text.

j) Bar Code Reader: Bar code readers are photoelectric scanner that reads the bar codes or vertical zebra striped marks printed on the product container and the computer automatically tells the prices of the product at the terminals.

k) Speech Recognition and Voice Response Devices: In this type of device the user speaks into a microphone which is attached to a digitizer. The dizitizer converts the analog sounds waves to “0” and “1”s which can be easily understood by the computer. Speech recognition devices are necessary because spoken commands are much quicker than typing. It helps to give command to a remote computer over telephone. It helps the computer usable to the blind people. In computer assisted learning environment it helps in the interaction between the man and machine.

l) Touchscreen: A touchscreen is a display that can detect the presence and location of a touch within the display area. The touchscreen has two main attributes. Firstly, it enables one to interact with what is displayed directly on the screen, where it is displayed, rather than indirectly with a mouse or touchpad. Secondly, it lets one do so without requiring any intermediate device such as a stylus that needs to be held in the hand. Such displays can be attached to computers or, as terminals, to networks.

m) Touchpad: A touchpad (also trackpad) is a pointing device consisting of specialized surface that can translate the motion and position of a user’s fingers to a relative position on screen. They are a common feature of laptop computers and are also used as a substitute for a computer mouse where desk space is scarce.

n) Light Pen: A light pen is similar to a mouse except that with a light pen one can move the pointer and select objects on the display screen by directly pointing to the object with the help of the pen.

o) Optical Mark Recognition (OMR): Optical Mark Recognition (OMR) is also called mark sensing. It is a technology where an OMR device senses the presence or absence of a mark such as pencil mark. OMR is used in test such as aptitude test.

2.2 Central Processing Unit: The CPU is the brain of any computer system; all major calculations and comparisons are made inside the CPU and it is also responsible for activating and controlling the operations of other units of a computer system. It guides, directs and controls a computer performance. It also executes the instruction given to it. The CPU consists of the ALU, control unit, registers, and basic I/O (and often other hardware closely linked with these). Early CPUs were composed of many separate components but since the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor.

2.2.1 Arithmetic and Logic Unit: The input devices are used to transfer the information into the memory unit of a computer. Information from the memory can be transferred to the ALU where comparison and calculation are done and the results are sent back to the memory unit. The set of arithmetic operations that a particular ALU supports may be limited to adding and subtracting or might include multiplying or dividing, trigonometry functions (sine, cosine, etc) and square roots. Some can only operate on whole numbers (integers) whilst others use floating point to represent real numbers with limited precision. An ALU may also compare numbers and return Boolean truth values (true or false) depending on whether one is equal to, greater than or less than the other. Logic operations involve Boolean logic: AND, OR, and NOT. These can be useful both for creating complicated conditional statements and processing Boolean logic.

2.2.2 Control Unit: It acts as a manager which controls all activities being carried out within the computer. The control unit strictly obeys the instruction given by us, follows the instruction in the same sequence and executes them one after another until the entire set of instruction is exhausted. Control Unit (CU) brings one instruction at a time from the memory, interprets it and obeys it by coordinating the working of all other units. The CU tells the input unit what is to be read and addresses the memory as to where it is to be stored. The CU ensures that according to the stored instruction the right operation is done on the right data at the right time. It manages and coordinates the entire computer system.


The simplified descriptions of the steps that are performed by the Control unit are given below. Some of these steps may be performed concurrently or in a different order depending on the type of CPU
a) Read the code for the next instruction from the cell indicated by the program counter (program counter is conceptually just another set of memory cells, it can be changed by calculations done in the ALU);
b) Decode the numerical code for the instruction into a set of commands or signals for each of the other systems;
c) Increment the program counter so it points to the next instruction;
d) Read whatever data the instruction requires from cells in memory (or perhaps from an input device). The location of this required data is typically stored within the instruction code;
e) Provide the necessary data to an ALU or register;
f) If the instruction requires an ALU or specialized hardware to complete, instruct the hardware to perform the requested operation;
g) Write the result from the ALU back to a memory location or to a register or perhaps an output device
h) Jump back to step one.

2.2.3 Memory Unit: It is the workspace area within the computer where the data and instructions are stored. It holds all data, instruction and results temporarily. It stores the data to be processed, the intermediate results and the final results until they are displayed. It contains the programs that are currently being run and the data, the programs are operating on. In modern computers, the main memory is the electronic solid-state Random Access Memory (RAM). It is directly connected to the CPU via a “memory bus” and a “data bus”. The arithmetic and logic unit can very quickly transfer information between a processor register and locations in main storage, also known as a “memory addresses”. The memory bus is also called an address bus or front side bus and both buses are high-speed digital “superhighways”. Access methods and speed are two of the fundamental technical differences between memory and mass storage devices. Main memories are of the following types

a) Random Access Memory (RAM): It is the key working area of the memory. It is possible to select randomly and use any location of this memory. It is also called the read/write memory because information can be read from RAM chip and can also be written into it. It is a volatile storage medium i.e. the contents of the memory are lost when power is switched off/cut, as it requires a steady flow of electricity to maintain its content. RAM is also quite expensive. RAM may be of VRAM, WRAM, NVRAM.

b) Read Only Memory (ROM): It holds permanent data or instruction that can only be read. That information is permanently recorded and cannot be changed by the programmer. It is non volatile in nature i.e. the contents of ROM are not lost when the computer is switched off. It contains instruction to get the computer started when the switch is on, holds instruction and data that control the various peripheral units of the computer such as graphic display, disk drives, etc. Most personal computers contain a small amount of ROM that stores critical programmes, as it is expensive to produce. Typically, ROM must also be completely erased before it can be rewritten, making large scale use impractical, if not impossible. ROM may be of the following types-

i) Programmable Read Only Memory (PROM): A PROM is a memory chip on which set of instructions or information can be stored, but it cannot be modified or wiped out later on. Like ROM its memory is also non volatile. To write data on a PROM one will need a special device called a PROM programmer or PROM burner. The difference between a PROM and ROM is that PROM is manufactured as blank memory where ROM is programmed during the manufacturing process.

ii) Erasable Programmable Read Only Memory (EPROM): EPROM is used widely in personal computers to enable the manufacturer to change the contents of PROM before the computer is actually skipped so that the bugs can be removed and new versions can be installed shortly before delivery. The EPROM is of two types - Electrically Erasable PROM (EEPROM), where high voltage electric pulses are used to erase the previous data or instruction and after that the disk can be reused and Ultra Violet Erasable PROM (UVEPROM), which retains its data or instruction until it is exposed to Ultra Violet light. The UV light clears its contents making it possible to reprogramme the memory. The difference between an EPROM and PROM is that while in PROM the data or instruction can be written only once and cannot be erased, in EPROM the content can be erased and reprogramming can be done.

c) Flash Memory: Many modern PCs have their Basic Input Output System (BIOS) stored on flash memory chip so that it can easily be updated if necessary. Such type of BIOS is sometimes called flash BIOS. Flash memory is also important for modem as it enables the modem manufacturer to support new protocols as they become standardized. EEPROM is similar to flash memory (sometimes called flash EEPROM). The principal difference between the two is that EEPROM requires data to be written or erased in byte at a time whereas flash memory allows data to be written or erased in blocks. This makes flash memory faster.

d) Cache Memory: It is a special type of internal memory used by many central processing units to increase their performance or “throughput”. Some of the information in the main memory is duplicated in the cache memory, which is slightly slower but of much greater capacity than the processor registers, and faster but much smaller than main memory. Multi-level cache memory is also commonly used—“primary cache” being smallest, fastest and closest to the processing device; “secondary cache” being larger and slower, but still faster and much smaller than main memory.

e) Virtual Memory: Virtual memory is a feature of an operating system (OS) that allows a computer to compensate for shortages of physical memory by temporarily transferring pages of data from Random Access Memory (RAM) to disk storage. So, in virtual memory, virtual address space is provided by the operating system that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer and so it is called as virtual memory.

2.3 Output Unit: An output device is any product or machine that is capable of bringing information for user view. It presents the processed data or information to the user. It can be a printed page, a picture in monitor, and so on. Anything which comes out of a computer system is the output of it. The common output devices are-

a) Monitor [Video Display Unit (VDU) / Cathode Ray Tube (CRT)]: It looks like a television. The advantage of having a video display unit is that as we write we can see what is being fed into the computer and by this way we can spot the mistakes and make the necessary correction. It also helps to display the stored information inside the computer system.

b) Liquid Crystal Display (LCD): CRT screens are relatively heavy and bulky, therefore unsuitable for small portable computer like note book. Considering this the screen of a portable computer is effectively replaced by a flat panel LCD screen which is smaller in size and lighter in weight.

c) Printer: A computer printer, or more commonly called the printer, is a device that produces a hard copy (permanent human-readable text and/or graphics) of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as computer peripherals, and are permanently attached by a printer cable to a computer which serves as a document source. The latest technology is combining printers with a scanner and/or fax machine in a single unit. The world’s first computer printer was a 19th-century mechanically driven apparatus invented by Charles Babbage for his Difference Engine.

d) Plotter: A plotter is a vector graphics printing device which operates by moving a pen over the surface of paper. Plotters are used in applications such as computer-aided design, though they are being replaced with wide-format conventional printers.

e) Speaker: Speaker output the music or speech from the programme. A speaker or loudspeaker converts an electrical signal to sound. The speaker pushes a medium in accord with the pulsations of an electrical signal, thus causing sound waves to propagate to where they can then be received by the ear.

f) Computer Output on Microfilm and Microfiche (COM): The output from the computer, instead of being printed is displayed on a high resolution cathode ray tube, and the output is obtained in microfilm or microfiche from which it is often used to store massive data in compact form. Then, when needed, with the help of a special microfilm reader it is used to read the output.

g) Speech Output Unit:  A speech output unit is one which reads string of character stored in a computer memory and converts it into spoken sentence. This type of speech output is very useful in many areas. Examples: A telephone where a message is given to the caller when the number dialed does not exist. It is also used in railway and airlines enquires.

3. Software: A computer cannot perform on its own. It needs to be exclusively instructed on what it has to do. The programmes written for a computer to perform different operation are called software and it can be defined as “the set of computer programme, procedures and associated documentation or complete set of instruction which enable the computer to obtain solution of a problem that resides in the memory or storage device of a computer”. The programme is a set of instructions written in computer language. Software is a general term that is used to describe only single programme or group of programme and makes the hardware run. It acts as an interface between the user and the computer. Computer softwares are generally classified into two broad categories: system software and application software.

3.1 System Software: System software is a set of one or more programs, designed to control the operation of a computer system. Generally, the system software supports the running of the other software, communicates with other peripherals devices, supports the development of other types of software and supervises the user of various hardware resources. System software is of the following types-

3.1.1 Operating System: An operating system (OS) is an integrated set of computer programmes that manage the hardware and software resources and the overall operation of a computer system. The operating system is designed to support the activities of computer installation. It acts as an interface between a user and the hardware i.e. all computer resources. It forms a platform for other system software and for application software. Its prime objectives are to improve the performance and efficiency of a computer system, increase the facility and the ease with which a system can be used. Most operating systems have a command line interpreter as a basic user interface, but they may also provide a Graphical User Interface (GUI) for ease of operation. Operating systems are mainly of two types-

i) Command / Character User Interface (CUI): In this type of OS the user has to type the commands at the command prompt mode, which will act as an input to execute the program. E.g. UNIX, MS DOS.

ii) Graphic User Interface (GUI): In this type, the user is able to select files, programmes or commands by pointing to graphical representations on the screen and thereby it avoids the typing of lengthy complex commands. E.g. Windows 8.

3.1.2 Language Processor: Men use their own language (High Level Language) to write their programmes because it is much easier to code in such languages. However, the computer is unable to understand such a High Level Language; it only understands its own language i.e. Machine Language (Binary Language). Therefore, it becomes necessary to process a HLL to LLL. The computer programme that performs this job is the language processor. The language processors are of the following types-

a) Assembler: In the 1950s to reduce programming complexity in Machine Language and to provide some standardization, assembly languages were developed. Assembly language is also known as Symbolic Language. Assembly language uses abbreviation or mnemonic code to replace the earlier 0s and 1s of machine language. i.e. it substitutes letters and symbols for the numbers in the machine language program. The function of an assembler is to translate an assembly code into the computer machine code / language.

b) Interpreter: This language processor converts a HLL program into machine language by converting and executing it line by line. If there is any error in any line, it reports it at the same time and the programme execution cannot resume until the error is rectified. For error debugging the interpreter is very useful as it reports the errors at the same time, but, once errors are removed then also interpreter is present in the memory. So, unnecessary usage of memory takes place in this case.

c) Compiler: It converts the entire HLL program at one go and reports all the errors of the programme along with the line numbers. After all the errors are removed, the programme is recompiled and after that the compiler is not needed in the memory as the object programme is available.

3.2 Application Software: These are the programmes written by the programmers to enable the computer to perform a specific task such as processing words, inventory control, handling calculation and figures, medical accounting, financial accounting, result preparation, railway reservation, billing, etc. It can be defined as “a set of programmes necessary to carry out operation for a specified application”. Application software can further be subdivided into three categories-

a) Packages: The application softwares that are designed for the individual user, so that they can be used in a manner that suits their needs and requirements are known as packages. Actually it is a bundle of essential features for carrying out a particular task. There are different packages available in the market. Some of the most common categories are given bellow-

i) Word Processing Software: It is the software that processes textual matter and creates, organized flawless documents. It provides a general set of tools for entering, editing and formatting text. A word processor has everything that a conventional type writer has; in addition, it also removes various barriers of the conventional type writers. eg. M.S. Word, Wordstar, WordPerfect, Softword, etc..

ii) Spreadsheet: An electronic spreadsheet is a programme that accepts data values in tabular form and allows the users to manipulate / calculate / analyze data in the desired manner. It can also generate graphs and charts to show the relationship among numbers. Eg. MS Excel, Quattropro, etc.

iii) Database Management System: A DBMS is software that can effectively store, manipulate and handle bulk of data. Eg. Foxpro, MS Access, Oracle, etc.

iv) Desktop Publishing Software: Desktop publishing packages handle page layout by combining the function of a traditional typesetter and a layout artist.

v) Graphics Software: The application software that manipulates images is known as graphics software.

vi) Multimedia Software: The software that incorporates images, text, sounds, animation, video sequences is known as multimedia software.

vii) Presentation Software: The application software that concentrates on professional looking visual aids is called presentation graphics software. Eg. Corel Draw, Macro Media, Director, MS. Power Point, etc.

b) Utilities Software: Utility software (also known as service program, service routine, tool, or utility routine) is a computer software designed to help, manage and tune the computer hardware, operating system or application software by performing a single task or a small range of tasks. Some utility softwares have been integrated into most major operating systems. Some of the utility softwares are mentioned below-

i) Text Editors: Text and Hex / Editors directly modify the text or data of a file. These files could be data or an actual programme.

ii) Backup Utility: Backup utilities can make a copy of all the information stored on a disk, and restore either the entire disk (e.g. in the event of disk failure) or selected files (e.g. in the event of accidental deletion).

iii) Compression Utility: Disk compression utilities can transparently compress / uncompress the contents of a disk, increasing the capacity of the disk.

iv) Disk Defragmenter: Disk defragmenters can detect computer files whose contents are stored on the hard disk in disjointed fragments, and move the fragments together to increase efficiency.

v) Antivirus Software: Anti-virus utilities scan for computer viruses and speed up the operation of the computer.

c) Customized Software: Customized software (also known as Bespoke software) is a type of software that is developed either for a specific organization or function that differs from or is opposite of other already available softwares. It is generally not targeted to the mass market, but is usually created for companies, business entities, and organizations.

4. Humanware: The trained computer professionals who, by their knowledge are able to run the computer and can perform different operations are known as Humanware. They are the persons who programme, design and operate a computer installation such as System Analyst, Programmer and computer operator. Without the human as a component of a computer, it is a dumb machine, only a human can make a computer usable. So, in simple, humanware is everything which helps human beings to handle (use and look after) the computer, controls, assembles the parts, make markings, instructions, and provide training and available for technical consultation.

5. Conclusion: The fundamental components of a general-purpose computer are Input Unit, Central Processing Unit (CPU) and Output Unit. The CPU consists of Arithmetic and Logic Unit (ALU), Control Unit and Memory. If memory is removed, the device we had would be a simple digital signal processing device (e.g. calculator, media player) instead of a computer.

Nearly all modern computers implement some form of the stored program architecture, making it the single trait by which the word “computer” is now defined. By this standard, many earlier devices would no longer be called computers by today’s definition, but are usually referred to as such in their historical context. The technologies used in computers have changed dramatically since the first electronic, general-purpose computers of the 1940s, but most still use the Von Neumann architecture.
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