2.1 How Does One Perceive The Computer?

Let us take a tour to the Computer Laboratory of our school. We may see an assemblage like the one shown in figure 2.1. But then we have also seen laptops. What differences do we notice? Let us gather information about devices that look different from these but still are computers.

Computer indeed derives its name from its essential function, namely, to compute. In a rudimentary sense any computation requires primary data and operations performed between them. This would mean receiving data, processing them, keeping in memory the data at different stages of operation, having some set of data that is fundamental to all operations and giving outcomes of operations. Hence, essentially a computer means a system of components (i) to input data and display output i.e. Input and Output devices; (ii) processing unit called the Central Processing Unit (CPU) and (iii) the Memory space which can be Read Only Memory (ROM) or Random Access Memory (RAM). One should realise that it is not the external appearance by which a computer is understood in as much as its functionalities.

In fact, the memory in terms of capacity (space) and speed, and the processing capacity of the CPU (speed with which it processes) characterise computers and distinguish them. Also the manner in which data is presented determines a classification of computers, namely, analogue or digital computers.

2.1.1 Classification of Computers

Computers can be classified into two major types based on their way of handling data. These are as follows:

1. Analogue Computers : They work on continuous variables like waves of the signal, their amplitude, etc.

2. Digital Computers : They work on the principle of binary digits i.e. 0 and 1. Any value or symbol is represented through a binary value.

Digital computers can also be classified according to their physical structure (size) and the purpose of their use.

Based on capacity, speed and reliability they are classified into three categories:

I. Microcomputer : The term “Microcomputer” was introduced with the advent of single chip large scale integrated circuit (LSIC) computer processors. It is the smallest single user computer and its CPU is a microprocessor. It can perform the same operations and can use the same type of instructions as other computers. These are the most frequently used computers, better known by the name “Personal Computers.”

Types of Personal Computer

In our everyday use, the term “computer” refers to digital computers, a typical example being the common personal computer (PC). These can be classified mainly as Desktop Computers, Laptop Computers, Palmtop Computers, Personal Digital Assistant (PDA), Tablet PC, etc.

Desktop Computers

Desktop computers are larger and not meant to be portable. They usually sit in one place on a desk or table and are plugged into a wall outlet for power. The case (chasis) of the computer holds the CPU. When this case lies flat on the desk then it is generally referred to as a Desktop Model (Figure 2.2) else when it stands vertically on the desk in the form of a tower, it is referred to as a Tower Model (Figure 2.3). The computer usually has a separate monitor. A separate keyboard and mouse allow the user to input data and commands. The main advantage of tower models is that there are fewer space constraints, which makes installation of additional storage devices easier.

Laptop

This is a small, portable computer (Figure 2.4), small enough that it can sit on the lap. Nowadays laptop computers are also called notebook computers.

Hand-held Computers/Palmtop

This computer is smaller than a laptop. It is so small that it literally fits in a palm, hence, referred to as palmtop (Figure 2.5). Because of their small size, most palmtop computers do not include disk drives. Compared to full-size computers, palmtops are severely limited, but they are practical for certain functions such as phonebooks and calendars. Sometimes these are also called pocket computers.

Personal Digital Computers/Assistants

Palmtops that use a pen rather than a keyboard for input are often called PDAs (Figure 2.6). This is a hand–held device that combines computing, telephone fax and networking features. PDA can function as a cellular phone, fax sender and personal organiser. PDA was pioneered by Apple, which introduced the Newton Message Pad in 1993.

It is a notebook slate-shaped mobile computer (Figure 2.7) which is equipped with a touchscreen or graphics table which allows the user to operate the computer with a digital pen or stylus or fingertip. A wireless keyboard can be connected to this for extensive text input. Other models of tablet PCs are convertible model (screen can rotate over the keyboard) and hybird model where the screen is fixed on the keyboard.

II. Mini Computer : It is a multiuser computer capable of supporting up to hundreds of users simultaneously. They are usually more powerful compared to microcomputers. Minicomputers are also known as mid range computers as their cost and computing powers lie in the middle range between micro and mainframe computers. Minicomputers are used for multiusers and interactive applications.

III. Mainframe Computer : A powerful multiuser computer, the mainframe computer, is capable of supporting many hundreds or thousands of users simultaneously. They are capable of faster processing and greater storage area. They are used in research organisations, large industries, large businesses and government organisations, banks and airline reservation where a large database is required.

2.1.2 Evolution of Computers

Table 2.1 gives a glimpse of the evolution of computers.

Table 2.1 : Evolution of Computers

The future is promising for the computer industry and its technology. The speed of processors is expected to double every year and a half in the coming years. As manufacturing techniques are further perfected, the prices of computer systems are expected to steadily fall. However, since the microprocessor technology will be advancing, its higher costs will offset the drop in the price of older processors. In other words, the price of a new computer will stay about the same from year to year, but the capacity of the technology will steadily advance.

A computer is basically a device that can respond to a specific set of instructions in a well – defined manner, execute a prerecorded list of instructions (a program) and quickly store and retrieve large amounts of data. Since a computer is understood more by its functions than its appearance, let us see what functions each of the components performs.

2.1.4 Organisation of the Computer System

A computer system (Figure 2.9) is usually defined as a combination of hardware and software. Computer hardware is the physical equipment. Software is the collection of programs that make the hardware do its job.

Each of the hardware and software components has undergone refinements and modifications depending upon the limitations encountered by the users and the endeavours to overcome these limitations to meet new requirements and challenges. Let us therefore understand the computer from two perspectives, namely, the general functions of its components and the evolution these components have undergone.

In order to accomplish a particular task the computer performs a series of jobs, namely, Input, Storage, Processing and Output, which it handles in a very special way as follows:

• It accepts the data (input) from the user.
  
• The computer has memory chips, which are designed to hold data until it is needed.
  
• It processes this data into information. The computer has an electronic brain called the Central Processing Unit, which is responsible for processing all data into information according to the instructions given to the computer.
  
• It then returns the processed information (output) to the user.
  

For performing each of the jobs listed above, every computer has special parts/components, for Input Device, Storage Device, Central Processing Unit and Output Device.

2.2.1 Input Devices

General Functions

It provides a means for the machine to communicate with its external environment by accepting (or reading) data and the program.

1. It converts the data based on the program into computer acceptable form through an input interface.

2. It supplies the converted instructions and data to the computer system for further processing.

The very scope of computers is enough to understand that there is no limit to the form and type of data that serves as an input. Sometimes the input is in the form of text in normal language, sometimes it is a picture, sometimes it is a voice input like a song. Selection of an appropriate input device will improve efficiency and keep human interaction minimal.

Types of Input Devices

On-line or Direct Data Entry Devices : These devices are under the control of the CPU and communicate directly with the CPU, that is, these are connected via cable to the CPU. This process of data entry may be time consuming and may have errors but by using these devices, we can avoid having to capture data from paper. These are also known as Interactive Input Devices and are classified as:

(a) Keyboard

(b) Locator devices

(c) Interface or pick devices.

(a) Keyboard

The most common input device is the keyboard (Figure 2.10). The modern keyboard generally contains 104 keys and is called the QWERTY keyboard because of the sequence of keys in the top row, which are similar to those of a standard typewriter. A keyboard is used for a wide range of tasks but is well known for its capability to enter data, which is copied from the original paper documents (such as name and address details on a form). Keyboards are also used for typing letters, memos, reports and other documents using word processing software.

(b) Locator devices

Locator devices are used to control the location or position of the cursor on the screen and they allow us to provide the necessary input. Generally, locator devices are good for gross movement but not for precision work. Different locator devices are discussed in Table 2.2.

(c) Interface/Pick Devices

Pick devices are used to select an object on the screen. The object may be text or graphic. Examples of pick devices are light pens and touch screens.

Light Pens

The light pen is a pen-like light-sensitive device that feeds a narrow electrical pulse generated by a pointing device directed towards the screen. It is connected by a wire to the computer terminal that detects the signals from the screen. Light pens may be used in place of a mouse or a keyboard, particularly for menu based applications where an option may be selected by pointing on it. They are used for Computer Aided Designing (CAD) and drawing purposes. Moreover, with the help of a light pen, engineers, architects or fashion designers can draw and edit the designs directly on the screen, though for editing a drawing keyboard may be used to select colours, lines with varying thickness, cropping or enlarging the drawings. It is also used to read bar codes from products that are available in big departmental stores.

Touch Screens

Touch screens are monitors on which the user makes selections by touching the screen, rather than moving a cursor to the location with external locator devices.

Touch screens are often used in situations where users are likely to have a low level of competence in using computer keyboards. Touch screens are sometimes used in restaurants, building societies and travel information systems.

Source Data Entry Devices : We have observed that in grocery stores, the clerks merely wave the product over a laser scanner/barcode reader, which automatically enters the product code and retrieves the corresponding price of the product automatically. The earlier practice was to enter the code/product name by using a keyboard (a time consuming process). The laser scanner/barcode reader comes under Source Data Entry Devices category, which increases the efficiency of a user.

These devices help us in entering the data directly from the source, without any human intervention.

The devices used in this category are:

(a) Video digitiser

(b) Digital camera

(c) Scanning devices

(d) Voice input

(e) Remote control

(f) Magnetic stripe reader

(g) Sound sensor – microphone

(h) MIDI instrument.

Source Data Entry Devices

(a) Video Digitiser

A video digitiser captures television pictures from devices such as a TV set, video camera or video recorder and converts them into a digital format that a computer can use for display, storage or general manipulation. A video digitiser is an ideal tool for capturing images from real-life scenes to incorporate into computer work. It is used to convert still or moving images into a digital format required for computerised presentation. Taking video pictures or frames from video files is known as ‘frame grabbing’.

A video digitiser may be required for video conferencing. (If a digital video camera is used there will be no need for a video digitiser.) It can also be used to make television advertisements and pop videos, etc.

Advantages

• Video digitisers allow us to capture real-life images which are often more appropriate than drawings.
  
• Captured images can then be transferred to paper.
  

Disadvantages

• A fast computer with a large memory capacity is required to cope with the large amount of data involved.
  

(b) Digital Camera

A digital camera (Figure 2.11) stores images (digital pictures) in the memory and does not use celluloid films as in the case of ordinary cameras. Each digital picture is made up of thousands of tiny dots, called pixels (picture elements), and the camera stores data about the colour of each dot. The picture quality is determined by the number of dots representing a specific picture. The resolution of the image is expressed in Dots Per Inch (DPI) defines. Higher the DPI, better the resolution . Most cameras allow the user to choose the resolution needed for a picture. Once the photograph is stored in the camera it can be transferred to a computer where it can be edited, printed or more permanently stored. Some cameras store images on a floppy disk or memory card so that they can be easily transferred. Other cameras are connected to the computer with a lead (cable) and special software is used to transfer the image.

Figure 2.11 : Digital Camera

Advantages

• There is no developing cost, no film is needed and we can insert images directly into a document on the computer.
  
• You can edit, enlarge or enhance the images.
  
• An image can be saved with multiple shots and the best one can be saved. That is, there is no wastage like a miss-shot in ordinary cameras.

Disadvantages

• Digital cameras are generally more expensive than ordinary cameras.
  
• When the memory is full they can be connected to a computer to download the stored images (or the images can be deleted from memory) before they are used again. This is not as convenient as simply changing a film.
  

(c) Scanning Devices

(i) Scanner

A scanner is a device which can capture still images or even text, to be stored and used on a computer. The scanner throws a light beam onto the paper and measures the amount of light reflected back by each part of the page. Like a digital camera, the page is split into tiny pixels (dots) and a number representing the colour of each dot is sent to the computer. Scanner software usually allows the user to choose between high and low resolution. Very high quality images take up a lot of memory.

Most scanners need to place the paper on a glass plate inside them for scanning similar to a photocopier machine. These are called flatbed scanners (Figure 2.12) and typically use a charge–coupled device (CCD) as a image senser.

Advantages

• Any image can be converted from paper into digital format and later enhanced and used in other computer documents.
  

Disadvantages

• Images take up a lot of memory space, but it is possible to reduce the size of the data file by reducing the resolution (number of dots per inch) or by using different ways of storing the data i.e., different file formats.
  

Whether to go for a flatbed or a drum scanner depends upon the objectives of our scanning. Table 2.3 gives the comparison of flatbed and drum scanners.

Hand-held digital cameras and camcorders are field scanners because they scan a whole 2-dimensional field at once.

(ii) Optical Mark Readers (OMR)

Optical Mark Readers are able to detect marks on paper. A pre-printed document is prepared for users to select certain areas to put a line or a mark. A document like this is scanned using reflected light to detect dark shadows.

Advantages

Simply selecting and marking a choice in the right location on an OMR form is easier than typing writing in data.

• Documents can be checked evaluated very quickly by scanning them with only a few chances of error (only 2–3% with modern OMR machines).
  

• Careful handling of forms is needed. Badly damaged, creased or folded forms may be rejected.
  
• They can only be used when the data to be input can be selected by marking with lines. Because every option must be catered for, the forms are sometimes not very easy for the user to understand. This may introduce errors.
  

(iii) Optical Character Readers (OCR)

Optical Character Readers (OCR) are devices that can detect patterns of marks which are in character format (numbers, letters, punctuation and some special characters like ‘ – , @ ’, etc.). Only printed characters can be recognised because it is not easy to cater for different styles of handwriting. The characters are converted by a scanner from picture format into coded characters that the computer can work with. This is normally in ASCII format (American Standard Code for Information Interchange). OCR software can be purchased separately for use with a standard A4 scanner. OCR is often used to scan text for use in a word processor.

Advantages

• When a computerised document has not been saved and only printed text is available, it is possible to use OCR to scan in the text for later editing and re-arrangement.
  

Disadvantages

• Some characters cannot easily be interpreted and errors occur during conversion. This is especially true if there are a lot of diagrams and characters laid out in a way the software cannot interpret.
  

(iv) Magnetic Ink Character Recognition (MICR)

Characters which have been printed using magnetised ink can be recognised using a Magnetic Ink Character Reader device. This type of data collection has limited use and it is mainly restricted to banking systems because of the expensive equipment involved. Cheques are pre-printed with some code and account number. When a cheque has been written the data input clerk also has to mark the cheque amount in magnetic ink.

MICR is used for Bank cheque processing.

Advantages

• Fast processing of large volumes of data.
  
• It is a fairly secure method of data collection as the shape of the characters cannot be changed without expensive equipment.
  
• This method of input is very reliable as the number of errors generated during the reading of documents is negligible.
  

Disadvantages

• MICR is expensive to use because special equipment is needed to create the characters as well as to read them.
  

(v) Bar–Code Readers

Bar-codes (Figure 2.15) are a series of thick and thin vertical lines grouped together. Bar-code readers (Figure 2.16) are the devices used to input data from such sets of Bar codes. The reader uses a laser beam to read the series of thick and thin lines which represent the barcode number. Almost every product we see on supermarket shelves has a barcode. The barcode is 13 digits long and there are four main divisions. The first part of the barcode (two digits) represents the country, the second part represents the manufacturer’s code (five digits), the third part represents the produce code (five digits) and there is a final digit, which represents the check digit. This last digit is a calculated digit to ensure that the barcode is read properly. If there is an error, the reader usually beeps and the operator must key in all the digits manually. When the barcode is read, it locates the correct product on the inventory file, stored on the computer’s disc. The price is read from the file and a sale is registered, producing a record on the customer’s receipt. The number filed in the inventory file is reduced by one.

Bar codes are used on library tickets, airport baggage labels, books, supermarket products, clothes and on many other retail items. Barcode readers are used in most situations where there is an Electronic Point of Sale terminal (EPOS terminal), for example: department stores, supermarkets.

Advantages

• Fast and accurate data entry.
  
• It is possible to store all the details about the country of origin and manufacturer as well as the product code itself. These are recorded in a standard format within the barcode.
  

Disadvantages

• If the barcode is damaged the barcode reader cannot read it. It is then time consuming to enter all the separate digits using a keypad.
  

(d) Voice input

We can now input data by simply talking into a microphone and using special software to recognise the voice. Users should be trained in the software regarding correct pronunciation of the words before the speech can be interpreted and transferred to the computer. Some computer systems can respond to voice commands and can carry out tasks because the spoken word is interpreted by the software and converted to instructions. These can be used for inputting text to word processing software, controlling devices such as electronically controlled doors and machines.

Advantages

• Voice input is extremely useful for those users unable to use keyboards or mouse.
  

Disadvantages

• The system must be able to recognise the voice of each user. ‘Teaching’ the software can be tedious and time consuming.
  
• Speech recognition software is still not very accurate.
  

(e) Remote Control

Remote control devices are used for transmitting data when the user is at some distance from the processor. Devices such as video recorders can accept data which is programmed into the system from a remote handset. Special keys for certain tasks or selections are usually found on the handset. An infrared sensor on the main processing unit picks up the signals when keys are pressed. It is used to enter data from a distance.

Advantages

• The device provides the user with the convenience of inputting and working, away from the processing unit.
  

Disadvantages

• Small remote devices can easily be misplaced.
  
• The unit and remote need to be quite near to each other and should have no other objects between them which may interfere with the signals sent.
  

(f) Magnetic Stripe Reader

Magnetic stripes are the dark-looking stripe that may be seen on the back of many plastic cards, for example, bank cards. The magnetic stripe holds data about the owner of the card, a bank card will contain details such as the bank account number (the card owner’s individual account number) and sort code (the code that identifies the branch of the bank where the account is held). The device (Figure 2.17) reads the data on the magnetic stripe and money can be taken from the correct account to pay the bill. The money is taken from the bank account, not from the card. Data on a magnetic stripe does not change and there is no balance recorded on the card. It may be noted that the card does not contain details of the credit limit. Magnetic stripe readers are often seen at supermarkets and in many different types of shops, in fact anywhere where there is a ‘point of sale’ (somewhere we pay for our purchases). When the data is read electronically, the point of sale is called EPOS (Electronic Point of Sale).

Advantages

• The card can be read many times without damaging the stripe.
  
• The data is not visible to people because a machine is needed to read the data (although with debit cards the bank details are actually displayed on the card as well).
  
• Putting magnetic stripes onto cards is not expensive so the cards are quite cheap to produce.
  
• The data from the stripe is read very quickly and more conveniently.
  

Disadvantages

• Magnetic stripes may get damaged or the magnetic stripe reader could break down.
  
• The biggest disadvantage of magnetic stripes is that ordinary equipment can read but not change the data. That is why Smart Cards are gaining more popularity. These cards contain a small chip (or tiny processor with some memory) instead of a magnetic stripe. The data on the chip may then be changed as the smart card is used.
  

(g) Sound Sensor Microphone

A sound sensor, which is a microphone (Figure 2.18), is a device that detects sound and inputs it to the computer, and then the sound which is analogue in nature is converted into digital format.

The person using the computer speaks into the microphone. Speech recognition software on the computer converts what the person has said into text. The text is displayed on the screen and can be saved as a word-processed file. Speech recognition is getting more reliable though it is necessary to spend some time ‘teaching’ the system to recognise the user speech. Remember, it still does not always make accurate conversions to text.

Some users record special sounds, both voice and music, from any source. These sounds may be used in computerised presentations, for example, multimedia teaching software.

Advantages

• People with a range of disabilities can be benefited from the use of microphones and speech recognition, for example, those who cannot use a keyboard. They can create word-processed documents by speaking instead of typing. Moreover, many devices in the home can now be controlled by voice.
  
• Recording our own sounds means that we can add spoken messages or music to our own presentations or even e-mails.
  

Disadvantages

• There should be no background noise while recording, otherwise sound may become distorted. Cutting down the background noise is sometimes difficult and the sound recorded using a microphone is often not very good.
  
• Sound sampling (converting the analogue sound to digital format using software) often produces very large data files.
  

(h) MIDI (Musical Instrument Digital Interface) Instrument

The music industry uses MIDI to input music directly into a computer so that it can be edited and developed, often by mixing it with other sounds which are input from a microphone. Some programs allow its user to enter a tune from a musical instrument, then convert this into written music, which can even be printed out.

Advantages

• Once the tune has been played on the musical instrument, all the details are held on the computer. These details may then be changed. It is possible to speed up the tune, slow it down or even make it sound like a completely different instrument.
  
• The data stored is very compact and takes up about 1/20th of the space taken up by even the lowest quality recorded sound samples.
  

Disadvantages

• A musician is required to play the instrument to acquire the input, so some musical knowledge is required.
  
• To take advantage of the digitally recorded sound, knowledge of software and music is required to edit recorded tunes.
  

2.2.2 Memory or Storage Devices

Memory is required in a computer to store programs and the data processed by programs.

Computer memory is made up of a large number of cells. Each cell is capable of storing one bit of information in the form of binary numbers.

Memory System

Memory in a computer system is required for the storage and subsequent retrieval of instruction and data. A computer system uses a variety of devices for storing instructions and data required for its operations. Normally, the information to be stored on a computer is classified in two basic categories – data and instructions.

Figure 2.20 : The Memory Hierarchy

Types of Memory

A memory system can be considered to consist of three types of memories. These are as follows:

1. Internal processor memories

2. Primary memory or main memory

3. Secondary or auxiliary memory

Any storage unit of a computer may have the following characteristics:

Storage capacity is the amount of information/data a storage unit can hold. Accessing the data to/from these memories may be fast or slow.

The speed and availability of inexpensive memory has had enormous impact on computer technology. The high speed memory devices are more expensive and occupy less space in comparison to the slow speed memory devices.

Internal Processor Memories

These consist of the small set of high-speed registers and high speed buffer memory (cache) which are internal to a processor and are used as temporary locations where actual processing is done.

Register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage available elsewhere. Processor registers are at the top of the memory hierarchy and provide the fastest way for a CPU to access data.

Cache memory is a small high speed buffer memory used to hold instructions temporarily during processing.

The CPU of a computer system commonly uses cache memory (Figure 2.21) where it holds or buffers the contents of the main memory because the CPU runs much faster than the main memory. Thus to reduce the waiting time of the CPU the cache is used. Cache memory reduces traditional system bottlenecks because system RAM is much slower than CPU. This prevents the processor from having to wait for a program and data from slower main memory.

Figure 2.21 : Cache Memory

A cache typically operates by retaining copies of blocks of storage, each containing recently used information. This memory (or caches) is usually transparent or invisible to the processor.

Primary Memory

It is a large memory which is fast but not as fast as an internal processor register. The processor directly accesses this memory. It is mainly based on integrated circuit.

The primary memory or the main memory is part of the main computer system. The processor or the CPU directly stores and retrieves information from it. This memory is accessed by the CPU, in a random fashion. That means any location of this memory can be accessed by the CPU to either read information from it, or store information in it. The primary memory itself is implemented by two types of memory technologies. The first is called Random Access Memory (RAM) and the other is read only memory (ROM). A more appropriate name for RAM is RWM (read write memory), the CPU can write and read information from any primary memory location implemented using RAM. The other part of primary memory is implemented using ROM which stands for Read Only Memory.

There are two types of built-in memory, permanent and temporary, known as ROM and RAM, respectively, details of each given below:

Read Only Memory (ROM)

As we know, in the computer terminology, read means transferring data instruction from an input source to the computers, main memory (or CPU) and write is transferring data/instruction from computer’s main memory to an output device. Therefore, read only means data/instruction can be retrieved from the ROM chip but cannot be modified.

Types of ROM

Basically, there are two types of ROM, namely, manufacturer programmed and user-programmed.

Manufacturer–Programmed Read Only Memory

Manufacturer-programmed ROM is one in which data is stored in it permanently by the manufacturer of the ROM. For example, a computer manufacturer may store the system boot program permanently in the ROM chip used on the motherboard.

User–Programmed Read Only Memory

User-programmed ROM is one in which the user can load and store “read-only” programs and data. Such a ROM is commonly known as PROM (programmable read-only memory), because, a user can program it. PROM (programmable read-only memory) is a memory chip on which we can store a program. But once the PROM has been used, we cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.

Other kinds of user-programmed ROM are EPROM and EEPROM. Both of these are special types of PROM. EPROM (erasable programmable read-only memory) can be erased by exposing it to ultraviolet light while EEPROM (electrically erasable programmable read-only memory) can be erased by exposing it to an electrical charge.

Random Access Memory (RAM)

RAM chips are meant for primary storage. They hold temporarily (a) software/program instructions and (b) data before and after processing.

“Random Access” means that any location can be referenced in the same time and in the same manner, as it is independent of the address or location in the memory. It is a volatile memory. It holds data and instructions, during their execution.

The additional RAM chip can be plugged into the special socket on the motherboard known as Single In-Line Memory Module (SIMM). Random Access Memory capacity ranges from 16 MB to 4 GB on personal computers.

Types of RAM

RAM chips are of two types, namely, static RAM (SRAM) and dynamic RAM (DRAM).

Static RAM (SRAM)

The SRAM can store data as long as power is applied, without the need for periodically rewriting the data into memory. Contents (memory cell) of this RAM will stay in a given state (store a bit) indefinitely, provided that power to the memory circuit is not interrupted. The main applications of SRAM are in areas where only small amounts of memory are needed or where high speed is required.

Advantage

SRAM can provide very high speed.

Disadvantage

SRAM is costly and has low power packing density.

Dynamic RAM (DRAM)

This memory stores data as charges on capacitors. With Dynamic RAM, the stored data will gradually disappear because of capacitor discharge, so that it is necessary to periodically refresh the data (i.e. recharge the capacitors). In the process of refreshing, the information is read from the memory cell and written back in the same position. Typically, each memory cell of a DRAM must be refreshed at least every 2 to 10 millisecond or its data will be lost.

Advantage

It has high capacity and power consumption is low.

Disadvantage

The need for refreshing of dynamic RAM because some external refreshing circuits is required.

Secondary or Auxiliary Memory

Auxiliary memory is much larger in size than main memory but is slower than the latter. It normally stores system programs and data files. These cannot be accessed directly by the processor.

Secondary or auxiliary memory, also known as secondary storage, is the memory that supplements the main storage. This is a long-term, non-volatile memory. The term non-volatile means it stores and retains the programs and data even after the computer is switched off. Unlike RAM which looses the contents when the computer is turned off and ROM to which it is not possible to add anything new, auxiliary storage device allows a computer to record information semi-permanently. This is to ensure that this information can be read later by the same computer or by another computer. Auxiliary storage devices are also useful in transferring data or programs from one computer to another. They also function as backup devices which allows backup of the valuable information that we are working on. So, even if by some accident our computer crashes and the data in it is in unrecoverable mode, we can restore it from your backups. The most common types of auxiliary storage devices are floppy disks, hard disks, magnetic tapes and magnetic disks.

Floppy Disk

Floppy disk (often called floppies or diskettes) is a soft magnetic disk. It is called floppy because it flops if we wave it (atleast the 5¼ inch variety does). The data on the floppy disk is organised in terms of tracks and sectors. Unlike most of the hard disks, floppy disks are portable because these can be removed from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity but are less expensive and are portable.

There are two basic sizes of a floppy, namely 5¼ inch and 3½ inch.

3½ inch : Floppy (Figure 2.22) is something of a misnomer for these disks as they are encased in a rigid envelope. Despite their small size, these floppies have a large storage capacity than their cousins – from 400K to 1.4MB of data. The common sizes for PCs are 720K (double density) and 1.44MB (high density).

Optical Disk

An optical disc is an electronic data storage medium that can be written to and read using a low-powered laser beam. Optical disk can store much more data, i.e. upto 6 GB. There are three basic types of optical disks namely, CD–ROM, WORM and Erasable.

CD-ROM : Like audio CDs, CD-ROMs come with data already encoded onto them. The data is permanent and can be read any number of times but CD-ROMs cannot be modified (Figure 2.23).

WORM : This term stands for “Write Once, Read Many” with a WORM disk drive. One can write data only once onto a WORM disk. After that, the disk behaves just like a CD-ROM.

Erasable : Optical disks that can be erased and loaded with new data are just like magnetic disks. These are often referred to as EO (Erasable Optical) disks.

Hard Disk

Hard disk is a magnetic disk (Figure 2.24) on which computer data can be stored. Hard disks hold more data and are faster than floppy disks. A single hard disk usually consists of several platters. Each platter requires two read/write heads, one for each side. All the read/write heads are attached to a single access arm so that they cannot move independently. Each platter has the same number of tracks. A track location that cuts across all platters is called a cylinder. For example, a typical 84 MB hard disk for a PC might have two platters (four sides) and 1,053 cylinders.

Magnetic Tape

Magnetic tape (Figure 2.25) is a magnetically coated strip of plastic on which data can be encoded. Tapes for computers are similar to the tapes used to store music. Some personal computers, in fact, enable one to use normal cassette tapes. Tapes are considerably cheaper than storing data on disks but accessing data from tapes is much slower than accessing data from disks.

2.2.3 Central Processing Unit

The central processing unit is one of the two most important components of the microcomputer. It is the electronic brain of the computer. In addition to processing data, it controls the functions of all the other components.

The Central Processing Unit (CPU) or the “Central Processor” carries out a variety of essential data manipulation (including arithmetic/logical calculations, comparisons, sorting, etc.) and controlling tasks in the computer. CPU consists of Arithmetic Logic Unit (ALU) and Control Unit (CU).

Every microprocessor contains a system clock. The speed at which the processor executes instructions is known as Clock Speed and it is measured in Megahertz (MHz).

Megahertz (MHz): a measure of frequency equivalent to 1 million cycles per second.

Arithmetic Logic Unit

The Arithmetic Logic Unit carries out as the name suggests Arithmetic and Logical operations on the data made available to it. Basic Arithmetic functions which an ALU can carry out are addition and subtraction. More powerful CPUs can support additional mathematical operations like multiplication and division. The logical operation which it can carry out are greater than, equal to, less than and comparison between two numbers. Besides these operations some processors also support operations which check if particular bits are on or off.

Control Unit

The Control Unit can be thought of as the brain of the CPU itself. It controls the computer based on the instructions it decodes, how other parts of the CPU and in turn, the rest of the computer systems should work in order that the instruction gets executed in a correct manner.

Register

The register is a special, high-speed storage area within the CPU. All data must be represented in a register before it can be processed. For example, if two numbers are to be multiplied, both numbers must be in registers, and the result is also placed in a register. (The register can contain the address of a memory location where data is stored rather than the actual data itself.)

2.2.4 Output Device

The output devices of the computer are used to display results in electronic or paper medium.

Common output devices which produce output that is easily understood are Printers and Monitors.

The results generated by the output devices may be of two types:

Hardcopy Output : Such outputs are non-electronic version/permanent, and thus may be used at later date as and when they are required. They are usually on paper and are generally used to produce reports. The devices which are used to produce hardcopy output are printers, graph plotters, computer output microfilm, etc.

Softcopy Output : Such outputs are electronic, thus they are available either on screen or may be stored digitally on some storage device. The softcopy may be directly seen on screen or may be stored on hard disk/ CD/ DVD for further use.

Other output devices used for specific purposes are Projector, Sound card, Speakers and Video card.

Printers

A printer is an output device that prints characters, images and symbols on paper. On the basis of the printing technique used, printers may be classified as impact or non-impact printers.

Impact printers are like typewriters, as they use some sort of striking/hammering action to press carbon or fabric ribbon against paper to create a character or a pattern. These devices are noisy and are not capable of producing high resolution graphics. The most common impact printers are character printers (e.g. daisy wheel, dot matrix) and line printers (e.g. chain printers, drum printers). Among impact printers, line printers print one line at a time, so, these printers are extremely fast. Impact printers are ideal to be used for carbon copies.

Dot-matrix Printers

The print head of Dot-matrix printers (Figure 2.26) uses small pins, which strike on the paper to create a character or image. They are more economical than most of other types of printers, with lowest running cost. These printers are not used much now-a-days. However, at places where invoices and delivery notes are produced with a copy and the quality is not important, these are still used.

Advantages

• The main advantage of dot-matrix printers now is the ability to produce carbon copies. They provide a cheap, reliable method of getting multiple copies of a printout.
  
• The other advantage is the low cost of printouts. It has low maintenance and running costs.
  

Disadvantages

• Dot-matrix printers are noisy, slow and produce a lower quality output.
  

Non-impact printers overcome the limitations of impact printers. They form characters or images without making print heads strike the paper. They use thermal, electrostatic, chemical or inkjet technology to produce printed output. They are fast and quiet in operation. The most common non-impact printers are inkjet printers and laser printers. These printers can come in two categories, one produces output with only one colour (i.e. black colour) and the other one produces output in multicolour.

Day–by–day, with advancement in the field of printing technology, the quality and speed of these printers are going up and prices are coming down. Colour printers are increasingly being used in homes and offices alike.

Inkjet Printers

Advantages

• Inkjet printers are relatively inexpensive to buy, especially if colour is needed.
  
• Inkjet printers are also lighter in weight than other printers and some are small enough to be carried around in a portable carry case.
  

Disadvantages

• Inkjet printers are slower than laser printers.
  
• If left unused for long periods of time the cartridges can easily dry out.
  
• Colour cartridges are expensive in comparison.
  

Laser Printers (Non–Impact Printer)

Laser printers produce high quality paper prints at a high speed (with an average speed as 6 to 12 pages per minute). They are almost noiseless and use dry magnetic ink technology.

Advantages

• Laser printers are noiseless, high speed and high quality printers.
  
• Laser printers are comparatively economic for large number of printouts.
  

Disadvantage

• Initial costs of laser printers are comparatively high.
  
• Size of laser printers is comparatively bigger.
• Repair cost and cartridges of these printers are generally expensive.
  

Plotter

A plotter (also known as a graph plotter) is an output device, which produces high quality diagrams on paper. Colour pens are used to draw lines on the paper which is placed in the plotter. Some plotters have a flat area (plotter base) to put the paper on. These are known as flatbed plotters (Figure 2.29). There is another type of plotters, the roller plotter which uses a large roll of paper mounted on a roller (Figure 2.30). Usually a plotter is capable of producing prints on very large size paper sheets but have a speed slower than printers. Graph plotters are used for drawing building plans, graphs and three-dimensional drawings. They are often used by architects and engineers designing machines, bridges, etc.

Control Devices (Lights, Buzzers, Robotic Arms, Motors)

Many other devices may be connected to the computer to produce output. For example, lights may be connected to a computer which will respond to electrical signals that tell them to switch on and off. Buzzers may also be used for sound as the computer sends an electrical signal to switch them on and off.

Control devices like traffic lights, buzzers and motors are used when the computer is controlling a situation such as regulating traffic lights or assembling cars.

Softcopy Output : Display Devices

Terminals

Terminal is a display device that may be used for entering data online from a remote location and for retrieving data. Depending upon the processing capability, terminals may be classified as intelligent or dumb terminals.

Intelligent Terminal refers to a terminal with its own memory processor and a firmware that can perform certain functions independently. They are usually personal computers, having their own local data processing capability, data storage and input/ouput capability. For a PC to be a terminal, a communication adapter (may be a modem) is essential. If terminals are communicating to one or several hosts and peers, then a network program is also needed. There are two types of intelligent terminals, namely, General purpose and Job-oriented. The PC is an example of General Purpose Terminal. Typical General Purpose application of terminal is the Internet connection. This is the most pervasive terminal application and comes with modem and browser software. Job-oriented terminals are designed and developed for specific tasks. Typical job-oriented application of terminal includes airline reservation system terminal, a fast forward terminal ATM and a terminal for monitoring of a hospital patient. In such terminals hardware and software are customised to fit the application.

Dumb terminals are meant to enter the data and to receive output from the main computer, as they cannot process data on their own. These terminals are connected to the main computer through a communication link.

Video Display System

Video Display System provides a visual link between the user and the computer. The videos subsystem of a PC consists of two main components:

(a) Monitor and

(b) Video adapter (also called video card or graphic adapter)

A monitor is a display device which can produce text and graphic as output. A monitor may use different display technologies like a Cathode Ray Tube (CRT) and Liquid Crystal Display (LCD). CRT contains a vacuum tube which is used for display on screen in a computer. LCD is a technology in which molecules of a liquid crystal line up in such a manner that it creates images on the screen by blocking or transmitting light on the screen.

As a voice recognition system allows the user to talk to a computer, a voice response system permits a computer to talk back to the user. Many organisations use audio-response systems to respond to human inquiries that are transmitted over telephone lines to the central computer.

Voice output is useful for people who are partially sighted. The computer can read word processed documents and communicate by sound output from the computer. Voice output is also useful for those who are physically handicapped and cannot speak. By typing on a keyboard they are able to communicate using the spoken word. Directory enquiries at Telecom Services use voice output to tell the caller the number that the computer system has found. Voice output is also used in multimedia presentations and provides a valuable means of communication.

• The user cannot see the result on the computer screen, but can know the result even when they are located far away from it, or when they are visually handicapped.
  

Disadvantages

• The quality of voice output is getting better but it still sounds like a set of small sounds strung together and is therefore obviously electronic and rather unnatural.
  
• Sound files used for voice output occupy a large amount of memory.
  

2.2.5 Computer Ports

The input/output device is a piece of hardware that can be used for both providing data to a computer and for receiving information from it. But how does the computer communicate with these external devices? There are input/output interfaces called ports (Figure 2.34).

Parallel Port : A parallel port transmits 8 bits of a byte of data in parallel as it contains eight or more data lines for control of I/O devices. It is commonly used to connect the printer to a computer. It is used for transmitting fast data over short distances because interference among multiple signals limits the cable to relatively short distances.

Serial Port : A serial port transmits one bit of a byte at a time as a single stream of bits. It is meant for transmitting slow data over long distances. Communication over a phone system is an example of serial communication. A serial port connects a variety of devices such as modems, scanners, barcode readers and device control circuits.

PS/2 Port : It is one of the serial port connectors used to plug mouse and keyboards into a personal computer. It consists of six pins in a small and round shaped socket.

Universal Serial Bus (USB) Port : USB port is the most popular port used to connect up to 127 peripheral devices such as digital camera, speaker, scanner, printer, plotter, modem, joystick, pen drive, etc. USB permits Plug and Play – i.e. it allows us to install device drivers as and when the devices are plugged on.

Small Computer System Interface (SCSI) Port : SCSI port allows data transmission in a “daisy chain” to up to 7 devices at a speed (32 bits at a time) much higher than serial and parallel ports. The devices that may be connected to SCSI ports are hard-disk drives, CD-ROM drives, scanners, backup units, network adapters etc. In daisy chain several devices are connected in series. If data has to reach the seventh device then it needs to go through, all the six devices first.

Fire Wire (IEEE 1394) Port : Fire Wire port is a relatively a new technology for high speed data communication. The need of this arose due to large audio and video data movement from multimedia devices. The devices connected via Fire Wire Port are Video Camera, External Hard Disk Drives, External CD/DVD Drives. Microsoft has developed device drivers to support IEEE1394 in all the versions after Windows 95. Linux also supports this port. It is extremely fast with Data Transfer Rate up to 400 Mb/sec.

2.2.6 Computer Equipment Maintenance

Computer maintenance entails a four-fold attack against deterioration of hardware, security threats, software bugs and performance degradation. Each can seem overwhelming or insignificant but, considering the importance of computer systems operations, the cost and time are trivial compared to the hassle of getting a system up and running after a crash.

In fact, we should upgrade the computer every three years or so, but we will have to transfer all the data and software from the original computer, whereas a hard-drive swap is a transparent upgrade.

Preventive Maintenance

It is important to maintain computer equipment and consumables. Routine or preventive maintenance ensures that the computer equipment will remain in good working order. Examples of preventive maintenance of computer equipment are the cleaning of the keyboard, monitor and mouse. There are a few important DOs and DON’Ts which, if adhered to, can save a lot of maintenance and repair cost. This is known as preventtive maintenance. The basic preventive maintenance features are given in Appendix 2.1.

Letters and telegrams have been the modes of communication for a very long time. They require setting up of post offices in various regions and then a hierarchical level of post officers and transport systems to carry the mails. Such a huge infrastructure is required for the mail delivery system. This was a sort of manual technology for mail delivery.

Then telephones were made more accessible to the common man. Communication through letters reduced considerably. The e-mail has simply revolutionised our lives, conquered time and space and communication is instantaneous and cheap.The scope of message that could be communicated through e-mail in comparison to postal or telephone mode is mind boggling.

Communication is essentially exchange of information and information by its own right has evolved as an independent technology. With the kind of information explosion that has happened globally and the logical thirst for knowledge, computers have an eminent role to play in view of their capacity to generate and store information as well as their essentiality in the broadband technology on which the Internet works.

2.3.1 Computer Technology in Education

Computer technology promotes independence and self-assessment. It stimulates the interest of the students. The various programs in the system help students in self-assessment. Word processor programs help students in organising their written work letter. Interactive computers also help students with their pronunciation. Computer technology assists people in learning languages. Teachers may evaluate their teaching methods with the help of computers. They may also effectively use computer technology to enhance their teaching skills. Computer technology assists people with learning disabilities. The physically challenged may use graphics software and word processor to express themselves. Computer technologies also benefit people with speech disability by replaying film clips of their speeches and helping in practising their speech skills.

2.4 SOFTWARE

A computer is a programmable device. A program is a stepwise organisation of the different steps in which an event happens. When these steps are predetermined and incorporated in a device we have actually programmed the device to perform the job. Our human brain also functions as a programmed device. But it is uniquely endowed with the power of altering a program with which it initially started if the situation demands. It is in this that the computer is different from the human brain. The computer does not have its own intelligence. It can as an obedient servant, execute instructions. But it has tremendous potential to execute them with an accuracy and speed far exceeding the domain human physical capabilities.Hence with increasing requirements, the human brain has worked out complex programs to make best use of the capabilities of a computer, at the same time overcoming its limitations.

Software is usually divided into two major categories:

System software

Application software.

2.4.1 System Software

Operating System : An operating system is a set of instructions that manages overall performance and functioning of the computer system by controlling the resources such as CPU, memory, I/O devices and overall flow of information within the system. It acts as an interface between the machine and its users. Some examples are Linux, MS/PC-DOS, Mac OS, Windows 95/98/NT/ME/Millienium/VISTA.

Language Processors : For program execution, instructions must be converted into the machine language. Language processors are used to convert assembly language and high-level language programs into machine level language. Examples: assemblers, translators and compilers.

Utilities : These are designed to aid, manage and tune the computer hardware, operating system or application software. These are also known as service programs, service routines, tools, or utility routines. Examples: disk defragmenter, disk compression, disk cleanup, disk checkup etc.

2.4.2 Application Software

Application software is a set of one or more programs designed to do a specific task such as processing of student admission examination result, pay calculation, payroll, general accounting, inventory control. Different organisations need different application programs. The special purpose programs are also known as “packages”.

A computer system comprises not only of hardware or software but of both. Hardware is generally controlled by the software. Software is a set of instructions called a program. British sp.–programme. Computer related spelt as program. In general, the word program means a sequence of steps, like come home, wash hands, have a cup of tea or lunch and like this we prepare a list of activities that we are going to do throughout the day. Similarly while designing a software, instructions need to be written in a proper sequence.

Software can also be classified as given below:

2.4.3 Shareware

Shareware software is copyrighted software that is distributed on a ‘try before we buy’ basis by paying a nominal shareware fee. Users who want to continue using the program after the trial period are encouraged to send a payment for the program to the program developer. Shareware is generally less expensive because often a single programmer develops it.

2.4.4 Freeware

Freeware programs are programs that are given away free of charge and often made available on the Internet. The programmer offers programs as freeware either for personal satisfaction or to assess its reception among interested users. Program developers often retain all rights to their freeware and users are not free to copy or distribute or sell it further.

Algorithm

When any job or task is written or described in a sequence of steps in plain English language, it is called an algorithm. It can contain simple words, which convey meaning, to do a task. For example, algorithm to add two numbers can be simply described in a few steps given below:

1. Start

2. Accept first number as A

3. Accept second number as B

4. Take a number C = A + B

5. Display C

6. End

As the computer is a machine and does not have a brain of its own we need to tell from where to begin and where to end. In the above example, use of ‘Start’ and ‘End’ is to indicate the beginning and end of the task.

Flowchart

Pictorial representation of these steps is known as flowchart. A flowchart uses a particular set of symbols shown in Table 2.7. A flowchart can help to identify bottlenecks or any inefficiencies in the program.

2.5 PROGRAMMING LANGUAGES

2.5.1 Machine Language (First Generation Computer Language)

After preparing an algorithm and a flowchart for a particular software it has to be written in the computer coded language. Unlike humans, computers can understand language of only 0s and1s (binary digits), i.e., binary number system (Number system is given in Appendix 2.2). A program written in 0s and 1s is called machine language program or binary language program. Machine language was the first generation (Figure 2.37) computer language.

A sample code in machine language is shown below:

ADD 011814 is written as

0001000000000010011101100001100

2.5.2 Assembly Language (Second Generation Language)

As it was very difficult to generate code in machine language, the assembly language was developed which consisted of small meaningful terms. This is considered to be the second generation language (Figure 2.38).

An example of an assembly language program for adding two numbers X and Y and storing the result in some memory location:

A machine cannot execute an assembly languages program directly as it is not in a binary form. An assembler is needed in order to translate an assembly language program into the object code executable by the machine.

This is illustrated in the figure 2.39:

Writing a program in assembly language is more convenient than in machine language. It is more readable. But assembly language has some complications associated with it like:

• Assembly language is not portable. It means that assembly language program written for one processor will not work on a different processor.
  
• Assembly language program is not as fast as machine language because it has to be first translated into machine (binary) language code.
  

Machine language and assembly language are referred to as low level languages since the coding for a problem is at the individual instruction level.

2.5.3 High Level Language (Third Generation Language)

We have heard about programming languages like C, PASCAL, COBOL, FORTRAN and BASIC. These are called high level languages (HLL) or third generation languages (Figure 2.40). The program shown below is written in BASIC to obtain the sum of two numbers.

The time and cost of creating machine and assembly languages was quite high and this was the prime motivation for the development of high level languages.

A high level source program must be translated first into a form which the machine can understand; this is done by a software called Compiler (Figure 2.41).

2.5.4 Fourth Generation Language

Most fourth generation languages (Figure 2.42) are non–procedural languages. The programmer does not have to give the details of procedure in the program, but instead, specify what is wanted.

Major fourth generation languages are used to get information from files and databases. They are query languages, which are used to answer queries or question with data from a database. For example:

SELECT Name, Address

FROM Personnel

WHERE Name = “Ekambaram Vasudevan”;

2.5.5 Fifth Generation Language

Fifth generation programming languages will permit the user to give commands in a more conversational way. These will use voice input devices rather than traditional keyboards or pointing devices.

Fifth generation languages are used mainly in artificial intelligence research. Example of fifth generation languages: PROLOG, OPS5, and MERCURY are the best known fifth generation languages.