CRT Monitors

A monitor consists of the display device- the picture tube along with the related circuitry that converts the signal send by the computer in a form that can be used by the display. The picture tube, called cathode ray tube or CRT, is a core of monitor.
It has many other elements such as a shadow mask and a phosphors coating that enables it to display picture along with the display electrons. The simplest way of explain the functioning of the monitor screen is by what we call CRT electron’s guns. The electrons are filtered through a shadow mask and hit the phosphors- coated screen to produce the image. This process is monitored and controlled by various display electronics.
The most important component in the component in the monitor’s electronic is the video amplifier increase the strength of single volt signals that are received from your PC, to the thousand of voltage that are required to drive the electron beam.
The Cathode- Ray Tube is a major component of any monitor. It essentially is a glass tube partially evacuated and filled with inert gas at a very low pressure. The cathode (a negatively charged electrode) beams a ray of electrons towards a positively charged the anode. At the end of their flight, the electron crash into a coating made from phosphor compounds that convert the kinetic energy of the electron into visible light- glowing to produce the picture.
Color CRTs use thousand of triangle painted across the inner surface of the tube. Every triad consists of three dots of the primary colours red, green and blue (RGB) arrayed next to each other. One triad of the dots makes up a ‘picture cell’ called a pixel.


To generate beam that light up the phosphor on the screen, a CRT uses one or more electron guns. An electron gun is an electron emmitter (a cathode) that draw the electron into sharp and high speed beams. These are three separate electron beam, one each for red, blue and green colors.
These beam have to go through a shadow mask, which is a layer between the phosphors triad and electron guns. The shadow mask cause the three beams to land on the phosphors in a distinct traingle so that each beam illuminates only the right – color dot. Each traid has a corrosponding hole in a shadow mask, which prevent the beam from illuminating the wrong color phosphors.

beams





Electron gun
Shadow Mask Screen

To move the beam across the breadth of the tube face, a group of powerful electromagnets are arranged around the tube, forming a yoke. They bend the electron beam in the course of its flight. Monochrome CRTs have a single electron gun that continuously sweeps across the screen. Most color tube has three guns arranged in a triangle. They emit their electrons continuously and the three resulting beam are steered by the yoke.

Flat Panel Monitors
There are several types of flat-panel of monitors, but the most common is the liquid crystal display monitor. The LCD monitors creates images with a special kind of liquid crystal that is normally transparent but become opaque when charged with the electricity.
One disadvantage of LCD monitors is that, unlike phosphors, the liquid crystal does not emit light, so there is not enough contrast between image and background to make them clear under all conditions. The problem is solved by backlighting the screen. Although this makes the screen easier to read, it requires additional power.
Another disadvantage of LCD monitors is their limited viewing angle – i.e. the angle from which the display images can be viewed clearly. LCD monitors use transistor for pixel but not the electron beam. The electricity coming from transistor determines the colour displayed by a pixel.
Depending upon the arrangement of transistors, flat-panel screens are divided into Passive Matrix LCD & Active Matrix LCD
Passive Matrix LCD is less expensive than Active matrix monitors. One Disadvantage is that, Passive matrix have narrow viewing area as compared to that the Active matrix. In a passive-matrix display, a transistor controls a whole row or column of pixels.
Active Matrix LCD technology assigns a transistor to each pixel, and each pixel is turned on and off individually. These enhancements allow the pixel to be refreshed much more rapidly. In an active-matrix display, each pixel is controlled by its own-transistor. Active matrix screens are much brighter & sharper than passive matrix screens.

3. Gas-Plasma Display:
It is like a neon bulb, in which the display uses a gas that glows & emits light in the presence of an electric current. At present, ELD & gas-plasma technologies are more expensive & they are not used as often as LCD technology.


Fig: Plasma display

A plasma display panel (PDP) is one type of flat panel display now commonly used for large TV displays (typically above 37-inch or 940 mm). Many tiny cells located between two panels of glass hold an inert mixture of noble gases. The gas in the cells is electrically turned into plasma which then excites phosphors to emit light. Plasma displays are commonly confused with LCDs, another lightweight flat screen display but with very different technology.
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Characteristics of Monitors
• Size
• Resolution
• Refresh Rate
• Dot Pitch
• Convergences

Size
The most important aspect of monitors is its screen size. A typical size of small VGA monitors is 14 inches. Just as the television set, monitors screen size are measured by the length of diagonal of their picture tubes
Refresh Rate
Display monitors must refresh many times per second. The refresh rate determines how many times per second the screen is to be refreshed. The refresh rate for a monitor is measured in Hertz (HZ) and is called vertical frequency or vertical refresh rate. The faster the refresh rate, the less the monitors flickers.
Convergence
Convergence refers to how sharply an individual colour pixel appears. If the dots are badly miss converged, the pixel will appear blurrily.
Resolution
The resolution of monitors indicates how densely the pixels are packed. The clarity or sharpness of a display screen is called its resolution. Pixel is short for “picture element”. The resolution of a computer monitors is classified by the number of pixel on the screen, expressed as matrix. For example, a resolution of 640 X 480 means that there are 640 pixels horizontally across the screen and 480 pixels vertically down the screen. The number of bit is used to represent single pixel that determine how many colour or shades of grey can be displayed. For example, an 8-bit colour monitors use 8 bit for each pixel, making it possible to display 2 to the 8th power (i.e. 256) different colours of grey.

Dot Pitch
Dot pitch is the distance between the two phosphors dots that makes up the single pixel. Each RGB triad contains three phosphors dot - red, green and blue. If these dots are not close enough together, the image on the screen will not be crisp. Smaller dot pitch means clearer and sharper pictures. In general dot pitch should not be greater than 0.28mm





ii. Printer
A printer is a peripheral which 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
When evaluating printers, following criteria must be considered-
Image Quality
Image quality, also known as print resolution, is usually measured in dots per inch (dpi). The more dots per inch a printer can produce, the higher its image quality. Most common print resolution is 300 or 600 dpi.
Initial Cost
Consumer printers cost $250 or less, but professional printers can cost thousands of dollars.
Speed
Printer speed is measured in the number of pages per minute (ppm) the device can print. A print speed goes up, so does cost.
Cost of Operation
The cost of ink or toner and maintenance varies with the types of printer. Many different types of printer paper are available, too, and the choice can affect the cost of operation.

1. Impact Printer
In impact printer, the hammer of the printer strikes against an ink ribbon to make marks on the paper. This includes. The working of an impact printer is exactly similar to a typewriter.
a. Dot Matrix Printers
This type of printer creates character from a set of dots. The print head consists of vertical array of pins. While printing, the head moves across the paper to the printing position & selected pins are activated to strike against an inked ribbon to form a pattern of dots on the paper. To print the full character, the print head moves a predetermined number of positions to complete the matrix. The matrix contains 7 rows & 5 columns. The numbers of pins in the print head are either 9 or 24, the bottom 2 pins being used to form a lower case letters.
It is a very versatile device because it is capable of printing in various languages with fronts of different types & sizes. It can also give graphic output. The major reason for its popularity is that dot matrix printers are cheaper than other printers. The speed of a typical dot matrix printer has 300-400 cps.
Advantages
1. They are generally cheaper as they use simple technology.
2. It only requires paper & the carbon ribbon.
3. It does not require air condition environment.
Disadvantages
1. They are noisy.
2. The quality of print is poor.
3. Not good for continuous printing




Fig: How a Dot Matrix Printer creates an image.

2. Non impact Printer
In Non-Impact printer, the ink is sprayed to the paper so that the letters or objects get printed. This includes Ink-Jet printer, Laser printer.

a. Ink-Jet Printer
An ink-jet printer produces a character by shooting small droplets of ink onto the paper. One simple technique is the thermal heating process. In this method, ink is heated from one end to more than 3000c & the vaporized ink forms a bubble. When the bubble expands, a drop of ink is forced out. The print quality of an ink-jet is generally 300 dpi to 600 dpi. An ink-jet printer can also generate colour graphics images.

Advantages
• High quality print.
• Price is lower than Laser printer
• Color ink-jet printers provide an inexpensive way to print full color documents.
• Performance: 2-4 page per minutes.
Disadvantages
• Expensive than dot–matrix printer.
• It needs special type of ink.
• Ink clogging (blocking) in the printer head is the main problem with an ink-jet printer.






















Fig: How an Ink-jet printer creates an image

b. Laser Printer
LASER – Light Amplification Stimulated Emission and Radiation
Laser printer uses a laser beam to produce an image on a drum, coated with photosensitive material. With this, certain parts of the drum surface get electrically charged and special ink is sprinkled on the drum, which sticks electro statically to the charged area of the drum, forming the image. As the drum rotates, the image is transferred onto the paper through a combination of heat and pressure.
Laser printer is reliable printers with high quality output. The resolution of laser printer ranges from 300 dpi to 1600 dpi. The standard speed of laser printer is 4 to 8 pages per minute.

















Fig: How a Laser printer creates an image.




iii. Speakers:
A speaker can be used for various sounds meant to alert the user, as well as music and spoken text. A speech output unit is one, which reads strings of characters stored in a computer’s memory and converts them into spoken sentences. Electronic chip have been designed which accepts ASCII characters of a sentence, combine sequence of words into sound, amplifies them and output them through a speaker.



6. CENTRAL PROCESSING UNIT (CPU)

The procedure that converts raw data into useful information is called processing. In the computer, processing is done by central processing unit (CPU). CPU is perhaps the most vital part of the computer. It is often regarded as “brain” of the computer. It is because CPU performs computer’s entire work of calculating and processing. In personal computer (PC), this job is done by microprocessor.

In addition to the processing, the CPU also controls all other units of the computer. It accepts inputs from input device, keep them into memory, processes them, and send outputs to the output device.

The CPU is made up of three units. They are:
a. Arithmetic and Logic Unit (ALU)
b. Memory Unit (MU)
c. Control Unit (CU)

a. Arithmetic and Logic Unit (ALU)
Arithmetic and Logic Unit (ALU) is the part of the CPU, where all the processing jobs are performed. As the name suggests, two types of operations are done inside the ALU – arithmetic operation and logical operation. Arithmetic operation means calculations like addition, subtraction, multiplication, and division on data. Similarly, logical operation means comparison of data like equal to, greater than, less than, and not equal.

All the operations in the ALU are done under directions of the control unit. Once data enter into memory, the data necessary for processing are transferred into the ALU. When processing completes, the data is again transferred back into the memory unit. This transfer of data from the memory unit to the ALU and vice-versa is also done under the directions of the control unit.

Main functions of the ALU are given below.
 Carries out arithmetic operations such as addition, subtraction, multiplication, and division
 Performs logical operations such as comparison among data (equal to, greater than, less than and not equal to).
 Works under the direction of the control unit

b. Control Unit (CU)
Control unit is the most vital part of the CPU. It reads every instruction stored into the memory. There after, it issues control signals necessary to execute the instruction. The control unit, actually, ensures that every instruction is read, understood, and executed in sequence.

The CU coordinates and controls all the parts of the computer. It is the nerve center of the computer. It coordinates and controls the interpretation, flow and manipulation of all data and information. It also controls the flow of data from input devices to memory and from memory to output device.

Main functions of the Control Unit are as hereunder.
 Receives inputs send by input device, and store them into memory unit
 Bring instruction from the memory unit, and decode the instruction (interpret what the instruction is saying)
 Controls movement of data in and out of the ALU.
 Directs the ALU to process data
 Control the movement of data and instruction in and out of the memory unit
 Controls movement of processed data (information) to output device
 Directs output device to produce information
 Directs and controls reading and writing of data on storage unit

Register
Registers are primarily used to store data temporarily during the execution of a program. They are used as temporary memory locations in the sense that the information or data is held in them only for as long as it is necessary for one instruction. Registers can be of different sizes & capacities.

Some registers are special purpose registers and some are general-purpose registers. General purpose registers store data and intermediate results during the execution of a program. General-purpose registers are also accessible to programmer through instructions. Special purpose registers like PC (Program Counter) holds the address of the next instruction to be executed and SP (Stack Pointer) holds the address of the first location of the stack.

Memory Unit (MU)
Memory unit is the area, where the computer keeps programs, files, data, and instructions. All these things are only stored temporarily. Memory unit is built inside the computer, so it is also known as internal memory.

The CPU needs data and instructions to operate the computer. These things must be stored in an area somewhere inside the computer. When data and programs are not in use, they must be kept permanently in another safe area. In computer, all these areas are called memory. Therefore, memory is the area in the computer, which holds data, instructions or programs.

As input unit sends data or instructions, the computer at first keeps them into the memory unit. When the computer completes processing of data, it again keeps them into memory unit. When a user launches a program, it is also loaded into and run from memory unit. Thus memory unit is a space, which is allocated for temporary storage of data and programs.

Memory is a part of computer (device) that is used to store data as well as instructions (programs). Physically, memory consists of chips either on the motherboard or on a small circuit board attached to the motherboard.

Followings are the functions of the memory unit.
 Stores data and instructions sent from input units
 Stores data and instructions before they undergo processing (or store raw data)
 Stores data and instructions during their processing
 Stores data and instructions after their processing (or store useful information)
 Loads and unloads programs run by users
 Loads files opened by users


a. Primary Memory
Primary memory is also called main memory. Primary memories are made up of semiconductor materials and consist of large number of 8-bit registers, arranged in a sequence (lines). A register can store eight binary bits.
i. Cache Memory
In computer, data moves between RAM and the CPU frequently. In fact, RAM is very much slower than the CPU. Hence, moving data between RAM and the CPU is a time consuming process. It makes the CPU often idle and waiting for data. A solution to this problem is a special type of memory called cache memory (pronounced cash).
The cache memory (also called RAM cache) exists between RAM and the CPU. A Cache memory is a high-speed memory placed in between central processing unit (CPU) and RAM to increase the speed of processing. The speed of processor is faster than the RAM; hence the cache memory is used in between CPU and main memory (RAM) so that the speed of operation of main memory and cache memory together can meet the speed requirement of the high speed CPU.

The cache memory is very small, expensive and has high speed. Today’s PCs have 256 KB, 512 KB or even 1024 KB cache memory. Cache memory is generally built directly into the microprocessor chip.

Actually, the cache memory stores only a few and frequently used data and instructions. When the CPU needs data, it first searches in the cache memory. If the data is not there, it searches RAM. This helps the CPU to work faster and increase processing speed. Below is the figure that shows how the cache memory works with the CPU and the main memory (RAM).





ii. Random Access Memory (RAM)
The read and write memory of a computer is known as RAM. The users of the computer can write information into RAM and read information from RAM. The main drawback of RAM memory is that it is a volatile memory, i.e., when the power goes off, the contents of RAM gets erased. RAM is available in the form of a chip with different memory capacity ranging, from 640 Kilobytes to 256 Megabytes. Increasing RAM capacity improves system performance.

Primary functions of RAM are:
1. Storage of the programs, data and other information that can be quickly accessed by the CPU at the time of execution.
2. Storage of a copy of main s/w program that controls the operation of the computer. This copy is loaded in main memory when the computer is turned on it stays as long as computer is on.
3. Temporary storage of a copy of the application program instruction (the specific s/w we are using in our business) to be retrieved by the CPU for the interpretations and the executions.
4. Temporary storage of the data that has been input from the keyboard or other input devices until instruction call for the data to be transferred into the CPU for processing.

There are two types of RAM:
• Static RAM
• Dynamic RAM

Static RAM (SRAM)
• A static RAM retains stored data and programs as long as power supply is on.
• Its cost is high.
• It is made up of flip flops and it stores the bit as a voltage.
• Speed is high.
• Produce more heat.
• Larger than DRAM

Dynamic RAM (DRAM)
• It loses the stored information in a very short time even though the power supply is on.
• Cost of DRAM is less (low price)
• Made up of transistors and logic gates, and stores the bit as a charge.
• Speed is lower than SRAM.
• Produce less heat.
• Smaller than SRAM.


Conventional/ Regular DRAM
This is the oldest and the slowest type of DRAM technology. This type of DRAM is now quite obsolete and is not used on any new systems, having been replaced by Fast Page Mode (FPM) DRAM and other new technologies. This type of DRAM has been used in PCs since original PC days. It is said to asynchronous meaning that it is not synchronized with system clock.

Fast Page Mode (FPM) DRAM
This type of DRAM is only slightly faster than conventional DRAM. Before the introduction of EDO DRAM it was the traditional DRAM used in PCs. It is pretty slow with access time of 120 ns however it is also found in 70 ns versions. It is not much used today due to its slow speed but it is almost universally supported. It is also asynchronous type.

Extended Data Out (EDO) DRAM
Economical and most common type of asynchronous DRAM is EDO DRAM. This DRAM is improvement of FPM DRAM and slightly faster than FPM DRAM. By switching from FPM to EDO, one can expect a performance improvement of 3 to 5 percent.

Synchronous DRAM (SDRAM)
This type of memory is relatively new and different kind of DRAM for PCs which is faster since it runs “in sync” with computer system clock. By running at the same speed, CPU and SDRAM move data much more efficiently than is possible with other DRAM. Rambus DRAM (RDRAM) and Double Data Rate SDRAM (DDR SDRAM) are examples of this type of DRAM which are now commonly used in the newer computer systems.

Video RAM (VRAM)
Another type of DRAM that is used to handle graphical data and displays images on screen, so it is also called display or graphic memory. VRAM: mounted on display card, is quite fast, as it needs to respond quickly to the CPU’s instructions and constantly refresh and update the screen image so that there is no flicker.

iii. Read Only Memory (ROM)
The read only memory (ROM) is a memory unit that performs the read operation only; it does not have write capabilities. The information stored in a ROM is made permanent during the hardware production and cannot be altered. ROMs are non-volatile memory, i.e. information stored in ROM is not lost even if the power supply goes off. So, ROMs are used for storing the programs to boot the computer handling the operating system and monitor program controlling a machine. They are slower than RAM. Actually, the ROM is built and assembled in the motherboard.

Typical uses of ROM are:
1. It allows storing permanently routines that are to be executed after the computer is turned on. These routines contain information for booting up the computer and performing computer’s diagnostics such as whether h/w devices are connected properly, whether rest of memory is functioning properly and so on.
2. It allows user to program critical and lengthy operations into micro programs that are fused or coded into ROM chip.
3. I t also allows manufacturers to store their information on ROM chips permanently that never altered.




There are various types of ROM:
a. PROM (Programmable Read Only Memory)
b. EPROM (Erasable Programmable Read Only Memory)
c. EEPROM (Electrically Erasable Programmable Read Only Memory)

PROM (Programmable ROM)
A PROM is a memory chip on which data can be written only once. Once a program has been written onto a PROM, it remains there forever and cannot be changed. They are manufactured as blank memory. To write data onto a PROM chip, you need a special device called a PROM programmer or PROM burner. The process of programming a PROM is sometimes called burning the PROM.

EPROM (Erasable Programmable ROM)
EPROM is a special type of memory that retains its contents until it is exposed to ultraviolet light for 10 to 20 minutes. The ultra-violet light clears its contents, making it possible to reprogram the memory. For erasing purpose, the EPROM chip has to be removed from computer.

EEPROM (Electrically Erasable Programmable ROM)
It is a special type of PROM that can be erased by exposing it to an electrical charge. The time required to erase this type of PROM is very short (few seconds). Unlike EPROM chips, EEPROMS do not need to be removed from the computer to be modified.

b. Secondary Memory
Secondary memory is also called secondary storage, auxiliary storage, mass storage or, simply storage. It holds large volume of data, programs, information and games for future uses. Secondary memory is non-volatile. It can keep data, programs, and information permanently even when electrical power is removed from the computer.

The secondary memory is used to store data, information and programs permanently. So, they are often referred as “storage memory”. The capacity of secondary memory is larger than the main memory. The two main categories of storage technology used today are magnetic storage and optical storage.

Magnetic Storage Devices
Magnetic storage devices uses surface coated with a magnetically sensitive material, such as iron oxide, chromium oxide, etc which reacts to a magnetic field. The orientation of a magnetic filed can be used to represent data. A magnet has one important advantage over transistor; it can be represent 0 and 1 without a continual source of electricity. The surfaces are coated with millions of tiny particles so that data can be stored on them. Each of these particles can act as a magnet, taking on a magnetic field when subjected to an electromagnet. The read/write heads contain electromagnets, which generate magnetic fields in the surface and thus magnetize the surface of the disk below the head. When binary 1 is to be stored, the information is sent to the head, and it magnetizes a spot below with left to right pole alignment SN. Similarly the binary 0 is stored with right to left pole alignment NS.

When formatting a disk, a set of magnetic concentric circles, called tracks are created. Most high-density diskettes have 80 tracks on each side of the disk. The tracks are numbered from the outermost circle to the innermost, starting with zero. Each track on a disk is also split into smaller parts known as sectors. All the sectors on the disk are numbered in one long sequence. Each sector can store up to 512 bytes.

Modern mass storage devices include all types of disk drives and tape drives. Mass storage is distinct from primary memory, which refers to temporary storage areas within the computer. Unlike main memory, mass storage devices retain data even when the computer is turned off.

Magnetic storage devices are of two types- Magnetic Disks and Magnetic Tapes

How data are represented on a magnetic storage device?
The purpose of a storage device is to hold the data even when the computer is turned off so that the data can be used whenever it is needed. Storage involves the process of writing data to storage media and reading data from the storage medium. The surface of the disks is all coated with a magnetically sensitive material, such as iron oxide, which reacts to the magnetic field.

When the iron particles are polarized, they become magnet, i.e. their particles have certain alignment. The alignment of different iron particles has different orientation of a magnetic field, which can be used to represent the data. The surface of magnetic disks and magnetic tapes are coated with millions of tiny particles so that data can be stored on them. Each of these particles can act as a magnet, taking on a magnetic field when subjected to an electromagnet.

The read/write head of a hard disk drive, floppy drive, and tape drive contains electromagnets, which generates magnetic fields in the strings of 1s and 0s by alternating the directions of the current in the electromagnets.

To read data from the magnetic surface, the process is reversed. The read/write head passes over the disk while no current is flowing through the electromagnet. Because the storage medium has a magnetic field but the head does not. The storage media charges electromagnet, which causes a small current to flow through the head in one direction or the other depending on the polarity of the field. The disk drive senses the direction of flow as the storage medium passes by the head, and the data is sent from the read/write head into memory.

How data is organized on a magnetic disk?
Magnetic disks are the most important and common secondary storage devices in a computer system. They are direct access or random access devices. A magnetic disk is a circular disk, which is made of metal (Aluminium) or a thin plastic (Mylar) coated with metallic oxide on both sides that can be magnetized. It allows the recording of data in the form of magnetized spots.

The data is stored on the disks in a number of invisible concentric circles called tracks. Tracks are divided into sectors and the number of sectors per track varies from computer to computer.

The read/write head is fixed to an arm that can move in and out over the surface of the disk, i.e., head can be moved quickly and directly to any disk location to store or retrieve data. So access of data in magnetic disk is faster.

Magnetic disks are available in the following forms: Floppy Disk, Zip Disk & Hard Disk

The common secondary or auxiliary memories used in computer are as follows:

i. Magnetic Tape
Magnetic tape looks like the tape used in music cassettes plastic tape with a magnetic coating. As in other magnetic media, data is stored as extremely small magnetic spots. Tapes come in a number of forms, including l/2-inch-wide tape wound on a reel, l/4-inch- wide tape in data cartridges and cassettes, and tapes that look like ordinary music cassettes but are designed to store data instead of music. The amount of data on a tape is expressed in terms of density, which is the number of characters per inch (cpi) or bytes per inch (bpi) that can be stored on the tape.



The highest-capacity tape is the digital audio tape, or DAT, which uses a different method of recording data. Using a method called helical scan recording, DAT wraps around a rotating read/write head that spins vertically as it moves. This places the data in diagonal bands that run across the tape rather than down its length. This method produces high density and faster access to data.

Tape now has a limited role because disk has proved the superior storage medium. Disk data is quite reliable, especially within a sealed module. Furthermore, as we will see, disk data can be accessed directly, as opposed to data on tape, which can be accessed only by passing by all the data ahead of it on the tape. Consequently, the primary role of tape today is as an inexpensive backup medium.

ii. Hard Disk
Hard disk is a secondary storage medium that stores large amount of data, is made from metal. It holds more data and is faster than floppy disk (capacity- 10 GB, 20 GB, 40 GB etc. and speed – 3600 rpm to 7200 rpm).

A single hard disk may consist of single or multiple platters (disks). Each platter requires two read/write heads, one for each side. All the heads are attached to a single access arm (comb like structure) so that they cannot move independently. Each platter has the same number of tracks and a track location that cuts across all platters is called a cylinder. The motor rotates the platters on the central spindle. The read/write head along with arm does not physically touch the disk surface; it floats above the surface of the disk and detects the data.

If there are 100 tracks per surface in a disk having 8 surfaces, then the number of cylinders is equal to 100 and each cylinder will have 8 tracks. If a certain amount of data is to be stored on the hard disk, the recording always starts from the first sector of the first track of the first cylinder. When the system has filled the track with data, it moves to the next surface on the same track i.e. from surface 1 to surface 2 at the same arm position. Thus no time is wasted because switching from one head to another is done electronically and no seek time is involved.




iii. Pen Drive
The pen drive is a portable USB flash memory device that can be used to quickly transfer audio, video, and data files from the hard drive of one computer to another. With a construction that is small enough to fit into a pocket, the pen drive derives its name from the fact that many of these USB drive devices resemble a small pen or pencil in size and shape.

Equipped with a large amount of memory capacity, the pen drive is considered to be an improvement on both the older floppy drive disks and the more modern compact disks that are often used to copy data and reload the files on a different hard drive. Even a pen drive with a relatively low storage capacity tends to provide plenty of space for a number of files. The types of files that can be loaded onto a pen drive are all the common types that can be housed on any hard drive. This makes it possible for persons to copy photos, spreadsheets, word processing documents, movie clips, music tracks, and just about any other type of file.


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Fig: Pen Drive

iv. Memory Card
A memory card (sometimes called a flash memory card or a storage card) is a small storage medium used to store data such as text, pictures, audio, and video, for use on small, portable or remote computing devices. Most of the current products use flash memory, although other technologies are being developed. There are a number of memory cards on the market, including the SD card (secure digital card), the CF card (CompactFlash card), the SmartMedia card, the Memory Stick, and the Multimedia Card (MMC). These cards are of varying sizes, and each is available in a range of storage capacities that typically corresponds directly to the price. The CompactFlash card is about the size of a matchbook, while the Multimedia Card and Secure Digital card are each about the size of a postage stamp.
Most available cards have constantly powered nonvolatile memory, which means that data is stable on the card, is not threatened by a loss of power source, and does not need to be periodically refreshed. Because memory cards are solid state media, they have no moving parts, and therefore, are unlikely to suffer mechanical difficulties. Earlier removable storage media, such as the PC Card, the smart card, and similar cards used for game systems can also be considered to be memory cards. However, the newer cards are smaller, require less power, have higher storage capacity, and are portable among a greater number of devices. Because of these features, memory cards are influencing the production of an increasing number of small, lightweight and low-power devices.
Memory cards offer a number of advantages over the hard disk drive: they're much smaller and lighter, extremely portable, completely silent, allow more immediate access, and are less prone to mechanical damage. In comparison, however, the hard disk still offers a compelling advantage: currently, a memory card (for example, CompactFlash) with a 192MB capacity typically costs more than a hard drive with a capacity of 4OGB.

Fig: Memory Card



v. Optical Disk
Optical disc is a secondary storage medium from which data is read and to which it is written by lasers. These storage devices works on a principle similar to magnetic storage devices, however, they use laser technology to read and write data.

A laser beam is projected on the reflecting surface to read data from the disc. By detecting the light intensity reflected from the surface, the information is stored on the disc. By detecting the light intensity reflected from the surface, the information stored on the disc can be accessed.
Data is laid out on a CD-ROM disk in a long, continuous spiral that starts at the outer edge and winds inward to the centre. Data is stored in the form of lands, which are flat areas on the metal surface, and pits, which are depressions or hollows. A land reflects the laser light into the sensor indicating 1 and a pit scatters the light indicating 0.

Types of optical Disks
1. Compact Disk Read Only Memory (CD-ROM)
CD-ROM is a type of optical disc capable of storing large amount of data. A single CD-ROM has the storage capacity of 600 floppy disks. It contains pre-recorded data that can be read only (i.e., can not be erased and modified/changed). A laser beam is used to write into and read data from CD-ROM.

CD-ROM discs are plastic discs coated with Aluminium on the surface. A layer of thin transparent plastic is further deposited on the disc. Information on a CD-ROM is written as a single continuous spiral, unlike magnetic discs with discrete cylinders and tracks.

The CD-ROMs take longer time to read data as compared to hard disks (slower than hard disk). These discs are specially used for software and multimedia system (to store data, sound and pictures).

A CD-ROM is prepared by using a high power laser to burn 1-micron (10 –6 of a meter) holes in a master disc.

How a CD-ROM drive reads data from the surface of a compact disc?
Data is stored in the form of lands, which are flat areas on the metal surface, and pits, which are depressions or hollows. A land reflects the laser light into the sensor indicating a data bit of 1 and a pit scatters the light indicating a data bit of 0.

To store data, the disk's metal surface is covered with tiny dents (pits) and flat spots (lands), which cause light to be reflected differently. When an optical drive shines light into a pit, the light cannot be reflected back. This represents a bit value of 0 (off). A land reflects light back to its source, representing a bit value of 1 (on).


CD-R (Compact Disc - Recordable)
It is also known as WORM (Write Once Read Many). With a WORM disc drive, you can write data onto a WORM disc, but only once. The writing process is normally slower than the reading process. After writing onto a WORM disc, it behaves just like a CD-ROM.


CD-RW (Compact Disc- Re-Writable)
CD-R/W disc is a new type of CD disc that enables the users to read and write data many times. With CD-R/W drives and discs, you can treat the optical disc just like magnetic disk, writing data onto it again and again.

2. Digital Versatile Disc Read Only Memory (DVD-ROM)
DVD-ROM is also called as Digital Video Disc – Read Only Memory. DVD-ROM is a high-density medium capable of storing a full-length movie on a single disc. It uses special data compression technology to store data on a disc.

DVD-R (Digital Versatile Disc- Recordable)
DVD-R lets us record data into a special recordable digital video disc, using DVD-RW drive. Once data are written, they can’t be erased and re-written.

DVD-RW (Digital Versatile Disc- Re-Writable)
DVD-RW lets us record, erase, and re-record data into a special recordable digital video disc, using DVD-RW drive. They can read and write all type of CDs and DVDs.

Drive Type Name of the Drive The drive can
CD-ROM Compact Disc – Read Only Memory Read CD-ROM and CD-R
CD-R Compact Disc - Recordable Read CD-ROM and CD-R. Write once on special discs (CD-R)
CD-RW Compact Disc - Re-Writable Read CD-ROMs, CD-R, CD-RW. Writes, reads, and re-writes on special discs (CD-RW).
DVD-ROM Digital Versatile Disc - Read Only Memory Read DVD-ROM, CD-R and CD-RW
DVD-R Digital Versatile Disc - Recordable Read DVD-ROM, CD-R, CD-RW and DVD-ROM. Write once on special discs named (DVD-R)
DVD-RW Digital Versatile Disc - Re-Writable Reads all CD formats. Reads DVD ROM, DVD-R, and DVD-RW. Writes, reads, and re-writes DVD discs

Limitation of optical disks
• Data access speed for optical disks is slower than magnetic disks.
• Optical disks require more complicated drive mechanism than magnetic disks.

Differences between optical disks and magnetic disks
Optical Disks Magnetic Disks
1. Optical disks can store much more data in a limited space than magnetic disks.
2. They are more reliable than magnetic disks.
3. Data access speed is quite slower than that of magnetic disks.
4. They require complicated mechanism to read from and write onto that of magnetic disks.
5. They are better storage medium for data archiving since they have data storage life in access of 30 years. 1. Magnetic disks can not store as much as Optical disk in a limited space.
2. They are less reliable than optical disks.
3. Data speed access is quite faster than that of optical disk.
4. They do not require complicated mechanism to read from and write to magnetic disks.
5. They are not as better as optical disks for data archiving.



3. Magneto- Optical (MO) Drives
MO drives are type of disk drive that employs both magnetic and optical technologies to obtain ultra high data density. MO dive uses both laser and magnetic read/write head to read from and write to. The laser heats up the diskette surface so it can be easily magnetized and also to allow region of magnetization to be precisely located and confined. MO disks are removable and can be both read and written to an unlimited no. of times. Typical cartridge is slightly larger than a conventional 3.5” magnetic diskette and looks similar. They can store data in range of 100MB up to several GBs. They are faster access medium than floppies and CD-ROMS but not as fast as HDD (Hard Disk drive). They are popular to backup files on a PC.

8. SOFTWARE

a. Introduction
Software is a set of electronic instructions that make computer perform tasks. In other words, software tells the hardware what to do. Without software, hardware is useless. Hardware needs instructions to process data into information. Those instructions are supplied from software.

Software is also referred to as a set of program. Computer uses varieties of programs. Some programs are developed for computer’s own use and internal maintenance. Other types of programs are used for calculating and processing data.

b. System Software
System software is a set of one or more programs, designed to control the operation and extend the processing capability of computer system. System software helps the computer perform specific functions like file management, memory management etc.


In general, a computer’s system software performs one or more of the following functions:

1. Supports the execution of other application program
2. Supports the development of other application program.
3. Monitors the effective use of various hardware resources, such as CPU, memory, peripherals, etc.
4. Communicates with and controls the operation of peripheral devices, such as printer, disk, tape, etc.

Hence, system software makes the operation of a computer system more effective and efficient. It helps hardware components work together, and provides support for the development and execution of application software (programs). The programs included in a system software package are called system programs, and the programmers who prepare system software are referred to as system programmers.

Some of the most commonly known types of system software are:

I. Operating Software (OS)
Operating System is an integrated set of programs i.e. used to manage the various resources and overall operations of computer system. It acts as an intermediary between the user and the computer hardware.

Definition:
1. Resource Allocator
Operating system manages and allocates difference resources like I/O devices.

2. Control Programs
It controls the execution of user programs and operations of I/O devices.

3. Kernel
OS is the most important part of the system software and the program which runs all the time.

OS goals
1) To execute program
2) To make the program easy to run
3) To use hardware as efficiently as possible.

OS Features
1) Provide user interface
The OS provide the interface with which the user interacts with the computer. There are two type of use interface:
i) Graphics User interface (GUI)
They are so called because we can use a mouse or any pointing device to point at graphical object on the screen.
The most modern operating system like Windows, Macintosh OS, OS/2, some version of UNIX.
In a GUI we can run multiple programs at a time in a separate window.
I
ii) Command Line Interface
In command line interface we use type written commands rather than graphical objects to execute tasks and process data. E.g. DOS, UNIX, LINUX

2) Running Programs
i) Basic Services
OS manages all other programs that run on PC. OS provides services to program and user. For eg: file management, memory management, printing etc. To provide services to program the OS makes system calls (system call are the request that the programmer call on the operating system to provide some information or services), requesting other hardware and system resources to perform task.
ii) Sharing information
Some OS like Windows enables programs to share information. Windows provides the clipboard, a special area in the memory that stores data out or copied form document so that we reuse as well.
iii) Multitasking
It is the capability of running multiple processes or program simultaneously.

3) Managing files
The OS keeps the tracks of all the files on each disk by maintaining a list of the contents of disks called file allocation table (FAT) which the OS creates when we format the disks. File management can be made easier by creating a hierarchical file system that includes folders and sub folders arranged in a logical order.

4) Managing Hardware
The OS uses Interrupt Requests (IRQs) (Interrupt Request is the request that the operating system sends the computer’s CPU to perform a specific tasks) to maintain organized communication with the CPU and other hardware devices.

Each hardware devices us controlled by a piece of software called driver which allow the OS to activate and use the device.

II. Utility Software

It is a program that performs a task i.e. not typically handled by an operating system. Utility program enhances the OS functionality. Utility programs are set of programs, which help users in system maintenance tasks, and in performing tasks or routine nature. Some of the tasks commonly performed by utility programs include formatting of hard disks, taking backup or files stored on hard disk on to a CD, sorting of the records stored in a file in a particular order based on some key fields, etc.

1) Device Driver
Device driver are the program that allows the operating system and other program to active and use the hardware device.

A driver typically communicates with the device through the computer bus or communications subsystem to which the hardware is connected. When a calling program invokes a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Drivers are hardware-dependent and operating-system-specific. They usually provide the interrupt handling required for any necessary asynchronous time-dependent hardware interface.

2) Antivirus
A utility program which helps protects a computer against being infected by a virus.

Antivirus (or "anti-virus") software is a class of program that searches your hard drive and floppy disks for any known or potential viruses. The market for this kind of program has expanded because of Internet growth and the increasing use of the Internet by businesses concerned about protecting their computer assets.
Some commonly used antivirus are: Kazpersky, AVG, Norton, NOD, etc.

c. Application Software
Applications software is a set of one or more programs, designed to solve a specific problem, or do a specific task. The programs involved in an application software package are referred as application programs, and the programmers who prepare application software are referred to as application programmers.
There are literally millions of application software available for a wide range of applications, ranging from applications such as word processing, inventory management, preparation of tax returns, banking, hospital, administration, insurance, publishing, to complex scientific and engineering applications, such as weather forecasting, space shuttle launching, oil and natural gas exploration, design of complex structures like aircrafts, ships, bridges, etc.
Application software tells the computer how to accomplish the task that the user requires such as creating documents or editing a graphic unit.
Some of the most commonly known application softwares are:
i. Word Processor
Word Processing is a term, which describes the use of hardware and software to create, edit, view, format, store, retrieve and prints the documents (written materials such as letters, reports, books etc). Word processing software enables us to do all these on the computer system.

Commonly Supported Features are entering text, editing text, formatting page style, formatting text, entering mathematical symbols, saving document, printing documents, checking spelling, grammar etc.

ii. Spread Sheet
A spread sheet package is a numeric data analysis tool, which allows us to create a kind of computerized ledger. A manual ledger is a book having rows and columns, which accountants use for keeping a record of financial statement. This is a tedious task and often takes a long time due to several iterations of formula calculations to come out with an acceptable and satisfactory financial statement.
A spreadsheet package offers considerable ease of performing such tasks by automating all arithmetic calculations, and making it easier to change certain numeric values and immediately seeing the effect of these changes across the worksheet.
Commonly supported features are rows and columns, cells, range of cells, cell contents, relative and absolute cell addresses, spreadsheet commands and spreadsheet graphics.

iii. Presentation Tool
These enables us to use a computer system for creating, editing, viewing, storing, retrieving and printing designs, drawings, pictures, graphs, sounds etc.
Commonly supported features are draw designs, paint drawings and pictures, present graphs, drag-and-drop objects, import objects, screen capture.