Evolution of computers
The history of computers gives us the basic idea of the technological development the
computers have gone through in the past, and leads to its projections in future. The Abacus
(Developed by ancient Romans) is the earliest digital computer and it represents numbers by
the position of the beads on a rack. Positioning the beads appropriately can carry out simple
and rapid addition and subtraction.
Ancestors of modern age computers were the earliest mechanical and electro-mechanical
devices. These can be traced as far back as the 17th century.
The first attempt towards automatic computing was made by Blaise Pascal, a French
mathematician. He invented the Mechanical Adding Machine in 1642. This device consisted
of a number of gears and chains and performed repeated additions and subtractions. It was
called Pascaline.
Later Charles Babbage invented two machines and he is called to be the father of modern
digital computers. He designed the following two computers –
1) The “Difference Engine” (1822)
He designed this machine based on the mathematical principle of finite differences and
used to solve calculations on large numbers and also used to solve trigonometric
functions and polynomials.
2) The “Analytical Engine” (1842)
It is a general purpose computing device and it could perform any mathematical
operation at a speed of 60 additions per minute.
Generation of computers
The history of computer development can be divided into distinct phases which are often
referred to as generation of computing devices and in computer terminology it is a step in
technology. Each generation of computers is characterized by a major technological
development that fundamentally changes the way computers operate, resulting in increasingly
smaller, cheaper, more powerful, efficient and reliable devices with decreasing energy
consumption and lesser generation of heat. Generally the term generation is used to distinguish
between varying hardware technologies only. But nowadays, it has been extended to include
both software and hardware. Each generation and the developments led to the current devices
that we use today.
1. First Generation of Computers (1940-1956) – Vacuum Tubes
This generation of computers used vacuum tubes in their circuitry and magnetic drums
for memory. A vacuum tube is a glass device that used filaments as a source of electrons
and could control and amplify electronic signals. Input was based on punch cards and
paper tape, and output was displayed on printouts. These could perform calculations in
milliseconds but they have some demerits as follows –
a) Very large in size taking up entire room
b) Very expensive to operate
c) Consumes a great deal of electricity
d) Generates a lot of heat resulting in malfunctioning
2. Second Generation of Computers (1956-1963) – Transistors
This generation of computers used Transistors in place of vacuum tubes. The transistor
was far superior to the vacuum tubes, allowing computers to become smaller, faster,
cheaper, more energy-efficient and more reliable than the first generation of computers.
Input was still based on punch cards, and output was displayed on printouts.
The main demerit is that it still generates a great deal of heat that could result in
computer damage.
3. Third Generation of Computers (1964-1971) – Integrated Circuits
This generation of computers used Integrated Circuits (IC) in place of transistors.
Instead of punched cards and printouts, users interacted with third generation of
computers through devices like keyboards and monitors. They also interfaced with an
operating system that allowed the device to run different applications at a time with a
central program that monitored the computer memory.
The main demerit is that it still generates a great deal of heat that could result in
computer damage
4. Fourth Generation of Computers (1971-Present) – Microprocessors
This generation of computers used Large Scale Integration (LSI) and Very Large Scale
Integration (VLSI). VLSI uses hundreds of thousands of components onto a single chip.
Some of the advantages are a) Fast processing in a small area and helped to diminish the size and price of
computers
b) Increased storage
c) Modular design, versatility, and compatibility
d) Sophisticated programs and languages for special applications
5. Fifth Generation of Computers (Present and Beyond) – Ultra Large Scale
Integration (ULSI)
This generation of computers used Ultra Large Scale Integration (ULSI). ULSI
increases hundreds of thousands of components remaining on a single chip to millions.
They are based on artificial intelligence and are still developing. They use parallel
processing and supercomputers is helping to make artificial intelligence as reality.
Some of the advantages are a) Very fast speed
b) Larger capacity storage like RAID and optical disk
c) Supports more complex applications
Keyboard
Keyboard devices allow input into the computer system by pressing a set of keys, mounted on
a board connected to the computer system. Keyboard devices are classified as –
1) General-purpose keyboards
2) Special-purpose keyboards
1) General-purpose keyboards
The most popular keyboard used today is the 101 – keys with a traditional QWERTY layout,
having an alphanumeric keypad, 12 function keys, different special function keys, numeric
keypad, and cursor-control keys. It is so called because the arrangement of its alphanumeric
keys in the upper left row.
a) Alphanumeric keypad – It contains keys for the English alphabets, 0-9 numbers, special
characters like * + – / [ ] etc.
b) 12 function keys – F1 – 12 keys and are a set of user programmable function keys.
c) Special-function keys – Designed for special functions assigned to each of these keys.
They are as follows –
i)
Shift – Used to enter capital letters or special characters defined above
the number keys
ii)
Spacebar – Used to enter a space at the cursor location
iii)
Ctrl – Used in conjunction with other keys to provide added
functionality on the keyboard
iv)
Alt – Like Ctrl key, it is also used to expand the functionality of the
keyboard
v)
Tab – Used to move the cursor to the next tab position
vi)
Backspace - Used to move the cursor one position to the left and delete
the character in that position
vii)
Caps Lock – Used to toggle between the capital letter lock feature on or
off. When it is “on”, it locks the keypad for capital letters only
viii) Num Lock – Used to toggle between the number lock feature on or off.
When it is “on”, it allows us to input numbers from the numeric keypad
ix)
Insert – Used to toggle between the insert and overwrite mode during
data entry. When it is “on”, entered text is inserted at the cursor location
x)
Delete – Used to delete the character at the cursor location
xi)
Home – Used to move the cursor to the beginning of the work area
xii)
End – Used to move the cursor to the end of the work area
xiii) Page Up – Used to display the previous page of the current document
xiv) Page Down – Used to display the next page of the current document
xv)
Escape – Used to negate the current command
xvi) Print Screen – used to print what is being currently displayed on the
screen
d) Numeric keypad – It consists of keys having numbers (0-9) and mathematical operators
(+ – * /). It is usually located on the right side of the keyboard and supports quick entry
of numerical data.
e) Cursor-Control keys – Defined by the arrow keys and used to move the cursor in the
direction of the arrow (up, down, left and right)
2) Special-purpose keyboards
These are standalone data entry systems used for computers deployed for specific applications
like keyboards used in Automatic Teller Machine (ATM) where keyboard is used for limited
functionality. Other examples are Point-of-Scale (POS) terminals at fast food joints,
Air/Railway reservation counters.
Mouse
It is a pointing device that a computer user pushes across a desk surface in order to point to a
place on a display screen and to select one or more actions possible from that position. The
mouse first became a widely used computer tool when Apple Computer made it as a standard
part of the Apple Macintosh. Todat, mouse is an integral part of the graphical user interface
(GUI) of any personal computer. The mouse apparently got its name by being about the same
size and colour of a toy mouse.
It has usually two buttons at the top: the left one is used most frequently and it lets the user
click once to select a particular position and double click to open a particular select object. The
second button on the right is used to open a pop-up menu for less-frequently needed capability.
In some models, we have a third button for additional capabilities.
Scanners
They are input devices used for direct data entry from the source document into the computer
system. They facilitate the capturing information and storing it in a graphical format for
displaying it back on the graphical screen. They consists of two components – one to illuminate
the page so that the optical image can be captured and the other to convert the captured
graphical image into a digital format for storing. The graphical image thus scanned can be seen
and processed directly by the computer.
Capturing information using scanners reduces the possibility of human error typically seen
during large data entry. The reduction in human intervention improves the accuracy of data and
provides for timeliness of the information processed.
There are two types of scanners – Contact and Laser. Both bounce a beam of light off an
image, and then measure the reflected light to determine the value of the image. Hand-held
contact scanners make contact as they are brushed over the printed matter to be read. Laser
based scanners are more versatile as they can read the printed data near the scanning area.
Hand-held scanners are used where the information to be scanned or the volume of the
documents to be scanned is very low. But they are cheaper as compared to flat-bed scanners.
Monitors
Monitors use a cathode ray tube (CRT) top display information. It resembles a television screen
and is similar to it in other aspects. It is typically associated with a keyboard for manual input
of characters. The screen displays information as it is typed and enabling a visual check of
input before it is transferred to the computer for further processing. It is also used to display
the output from the computer and hence it is termed as a display device. It is the most
commonly used i/o device today and is also known as a soft copy terminal. A printing device
is required to produce a hard copy on a piece of paper.
Speaker
A loudspeaker or speaker is an electroacoustic transducer, a device which converts an electrical
audio signal into a corresponding sound and is used as an output device in our computers. The
first crude loudspeakers were invented during the development of telephone systems in the late
1800s, but electronic amplification by vacuum tube beginning around 1912 made loudspeakers
truly practical. By the 1920s they were used in radios, phonographs, public address systems
and theatre sound systems for talking motion pictures.
The most popular speaker used today is the dynamic speaker, invented in 1925 by Edward
W. Kellogg and Chester W. Rice. The dynamic speaker operates on the same basic principle
as a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an
alternating current electrical audio signal input is applied through the voice coil, a coil of wire
suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move
rapidly back and forth due to Faraday's law of induction, which causes a diaphragm (usually
conically shaped) attached to the coil to move back and forth, pushing on the air to create sound
waves. Besides this most common method, there are several alternative technologies that can
be used to convert an electrical signal into sound.
Printers
Printers are used for producing output on paper. The printers can be classified into categories
based on –
a) Printing technology
b) Printing speed
c) Printing quality
Printing Technology
Printers are classified as impact and non-impact printers based on the technology they use for
producing output.
a) Impact printers
They use the standard typewriter mechanism where a hammer strikes through an
inked ribbon. These printers have a mechanism that touches the paper in order to
create an image. Eg. Dot matrix printers and character printers.
b) Non-impact printers
They don’t touch the paper when creating an image. They use chemical, heat or
electrical signals to etch symbols on paper. Eg. Inkjet, laser, and thermal printers.
Printing Speed
It defines the number of characters printed in a unit of time. Based on the speed, these
printers may be classified as –
a) Character printers – prints one character at a time
b) Line printers – prints one line at a time
c) Page printers – prints entire page at a time
Printing speeds are therefore measured in terms of characters-per-second or cps for a
character printer, lines-per-minute or lpm for a line printer, and pages-per-minute or
ppm for a page printer.
Printing Quality
Printing quality is determined by the resolution of printing and is characterized by the number
of dots that can be printed per linear inch, horizontally or vertically. It is measured in terms of
dots-per-inch or dpi. Printers can be classified as near-letter-quality or NLQ, letter-quality or
LQ, near-typeset-quality or NTQ, and typeset-quality or TQ based on their printing quality.
The resolution for NLQ, LQ, NTQ, and TQ are 300 dpi, 600 dpi, 1200 dpi, and 2000 dpi
respectively. NLQ and LQ printers are used for ordinary printing in day-to-day activities, while
NTQ, and TQ are used to produce top-quality printing, typically required in publishing
industry.
Type of printers
Various type of printers available in the market are –
i)
Dot Matrix printer
These printers were the most popular impact printers used in personal computing.
They use a print head consisting of a series of small pins to strike a ribbon
coated with ink, causing the ink to transfer to the paper at the point of impact.
Characters are thus produced in a matrix format. The shape of each character i.e.
the dot pattern is obtained from information held electronically.
Features –
a) Low cost
b) Attractive in the personal computer market
c) Printing speed is 40-1000 cps
The major disadvantage of this printer is that the printing quality is low.
ii)
Inkjet Printer
These printers use a series of nozzles to spray drops of ink directly on the paper.
These are the non-impact printers. The print head consists of a number of tiny
nozzles that can be selectively heated up in a few microseconds by an IC –
register. When this happens, the ink near it vaporizes and is ejected through
the nozzle to make a dot on the paper placed in front of the print head. The
character is printed by selectively heating the appropriate set of nozzles as the
print head moves horizontally.
Features –
a) Dots are extremely small usually 50-60 microns in diameter
b) Dots are positioned very precisely with resolutions of upto 1440×720 dots
per inch (dpi)
c) Dots can have different colors combined together to create photo-quality
images.
The major disadvantages are –
a) Ink cost is expensive
b) Slower than dot-matrix printers
These are generally used by the people or organizations where printing speed is not
the most important factor.
iii) Laser Printer
These printers use dry ink (toner), static electricity, and heat to place and bond the
ink onto the paper. They use a combination of laser and photocopier technology.
Printing is achieved by deflecting the laser beam onto the photosensitive surface of
a drum after which the latent image attracts the toner to the image. The toner is then
electrostatically transferred to the paper and fixed into permanent image. They are
capable of converting computer output into print page by page. Since characters are
formed by very tiny ink particles, they can produce high quality images (text and
graphics)
Features –
a) High quality printing
b) Silent in use
c) Faster in printing than other printers
d) High Speed and printing range is 10 – 200 pages per minute depending on
the model
The major disadvantage of this printer is that it is the expensive one.
Hard Disk
A hard disk drive (HDD) is a data storage device used for storing and retrieving digital
information using rapidly rotating disks (platters) coated with magnetic material. An HDD
retains its data even when powered off. Data is read in a random-access manner, meaning
individual blocks of data can be stored or retrieved in any order rather than sequentially. The
first hard disks, which first showed up in the 1950s on mainframe computers, were 20 inches
in diameter and held only a few megabytes of data. Hard disks were originally called as fixed
disks and Winchester drives and became known as hard disks later to differentiate them from
floppy disks.
Introduced by IBM in 1956, HDDs became the dominant secondary storage device for generalpurpose computers by the early 1960s. Continuously improved, HDDs have maintained this
position into the modern era of servers and personal computers. More than 200 companies have
produced HDD units, though most current units are manufactured by Seagate, Toshiba and
Western Digital.
The two most common form factors for modern HDDs are 3.5-inch, for desktop computers,
and 2.5-inch, primarily for laptops.
As of 2015, the primary competing technology for secondary storage is flash memory in the
form of solid-state drives (SSDs), but HDDs remain the dominant medium for secondary
storage due to advantages in price per unit of storage and recording capacity. However, SSDs
are replacing HDDs where speed, power consumption and durability are more important
considerations.
Major components of a hard disk drive are as follows –
i)
ii)
iii)
iv)
v)
vi)
vii)
viii)
Disk platters
Spindle and spindle motor
Read/Write heads
Head actuators
Air filter
Logic board
Connectors and jumpers
Bezel
Each hard disk is organized into the following organizing building blocks –
i)
ii)
iii)
Track – they are concentric areas on the disk that complete one circumference of
the disk. On a disk drive there can be 1,000 tracks or more. The first track where
the data is written first, is along the outer edge of the disk. Tracks of the same
number on all platters on the drive form a cylinder.
Sectors – Sectors divide the disk into the number of cross-sections that intersect all
of the tracks on the disk. A sector is typically 512 bytes in length, and disk drives
have from 100 to 300 sectors per track.
Cylinders – Cylinders reflect how the read/write heads move in and out of the disk
platters in unison. This grouping technique is unique to hard disks. It is the logical
grouping of the same tracks on each disk surface.
iv)
v)
Clusters – A cluster is formed from group of sectors. This logical grouping is used
by OS to track data on the disk. There are normally around 64 sectors to a cluster,
but the size of the disk drive and the OS in use determine the actual number of
sectors in a cluster.
Multiple zone recording – This technique is used on some disks to eliminate the
effect of the shape of the disk in recording sectors. More sectors are placed on tracks
closer to the outer edge of the disk and fewer sectors are placed on the tracks closer
to the inside edge.
Multiple disks are usually stacked and used together to create disk storage systems having large
capacities. The multiple disks are fixed on a central shaft, one below the other to form a disk
pack. This is then mounted on a disk drive that has a motor to rotate this disk pack about its
axis. The disk drive has an access arm assembly with a separate read/write head for each surface
of the disk pack. The access arms for all the disk surfaces move together. So a disk system is
addressed by the disk number, the disk surface, sector number, and the track within the sector.
Usually the upper surface of the topmost disk and the lower surface of the bottom-most disk
are not used as they are prone to getting scratched easily. For faster access of data from the
disk packs, a concept called cylinder is used. A set of corresponding tracks on all the recording
surfaces on the disk pack together form a cylinder. If there are 100 tracks on each disk surface,
there are 100 cylinders in the disk pack.
Cylinder-based organization provides faster data access. The related records of the file can be
stored on the same cylinder (on multiple disks of a disk pack) and subsequently with one
movement of the access arm, all records on say cylinder 5 (5th track of every recording surface)
can be simultaneously read.
The figure shows a disk pack with four disks.
The storage capacity of a disk system can determined as –
Storage capacity = No. of recording surfaces × No. of tracks per surface × No. of sectors
per track × No. of bytes per sector
FAT
A file allocation table (FAT) is a file system developed mostly for hard drives that
originally used 12 or 16 bits for each cluster entry into the file allocation table. It is stored on
a hard disk or floppy disk that indicates the status and location of all data clusters that are on
the disk. It is used by the operating system (OS) to manage files on hard drives and other
computer systems. It is found in flash memory, digital cameras and portable devices. The
FAT file system is supported by nearly all OSs installed on personal computers (PCs). It is
used to store file information and extend the life of a hard drive. Most hard drives require a
process known as seeking; this is the actual physical searching and positioning of the
read/write head of the drive. The FAT file system was designed to reduce the amount of
seeking and thus minimize the wear and tear on the hard disc. Although first produced for
hard disc drives, FAT was later used for storing information on portable media such as flash
memory cards. If the file allocation table is damaged or lost, then a disk is unreadable.
The FAT12 is the file system on a floppy disk. The number “12” is derived from the
fact that the FAT consists of 12-bit entries.
The storage space on a floppy disk is divided into units called sectors. In larger
storage devices, a bunch of sectors form a cluster. However, for the floppy disk, the number
of sectors in a cluster is one. Also, the size of a sector (and hence a cluster) is 512 bytes for a
floppy disk.
Disk organization
A floppy disk layout (FAT-12) consists of four major sections: the boot sector,
FAT tables, root directory, and data area:
Figure 1 Disk organization of the FAT12 file system
•
The boot sector consists of the first sector (sector 0) on the volume or disk. The boot
sector contains specific information about the rest of organization of the file system,
including how many copies of the FAT tables are present, how big a sector is, how many
sectors in a cluster, etc.
•
FAT tables contain pointers to every cluster on the disk, and indicate the number of
the next cluster in the current cluster chain, the end of the cluster chain, whether a cluster is
empty, or has errors. The FAT tables are the only method of finding the location of files and
directories on the rest of the disk. There are typically two redundant copies of the FAT table
on disk for data security and recovery purposes. On a floppy, since a cluster consists of just
one sector, there is a FAT entry pointer to every sector on the disk.
•
The Root directory is the primary directory of the disk. Unlike other directories
located in the data area of the disk, the root directory has a finite size (For FAT12, 14 sectors
* 16 directory entries per sector = 224 possible entries), restricting the total amount of files or
directories that can be created therein.
•
Data Area is the first sector or cluster of the data area corresponds to cluster 2 of the
file system (the first cluster is always cluster 2). The data area contains file and directory data
and spans the remaining sectors on the disk.
Why do we need a FAT?
The directory entry has a field called the First Logical Cluster field which
specifies where the file or subdirectory begins. Since files and directories can be larger
than a sector, a directory or file may have to be stored across more than one sector. The data
sectors belonging to a file or a directory are not always stored in contiguous locations in
memory. A FAT therefore is used to keep track of which sectors are allocated to which
file.
To retrieve the entire contents of a file, for example, the First Logical Cluster field
would point to the sector number that holds the first 512 bytes of data. The data from this
sector needs to be read in. To determine if there is more data, one must examine the FAT
entry that corresponds to the First Logical Cluster. By examining the FAT entry value, it can
be determined if there is another sector allocated to this file. If there is, then the logical sector
value is translated to physical sector value and the data from that sector is read in. Next, the
FAT entry for the second data sector is examined to see if it is the end of the file. If not, the
process is continued.
Therefore, the FAT allows the access of data stored in non-contiguous sectors of the
storage device.
In the Figure, File1.txt is stored in logical sectors 2, 4, 6 and 7. The directory entry
field “Start Cluster” i.e. First Logical Cluster field points to sector number 2 which is the first
data sector. In the FAT, the value at FAT entry 2 is 4, indicating that the next data sector of
File1.txt is stored in logical sector 4. The last sector is sector 7, which is evident as the FAT
entry 7 holds the EOC value.
In a FAT folder structure, files are given the first available location on the volume. The
starting cluster number is the address of the first cluster used by the file. Each cluster contains
a pointer to the next cluster in the file, or an end-of-file (EOF) indicator at (0xFFFF) which
indicates that this cluster is the end of the file. These pointers and end-of-file indicators are
shown in the above Figure.
Figure: Files on a FAT Volume
The above figure shows three files in a folder. File1.txt is large enough to use three
clusters. File2.txt is a fragmented file that also requires three clusters. The third file, File3.txt,
fits completely in one cluster. In each case, the folder entry points to the first cluster of the file.
In this section, the choice of the value “12” is explained followed by a description on
how a 12-bit value is stored in the FAT.
The space on a floppy disk = 1.44 MB.
The number of bytes in a sector = 512 bytes
The number of sectors in 1.44 MB = x ≈ 2812
Therefore, the minimum number of bits required to address “x” sectors = 12 bits (211
< 2812)
It can be seen from the above computations that 12 bits is the minimum number of bits
needed to access the entire 1.44M space of a floppy disk.
FAT12 is the original implementation of FAT and is intended for very small media.
The file allocation table for FAT12 is smaller than the file allocation table for FAT16 and
FAT32, because it uses less space for each entry.
FAT16 is not recommended for volumes larger than 511 MB because, when relatively
small files are placed on a FAT16 volume, FAT uses disk space inefficiently. We cannot use
FAT16 on volumes larger than 4 gigabytes (GB), regardless of the cluster size.
FAT16 supports volumes up to 2 GB, whereas theoretically FAT32 can manage
volumes up to 2 TB. The FAT32 on-disk format and features on Windows 2000 are similar to
those on Windows 95 OSR2 and Windows 98.
FAT32 uses 4 bytes per cluster and FAT16 uses 2 bytes per cluster within the file
allocation table.
Difference between FAT12, FAT16 and FAT32
Attribute
FAT12
FAT16
FAT32
Used for
Floppies, small
Small to large hard
Large to very large
hard drives
drives
hard drives
12 bits
16 bits
32 bits
~4,096
~65,536
~268, 435, 456
512 B to 4 KB
2 KB to 32 KB
4 KB to 32 KB
Size of each FAT
entry
Maximum number of
clusters
Supported cluster size
Maximum volume size
16 MB
2 GB
2 TB
NTFS
NTFS stands for New Technology File System developed by Microsoft and this took
over FAT as the primary file system being used in windows. Starting with Windows NT 3.1,
it is the default file system of Windows family.
NTFS has several improvements over FAT and HPFS (High Performance File
System), the system that it suspended, such as improved support for metadata, and the use of
advanced data structures to improve performance, reliability, and disk space utilization, plus
additional extensions, such as security access control lists (ACL) and file system journaling.
Difference between FAT32 and NTFS
FAT32
NTFS
FAT32 provides very little security. A user
NTFS allows the use of NTFS Permissions.
with access to a drive using FAT32 has
It's much more difficult to implement, but
access to the files on that drive.
folder and file access can be controlled
individually, down to an extreme degree if
necessary.
FAT32 must be used for any drives that
must be accessed when the computer is
NTFS volumes are not recognized by
Windows 95/98/Me.
booted from Windows 95/98 or Windows
Me.
FAT and FAT32 volumes can be converted
NTFS cannot be converted to FAT32
to NTFS volumes.
without reformatting.
It does not support disk quotas.
NTFS supports disk quotas, allowing you to
control the amount of disk usage on a per
user basis.
FAT32 supports no file compression
NTFS supports file compression.
It is not very reliable since it does not support It is highly reliable since it supports bad
bad cluster mapping.
cluster mapping and transaction logging.
Defragmentation
Defragmentation is the process of locating the noncontiguous fragments of data into which a
computer file may be divided as it is stored on a hard disk, and rearranging the fragments and
restoring them into fewer fragments or into the whole file. Defragmentation reduces data
access time and allows storage to be used more efficiently.
Some operating systems automatically defragment storage periodically; others require that the
user occasionally use a special utility for this purpose. Windows 98 comes with a built-in
defragmenter as a "system tool" that the user can run. Windows NT did not come with a
defragmenter because its file system, NTFS, was designed to minimize fragmentation;
however, NT users often find one necessary and several vendors provide defragmenters.
Windows 2000 comes with a "light" version of the Diskeeper defragmenter; some users
(especially corporate users) use Diskeeper or some other full-function defragmentation
program to manage storage efficiency and performance. Windows XP comes with a utility
called "Disk Defragmenter."
Disk Error
A hard disk drive failure occurs when a hard disk drive malfunctions and the stored information
cannot be accessed with a properly configured computer. A disk failure may occur in the course
of normal operation, or due to an external factor such as exposure to fire or water or high
magnetic fields, or suffering a sharp impact or environmental contamination, which can lead
to a head crash.
Hard drives may fail in a number of ways. Failure may be immediate and total, progressive, or
limited. Data may be totally destroyed, or partially or totally recoverable.
Earlier drives had a tendency toward developing bad sectors with use and wear; these bad
sectors could be "mapped out" so they were not used and did not affect operation of a drive,
and this was considered normal unless many bad sectors developed in a short period of time.
Some early drives even had a table attached to a drive's case on which bad sectors were to be
listed as they appeared. Later drives map out bad sectors automatically and invisibly to the
user; S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology) information logs
these problems. A drive with bad sectors may usually continue to be used.
Some hard drives simply fail because of worn out parts, others fail prematurely. HDD
manufacturers typically specify a Mean Time Between Failures (MTBF) or an Annualized
Failure Rate (AFR) which are population statistics that cannot predict the behavior of an
individual unit. These are calculated by constantly running samples of the drive for a short
amount of time, analyzing the resultant wear and tear upon the physical components of the
drive, and extrapolating to provide a reasonable estimate of its lifespan. HDDs typically fail
within a short time if there is a defect present from manufacturing. If an HDD proves reliable
for a period of a few months after installation, the HDD has a significantly greater chance of
remaining reliable. Therefore, even if a HDD is subjected to several years of heavy daily use,
it may not show any notable signs of wear unless closely inspected. On the other hand, an HDD
can fail at any time in many different situations.
Components of the electronic circuitry may fail making the drive inoperable.
Bearing and motor failure may fail or burn out, and bearings may wear enough to prevent
proper operation.
The most notorious cause of HDD failure is a head crash, where the internal read-and-write
head of the device, usually just hovering above the surface, touches a platter, or scratches the
magnetic data-storage surface. A head crash usually incurs severe data loss, and data recovery
attempts may cause further damage if not done by a specialist with proper equipment.
Another cause of failure is a faulty air filter. The air filters on today's HDDs equalize the
atmospheric pressure and moisture between the HDD enclosure and its outside environment.
If the filter fails to capture a dust particle, the particle can land on the platter, causing a head
crash if the head happens to sweep over it. After an HDD crash, particles from the damaged
platter and head media can cause one or more bad sectors. These, in addition to platter damage,
will quickly render a HDD useless.
During normal operation, heads in HDDs fly above the data recorded on the disks. Modern
HDDs prevent power interruptions or other malfunctions from landing its heads in the data
zone by either physically moving (parking) the heads to a special landing zone (an area of
the platter usually near its inner diameter) on the platters that is not used for data storage,
or by physically locking the heads in a suspended (unloaded) position raised off the platters.
Some early PC HDDs did not park the heads automatically when power was prematurely
disconnected and the heads would land on data. In some other early units the user would run a
program to manually park the heads.
Disk Cleanup
Disk Cleanup (cleanmgr.exe) is a computer maintenance utility included in Microsoft
Windows designed to free up disk space on a computer's hard drive. The utility first searches
and analyzes the hard drive for files that are no longer of any use, and then removes the
unnecessary files and helps us remove them to cleanup some space and make our computer run
a bit faster. There are a number of different file categories that Disk Cleanup targets when
performing the initial disk analysis:

Compression of old files

Temporary Internet files

Temporary Windows files

Downloaded program files

Recycle Bin

Removal of unused applications or optional Windows components

Setup log files

Offline web pages (cached)
Floppy Disk
The disks used with a floppy disk drive are small removable disks made of plastic coated with
magnetic recording material. There are two sizes commonly used with diameters 51/4 and 31/2
inches.
1. The 51/4 disk is a 51/4 inch diameter floppy disk. Earlier such disks recorded data only
on one side and were called single-sided (SS) disks. Today both surfaces are used for
recording and are called double-sided (DS) disks. These are available in two capacities
– double density (DD) and high density (HD), where density refers to the number of
bits that can be stored per square inch area. The capacity of the disk is 1.2 MB and has
96 total no. of tracks.
2. The 31/2 disk is a 31/2 inch diameter floppy disk. These record data on both sides and
are therefore double sided disks. These disks come in three different capacities – double
density, high density, and very high density. These are smaller and can store more data
than the 51/4 inch disks. The capacity of the disk is 1.44 MB and has 80 total no. of
tracks.
Sliding metal piece
cover (Flap)
Label for identification
Write-protect tab
Fig. 31/2 inch Floppy Disk
The storage capacity of the disk can be calculated as –
Storage capacity = No. of recording surfaces × No. of tracks per surface × No. of sectors
per track × No. of bytes per sector
Virtual Memory
A process was loaded into the main memory for execution only when sufficient free memory
was found to be available to load the entire process. This scheme suffers from the following
two limitations –
1) A process cannot be loaded and has to keep waiting for its execution to start until
sufficient free memory for loading the entire process becomes available.
2) A process cannot be loaded in a system whose main memory size is less than total
memory required by the process.
Virtual Memory is a memory management scheme that overcomes the above mentioned
limitations by allowing the execution of processes that might not be completely loaded in the
main memory i.e. it does not require the entire process to be in main memory before the process
can execute.
In a virtual memory system, all processes are partitioned into pages and reside on the on-line
secondary storage. The physical memory is also portioned into page frames of the same size.
Now instead of swapping the entire process before its execution can start, a swapping algorithm
(called demand paging) is used that swaps in only those pages of a process that are currently
needed in the main memory for continuing the process’s execution.
Cache Memory
Cache memories are small, fast memories placed between the CPU and the main memory. They
are faster than the main memory with access times closer to the speed of the CPU. Although
caches are fast, they are also very expensive and so are used only in small quantities. Cache
memories are therefore intended to provide fast memory retrieval without sacrificing the size
of the memory.
Though the memory is small, it is used in increasing the overall speed of the memory due to
the property of locality of reference.
The memory references at any given interval of time tend to be confined to a few localized
areas in the memory. This phenomenon is known as the property of locality of reference. Based
on the locality of reference, the cache contains a copy of certain portions of main memory. The
memory read or write operation is first checked with the cache and if the desired data is
available in the cache it is used by the CPU directly. Otherwise, a block of words is read from
the main memory to cache and the CPU then reads these words from the cache.
Fig. Functioning of the Cache Memory
Optical Media or Disks
Optical disks are storage devices with huge storage capacity. It is a relatively new storage media
and uses laser beam technology for reading and writing of data. Optical disks consist of one
large track that starts from the outer edge and spirals inward towards the center. It is split up
into sectors but these are of the same length regardless of its location on the track. Data is
packed at maximum density over the disk.
The storage capacity of an optical disk is determined as –
Storage Capacity = Number of sectors × Number of bytes per sector
(We don’t consider the number of tracks as there is only one track)
The technology used in optical disks uses laser beams to write and read data as opposed to the
read/write head used in magnetic disks. Data is recorded by etching microscopic pits (burning
surface) on the disk surface. A high intensity laser beam is used to etch the pits while a low
intensity laser beam is used for data retrieval.
The optical disks that are becoming increasing popular in various computer applications are
CD-ROMs, Erasable Optical disks, and DVDs.
CD - ROM
The CD-ROM (Compact Disc Read-only Memory) is a direct extension of the audio CD. It is
usually made from a resin named polycarbonate that is coated with aluminium to form a highly
reflective surface. The information on a CD-ROM is stored as a series of microscopic pits on
the reflective surface (using a high intensity laser beam). The process of recording information
on these disks is known as mastering. This is so called because this master disk is then used to
make a die, which is used to make copies.
The information is retrieved from a CD-ROM using a low powered laser beam, which is
generated in an optical disc drive unit. The disk is rotated and the laser beam is aimed at the
disk. The intensity of the laser beam changes as it encounters a pit. A photo-sensor detects the
change in intensity, thus recognizing the digital signals recorded on the surface of the CDROM and converts them into electronic signals of 1s and 0s.
The information stored in the CD-ROM can be read only. It cannot be modified in any way. It
is therefore useful for applications in which there is a database of information that is useful as
it is and does not need changing in any way, eg. A directory such a Yellow Pages. CD-ROMs
are useful for distributing information to a large number of users.
Advantages of CD-ROM:
1) Large storage capacity for information or data
2) Fast and inexpensive mass replication
3) Suitable for archival storage as they are removable disks
Disadvantages of CD-ROM:
1) They are read-only and cannot be updated
2) Access time is greater than that of magnetic disc
DVD
Digital Versatile Discs is an optical disk storage format, which is used for digital representation
of movies and other multimedia presentations that combine sound with graphics. DVDs are
similar to Compact Discs (CDs) but with much greater storage capacity than that of CDs. A
DVD uses 5-inch disc storage capacity ranging from 4.7 GB to 17 GB. A DVD can store entire
movie, or several hours of audio, or computer data of 5 GB or more. We can use DVDs for
recording purpose on both sides as well as in dual layers, which provides us two individual
recordable layers on a single-sided DVD disc.
BIOS
The Basic Input/Output System (BIOS) is a built-in software. It is placed in the ROM chip on
our motherboard. It is also called as ROM BIOS. BIOS performs Power-On Self-Test. It is a
diagnostic testing tool that checks whether our hardware devices are working properly or not.
Modern PCs contain flash BIOS. In the flash BIOS, the BIOS have been stored on a flash
memory chip that is rewritable memory chip. We can update the flash BIOS, if needed in future.
It acts as an interface between the OS and the hardware components, when the system is
booting and running.
Some of the manufacturers of BIOS are –
1. American Megatrends Inc. (AMI)
2. Award
3. MR BIOS
4. Phoenix Technologies
Functions –
1. Ensures the basic operability of each component by using the list of all the hardware in
the system.
2. Identifies and configures new hardware including hard drives and floppy drives.
3. Finds a valid operating system and transfers the control of the system to that operating
system after the boot functions have been completed.
SMPS (Switched-Mode Power Supply)
It is an electronic power supply unit. It helps us to convert input power to a controlled voltage.
A computer works on DC power supply, however, the power socket provides us with AC power
supply. One of the main functions of SMPS is to convert the AC power into DC power.
Power connector is an electrical connector that is used to carry DC or low frequency AC power.
With the help of power connectors, the power supply unit provides current to the different
components of a system which includes motherboard, hard drive, floppy drive, and other
components. The different of power connectors are –
1. AT Power Connectors
AT (Advanced Technology) power connectors have two 6-wire connectors named P8
and P9 which are used to connect to the motherboard with black wires facing each other.
The different form factors are PC/XT, AT, Baby AT, and LPX and use a pair of 6-wire
connectors known as AT style connectors.
2. ATX Power Connectors
ATX (Advanced Technology Extended) power is a 20-pin connector which outputs 6
types of voltages. ATX style power connectors are used by ATX/NLX form factors.
3. Standard Peripheral Power Connectors
It is also known as 4-pin Molex power cable or 4-pin molex. The 4-wire connectors are
used by the disk drive power connectors.
Pin Number
Color
Pin 1
Yellow
+12V
Pin2
Black
Ground
Pin3
Black
Ground
Pin4
Red
+5V
4. Floppy Drive Power Connectors
It is known as mini-plug and are used in 3.5 inch floppy disk drive.
Description
RJ45 Connector
The standard connector for unshielded twisted pair cable is an RJ45 connector. This is
a plastic connector that looks like a large telephone-style connector. A slot allows the RJ-45 to
be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a
standard borrowed from the telephone industry. This standard designates which wire goes with
each pin inside the connector.
NIC Card
The network interface card (NIC) provides the physical connection between the
network and the computer workstation. Most NICs are internal, with the card fitting into an
expansion slot inside the computer. Some computers use external boxes which are attached to
a serial port or a SCSI port. Every NIC has a MAC address (Medium Access Control), which
is a binary address unique to a single network. This MAC address is 48-bit long and used to
uniquely identify each node of a network.
NICs are major factor in determining the speed and performance of a network.
The NIC has a driver, which is a software component that manages the NIC. The NIC
driver serves as a crucial role in the networking architecture.
Network Cable
Cable is the medium through which information usually moves from one network device to
another.
There are several types of cables which are commonly used with LANs. They are –
1. Twisted Pair cable
2. Coaxial Cable
3. Fiber Optic cable
1. Twisted Pair Cable
It contains two cable wires that uses copper as a conductor. These wires have a plastic
insulation and are twisted. One twisted cable wire carries the signal to the receiver and
the other cable wire acts as a ground reference. At the other end, the receiver obtains
the difference in the values of the signal between two cable wires. The cable wires are
twisted to provide protection against crosstalk and noise. It is generally used for voice
and data communication.
Twisted pair cable comes in two varieties – shielded and unshielded. The shielded
twisted pair cable has a metal shield covering each pair of insulated cable wires. This
type of shielding protects the cable from external electromagnetic interferences and
crosstalk. STP is suitable for environments with electrical interference, but the extra
shielding can make the cables quite bulky.
Unshielded twisted pair cable is the most popular cable and is generally the best option
for school networks. The cable has four pairs of wires inside the jacket. Each pair is
twisted with a different number of twists per inch to eliminate interference from
adjacent pairs and other electrical devices. The tighter the twisting, the higher the
supported transmission rate and the greater the cost per foot. It is generally used in
Ethernet networks and telephone systems.
The main disadvantage of UTP is that it may be susceptible to radio and electrical
frequency interference.
2. Coaxial cable
Coaxial cable consists of a two conductors. One is the inner conductor of solid copper
and the other is the outer conductor. The outer conductor acts as a shield against
noise. There is insulating plastic between the conductors and the whole cable is
covered with plastic jacket. The coaxial cable can carry signals of higher frequency
ranges than twisted pair cables. These cables are used in cable TV network and
traditional Ethernet LANs.
3. Fiber Optic cable
It is made up of glass or plastic and the signal propagates in the form of light. The inner
core is surrounded by a cladding of glass or plastic that is less dense than the core. It
contains greater bandwidth than the metallic cables. Due to greater bandwidth, these
cables can carry more data. Fiber optic cables are thinner and lighter than metallic
cables. The main disadvantage of this type of cable is that it is much more expensive
on install.
Switch
It is a network hardware device and is used to connect multiple computers in a LAN.
They are generally used in areas of high network traffic. A switch is able to inspect the
received data packet and determines its source and destination devices and forward it
accordingly. So, on a busy network, a switch performs better work than a hub.
Hub/Concentrator
A concentrator is a device that provides a central connection point for cables from
workstations, servers, and other peripherals to form a small network. Hubs are multislot
concentrators into which can be plugged a number of multi-port cards to provide additional
access as the network grows in size. Some concentrators are passive, that is they allow the
signal to pass from one computer to another without any change. Most concentrators are active,
that is they electrically amplify the signal as it moves from one device to another. Active
concentrators are used like repeaters to extend the length of a network.
In a network, the hub broadcasts the data packets to all the computers in the network
without determining the source and destination devices of the data.
Repeater
The repeater electrically amplifies the signal it receives and rebroadcasts it. Repeaters
can be separate devices or they can be incorporated into a concentrator. They are generally
used when the total length of the network cable exceeds the standards set for the type of cable
being used.
I/O Box Connector
It is a connector mounting RJ45 connector into keystone wall plate, face plate,
surface-mount box, or a patch panel with rectangular holes, called ports. One end of the patch
cord is connected to the I/O box connector and the other end is connected to the host machine
for sharing resources or information in LAN.
Patch panel
A patch panel, patch bay, patch field or jack field is a device or unit featuring a
number of jacks, usually of the same or similar type, for the use of connecting and routing
circuits for monitoring, interconnecting, and testing circuits in a convenient, flexible manner.
Patch panels are commonly used in computer networking, recording studios, and radio and
television.
Patch cord
A patch cable or patch cord or patch lead is an electrical or optical cable used to
connect one electronic or optical device to another for signal routing. Devices of different types
(e.g., a switch connected to a computer, or a switch to a router) are connected with patch cords.
Patch cords are usually produced in many different colors so as to be easily distinguishable,
and are relatively short, perhaps no longer than two metres. Types of patch cords include
microphone cables, headphone extension cables, XLR connector, Tiny Telephone (TT)
connector, RCA connector and ¼" TRS phone connector cables (as well as modular Ethernet
cables), and thicker, hose-like cords (snake cable) used to carry video or amplified signals.
However, patch cords typically refer only to short cords used with patch panels.
Patch cords can be as short as 3 inches, to connect stacked components or route signals
through a patch bay, or as long as twenty feet or more in length for snake cables. As length
increases, the cables are usually thicker and/or made with more shielding, to prevent signal loss
and the introduction of unwanted radio frequencies and electromagnetic interference.
Patch cords are often made of coaxial cables, with the signal carried through a shielded
core, and the electrical ground or earthed return connection carried through a wire mesh
surrounding the core. Each end of the cable is attached to a connector so that the cord may be
plugged in. Connector types may vary widely, particularly with adapting cables.
Patch cords may be:

Single-conductor wires using, for example, banana connectors

Coaxial cables using, for example, BNC connectors

Shielded or unshielded Cat5, Cat5e, Cat6 or Cat6A cables using 8P8C (RJ-45) modular
connectors with straight-through T568A or T568B wiring.

Optical fiber cables
A patch cord is always fitted with connectors at both ends.
IP Address
An IP-address is a unique 32-bit logically assigned binary number of four octets, which
is represented in decimal notation, designated to a particular device. Hosts and routers used in
the TCP/IP protocol suite are identified by the IP-address. IP- address is used to send and
receive information from one host to another through a physical medium such as, cable on the
network. IP – address is divided into five type of classes: A, B, C, D, and E and each class has
its specific range. The first four bits of an IP- address determines the class into which an IPaddress fits. The IP-address consists of two parts –
1. Network ID: identifies the network to which the host is connected. It is same for all
the hosts on a particular network.
2. Host ID: identifies the host on the network. It is different for different hosts on the
network.
Eg. 192.168.75.38 (Network ID: 192.168.75
Host ID: 38)
Rules regarding the identification of IP address:
1. An IP – address cannot be duplicated
2. All hosts bits cannot be set to zero
3. All host bits cannot be set to one
4. 127 cannot be taken as the first octet because it is reserved as a loopback address (
Address that acts as a test mechanism for network devices and protocols)
Types of IP-addresses
Class A: If the first bit is zero, IP-address belongs to Class A address. Class A addresses are
used when there are several hosts communicating on a large network. It uses the first octet, in
which the first bit is set to zero, and the other seven bits are all set to one. These networks use
the first 8 bits of the IP – address to identify the network address and the remaining 24 – bits
are used to identify the individual hosts. Class A has upto 224 different host IDs.
Class B: If the first bit is one and the second bit is zero, IP-address belongs to Class B address.
Class B addresses uses the first two octets or 16 bits of an IP – address to identify the network
address and the remaining 16 bits are used to identify the host ID. Class B has upto 216 different
host IDs. Class B addresses are used in medium to large scale organization.
Class C: If the first two bits are one and the third bit is zero, IP-address belongs to Class C
address. Class C addresses uses the first three octets or 24 bits of an IP – address to identify the
network address and the remaining 8 bits are used to identify the host ID. Class C has upto 2 8
different host IDs. Class C addresses are used in LANs.
Class D: If the first three bits are one and the fourth bit is zero, IP-address belongs to Class D
address. Class D addresses are not assigned to hosts but are designed for multicasting
communication.
Class E: If all the four bits are one, IP-address belongs to Class E address. Class E addresses
are not assigned to hosts. These types of addresses are not used for general applications but are
reserved for experimental purpose.
In addition to five types of IP addresses, following are the other types of IP addresses –
1. Unicast IP address: Helps in one-to-one communication. It uniquely identifies a host
in the network when a data packet is sent from a host to the destination address.
2. Multicast IP address: Helps in one-to-many communication, when a data packet is
sent from a host to a group of hosts and not to a single destination address.
3. Broadcast IP address: Helps in one-to-all communication. A broadcast allows a data
packet to be sent from a host to all other hosts on the network.