Computer Notes Chapter- 9 SECONDARY STORAGE DEVICES ! Sequential and direct-Access Devices ! Sequential Access Device ! Magnetic Tape ! Tape Drive ! Direct-Access Devices ! Magnetic Disk ! Types of Magnetic Disks ! FLOPPY DRIVE ABCs ! FLOPPY HISTORY ! Hard disk drive ABCs ! Optical Disk ! Optical Disk Drive ! Types of Optical Disks ! CD-ROM ! WORM DISK ! BUS ! Data Bus ! Address Bus ! Control Bus !
SECONDARY
STORAGE DEVICES
Several
different devices can be used as secondary storage device, but the one selected
for a particular application, mainly depends upon how the stored information
needs to be accessed. There are two methods of accessing information-sequential
or serial access, and direct or random access. A sequencing –access storage
device is one in which the arrival at the location desired may be preceded by
sequencing through other location, so that access time varies according to location
. in other words, information on a sequential-access device can only be
retrieved in the same sequence in which it is stored. Sequential processing is
quite suitable for such applications like preparation of monthly pay slips, or
monthly electricity bills, etc. These direct-access devices are also called
random-access devices, because the literally available at random ,i.e., it is
available in any order. Hence, a random-access storage device is one in which
any location in the device may be selected at random, access to the information
storage device may be selected at random, access to the information stored is
direct, and approximately equal access time is required for each location. This
analogous to a music CD, if 10 songs are recorded on a music CD, and if you
want to listen to (or access) the 6th son, you need not listen to or
fast-forward the 5 songs, which come before your favorite song. All you have to
do is selected track number 6 on the player, and player directly moves the
pickup arm across the CD to the groove where the 6th song begins,
and starts playing. Magnetic and optical disks of various types are examples of
direct-access storage devices.
Sequential
Access Device:-
Magnetic
Tape:
Magnetic tape is the most popular storage
medium for large data, which are sequentially accessed and processed. The
magnetic tape medium is a plastic ribbon, which is usually ¼ inch wide and 50
to 2400 feet long. It is coated with magnetically recording material, such as
iron oxide or chromium dioxide . Data are recording on the tape in the form of
tiny invisible magnetize and non-magnetized spots (representing 1s and 0s) on
the coated surface of the tape. The tape ribbon is itself stored in reels or a
small cartridge or cassette.
Tape
Drive:-
A magnetic tape drive is used for storage
and retrieval of data, which is stored on magnetic tape medium. The tape drive
is different for for tape feels, tape cartridge, and tape cassettes. However,
all of them work on a similar mechanism like the audio tape recorders or videocassette
recorders (VCR) found in our homes. That is, the tape drive has read/write
heads, and as the tape ribbon passes under the read/write heads, the data can
be either read and transmitted to primary storage, or transmitted from primary
storage and written to the tape by giving suitable commands to the tape drive.
Instead of play and record, read and write commands are used with the tape
drive. Just as in the case of an audio tape recorder or videocassette recorder,
a magnetic tape reel or cartridge or cassette has to be first loaded on to a
tape drive for processing. Once loaded, the magnetic tape is said to be online,
that is, it can now be used for storage or retrieval of data by the computer
system. When processing is complete, the tape is removed from the tape drive
for off-line storage, that is, it is stored away from the computer system,
Data on it are not accessible to the
computer system, until it is loaded again on the tape drive.
Direct-Access
Devices:-
Magnetic
Disk:-
Magnetic disk is the most popular storage
medium for direct-access secondary storage. Due to their random access
capability, magnetic disks are the most popular on-line secondary storage
device.
A magnetic disk is a thin, circular
plate/platter made of metal or plastic, which is usually coated on both sides
with a magnetically recording material, such as iron oxide.
Storage Organization: For
data recording, the surface of a disk is divided into a number of invisible
concentric circles, called tracks. The tracks are numbered consecutively from
outermost to innermost, starting from zero.
Each track is further
subdivided into sectors. For this, in additions to the concentric circles, the
disk surface is also divided into invisible pie-shaped segments. Hence, if
there are eight such pie-shaped segments, each track will be divided into eight
parts, and each of these eight portions of a track is called sector.
Types
of Magnetic Disks:-
All magnetic disks are round platters. They
come in different type of packaging, and can be made of rigid metal or flexible
plastic. Based on these differences, there are many different types of magnetic
disk available today. However, all of them may be broadly classified into two
types-floppy disk and hard disks. Floppy disks are individually packaged in
protective envelops or plastic casog, whcroas hard disks may be packaged
individually or in multiples, in cartridges or contamination-free containers.
Depending on the type of packaging, hard disks are further classified into
zip/Bernoulli disks, disks packs, and Winchester disks. The four type of
magnetic disks are described below.
FLOPPY
DRIVE ABCs:-
Originally
created in 1967 by IBM, the floppy reference is derived from the floppy media
that is encased within the protective casing. This media is a magnetic medium
and is written to much like the method used by hard disks drives. Floppy drives
have been found in computers for several years and are commonly still being
used today. The next generation of floppy drives will more than likely be the
LS 120 diskette drives which are already being included in many computers now.
FLOPPY
HISTORY:-
8” Disks- The first disk was introduced in
1971. The disks was 8” in diameter with a magnetic coating, enclosed in a
cardboard case with the capacity of one megabyte. Conversely to hard disks, the
heads actually touch the disk, like in a cassette or video player, which
actually the media.
5.25”
Disks-60KB first Created in 1976 and later becoming a standard in 1978.
1. 5.25” Single Side-160KB
2. 5.25” Double Side-360KB
3. 5.25” Double Side High Density-1.2MB
3.5”
Disk-Created by IBM in 1984, which even today are still commonly used. Most 3 ½
“ newer disks have a capacity of up to 1.44MB.
1. 3.5” Double Density- 720KB
2. 3.5” High Density – 1.44MB
3. 3.5” Extended Density (IBM ONLY)-2.88MB
Hard
disk drive ABCs:
The hard disk drive is the primary storage
location where data is permanently stored. Below is an illustration of what the
inside of the hard disk drive may look like. The four main components of a hard
disk drive are the platters, head arm, and the head actuator.
The majority of computer hard disk drives is permanently stored in an
internal drive bay at the front of the computer and is connected with one
ATA/SCSI cable and power cable, Unlike other drives, the hard disk drive is the
only drive that is physically accessed by the user like the floppy disk drive
or the CD ROM drive.
Optical
Disk:
As compared to magnetic tape and magnetic
disk, optical disk is a relatively new secondary storage medium, during the
last few years, it has proved to be a promising random access medium for high
capacity secondary storage, because it can store extremely large amount of data
in a limited space.
An optical disk storage system consists of
a rotating disk, which is coated with a thin matter or some other material that
is highly reflexive. Laser beam technology is used for recording/ reading of
data on the disk. Due to the user of laser beam technology, optical disks are
also known as laser disk or optical laser disk.
Optical
Disk Drive:
An optical disk has to be mounted on an
optical disk, before it can be used for reading or writing of information. An
optical disk drive contains all the mechanical, electrical and electronic
components which they the disk is kept, the read/write laser beams assembly,
and the monitor to rotate the disk.
Types
of Optical Disks:
All optical disks are round platters. They
come in different sizes and capacities. The two most popular types of optical
disk in use today are CD ROM and WORM disks. They are described below.
CD-ROM:
CD-ROM stands for compact Disk-Read Only
Memory. It is a spin-off of music CD technology, and words much like the music
CDs used in music systems. In fact, if you have a soundboard and speakers
connected to your can play music CDs with Your computer.
The CD-ROM disk is a shiny, silver color
metal disk of 5 ¼ - inch (12cm) diameter. It has a storage capacity of about
650 Megabytes. It so called, because of its enormous storage capacity on a
compact size disk, and because it is a read only storage medium. That is, these
disks come per-recorded, and the
information stored on them cannot be altered.
WORM
DISK:
Worm stands for write-once read many. WORM
disks allow the users to create their own CD-ROM disks by using a CD recordable
(CD-R) drive, which can be attached to a computer as a regular peripheral
device. WORM disks. Which look like standard CD-ROM disks, are purchased blank
and encoded using a CD-R dive. The information recorded on a WORM disk by a
CD-R drive can be read by any ordinary CD-ROM drive. As the name implies, data
can be written only once on a WORM disk, but can be read many times That is, as
with a CD-ROM disk, once data has been etched on to the surface of a WORM disk,
it becomes permanent, which can be read, but never altered, Moreover, writing
on a WORM disk cannot be done in multiple sessions and all the data to be
recorded have to be written on the disk surface in a single recording session.
The same laser beam technology as discussed before is used for recording and
reading of data.
BUS:
The system bus carries bits between CPU and
only one peripheral at a time. In computer technology best three main buses.
1.
Data Bus
2.
Address Bus and
3.
Control Bus
Data
Bus:
It an electrical path connects the CPU,
memory and other devices on the motherboard. In real sense bus is a group of
parallel lines. The number of lines in the bus affects the speed at which data
can travel between hardware components. Since each wire can transfer one bit at
a time, 16-wire bus move 16 bits at a time, which are 2 bytes. Similarly a 32
bit bus can transfer 4-bytes at a time.
Address
Bus:
It connects only the CPU and memory and
helps in location memory address faster. The number of lines in it determines
the maximum number of memory address. Today most CPU has 32-bit address buses
that can address 4GB (over 4 billion bytes) of memory.
Control
Bus:
It controls the direction, flow, origin and
signals for every operation it performs. These signals are used to identify a
device type with which the CPU intends to communicate
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