Transcript Sec (1.3): Mass storage (secondary storage) File storage and retrieval: Field: set of characters Record: set of fields File: set of records Data base: set of.

Sec (1.3):
Mass storage (secondary storage)
File storage and retrieval:
Field:
set of characters
Record: set of fields
File: set of records
Data base: set of files
Mass storage (secondary
storage)
Like magnetic disk, CD, DVD, magnetic tapes and flash drives
Advantages of mass storage over main memory:
1. include less volatile
2. large storage capacity
3. low cost
4. many cases, the ability to remove the storage medium from the machine for
archival purposes
Disadvantages:
A major disadvantage of the mass storage systems is that they typically
require mechanical motion and therefore require significantly more time to
store and retrieve data then a machine's main memory, where all activities
are performed electronically
On-line and Off-line:

Means that the devices that can be either
attached or de-attached from a machine

On-line: means that the device or information is
connected readily available to machine without
human intervention

Off-line: means that human intervention is
required before the devices or information can
be accessed by the machine.
Magnetic system:
It is significant to two kinds:
1.
sequential access secondary storage
2.
Direct access secondary storage

sequential access secondary storage
searching for a certain data means starting from the beginning of
the file sequentially till the data is reached ( like magnetic reel tape
and cassette )

Direct access secondary storage
Data can be reached directly without going through the stored
data (like magnetic disk )
Magnetic system: sequential
access secondary storage
1 ) Reel Magnetic Tape:
 Used in mainframe computers
 It has a plastic strip coated with one side with magnetized material
(Chromium Dioxide)
 It is width 1/2 inch and it is length commonly 2400 feet (some reel
with 300, 600 or 1200 feet )
 The strip is divided into 7 or 9 tracks (9 common )
 Strip with 9 tracks stores about 6250 characters per inch.
 In strip with 18 tracks, 38000 characters can be stored in 1 inch
The devices that reads the tape is called tape drive it consists of:
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Two reels one for source and the other for destination
R/W head which reads or writes the data on the strip
Control unit which control in reading and writing operations
Magnetic system: Direct access
secondary storage
2 ) magnetic disk
 Note: the capacity of disk storage system
depends on the number of disk used and the
density in which the tracks and sectors are
placed
 Lower-capacity
system consists of a single plastic
disk known a diskette or floppy disk (off-line storage).
3 1/2 inch have a capacity of only 1.44 MB
 High capacity system like hard-disk (consist of 5-10
rigid disks mounted on a common spindle )
Mass Storage: Magnetic Disk
Systems

Magnetic disks: the most widely used storage medium in
computers today

Data is stored by magnetizing particles on the storage
medium.

Capacity depends on # of disks & density of tracks and sector
placed on
Mass Storage: Magnetic Disk
Systems
Two common types:
 Floppy
disks: Diskette (low-capacity)
 Hard disks: (high-capacity)
Floppy Disks and Drives

Floppy disk characteristics

Single plastic flexible disk

Typically 3½ inches in diameter.

Typically hold 1.44 megabytes.

Inserted into floppy disk drive to be read from or written to.
Floppy Disks and Drives,
Cont’d.

Disk is divided into tracks,
sectors, and clusters.
Floppy Disks and Drives,
Cont’d.

Using floppy disks
 Must be inserted into the proper drive in the proper direction.
 Should not be removed when the disk is being accessed.

High-capacity removable magnetic disks and drives
 Zip disks (750 MB)
 SuperDisks (240 MB)
Hard Disk System:
Hard disk characteristics
 Usually (5-10) rigid disks mounted on a common spindle and
combined into a disk pack
 Disk drive - a device that allows data to be read from or written to
a disk
 Disk drive for personal computers contained within computer
housing
 Can be internal or external.
 Large computer systems may have several external disk drives
 Capacity in gigabytes
 Accessing files much faster than accessing files on diskettes
Disk Packs
Each disk has its own
access arm with
read/write head
 Most disk packs
combine disks,
access arms, and
read/write head

Reading/Writing Data


Access arm moves read/write
head over particular location
Read/write head hovers a few
millionths of an inch above disk
and doesn’t touch the surface of
the disk.
 If head touches platter, a
head crash occurs and data is
destroyed
 Data can be destroyed if head
touches miniscule foreign
matter on surface of disk
How Data Is Organized
Organized into tracks, sectors, clusters, and
cylinders
Track
Sector
Cluster
Cylinder
Disk Access Speed
Access time - the time needed to access
data on disk
 Three factors

 Seek
time
 Head switching
 Rotational delay

Once data found, next step is data transfer
Optical Systems:
Like Compact Disk (CD):



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These disks are 12 cm (approximately 5 inches)
in diameter and consist of reflective material
covered with a clear protective coating
The capacity of the CD about 600 to 700 MB.
Information is recorded on them by crating
variations in their reflective surfaces.
This information can be retrieved by means of a
laser beam. That monitors irregularities on the
reflective surface of the CD.
Optical Systems:
CD-DA: (Compact Disk Digital Audio):
Which technology applied to audio recordings
Information on these CDs is stored on a single track that spiral
around the CD, this track divided into units called sectors. Each with
its own identifying marking s and a capacity of 2KB of data
Note: the distance around the spiraled track is greater from the outer
edge of the disk than the inner portion
Note: to maximize the capacity of a CD: information is stored at a
uniform linear density over the entire spiraled track which means
that more information is stored in a loop around the outer portion of
spiral than in a loop around the inner portion
Mass Storage: Optical Systems


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Provides inexpensive and compact storage with greater
capacity ( approx. 5 inches diameter)
Information is recorded by creating variation in reflective
services
Information is retrieved by a laser beam that monitors
irregularities on the reflective surface
Laser scans disk and picks up light reflections from disk
surface
Optical Systems: Compact
Disks
Categorized by read/write capability:

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CD-ROM - drive can only read data from CDs
 CD-ROM stores up to 700 MB per disk
 Primary medium for software distribution
CD-R - drive can write to disk once
 Disk can be read by CD-ROM or CD-R
drive
CD-RW - drive can erase and record over
data multiple times
 Some compatibility problems trying to
read CD-RW disks on CD-ROM drives
Optical Systems: Digital Versatile
Disk (DVD)


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Constructed from multiple, semi-transparent layers
 DVD drive can read CD-ROMs
 Capacity up to 17GB
 Allows for full-length movies
 Sound is better than on audio CDs
Several versions of writable and rewritable DVDs exist
Which are constructed from multiple layers that serve as
distinct surfaces when viewed by precisely focused laser
provide storage capacities of several GB. Such disks are
storing lengthy multimedia presentations, including entire
motion pictures
Mass Storage: Flash memory
media

Bits are stored by sending electronic signals
directly to the storage medium where they
causes electrons to be trapped in tiny
chambers of silicon dioxide.

Chip-based storage.

This technology is suitable for off-line
storage of data
Flash memory media
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
Note: data stored in flash can be accessed in
small byte-size units as in RAM applications.
Repeated erasing slowly damages the silicon
dioxide chambers, meaning that current flash
memory technology is not suitable for general
main memory application
Since flash memory is not sensitive to physical
shock, it is potential in portable applications is
enticing
Flash memory media
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Flash drives, with capacities of up to a few GB, are
available for general mass storage applications
These units are packaged in small plastic cases
approximately three inches long with the removable cap
on one end to protected the unit's electrical connector
when the drive is off-line
Commonly used with digital cameras, digital music
players, handheld PCs, notebook computers, smart
phones, etc.
Flash memory media

Flash memory media, cont’d.

Common forms of flash memory:

Flash memory sticks.

Flash memory cards—CompactFlash, Secure Digital
(SD), MiniSD, MultiMedia (MMC), SmartMedia, and xD
Picture cards.

Flash memory drives, such as USB flash drives.