DATA STORAGE
Unit Overview
In this unit, the student will differentiate between computer memory and secondary
storage. The student will also identify and explain the most common secondary storage
options available for personal computers.
Key Terms
Computer Memory: Computer
Secondary Storage: Secondary
memory or primary storage is a
storage is storage that is outside of
temporary storage place for data and
the computer itself.
programs that are being processed.
Access Time: Time required
Bit: A bit or binary digit is the most locating and transferring the data
basic unit of computer storage. from secondary storage to the
computer’s memory.
Writing/Recording: The Reading: The process of retrieving
process of storing data. data.
Memory vs. Secondary Storage
In order to fully understand computer storage, there must be a distinction made between
computer memory and secondary storage. Computer memory or primary storage is a
temporary storage place for data and programs that are being processed. Computer
memory is volatile meaning that when the computer’s power is turned off, the data and
programs saved in the computer’s memory are erased. RAM is one type of computer
memory.
Secondary storage is storage that is outside of the computer itself. It provides a more
permanent form of storage. Secondary storage saves data and programs when they are
not being processed. It is non-volatile, which means its contents are not erased when the
computer’s power is turned off. Secondary storage is also known as auxiliary storage.
The hard disk is an example of secondary storage.
The bit or binary digit is the most basic unit of computer storage. A bit has two
conditions either 0 or 1. Usually, these values are representative of true or false. A byte
is a group of eight bits that have been joined together to store more complex forms of
data.
Secondary/Auxiliary Data Storage Devices
Secondary storage provides a method for semi-permanently storing information or
software outside of the computer. Storage includes both the media and the methods for
storing data. Data is written or recorded on storage medium for later use. The data is
later read or retrieved by the storage device. This explains why storage devices can be
classified as either input or output devices depending upon their current function.
Magnetic Disk Storage
Floppy disks, hard disk drives, and removable disk cartridges are all types of magnetic
media. Magnetic disk storage represents data as magnetized spots on the surface of a
spinning disk. The appearance of a magnetized spot represents a 1 bit, while the absence
of a spot represents a 0 bit. When data is read from the disk, the magnetized data is
translated into electrical impulses that are sent to the processor. This process is reversed
when data is written to the disk.
Floppy Disk
Floppy disks have evolved since the early 1970’s when they were first introduced. At
that time, floppy disks were eight inches wide, flexible, and thin. The floppy disk
gradually became smaller, first being reduced to 5 ¼ inches wide and later to the 3 ½ inch
wide disks that we use today. Floppy disks are made up of a round piece of oxide-coated
Mylar that is sandwiched between paper liners and surrounded by a hard plastic case.
A metal shutter protects the disk
surface from dirt and fingerprints; the
shutter slides out of the way when the
disk is inserted into the drive so that
the read/write heads can reach the
disk.
A small plastic slider can be slid to
unblock a hole in the corner of the
diskette to write-protect the disk so
data can’t be accidentally erased.
High-density floppy diskettes hold 1.44 megabytes of data. The access time is much
slower than for a hard disk, and they are somewhat unreliable. Many new computers
don’t have a floppy drive, but you can purchase an external drive to plug in if you need
to.
In order to use a floppy disk for storage, it must be formatted. During formatting, all data
contained on the disk is erased, the disk surface is checked for defects, and the tracks,
cylinders, and sectors on the disk’s surface are defined. When the diskette is slid into the
disk drive, the metal shutter slides to the side exposing the recording surface. The floppy
drive motor slides into the metal hub and rotates the disk surface. The read/write head
moves over the disk surface to read or write data.
Zip Disk
A Zip disk is similar in size to a floppy disk, but
thicker. It is basically a “super floppy”, but the
higher construction tolerances and smaller
read/write heads allow the Zip disk to hold more
data than a floppy disk. The first Zip disks held 100
MB. However, the newer model Zip drive now
uses 750 MB disks.
Caring for Floppy Disks
To prevent unnecessary damage to your floppy disk follow these guidelines:
• Store floppy disks vertically.
• Avoid extreme temperatures.
• Use caution when inserting or removing diskettes from the
drive. Don’t force them in or out of the drive.
• Store diskettes away from food, drinks, and smoke.
• Store diskettes away from magnets.
• Never touch the recording surface of the diskette.
USB Flash Drive
A USB Flash Drive is a portable solid state memory
device that plugs into a USB port on the computer.
The USB Flash Drive is also called a key drive,
pocket drive, thumb drive, and a pen drive. They
have replaced floppy diskettes and Zip disks. The
storage capacity varies, but anything from 16 MB to
over a gigabyte are available.
Flash Memory Cards
The same kind of flash memory used in the USB flash drives are used in small memory
cards such as a Secure Data SD card and a Compact Flash card, as shown in the figure.
These cards are used by PDA’s, digital cameras,
MP3 music players, and other digital devices. A
flash memory card reader can be attached to a
computer to read and write data to these cards, as
well. These memory cards come in a variety of
storage capacities from tens of megabytes to over
a gigabyte.
Hard Disk
The hard disk consists of a hard magnetic oxide-coated aluminum platter. The hard disk
uses the same technology as the floppy diskette to read and write data. The hard disk’s
top and bottom surfaces can be used to record data. Some hard disks contain several
platters, which are combined into disk packs. Usually, the access arm and read/write
heads are sealed in an airtight module to prevent damage from foreign matter such as
dust, smoke, or fingerprints.
The hard disk drive is used to read or write
hard disk platter
data to the hard disk. The access arm moves
the read/write head into position so that it
hovers over the hard disk to read or write data
on its surface. If the read/write head touches
the disk surface, data is destroyed. This is
access arm called a head crash.
read/write head
Hard disks are also formatted before they can store data. The hard disk can be partitioned
or divided into areas before formatting takes place. Partitions can be used to separate
information for different users and to install more than one operating system on a single
computer. Hard disk partitions are typically identified by the letters.
Megabytes, gigabytes, or terabytes are the units used to measure the hard drive’s
capacity. The capacity of hard drives has increased dramatically in recent years. Most
hard drives are forty or more gigabytes. The table below shows the units of measurement
associated with data storage.
Number of
Term Abbreviation Number
Bytes
Kilobyte KB 1,000 Thousand
Megabyte MB 1,000,000 Million
Gigabyte GB 1,000,000,000 Billion
Terabyte TB 1,000,000,000,000 Trillion
It generally takes the hard drive about ten to twenty milliseconds to locate and transfer
the data from the hard disk to the computer’s memory. This access time is much faster
than the floppy disk drive, which takes about 150 milliseconds to perform the same
operation. The hard disk controller manages the flow of data to and from the hard disk.
The hard disk controller can be a separate board installed on the computer or it can be
built into the hard drive. The most common types of hard disk controllers are IDE
(Integrated Drive Electronics) and SCSI (Small Computer System Interface).
Caring for the Hard Disk Drive
Hard Disks are not as susceptible to damage as floppy disks because they are sealed
within the computer’s systems box. Nonetheless, there are some precautions that you
should take to protect the data stored on your hard disk drive.
• When the computer is on, be careful not to bump or knock over the computer’s
systems box where the hard disk drive is located.
• Follow the proper procedure for shutting down the computer. Avoid rapidly switching
on and off the power to your computer.
• Avoid extreme heat and keep magnets or anything magnetized away from the
computer’s hard disk drive.
The Removable Hard Disk
Removable hard disks are externally or internally installed hard drives that use
removable tapes to store data. Removable hard disks are most commonly used to back up
the data stored on the computer’s hard disk drive. The capacity of the disk cartridge used
with the removable hard drive can be up to two gigabytes. This can be a viable option for
data security, since your work can be stored on the removable disk and locked up or
taken with you at the end of the work day without leaving data on the computer.
Magnetic Tape
In the past, magnetic tape was commonly used to store large amounts of data. Magnetic
tape can be in the form of a cartridge or reel-to-reel. Data is recorded on magnetic tape
sequentially in long tracks, angles, or channels depending on the method used to record
the data. The magnetic tape unit uses a read/write head to record data, just like other disk
drives, but it also has an erase head that first erases any previously recorded data
contained on the tape. The popularity of magnetic tape declined as disk storage provided
greater reliability and increased access speed.
Data Organization on a Disk
Whether you are using a hard drive, a floppy drive, or a removable hard drive, data is
organized on the media using the same process. Tracks, sectors, clusters, and cylinders
are used to organize data. When accessing data on a track, the surface number, track
number, and sector number are used.
Tracks
Tracks are circular rings on the disk surface. The read/write head hovers over the tracks
to write data to the disk’s surface. A floppy disk has eighty tracks, while a hard drive can
have over one thousand. The number of tracks on the disk’s surface is influenced by the
access arm’s placement of the read/write head and the disk’s capacity.
Sectors
A disk sector is a triangular shaped portion of the disk that holds
an established amount of data.
A track sector is a portion of the track within a sector. The track
sector holds 512 bytes of data.
Clusters
Clusters are comprised of a set number of neighboring track
sectors. Most clusters contain between 2 and 8 track sectors. This
is determined by the operating system. The operating system
treats a cluster as a single storage component. The cluster is the
smallest storage component on a floppy disk. Therefore, small
files are given an entire cluster for storage purposes.
Cylinders
To speed up the access time and write time of data, large files are often saved to tracks
within the same cylinder. A cylinder is comprised of a
group of tracks that have the same numbers and
positions on the top and bottom of the disk’s surface.
So, in essence, when data is recorded to a cylinder it is
being saved vertically. This reduces the movement of
the access arm and makes the reading and writing of data more efficient.
Optical Disk Storage
There are a variety of optical disk storage devices. The most common are the CD-ROM
(compact disk read-only memory) drive, the CD-R (compact disc-recordable) drive, the
CD-RW (compact disk re-writable) drive, and the DVD-ROM (digital versatile disk)
drive. Optical disk storage uses laser beams to read and
write data to optical disks. High-powered laser beams are
used to burn pits into the surface of the plastic optical
disk, while low-powered laser beams are used to read the
data that is then translated by the computer into 0’s or 1’s
based on the reflection or absorption of the laser light.
The CD-ROM drive is used to read data from CD-ROMs. CD-ROMs have a capacity of
up to seven hundred megabytes. They are commonly used for saving software
applications that are far too large to be stored on floppy disks.
The CD-R drive equipped with specialized
software allows the user to write data on special
CD-R disks. One drawback to this technology is
that data can only be written once. So you must be
careful not to make mistakes because you cannot
erase them. However, once data has been saved to
a CD-R disk, it can be read from a regular CD-
ROM drive, as well as a CD-R drive.
Using specialized software and CD-RW disks, the user can erase and write over data
many times with a CD-RW drive. While the CD-RW drive provides greater flexibility,
the CD-RW disk is not always able to be read by regular CD-ROM drives.
The DVD-ROM or Digital Versatile Disk’s capacity of up to 17 gigabytes makes it ideal
for storing full length movies. DVD’s have a larger storage capacity than CD-ROM’s.
The DVD technology is similar to the CD-ROM, but with a short wavelength laser
capable of reading minuscule marks that stand for data. DVD-ROMs are read using a
DVD-ROM drive, which is also capable of reading CD-ROMs. DVD-RAM is the
writable version of the DVD. DVD-RAM’s can only be read by DVD-RAM drives.
Caring for Optical Disks
To prevent unnecessary damage to your optical disks follow these
guidelines:
• Do not touch the surface of the optical disk. Handle it by
picking it up by the edges or center hole.
• Return an optical disk to its case immediately after use.
• Avoid extreme temperatures.
• Do not write on the label side of the optical disk with a pencil,
pen, or marker.
Benefits of Secondary/Auxiliary Data Storage
Secondary storage is beneficial to the computer user because:
• It allows large amounts of data to be stored in a relatively small space.
• It provides a safe and reliable way to store data or distribute software.
• It is a cost effective data storage option that allows users to quickly locate and access
data.