Conceptual framework storage devices (2)

Unit 1
Computer Application and Management Information System; NMBA 018 1

 Storage Devices
 Primary Memory
 Secondary Storage
◦ Magnetic storage devices
◦ Optical storage devices
◦ Flash Memory
 Cache Memory
Computer Application and
Management Information System;
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 used to store the data, instructions and
information before, during and after the
processing by ALU
 Types – Primary and Secondary
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 work area within the computer
 Three types:
◦ Read Only Memory (ROM)
◦ Random Access Memory (RAM)
◦ Complementary Metal Oxide Semiconductor
Memory (CMOS)
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 An essential component of the memory unit
 This memory is permanent and is not erased
when system is switched off
 Memory capacity of ROM varies from 64 KB to
256 KB (1 Kilobyte = 1024 bytes)
 Basic Input Output System (BIOS) - Most
important program of ROM
 Activates the hardware in communicating with
the system and application software
 Types of ROM: Mask ROM, PROM, EPROM,
EEPROM and EAPROM
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 Mask ROM
◦ basic ROM chip
◦ information stored at the time of its manufacturing
◦ cannot be altered or erased later on
 Programmable Read Only Memory (PROM)
◦ information stored by programmers after its
manufacturing
◦ cannot be altered or erased later on
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 Erasable Programmable Read Only Memory
(EPROM)
◦ similar to PROM
◦ information can be erased and it can be
reprogrammed
 Electrically Erasable Programmable Read Only
Memory (EEPROM)
◦ similar to EPROM
◦ information can be erased
 Electrically Alterable Read Only Memory
(EAPROM)
◦ information stored in EAPROM can be altered later
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 Another important component of Memory
Unit
 Used to store the data and instructions
during the execution of programme
 Temporary and Volatile - is erased when
computer is switched off
 RAM is a read/write type of memory
 The memory capacity of RAM varies from 640
KB to several megabytes (1 Megabyte = 1024
KB) with different models of Pc
 Types of RAM: Dynamic and Static RAM
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 Static RAM (SRAM)
 memory retains its contents as long as power is
being supplied
 data is lost when power gets down
 SRAM chips use a matrix of 6-transistors and no
capacitors. Transistors do not require power to
prevent leakage, so SRAM need not have to be
refreshed on a regular basis.
 Because of the extra space in the matrix, SRAM
uses more chips than DRAM for the same amount
of storage space, thus making the manufacturing
costs higher. So SRAM is used as cache memory
and has very fast access.
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 It has long life
 There is no need to refresh
 Faster
 Used as cache memory
 Large size
 Expensive
 High power consumption
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 Dynamic RAM (DRAM)
 DRAM must be continually refreshed to maintain
the data
 done by placing the memory on a refresh circuit
that rewrites the data several hundred times per
second.
 used for most system memory because it is
cheap and small.
 All DRAMs are made up of memory cells which
are composed of one capacitor and one
transistor.
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 It has short data lifetime
 Need to be refreshed continuously
 Slower as compared to SRAM
 Used as RAM
 Lesser in size
 Less expensive
 Less power consumption
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 Complementary Metal Oxide Semiconductor
 Used to store the system configuration, date,
time and other important data
 When computer is switched on, BIOS matches
the information of CMOS with the peripheral
devices and displays error in case of
mismatching
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 To store Large files and databases
 Data and instructions from the secondary
storage are moved into the primary memory
for the CPU to access them
 also called auxiliary storage
 may be serial access or direct access devices
 Types: Magnetic Disks, Hard Disks, Magnetic
Tapes
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• Magnetic media
• Tape
• Disks
• Optical Media
• Compact Discs
• CD-R, WORM (Write Once, Read Many)
• CD-RW
• DVD
• DVD-R
• DVD-RW

• What is magnetic tape?
• Thin layer of material capable of storing a
magnetic signal
• Usually contains Iron Oxide
• Protected by
backing layer
called the
“Substrate”
Magnetic Tapes

• Cassette tapes
• VHS video tape
• Computer Tape
• 8-track
• DAT
Examples of Magnetic Tapes

• Inexpensive to store large amounts of information.
• A reel of tape could store as much as 225 MB in
the early 1980s.
• No other storage technology could compare to
the price of tape
• Sequential access – Slow access
• Tapes are highly susceptible to magnetic fields and
changes in temperature and humidity
• Tapes are also susceptible to physical damage
• Tape miss-feeds
• Magnetic particle instabilities
• Substrate deformation
Pros and Cons of Magnetic
Tapes

Magnetic Tape Uses Today
• Today, tape based systems are usually used
for backup purposes only.
• Tape still provides the most cost effective
method of storing larger amounts of backup
information
• Reliable as long as temperature is kept low
and humidity conditions are kept right.
• Although hard disks are increasingly used as a
backup mechanism, tapes are portable.
• Backups can be taken off-site for greater
risk prevention

• Today, most people use Hard Disks for
secondary storage
• The basic technology used in hard disks is
similar to that of magnetic tape
• Magnetic material is layered onto a high-
precision aluminum disk
• The disk head can move to any point on
the platter almost instantly compared to tape
• With tape, the head touches the tape. With
disks, the head never touches the platter
• Tape moves at approx 5 cm/s. Disk
platters move at up to 7500 cm/s (272
km/h!)
Hard Disks

• To increase capacity, a hard disk will usually
contain several platters
Hard Disks

• The heads never touch the platters, but they
are very close. This makes hard disks
susceptible to mechanical shock.
Hard Disks

• Each platter is broken up into tracks and
sectors
• Tracks are concentric circles on the disk
• Each track is broken up into a series of
sectors
Track (yellow
ring)
Sector (yellow
ring
between the
lines)
Storing information on a Hard Disks

Sectors and Blocks
• Sectors are further broken up into blocks
• A block is a fixed size unit of storage
• 512 bytes/block is most commonly used
• 1024 bytes/block is common with SCSI
disks
• 2048 bytes/block is used with CDs
• If the user stores onto the hard disk a file
which is larger than the block size, then
multiple blocks are used.

Blocks and Files
• If a file takes up multiple blocks, it is
necessary to keep track of which blocks
comprise that file
• Each block is assigned an address
• The location of a “file” is stored in what is
called a “File Allocation Table” (or FAT)
• When the hard disk is formatted, several
blocks are reserved so that the Operating
System can manage where files are stored on
the disk
• FATs are often used to keep track of the
filename and directory as well.

Filesystems
• Files are managed within a “filesystem”
• The filesystem defines how and where files
are stored within a hard disk (or partition)
• Common filesystems include:
• FAT16 (MSDOS)
• VFAT (Windows 95)
• FAT32 (Windows 98)
• NTFS (Windows NT)
• UFS (UNIX)
• ext2/ext3 (Linux)
• ISO9660 (CD Roms)

Filesystems
• When a disk is formatted, a filesystem is
placed on the disk
• The filesystem reserves space for the FAT.
• The remaining space is available for files
• When a file is to be saved into the filesystem,
the system looks for the best location to save
the file
• It is usually best to save the file in
contiguous blocks.
• If the disk is nearly full, it may not have
enough contiguous blocks to save the file.
In that case, the filesystem will place the file
wherever it can find space.

Deleting Files
• In order to make the delete operation faster,
when a file is deleted ONLY the FAT is updated.
• The actual data still remains on the disk
• This is why it MAY be possible to recover a
file which has been removed.
• The FAT is told that the blocks where the file
was stored are now available for writing
• The data will remain until another file is
stored in the same blocks.
• As more files are written on the disk, it is less
likely that a delete file can be recovered.

Hard Disk Fragmentation
• When files are deleted, their blocks are open
for writing.
• What if the next file which is written in the
same blocks is larger than the file which was
originally written there?
• The system has to break the file up into
pieces throughout the filesystem
• This is called fragmentation. It considerably
slows the access of files.
• Most modern Filesystems contain automatic
defragmentation utilities.

Hey! Wait a minute!
• When you purchase a hard disk, the
manufacturer indicates its capacity.
• When you format that hard disk, you’ll notice
that its capacity is less than what the
manufacturer told you.
• Formatting the disk requires space for the
FAT
• Manufacturers use decimal numbers to
represent number of bytes
• 1GB = 1,000,000,000 bytes
• There is a class action suit currently
filed against the major disk
manufacturers because of this

Floppy Disks
• Floppy disks are similar to hard disks
• Because the medium is “floppy”, the disks
cannot operate at the same speeds as a hard
disk.
• Floppy disks are older technology which
haven’t received a lot of attention since the late
1980s.
• Their capacity hasn’t increased much
• Many computers today do not even come with
floppy disk drives anymore.

Floppy Disk History
• The first floppy disks were 8 Inches in
diameter
• The held about 256 K
• The next generation were 5.25 inches in
diameter
• Could hold up to 1.44 MB
• The next generation were 3.5 inches in
diameter
• These disks are held within a hard plastic
case
• A spring loaded flap protects the disk from
dust and greasy fingers.
• Could hold up to 2.88 MB

Zip / Jaz Disks
• Similar to floppy disks
• They are removable
• Their platters are made of a hard material
• Can operate at higher speeds than
floppies.
• Newer standards hold more data
• Most people have outgrown floppy disks

Optical Media
• Optical disks are very much like hard disks
• Hard disks store information using magnetic
material
• Bits are stored by changing the magnetic
properties of the magnetic material
• Bits are read by picking up the tiny
magnetic field with a read head
• Optical disks store information as pits in a
physical medium
• A laser is used to determine if a pit is
present or not.

CD ROM
• CD Roms use the same technology as audio
Compact Discs.
• A master disc is created.
• Copies of the disc are created through a
pressing process
• The discs are aluminum sandwiched
between plastic
• CDs are single sided.
Label
Acrylic
Aluminum
Plastic

CD-R
• CD Roms must be pressed. They are read only
• CDR discs can be written once and read many
times
• CDRs are made out of aluminum and plastic,
but also contain a dye layer
• This dye is modified by a laser when the disc is
being written
• The laser heats up the dye and it becomes non-
reflective
Label
Acrylic
Aluminum
Plastic
Dye

Storing Information on a CD
• Because CDs were originally intended for
audio output, there is a single track of data
which spirals out from the center of the disc

CD-RW
• CD-RW is similar to CD-R
• The main difference is that the dye can be
made reflective again through an erase process
• In this way, CD-RW discs can be written many
times
• Too much erasing, and the dye starts to fade.
Label
Acrylic
Aluminum
Plastic
Dye

DVD – Digital Versatile Disk
• DVDs hold approx 7 times the information that
CDs do in the same amount of storage space
• DVDs come in 3 types
• Single Sided/Single Layer (4.7 GB)
• Single Sided/Double Layer (8.5 GB)
• Double Sided/Double Layer (17 GB)
• DVD uses a laser with a shorter wavelength so
the pits are smaller
• More pits can be stored on a DVD
• Narrower track

DVD – Multiple Layers
• Each side of a DVD can contain 2 layers,
doubling the amount of data that can be stored
• One layer is semi-transparent. The laser can be
focused “through” that layer onto the second
layer.
• If you took the track off of a single layer of a
DVD and stretched it out into a straight line, it
would be 7.5 miles long!
• If you did the same with a double layer, double
sided disc, the track would stretch to over 30
miles!

DVD-R and DVD-RW
• These two technologies are still relatively new
and still fairly expensive.
• DVD-R and DVD-RW use similar techniques as
CD-R and CD-RW
• A dye layer is used for data writing
• The dye layer in DVD-RW is erasable

 Referred to as a USB flash drive, data stick,
pen drive, keychain drive, thumb drive or
jump drive
 A portable drive that is often the size of
thumb that connects to the computer USB
port.
 only contains an integrated circuit memory
board capable of storing information and
has no movable parts.
 can be used like any drive on computer
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 very high speed semiconductor memory
which can speed up CPU
 acts as a buffer between the CPU and main
memory
 is used to hold those parts of data and
program which are most frequently used by
CPU
 The parts of data and programs are
transferred from disk to cache memory by
operating system, from where CPU can access
them.
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 Advantages
◦ Cache memory is faster than main memory.
◦ It consumes less access time as compared to main
memory.
◦ It stores the program that can be executed within a
short period of time.
◦ It stores data for temporary use.
 Disadvantages
◦ Cache memory has limited capacity.
◦ It is very expensive.
NMBA 018 45

 Presentation on:
◦ RAM, ROM and Cache Memory
◦ Types of Secondary Devices
NMBA 018 46

Conceptual framework storage devices (2)

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