Transcript 3-storage-devices4

Storage Devices
Bits & Bytes
Bits & Bytes
 Data consists of bits and bytes.
 A bit is the smallest unit of data. The word bit comes
from Binary Digit.
 A bit is represented by a 0 or 1. (The 1 actually
represents a small flow of current, and the 0
represents the absence of current).
 A byte is eight bits stuck together in a chunk, one
after the other, e.g.
01110001
is a byte
 Numbers that consist of just ‘0’s and ‘1’s are called
binary numbers.
Bits & Bytes
 As computers can only understand numbers, letters
are stored and transmitted as numbers.
 Each letter is stored and transmitted as a single
unique byte, called its ASCII code.
 ASCII stands for American Standard Code for
Information Interchange.
 The ASCII code for the letter A is 65, but this is
stored as a single byte of binary numbers…
01000001
 When you press the ‘A’ key on a keyboard, it is this
series of ‘0’s and ‘1’s that is sent to the computer.
The computer recognises the series of ‘0’s and ‘1’s
and displays the letter ‘A’.
Units of Data
 Units of data increase by a factor of a thousand.
(Actually 1024, but 1000 is used for simplicity)
 A bit is a 0 or 1
 A byte is eight bits
 A kilobyte is a thousand bytes (1000)
 A megabyte is a million bytes (1000,000)
 A gigabyte is a billion bytes (1,000,000,000)
 A terabyte is a trillion bytes (1,000,000,000,000)
Memory or Storage?
 Storage can be permanent or temporary.
 Data in permanent storage is retained after the
computer is switched off, whereas data in temporary
storage is lost when the computer is switched off.
 Temporary storage is called memory - this is RAM.
Typical RAM size is about 1 or 2Gb, but some of the
latest motherboards can hold 8, 16 or even 32Gb.
 Permanent storage is often called backing storage or
just storage - the most common form is a hard drive.
Hard drive capacity varies from around 60Gb to over
a terabyte. This presentation is about storage
devices, such as hard drives.
Storage Devices
 Backing storage is the permanent store of data on an
internal hard drive, external hard drive, CD or DVD,
memory stick, Zip disc, floppy disc, etc.
Hard Drive
Zip Disc
Floppy Disc
USB Memory Stick
CD, DVD
Hard Drive
 There is a hard drive in every computer – it is the
main storage device. The hard drive is the entire
device which contains the hard disc, but the two
terms are used to mean the same thing.
 A hard disc consists
of a collection of
metal platters that
have a magnetic
coating on them.
 The platters spin very quickly.
Hard Drive
Head
Sectors
Hard
Disc
 This is just one platter. A hard drive will have many
double-sided platters, each with a head which can
read the top and bottom of the disc.
How a Hard Drive Works
 When you open
a file, you tell
the operating
system to
access the file
from the disc.
 The operating
system sends
a message to
the hard drive
controller to
collect the file
from the disc.
How a Hard Drive Works
 The file may be
spread over
many different
‘sectors’
 The disc head
collects all the
pieces together
and puts them
together as
one file.
 The actual
speed of a hard
disc is many
times faster
than speed 3.
Hard Drive: Advantages
 Hard drives have a large storage capacity.
 They are cheap and reliable.
 Very quick at writing and reading data.
Hard Drive: Disadvantages
 They can fail without warning, resulting in the loss
of the data on the disc. (This shows the importance
of backing up).
 Much slower than RAM, although for most home
computing, this is not a problem.
USB Memory Stick
USB Memory Stick
 A USB memory stick (pen drive, or flash drive) is
the most common portable storage device.
 A memory stick consists of
a small circuit board full of
transistors. This is encased
in a plastic or metal case.
 The capacity varies from 256Mb to up to 64GB,
although 1Gb, 2Gb and 4Gb are the most common.
 They are very portable and many people have them
on their key-ring. They are available in many
different colours and designs, and have
become fashion accessories.
USB Memory Stick: Advantages
 Very cheap, reliable and long lasting: They are ‘solid
state’ which means there are no moving parts.
 Some of the better quality memory sticks are very
quick at transferring data.
 No special software is needed to use them. Can be
used to transfer data between different computers.
 Some now come with special software to protect the
data with a password.
USB Memory Stick: Disadvantages
 Can be easily lost or left around. Caps fall off.
 Cheaper models can be very slow at transferring data
 Capacity is limited to around 64Gb, but this is
increasing all the time.
Floppy Disc
 Floppy discs are almost completely obsolete,
although some computers still have a floppy drive.
 They have to be formatted
before use.
 The magnetic disc inside
is floppy, but the outer
casing is rigid.
 They have a capacity of
1.4Mb, although you can
get high capacity versions
which hold over 2Mb.
Floppy Disc: Advantages
 Cheap, lightweight and portable.
 Can be used for very small files.
 No special software needed to use them.
 Some computer technicians use them for rebooting
the computer when the operating system crashes.
Floppy Disc: Disadvantages
 Almost completely obsolete.
 Data capacity tiny by modern standards.
 Data transfer extremely slow.
 Few new computers these days have floppy drives.
Zip Disc
 A zip disc is like a large, chunky floppy disc.
 Zip discs have a hard
plastic outer casing with
a magnetic disc inside.
 Originally they stored 100Mb, but later, 250Mb and
750Mb versions were developed. People mostly
preferred rewritable CDs and USB memory sticks.
Zip Disc: Advantages
 Light and portable.
 Greater capacity and much quicker than a floppy.
Some of the later discs stored as much as 750Mb.
Zip Disc: Disadvantages
 Almost completely obsolete.
 Data capacity small compared to USB memory
sticks, or DVDs.
 Data transfer very slow, compared to USB memory
sticks.
 No new computers have zip drives, although they
can still be bought and installed.
CD
CD
 A CD (compact disc) is an optical medium. This
means that data is written to it, and read from it
using a laser.
 CDs have a capacity
of about 700Mb.
 There are three main varieties: CD-Rom (CD-Read
Only Memory), CD-R (CD-Writable) and CD-RW (CDRewritable)
CD-Rom
 CD-Roms already have the data written on them.
They cannot be edited or erased.
 Software and music is sold on discs like this, so that
the user cannot accidentally erase the disc.
CD-R
 CD-Rs can be written to once only. After that, the
data cannot be erased or edited, but can be read
many times. These discs are also known as WORM
discs (Write Once Read Many)
CD-RW
 The data on a CD-RW can be changed many times.
 This makes it suitable as a backup medium, as the
disc can be used again and again.
CD: Advantages
 A useful storage capacity which fits the purpose 700Mb fits most software and all music albums.
 CD-Roms and CD-R discs cannot be accidentally
erased.
 Read speeds are very fast.
 Every computer has a drive which will read them.
 External CD-Drives are available.
CD: Disadvantages
 Write speeds are quite slow. Need ‘burning’ software
to write to them.
 Can be scratched easily.
DVD
 A DVD (Digital Versatile Disc) is an optical disc like
the CD, but with much higher capacity. There are
many different types, but the standard DVD can
store about 4.7Gb.
 There are various double sided, and even dual
layered DVDs with increased capacity.
 DVDs can also be writable or re-writable.
 Blu-ray discs (BD) are the latest optical disc. They
have a capacity of around 50Gb. Blu-ray discs are
used by businesses for backing up, and Blu-ray
drives are available on personal computers.
DVD: Advantages
 DVDs have a high storage capacity, much higher
than a CD.
 DVD drives will read CDs so you only need one drive
for both media.
 Data transfer is fast, making them ideal for backing
up.
 Capacity is ideal for storing normal definition films.
DVD: Disadvantages
 Too many different types makes it difficult for users
to know which disc to buy for their system.
 Can be easily scratched like the CD.
 Not enough storage capacity for high definition films.
Magnetic Tape
 Magnetic tape consists of a plastic tape covered in a
magnetic coating. There are many different
standards of magnetic tape…
The older
reel-to-reel tape
Cartridge tapes have
a capacity of up to 800Gb
Magnetic Tape
 Magnetic tape is a serial access device (also called
sequential access if the files are saved in a logical
order). This means that if you want to access a piece
of data that is stored in the middle of the tape, you
cannot do it directly, you have to wind the tape all the
way to the middle which takes time.
 Magnetic tape is mainly used for archiving and
backing up data that is already stored on hard drives,
so serial access is not a major problem.
 Magnetic tape has a long storage life of between 30
and 100 years.
Magnetic Tape: Advantages
 Large storage capacity, up to 800Gb.
 Cost per Gb storage is low.
 Magnetic tapes have a long storage life of between
30 and 100 years.
Magnetic Tape: Disadvantages
 Can only be used for archiving or backing up, due to
serial access.
 Gradually being replaced by hard drives, due to their
low price, increased capacity and speed of data
transfer.
Direct Access v Serial Access
 Hard drives, flash drives and almost all other storage
devices are direct access. This means that data
stored anywhere on the device can be accessed
in an equally short space of time.
 Magnetic tape is a serial access or sequential access
device. This means that you have to wind through it
to reach a piece of data that you require.
 Direct access is much quicker and more efficient.
Serial access devices, such as magnetic tapes are
only used for backup and archive purposes.
Keyword Activity 1
Drag the missing words to the correct place in the
sentences…
Keyword Activity 2
Drag the missing words to the correct place in the
sentences…
Wordsearch
Crossword
Quiz
Objectives
After viewing this presentation, you should…
 Understand bit, byte, kilobyte, megabyte and
gigabyte.
 Be able to explain the difference between memory
and storage.
 Be able to explain the difference between RAM and
ROM.
 Be able to explain the difference between direct
access and serial access.
 Be able to list a variety of backing storage devices,
and to know their approximate capacity, their uses,
advantages and disadvantages.
End of Show
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The Maths of Binary Numbers
 This is for extension only, treat with caution…
Binary Numbers
 Computers can only understand binary numbers, not
our ordinary numbers, called base 10.
 We can convert between ordinary base 10 numbers
and binary. Each place in a binary number stands for
a power of 2…
 The ‘power’ is the number of 2’s multiplied together…
2
2 =2x2=4
3
2 =2x2x2=8
power
4
2 = 2 x 2 x 2 x 2 = 16
 By convention…
0
2 =1
and
1
2
=2
To Convert from Binary to Base 10
 Each place in a binary number stands for a power
of 2. A ‘1’ adds the correct power of 2 to the number.
A ‘0’ adds nothing to the number, e.g.
01100001
7
6
5
4
3
2
1
0
2
2
2
2
2
2
2
2
0
64
32
0
0
0
0
1
Decimal Value = 0 + 64 + 32 + 0 + 0 + 0 + 0 + 1
Decimal Value = 97
 Example: Change 11001010 into base 10…
 Answer: You should get 128 + 64 + 8 + 2 = 202
To Convert from Base 10 to Binary
 We break the base 10 number down into powers of
two, starting with 128, then 64, then 32…etc.
 Example: Change 146 into binary…
146 = 128 + 0 + 0 + 16 + 0 + 0 + 2 + 0
7
6
5
4
3
2
2
2
2
2
146 = 1
0
0
1
0
2
2
0
1
0
2
2
1
0
 Example: Change 67 into binary…
 Answer: You should get 0 0 0 1 0 0 0 1 1