Session Objectives#7
COULD select and justify suitable storage devices/media for given situations
SHOULD explain the need for secondary storage and the use of buffers and interrupts
in the transfer of data.
MUST describe common secondary storage technologies
Create coded messages converting decimal to binary.
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Secondary Storage
We know that primary storage consists of both RAM and ROM.
The RAM (Memory) is volatile and is wiped when the power is
turned off.
ROM is NOT volatile BUT IS read-only, therefore it can only
store preloaded instructions (the boot strap program, for
example) and nothing else can be saved on to it.
We don’t want to have to rewrite programs and data each time we
want to process something, therefore a computer needs the
ability to store data when the power is turned off
SECONDARY STORAGE is the additional storage facility added to a
computer system to store data and programs when the power is
switched off.
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Storage Media
These are the materials which hold the data whereas a storage
device is the hardware which reads or writes on to the media.
THINK of media as a CD and the device is the CD player.
STORAGE MEDIA is grouped into 3 types:
1. Magnetic Hard Disk
2. Optical Disk
3. Flash Memory (Solid State Memory)
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Storage Investigation
Use the internet to research and investigate the characteristics
of the different types of storage media and then attempt to
answer the following questions:
1. State which type of secondary storage is most appropriate for
use in the following situations. Give reasons:
a) Storing tracks on a MP3 player
b) Transferring work from a school computer to home
c) Distributing a movie
d) As the storage area on a school network.
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Storage Media
Magnetic Hard Disk
The magnetic hard disk is a magnetised rigid plate or stack of
plates with heads to read the data as the platter spins around.
It is a reliable and cost effective way of providing high
capacity storage at a low cost. The devices can be internal HDDs
(Hard Disk Drives) or portable external HDDs which can be used
to transfer or back up large amounts of data.
Optical Disk
The CD and DVD use light from lasers to detect reflections from
the data area on the CD/DVD surface. The ROM version is
obviously read only, whereas RW (rewritable) can have data
written (or burnt) onto them. They are low capacity ( 700MB for
CD & 4.7 GB for DVD) and cheap BUT they are also quite slow at
transferring data and less reliable as they are prone to damage,
either by direct sun light or physical damage such as scratches.
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Storage Media
Flash Memory (Solid State Memory)
Solid state flash memory has much quicker access times than
magnetic disks and are more reliable, as they have no moving
parts which means they are less prone to damage through knocked
or dropped. It is however comparatively more expensive than
magnetic storage and are therefore only used in smaller, more
affordable sizes, such as flash memory sticks or as SSHDs for
notebooks and slim line laptops (This is also because they can
be smaller in size as they do not need room for a rotating
disk(s).
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Data Transfer
Data transferred from memory to other devices, printers, hard
disks etc through the use of buffers and interputs.
Buffers are used to store data sent from main memory where it
waits until used by the device (either written on disk or
printed, for example). This frees up the processor to carry on
with other processes. Buffers are only quite small so more data
may need to be fetched from memory to finish off the job. This
is done through the use of interrupts.
Interrupts are requests sent from a buffer to the processor
asking for more data to be sent to complete a particular job.
The processor must then stop what it is doing and complete the
request.
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Data Transfer
Processor
Buffer
DATA
Write
Read
Devices, HDD,
printers, mic,
web cam
INTERRUPT
Buffers are generally regions of physical memory found in peripheral devices but can also
be allocated to RAM by software, creating virtual data buffers.
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