Unit 1 System architecture, memory and storage

4

GCSE OCR Computer Science J276

Storage

Unit 1 System architecture, memory and storage

•  Discuss the need for secondary storage including optical, magnetic and solid state storage

•  Evaluate suitable storage devices and storage media for a given application using the following characteristics: §  capacity, §  speed, §  portability, §  durability, §  reliability, §  cost

Objectives

Storage Unit 1 Systems architecture, memory and storage

Secondary storage •  What do you already know about secondary storage

and its role within computer systems?

MAIN MEMORY

SECONDARY STORAGE

CENTRAL PROCESSING UNIT (CPU)

INPUT DEVICES

OUTPUT DEVICES


Secondary storage •  Are secondary storage devices volatile or non-

volatile?

•  Why do we need secondary storage?

•  Why are there so many different types of secondary storage?


Uses of secondary storage •  Secondary storage is non-volatile – it keeps its

content when the power is switched off

•  It has many different uses – for example: •  Programs and data are stored on hard disk

•  CDs may be used to distribute software, music, e-books etc.

•  Memory sticks may be used to transport data from one place to another

•  Magnetic tape or external hard drives may be used for backup

•  What other uses are there for secondary storage devices?


Storage types •  Primary storage

•  RAM and ROM

•  Secondary storage •  Hard Disk Drive (HDD) •  Solid State Drive (SSD)

•  Offline secondary storage •  Compact Disc (CD) or Digital Versatile Disc (DVD) or BluRay •  USB Flash memory •  Removable HDD •  Magnetic tape


Storage methods •  Magnetic: Mechanical parts move over the disks

surface to read and write data magnetically

•  Optical: Lasers read and write data using light

•  Solid State: Data is recorded onto solid memory chips without any moving parts


Magnetic disks Basic features:

•  Disk contains concentric circles called tracks

•  Each track is divided into sectors

•  Disk heads mounted on mechanical arms read and write the data


Inside a hard disk


Magnetic storage: hard drives •  Fixed hard drives are built into virtually all

PCs and laptops •  They have a very large storage capacity, up to 6TB or more

•  Portable hard drives can be connected to a computer via a USB port

•  They are used for backing up or transporting data

•  Some portable music players have tiny hard drives no bigger than a small coin

•  They can store several GB of data


Magnetic storage •  Advantages:

•  Cheap, vast storage capability, fast write speed

•  Disadvantages: •  Lots of mechanical parts, durability an issue, sealed unit due

to disk head and platter precision and not very portable

•  Uses: •  Personal computers, storage of vast quantities of data

•  Capacity: •  500GB - 6TB or greater


Optical storage •  Basic features:

•  Data is stored as pits and lands burnt into a spiral track circulating outwards from the centre

•  A laser beam passes over the pits and lands the level of reflection is measured

•  From this signal, 0s and 1s can be derived from reflections or no reflections


How CDs work


Optical storage •  Advantages:

•  Cheap, very easily portable, takes up little space physically •  Disadvantages:

•  Less storage capacity compared to other types •  Easily damaged / scratched, requires a CD reader •  Slow write speeds

•  Uses: •  Songs, videos and other multi-media storage, backup and

archiving of data •  Capacity:

•  CD-ROM – up to 720Mb •  DVD – up to 8.4Gb •  Blu-Ray – up to 50Gb


CDs, DVDs and BluRay •  Why are the capacities of these discs different given

they are all the same physical size? •  Microscopic view of the surface of a CD Rom


Pit size and laser wavelength


Solid State Drives (SSD) •  Basic features:

•  Solid-state disks use non-volatile flash memory to store information

•  Very fast burst read/write speeds due to data being physically close and easy to recover

•  No mechanical or moving parts


How flash memory works •  Large electric current used to force electrons

through a barrier and trap them on the other side

•  They remain on the other side until “flashed” with a new current, hence the name

•  Trapped (charged) or not trapped = 0 or 1

Electrons trapped in this layer

Electrons detected here

Current applied and electrons forced through barrier

Barrier


Advantages /disadvantages of SSDs

•  Uses: •  Portable computers •  Military usage

•  Capacity: •  100GB – 16TB

•  Advantages: •  Highly durable, no moving parts, very fast read/write speeds,

no noisy fan, faster start up times •  Disadvantages:

•  Expensive at present, less storage capacity / physical size than traditional hard disks


Flash memory •  Low cost, portable, no moving parts, durable

•  This makes them ideal for a range of offline devices:

•  Cameras

•  Mobile phones

•  USB memory sticks


Worksheet 4 •  Complete Task 1 on the worksheet


Data capacity •  Different storage devices have a range of

storage capabilities

•  A higher capacity will allow greater data storage •  How do you decide which storage device to use?

•  How do you calculate storage requirements?


Data capacity •  When you know how much data you need to store,

you can decide which storage device(s) would be most appropriate

•  If you have 300GB of data to store, is optical storage appropriate?

•  If not, why not? What could you use instead?

•  Why is this particular device appropriate?


Calculating data capacity •  Knowing the capacity required will enable us to

make an informed decision as to which device to use

•  If we have 500 files that are 300MB each, we need a total of 150GB

•  Which storage devices are suitable for this amount of data?


Worksheet 4 •  Complete Task 2 and 3 on the worksheet


Plenary •  Three different types of storage device have

been discussed

•  Each has its own advantages for a particular application

•  Technology is constantly moving forward, resulting in higher capacities, speeds and durability

•  Prices are continually falling

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Unit 1 System architecture, memory and storage

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