Secondary Storage (Cambridge (CIE) O Level Computer Science)

Revision Note

James Woodhouse

Written by: James Woodhouse

Reviewed by: Lucy Kirkham

Secondary Storage

  • Storage devices are non-volatile secondary storage, that retain digital data within a computer system

  • They provide a means of storing, accessing, and retrieving data, which can include software applications, documents, images, videos, and more

  • There are 3 types of storage:

    • Magnetic

    • Solid-state (flash memory)

    • Optical

  • Computer systems need both primary and secondary storage to operate

  • Both types of storage play a crucial role in the operation of a computer system

  • A quick comparison of primary and secondary storage shows:

Primary

Secondary

Volatile (with the exception of ROM)

Non-volatile

Small capacity

Large capacity

Why do you need secondary storage?

  • A computer needs secondary storage for long term storage of programs and data that are currently not in use 

  • Secondary storage is needed as ROM is read only and RAM is volatile

  • Secondary storage holds the programs and data whilst the computer is turned off (non-volatile)

  • Performance of secondary storage is slower than primary storage but capacity is much higher which makes it perfect for backup & archive of data files

What are the characteristics of secondary storage?

  • Capacity - What is the maximum amount of data that can be stored?

  • Speed - How fast can data be read from and written to? (R/W)

  • Cost - How much does it cost?

  • Portability - How easy is it to move around? What is the physical size? Weight?

  • Durability - How robust is the storage?

  • Reliability - What is the probability that the device will consistently perform its function?

Worked Example

A games console has secondary storage.

State, using an example, why the games console needs secondary storage [2]

How to answer this question

  • They keyword is 'why', why does a games console need secondary storage? give an example, do not just describe what secondary storage is!

Answer

  • To store data once the power is turned off / permanently // for non-volatile storage

  • Examples could include:

    • Storing games

    • Save games

    • User data/profiles

    • Downloaded content/applications/updates

Magnetic Storage

What is magnetic storage?

  • Magnetic storage is a type of non-volatile media that uses magnets (polarity) to store binary 0s and 1s

  • A magnetic hard disk is made up of several metal discs coated with a magnetic material

    • These are called platters

    • Iron particles on each platter are magnetised to represent a 0 or 1

Diagram of a hard disk showing labeled components: read head, platter, track, and sector. The read head hovers above the platter and sectors are segments of the platter.
  • Each platter is divided by concentric circles creating several tracks and wedge shaped sectors

  • Where they intersect is a track sector

Diagram of a hard drive showing platters, tracks, sectors, and disk read-and-write heads. Labels explain components, including magnetic layers and their role in data storage.
  • The hard drive spins the metal disk(s) at a high speed (typically around 5400-7200 RPM) using a motor

  • A read/write arm, controlled by an actuator, moves the head over the surface of the disc to the location of the data

  • The data is read/written using electromagnets

Advantages

Disadvantages

Capacity - High storage

Durability - Moving parts can get damaged if dropped

Cost - Low per gigabyte

Portability - Heavy & bulky making them less convenient for transport

Speed - Moderate read/write access 

Reliability - Prone to mechanical failure

 

Noise - Loud (spinning disks)

Solid-state (Flash Memory)

What is solid state (flash memory)?

  • Solid-state (flash memory) storage is a type of non-volatile media that uses electronic circuits to store binary 0s and 1s

  • Examples of solid state storage devices include:

    • Solid state drives (SSD) - replacing magnetic hard drives as a computers primary secondary storage device as capacity increases and cost decreases

    • USB flash memory - most common form of portable storage device

  • It uses NAND and NOR gates in electrical circuits to persistently control the flow of electrons

Diagram showing sections of a floating gate transistor: control gate (orange), oxide layers (gray) above and below floating gate (orange), and oxide layer with electrons (blue dots).
  • Solid-state (flash memory) is memory made up of tiny cells that can contain one bit of data (1 or 0)

  • Each cell contains a transistor that acts as a switch that can be turned on or off

  • The transistor contains two main parts:

    • Control gate - top layer of transistor, connects to circuit and controls if current can flow through the transistor

    • Floating gate - can hold a charge (like a tiny rechargeable battery) and is sandwiched between two layers of insulating material (Oxide)

  • To store data a charge is placed on the floating gate

    • A high voltage is applied to the control gate, which allows electrons to be pushed through the oxide layer and onto the floating gate

  • To remove data, a high voltage charge is applied in the opposite direction, pulling the electrons off the floating gate

Advantages

Disadvantages

Capacity - Medium/high storage

Cost - Very high per gigabyte

Speed - Very fast read/write access

Reliability - Limited read/write cycles

Durability - No moving parts

 

Portability - Small and no moving parts

 

Noise - Silent 

 

Optical Storage

What is optical storage?

  • Optical storage is a type of non-volatile media that uses lasers to burn the surface of a disk, creating pits and lands suitable for storing binary 0s and 1s

  • Examples of optical storage include:

    • Blu-rays have the largest capacity

    • CDs have the lowest capacity

    • CD-R are read-only (you cannot save data on to them)

    • CD-RW can be written to and read from

    • DVD-RW can be written to and read from

A rectangular waveform diagram with labeled sections 'LAND' and 'PIT'. Below the waveform is a binary sequence: 0001000000000010000100000000100000000001000.
  • All optical devices work by shining a laser at the disk and processing the reflection

  • An arm moves the laser across the surface of the disk

  • In CD-Rs a laser burns the data, permanently on to the disk, by creating pits and lands

  • The laser is also used to read the data from the pits and lands

  • When the laser light hits the point where the pit changes into a land or vice versa the light scatters and is not reflected back as well.

  • This is captured by a sensor and can be interpreted as a change in the binary value

Advantages

Disadvantages

Cost - Very low per gigabyte

Capacity - Very low

Durability - No moving parts

Speed - Very slow read/write access

Portability - Small and no moving parts

Reliability - Prone to scratches

Noise - Silent 

 

Worked Example

Zarmeen has purchased a new tablet computer. The tablet has an internal secondary storage device.

The storage device is a solid state device.

  1. Give three benefits of the tablet having a solid state device instead of a magnetic device [3]

  2. Give two drawbacks of the tablet having a solid state device instead of a magnetic device [2]

Answers

  • Benefits

    • Faster access / read/write speeds

    • Smaller in physical size

    • More durable // no moving parts

  • Drawbacks

    • More expensive (per gigabyte)

    • Smaller capacity (usually)

    • Limited number of read/write cycles

Guidance

  • Portable is not enough on it's own

  • No moving parts is not enough on it's own

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James Woodhouse

Author: James Woodhouse

Expertise: Computer Science

James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.

Lucy Kirkham

Author: Lucy Kirkham

Expertise: Head of STEM

Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.