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CIE IGCSE Computer Science Your notes
3.3 Data Storage
Contents
Primary Storage
Secondary Storage
Virtual Memory
Cloud Storage
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Primary Storage
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Primary Storage
Primary storage is directly accessible by the CPU , it includes RAM, Cache, and ROM
The purpose of Random Access Memory (RAM) is to store data, instructions and software (including
parts of the OS) currently in use
RAM is also called main memory
RAM has faster read/write speeds than secondary storage
RAM is volatile which means it is temporary memory and all data will be lost when the computer is
switched off
The larger the main memory, the more data the computer access quickly, which can improve the
computer's performance
A standard desktop computer has between 4GB and 16GB of RAM
The parts of the operating system currently in use will be stored in RAM
The purpose of cache is to store frequently/recently used instructions and data
Cache has faster read/write speeds than RAM
There are different levels of cache
Level 1 cache is situated within the CPU
Level 2 and 3 cache are situated on the motherboard
If you increase the amount of cache you can store more frequently used data and increase the
performance of your computer
If you have too much cache it will decrease the data access speeds because the data
requested will take longer to find
Cache is volatile and all data will be lost when the computer is switched off
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Your notes
A diagram showing the parts of a computer system which are classed as primary storage
The role of Read-only memory (ROM) is to store the computer’s boot-up sequence
The boot-up sequence is a set of instructions the computer executes when then computer is
initially switched on
This sequence will run a series of checks to ensure all of the system hardware is working
It will then load the operating system which will move from secondary storage to RAM
ROM is non-volatile which means data will be stored when the computer is switched off
ROM is read-only which means data cannot be written to ROM
The difference between ROM and RAM
RAM ROM
Volatile Non-volatile
Can read and write data to RAM Read-only
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Exam Tip
Your notes
Make sure you're specific about which component you're referring to in exam questions and give
the proper name e.g. RAM rather than memory unit
Worked example
Describe what is meant by main memory and how it is used in the Von Neumann model for a computer
system
[3]
Any 3 of:
Memory is RAM [1]
It is primary storage [1]
It is volatile memory [1]
It holds currently in use data/instructions [1]
It is directly accessed by the CPU [1]
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Secondary Storage
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Secondary Storage
Secondary storage is not directly accessed by the CPU
It is used for permanent storage of data
Secondary storage is non-volatile. This means data is stored even when power is lost to the device
Secondary storage is needed as primary storage (such as RAM ) is volatile and will not retain data when
the computer is turned off
Unlike ROM, secondary storage can be read from and written to. ROM is read only
Common examples of secondary storage include hard disk drives (HDDs), solid-state drives (SSDs),
external hard drives, USB flash drives, CDs, DVDs, and Blu-ray discs. These can be used to store data
to transfer it to another computer
Data access speeds for secondary storage devices are typically slower than primary storage (such as
RAM)
Secondary storage devices often have large capacity (can store large amounts of data)
Software such as the operating system is saved permanently on secondary storage. The instructions
and data that makes up the software then moves to RAM when it is in use
Secondary storage devices can be internal (inside the computer) and external (outside the computer)
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Your notes
Diagram showing different internal and external secondary storage devices
Comparison of primary and secondary storage
Primary storage Secondary storage
Volatile / temporary Non-volatile /permanent
RAM and cache can be read from and written to
Secondary storage can be read from and written to
ROM is read-only
Can be directly accessed by the CPU Cannot be directly accessed by the CPU
Smaller capacity Larger capacity
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Data access speeds faster than secondary storage Data access speeds slower than primary storage
Stores data and instructions Stores data and instructions Your notes
Worked example
Which of the following is an example of secondary storage?
A Random access memory
B Read only memory
C Solid State Drive
D Blu-ray disc
C and D [1]
Random access memory and read only memory are both primary storage and can be accessed
directly by the CPU. Solid-state drives and blu-ray discs are classed as secondary storage. Data
can be permanently stored on them but they cannot be accessed directly by the CPU
Exam Tip
When referring to a USB memory stick, don't refer to it as USB as this could be anything that is
plugged in with USB and not necessarily a storage device
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Types of secondary storage
There are three types of secondary storage - magnetic, solid state and optical Your notes
Exam Tip
You need to be able describe the operation of each type of secondary storage. You may also
need to recommend a type of storage for a given scenario and justify your choice
Magnetic
Magnetic hard disk drives have been the main type of internal secondary storage in personal
computers for many years however solid state drives are increasingly popular due to their durability,
low power consumption and high read/write speeds
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
A diagram showing the structure of a magnetic hard disk
Each platter is divided by concentric circles creating several tracks and wedge shaped sectors. Where
they intersect is a track sector
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Your notes
A diagram showing how each platter is divided by tracks and sectors
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
Solid State
Solid State secondary storage is made of transistors that are arranged in a grid layout
It uses NAND and NOR gates in electrical circuits to persistently control the flow of electrons
Current flowing is 1, no current is 0 - which is how electrical flow can represent binary values
A NAND gate will take in two inputs and produce an output (1/electrical current) as long as both inputs
are not 1
The truth table truth table for a NAND gate
InputA InputB Output
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0 0 1 Your notes
0 1 1
1 0 1
1 1 0
A NOR gate will produce an output (1) only when both inputs are 0
The truth table for a NOR gate
InputA InputB Output
0 0 1
0 1 0
1 0 0
1 1 0
Optical
Optical devices include CDs, DVDs and Blu-rays
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
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
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Your notes
A diagram showing how data is burnt on to optical discs to create lands and pits which can then be used
In CD-RW the chemical composition of the disk is changed to represent 0s and 1s. This change can be
overwritten
Exam Tip
Make sure you have a greater understanding of the operation of the different types of storage
Worked example
Describe how a magnetic storage device stores data
[6]
Any 6 of:
Storage device has platters [1]
Platters/disk divided into tracks [1]
Storage platter/disk is spun [1]
Has a read/write arm that moves across storage media [1]
Read/writes data using electromagnets [1]
Uses magnetic fields to control magnetic dots of data [1]
Magnetic field determines binary value [1]
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Virtual Memory
Your notes
Virtual Memory
The purpose of RAM is to store current data and program instructions in use
The amount of physical RAM available is limited. The average modern desktop computer has between
4GB and 16GB of RAM. Some programs are larger than this
Virtual memory is part of secondary storage which acts as RAM when RAM is full
It prevents the computer system crashing when RAM is full
Data that needs to be accessed by the CPU directly is switched by the operating system from virtual
memory into RAM and an inactive page from RAM replaces it in virtual memory. This is called page
switching
A diagram showing page switching between RAM and virtual memory
The table describes the steps that are taken when a program is opened on a computer
Step Description
1 The memory manager software built into the Operating System
checks to see if there is sufficient space in RAM to move the new
program’s instructions into RAM, so that they can be directly
accessed by the CPU
2 If RAM is full, the memory manager will move out the most inactive
pages of data and put them in virtual memory until needed
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3 The new program instructions are moved into RAM
Your notes
Worked example
Sandy is playing an open world role playing computer game set in a large 3D environment. This requires
the use of virtual memory. Explain why
[3]
To extend the RAM capacity [1]
To stop the computer game from crashing when the physical RAM is full [1]
To allow the computer to process the large amount of data required [1]
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Cloud Storage
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Cloud Storage
Cloud storage is the concept of storing data on remote servers, that can be accessed via the internet,
instead of on a local storage device (such as an SSD or HDD)
The physical servers, where the data is stored, are owned and managed by a hosting company
Cloud storage warehouses often have thousands of servers containing harddrives
The hosting company provides services such backing up the data, managing access to the data as
well as general security
The data can be accessed at any time, on any device, as long as there is an internet connection
The data is accessible using a web browser or dedicated software that connects to the cloud storage
service
There is potentially limitless storage, although it does need to be paid for
Online storage means it is easy to share data and collaborate with others
If the internet connection is slow/lost the access to your data is affected
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Your notes
A diagram showing different devices connecting to remote servers
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Advantages of cloud storage
Cloud storage offers several advantages over storing data on a local secondary storage device Your notes
Benefit Explanation
Accessibility
Data can be accessed from anywhere
Data can be accessed by anyone with the relevant
permissions, making it quick to share files and collaborate
with others
Data can be accessed on any device with an internet
connection
Scalability Cloud storage providers allow customers to increase or
decrease their storage capacity as needed
Reliability
Cloud storage providers often use multiple servers to store
and backup data, reducing the risk of data loss due to
hardware failure
Security
Cloud storage providers offer advanced security features,
such as data encryption and multi-factor authentication multi-
factor authentication, to protect user data from unauthorised
access
There is no need to hire specialist staff as IT services being
provided by the cloud storage provider
You also do not need to purchase local secondary storage devices
One cloud storage centre is more environmentally friendly than millions of individual servers
There are some drawbacks to using cloud storage
If the user has a slow or unreliable internet connection they would have problems accessing their
files
If you are storing a lot of data it can be expensive
You have to pay the cloud storage provider
You may need to commit to an annual subscription
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You may have to pay for high download and upload data transfer limits
Data being sent over the internet has the potential to be intercepted
Security is managed by the cloud storage provider which means the user does not have complete Your notes
control over it but is still legally responsible for ensuring any personal and sensitive data is secure
Worked example
A school currently stores all of their data on local servers. They are considering upgrading their system
to utilise cloud storage.
Explain two disadvantages to the school of storing their data in the cloud.
[4]
You need a constant internet connection [1]
if the internet connection failed, teachers would not be able to access lesson files [1]
if the internet connection failed, teachers would not be able to access student data/take
electronic registers [1]
if the internet connection failed, students would not be able to access their work/coursework [1]
Reliant on third party to carry out security procedures [1]
but the school is still legally responsible if data is accessed by unauthorised people [1]
Data stored on the cloud could be hacked or intercepted [1]
Explain two advantages to the school of storing their data in the cloud.
[4]
Additional storage can be purchased if needed [1]
The cloud hosting company provides back up [1]
The cloud hosting company provides security [1]
Data can be accessed from anywhere and from any device provided there is an internet
connection [1]
So teachers would be able to access data from home [1]
So students would be able to access work from home [1]
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