Topic 3: Hardware - 3.3
Data storage
Storage Devices - additional notes
https://www.computerscience.gcse.guru/theory/storage-devices#flashmemory
quiz
https://www.computerscience.gcse.guru/quiz/input-output-or-storage
Why do you need primary storage?
● A computer needs primary storage because access times are considerably
faster than secondary
● This means the time taken to complete operations such as the Fetch-Execute
Cycle is dramatically reduced
● Primary storage holds the data and instructions that the CPU needs to access
whilst the computer is turned on
● Due to the fast access times, primary storage is used as short-term, working
memory, in hardware that is directly connected to the CPU such as RAM, and
components that reside inside the CPU such as Cache and Registers
● Performance of primary storage means a much higher cost which limits the
amount that is used
● For example, RAM is commonly purchased in 16 or 32 gigabytes whereas
secondary storage such as a hard drive is in terabytes
What is ROM?
● ROM (Read Only Memory) is primary storage that holds the first
instructions a computer needs to start up (Bootstrap)
● ROM contains the BIOS (Basic Input Output System)
● ROM is a small memory chip located on the computer's motherboard
● ROM is fast memory, much faster than secondary storage but slower
than RAM
● ROM is non-volatile which means the contents of ROM are not lost
when the power is turned off
● ROM is read only which means data can only be read from
● In comparison to RAM, it has a much smaller capacity
What is RAM ?
● RAM (Random Access Memory) is primary storage that is directly
connected to the CPU and holds the data and instructions that are
currently in use
● RAM is volatile which means the contents of RAM are lost when the power
is turned off
● For the CPU to access the data and instructions they must be copied from
secondary storage
● RAM is very fast working memory, much faster than secondary storage
● RAM is read/write which means data can be read from and written to
● In comparison to ROM, it has a much larger capacity
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
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?
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
Memory and Storage
What Are HDDs?
● Hard Disk Drives (HDDs) are one of the most common
methods for storing data on computers.
● Data is stored in digital format on the magnetic
surfaces of spinning disks called platters.
● Platters are typically made from aluminium, glass, or
ceramic materials.
What Are HDDs?
HARD DISK DRIVE
Key Components and How They Work
● An HDD contains multiple platters that spin very fast—
at about 7000 times per second (or 7000 RPMRevolution Per Minute).
● Read-write heads (made of electromagnets) read data
from or write data to the platters.
● Each platter has two surfaces for data storage (top and
bottom), and multiple read-write heads access all
surfaces.
● Heads move quickly: from the center to the edge of the
disk (and back) about 50 times per second.
Data Organization on Platters
● Data is stored in sectors and tracks on the platter
surfaces.
● A sector on a track holds a fixed number of bytes
(e.g., typically 512 bytes or 4KB in modern HDDs).
● When storing a file, the HDD allocates the required
number of sectors—but these sectors may not be
adjacent (leading to fragmentation over time).
Access Speed and Limitations
● HDDs have slower data access compared to RAM due to
mechanical parts.
● Applications often require read-write heads to constantly seek
specific blocks of data, causing many head movements.
● Latency is the delay as a specific data block on a track
rotates under the read-write head—makes access even
slower.
● Users notice this as "Please wait" messages or "not
responding" errors.
Fragmentation and Defragmentation
● Over time, HDDs fill with deletions and edits, causing
fragmentation: sectors for files become scattered and nonadjacent.
● Fragmentation worsens performance—data access takes
longer and longer.
● Defragmentation software fixes this by reorganizing ("tidying
up") sectors into contiguous blocks.
● https://www.youtube.com/watch?v=AtRIOUZuI2c
Data Reading Process
● Data in a sector is read sequentially (in
order).
● But accessing the sector itself is direct: readwrite head moves to the right track and waits
for rotation (random access within limits).
Removable HDDs
● Removable HDDs are external HDDs connected via
USB ports.
● Uses: Back-up storage or transferring files between
computers.
Solid State Drives (SSDs)
Solid State Drives (SSDs)
solve HDD latency issues
with no moving parts,
making data access
uniform and fast.
Key Advantages Over HDDs
● No latency issues (unlike
HDDs) - all data retrieved at
same fast rate.
● No moving parts eliminates
mechanical delays and
failures.
● Data stored electronically
using NAND/NOR chips, not
magnetic surfaces.
How SSDs Store Data
●
Solid State Drives (SSDs) store data using millions of tiny electronic components called
transistors.
●
Each transistor stores a binary value (0 or 1).
●
Charged transistor = 1, uncharged transistor = 0.
●
Each transistor has two parts: a control gate and a floating gate.
●
When voltage is applied, electrons move into the floating gate and become trapped by an
insulating (dielectric) layer.
●
Because the electrons stay trapped even when power is off, SSDs are non-volatile (data is not
lost when the device is turned off).
●
The transistors are arranged in a matrix/grid structure to store large amounts of data.
●
Over a long time, a small amount of charge may leak, so SSDs should be used occasionally to
Major Benefits of SSDs
● More reliable - no mechanical
parts to fail.
● Lighter weight - ideal for laptops.
● Instant access - no spin-up time.
● Lower power consumption + runs
cooler - perfect for portable
devices.
● Very thin profile due to no moving
parts.
● Much faster data access than
HDDs.
Main Drawbacks of SSDs
● Limited write endurance:
Rated ~20GB writes/day for 3
years.
● Cannot overwrite directly must erase first, then write
new data.
● Still avoided in high-write
servers (though improving
rapidly).
Technical Limitations Explained
● Erase-before-write process slows some operations.
● Endurance rating (20GB/day × 3 years) limits heavy-write applications.
● Manufacturers improving durability for servers/cloud storage.
How do HDDs and SSDs work?
How does a HDD work?
●
●
HDDs use magnetism to store/represent
●
SSDs have no moving parts
binary
●
Data is saved using integrated circuits and
●
Data can be accessed at virtually equal
Data is saved within tracks and sectors on the
speeds from anywhere on the disk (very handy
disk surface, and read/written from the tip of
for fragmented files)
the actuator arm
●
NAND flash memory
It is mechanical with moving parts (a spinning
disk platter and swinging actuator arm)
●
How does a SSD work?
It can take time to reach the data, especially if
it is fragmented (files split into different areas
of the disk)
Advantages and disadvantages of SSDs and HDDs
Advantages of HDDs
● Cheaper per unit of storage
● Huge capacities available and
affordable
● No limit to write cycles
Disadvantages of HDDs
● Not as robust (e.g. might break if
dropped)
● Not as fast
● Takes time to get to full speed
Advantages of SSDs
● More robust
● Faster, especially for fragmented data
● Uses less power
● Generates less heat
● Lighter
● Often a smaller form factor
● Quieter
Disadvantages of SSDs
● More expensive per unit of storage
● Limited number of write cycles
Youtube links and quizzes
Quiz: Input, Output or Storage
https://www.computerscience.gcse.guru/quiz/input-output-or-storage
SSD vs Hard Drive vs Hybrid Drive
https://www.youtube.com/watch?v=1cyMTl_QXSc
Common types of secondary storage devices
https://www.youtube.com/watch?v=DMq_DV1kWuA&t=504s
Every Storage Device Explained in 8 Minutes
https://www.youtube.com/watch?v=1vO9l1idWac&t=363s
General comparison video for better understanding
https://www.youtube.com/@bytesizedexplainer/videos
Videos to watch to understand how secondary storage
works.
How does an optical disk drive works?
https://www.youtube.com/watch?v=rBSRvjKdTh8
Hard Disk working in Animation
https://www.youtube.com/watch?v=1DPSInluopc
SSD vs Hard Drive vs Hybrid Drive
https://www.youtube.com/watch?v=1cyMTl_QXSc
Memory Sticks (Flash Drives / Pen Drives)
● Memory sticks use solid-state technology to store data.
● They usually connect to a computer through a USB port.
● They are small, lightweight, and portable, making them useful for transferring
files between computers.
● They can also be used as small backup devices for files such as music, photos,
or documents.
● Some expensive software uses a memory stick called a dongle.
● The dongle contains special files needed to run the software.
● Without the dongle, the software will not work, which helps prevent illegal
copying or unauthorised use.
CD/DVD – Optical Storage
● CDs and DVDs are optical storage devices.
● They use laser light to read and write data on the surface of the disk.
● Data is stored using a thin layer of metal alloy or light-sensitive dye.
● The disk has one spiral track that runs from the centre to the outer edge.
● As the disk spins, the optical head moves along the spiral track to read
or write data.
● Like a hard disk, the disk is divided into sectors for direct access to data.
How Data is Stored -CD/DVD – Optical Storage
● Data is stored as pits and lands on the spiral track.
● A red laser reads the difference between pits and lands.
● The outer part of the disk moves faster than the inner part
when spinning.
● CDs and DVDs can be:
○ R (Recordable) – can be written once only.
○ RW (Rewritable) – can be written and erased many
times.
Differences Between CD and DVD
● DVDs can store more data than CDs.
● DVDs can use dual layers (two recording layers) to increase storage
capacity.
● The layers are separated by a transparent spacer and a thin reflector.
● The laser focuses at slightly different depths to read each layer.
CDs VS DVDs
Laser and Storage Capacity
● DVDs use a 650 nm wavelength laser.
● CDs use a 780 nm wavelength laser.
● DVDs have smaller pits and narrower tracks, allowing more data to be
stored.
● Shorter wavelength lasers allow greater storage capacity.
Summary Difference: nm- Nanometers
● CD: 780 nm (Infrared) = Lower density, larger pits.
● DVD: 650 nm (Red) = Higher density, smaller pits.
Blu-ray discs
● The technology uses a blue laser, is a type of optical storage device.
● They use laser light to read and write data on the surface of the disc.
● CDs and DVDs use a red laser, whereas Blu-ray discs use a blue-violet laser
● They use a blue-violet laser, which has a shorter wavelength than the red laser
used in CDs and DVDs.
● Because of this shorter wavelength, data can be stored more densely on the
disc.
● This allows Blu-ray discs to store much more data than CDs or DVDs.
● Blu-ray is commonly used for high-definition (HD) video storage and movies.
Interactivity in Blu-ray
Blu-ray discs allow more interactive features than DVDs, such as:
● Recording high-definition TV programs
● Skipping quickly to any part of the disc
● Creating playlists of movies or TV programs
● Editing or re-ordering recorded programs
● Automatically finding empty space on the disc
●
Accessing websites to download subtitles and extra features
Uses of Optical Storage (CD, DVD, Blu-ray)
● Used for backup storage (photos, music, multimedia files)
● Used to transfer files between computers
● Software such as printer drivers may be supplied on CDs
or DVDs
● These discs are often read-only when used for software
distribution.
Optical Storage until 3.55
Virtual Memory
● One of the problems associated with memory
management is when processes run out of RAM.
● If the amount of available RAM is exceeded due to
multiple programs running, it can cause a system crash.
● This problem can be solved by using the hard disk drive
(HDD) or solid-state drive (SSD) as additional memory.
● This is known as virtual memory.
● RAM is the physical memory.
● Virtual memory = RAM + swap space on the HDD/SSD.
● To execute a program, data is loaded into memory from
the HDD/SSD whenever required.
https://www.savemyexams.com/igcse/computer-science/cie/23/revision-notes/3-hardware/data-storage/virtual-
Without Virtual Memory
● Suppose there are five programs (1 to 5) in memory,
all requiring access to RAM.
● All programs must fit completely into RAM.
● If the total memory required exceeds the available
RAM:
○ There is not enough space to load all programs.
○ The system runs out of RAM.
○ This can cause the computer to crash or stop
working.
Virtual Memory (Concept & Working)
● The CPU uses virtual memory to allow all five programs (1 to 5) to access
RAM as required. This involves moving data out of RAM into the HDD/SSD
and bringing required data back into RAM when needed.
● Virtual memory moves the oldest data out of RAM into the HDD/SSD to free
space. This frees address space , allowing other program to be loaded into
RAM and gain access.
● A 32-bit map is updated to reflect these changes, showing which data is
stored in RAM and which is stored in HDD/SSD. This ensures that programs
can still access their data correctly.
● This process continues as long as multiple programs are running, preventing
RAM from being over-utilised. Virtual memory gives the illusion of unlimited
memory, since data can always be moved in and out of secondary storage
even when RAM is full.
Paging & Performance
● In operating systems, paging is used to manage virtual memory.
Data is divided into pages, which are fixed-length contiguous blocks.
These pages are moved between HDD/SSD and RAM as required.
● This mechanism allows efficient transfer of data blocks, but
accessing data from virtual memory is slower than accessing
RAM. Therefore, the larger the RAM, the faster the CPU can
operate, as less swapping is required.
Advantages of Virtual Memory
● Programs can be larger than physical memory and
still be executed.
● Memory is used more efficiently since unused data
(e.g., during error handling) does not occupy RAM
unnecessarily.
● Reduces the need to install additional, expensive
RAM (although this has practical limits).
Virtual Memory and Cloud Storage
Disk Thrashing & Solutions
●
A major drawback when using HDD for virtual memory is disk thrashing. This happens when
main memory is full and data is constantly swapped in and out, leading to excessive
read/write operations by the disk head.
●
As a result, the system spends more time moving data than executing processes, causing a
significant reduction in performance. In extreme cases, the system may reach the thrash
point, where processing almost stops because of continuous data movement.
●
Disk thrashing can also lead to premature failure of the hard disk due to excessive head
movements.
●
It can be reduced by:
○
Installing more RAM
○
Running fewer programs at the same time
○
Reducing the size of the swap file
○
Using an SSD instead of HDD, as it is faster and has no moving parts
CLOUD STORAGE
Cloud storage is a method of data storage where data is stored on
remote servers instead of a local computer.
The same data is stored on more than one server in case of
maintenance or repair. This allows clients to access data at any
time. This is known as data redundancy.
The physical environment is owned and managed by a hosting
company. It may include hundreds of servers in many locations.
Cloud Storage
Types of Cloud Systems
Public Cloud: A storage environment where the customer/client and
cloud storage provider are different companies.
Private Cloud: Storage provided by a dedicated environment behind a
company firewall. The customer/client and cloud storage provider are
integrated and operate as a single entity.
Hybrid Cloud: A combination of public and private cloud. Some data
resides in the private cloud. Less sensitive or less commercial data
can be accessed from a public cloud storage provider.
Benefits and Drawbacks of cloud storage
Data Security in Cloud Storage
When companies transfer large amounts of confidential data to a cloud
service provider, they give up some control over data security.
This raises important questions:
1. What physical security is there in the building where data is
stored?
2. How well can the provider handle natural disasters or power
failures?
3. What safeguards are in place for employees? Can staff misuse
their access to view or sell confidential data?
Potential Data Loss in Cloud Storage
There is a risk that important and irreplaceable data could be lost from cloud storage.
Hackers may gain access to accounts or carry out pharming attacks, leading to data loss
or corruption. Users must ensure that strong safeguards are in place to reduce these
risks.
Pharming attacks trick users into giving away login details by redirecting them to fake websites,
leading to unauthorised access to cloud-stored data.
Examples of security breaches:
1. The XEN security threat forced cloud operators to reboot servers due to a problem in the
XEN hypervisor (software/firmware/hardware that creates and runs virtual machines).
2. A cloud provider permanently lost data during a routine back-up procedure.
3. The celebrity photo hacking incident where over 100 private photos were leaked after
hackers accessed cloud accounts.
4. In 2016, the National Electoral Institute of Mexico suffered a breach where 93 million voter
records became publicly available, with data also linked to a cloud server outside Mexico.
3.4
Network
hardware- Network Interface Card (NIC)
A Network Interface Card (NIC) is a hardware component that allows a device
(computer, laptop, etc.) to connect to a network such as the internet.
●
●
●
It is usually built into the device hardware.
Every NIC has a unique MAC (Media Access Control) address assigned
during manufacturing.
This MAC address helps identify the device on a network.
Wireless NIC (WNIC):
●
●
●
Works the same as a NIC but uses wireless connection instead of cables.
Uses an antenna to communicate with networks using microwaves.
Can be:
○ Plugged into a USB port, or
○ Built into the device as an internal integrated circuit
Wireless Networks with Wireless NIC (WNIC):
Media Access
Control
(MAC)
A MAC address is a unique identifier assigned to a device’s Network Interface Card
(NIC).
● It consists of 48 bits
● Written as six groups of hexadecimal digits
Format & Structure:
NN – NN – NN – DD – DD – DD
→ First 3 groups (NN): Manufacturer’s code
→ Last 3 groups (DD): Device serial number (unique)
Example:
● 00 – 1C – B3 – 4F – 25 – FF
○ First 6 digits → Identify manufacturer (e.g., Apple)
○ Last 6 digits → Unique device number
Important:
Media Access
Control
(MAC)
There are two types of MAC address:
1. Universally Administered Address (UAA): Set by the manufacturer at
the factory; this is the most common type and is rarely changed.
2. Locally Administered Address (LAA): Can be changed by the user or
organisation, but it must remain unique to avoid network issues.
Reasons for changing a MAC address include:
1. Some mainframe systems require all MAC addresses to follow a strict format, so
devices may need their MAC address changed to match.
2. To bypass MAC address filtering on a router or firewall, where only specific MAC
address formats are allowed.
3. To overcome network restrictions by emulating an allowed (unrestricted) MAC
address.
Internet Protocol (IP) Address
When a device connects to a private network,
the router assigns a private IP address.
● This address is unique within that network
● It may be the same as an IP address on another network
When the router connects to the internet:
● It is given a unique public IP address by the Internet Service
Provider (ISP)
● No other device on the internet shares this public IP
address
● All devices on the network:
○ Share the same public IP address
○ Have different private IP addresses
Internet Protocol (IP) Address
Why IP Addresses are Needed -The internet works using protocols (rules).
● These rules must be followed by senders and receivers of data
● An IP address is required for communication between devices
Pv6:
Versions of IP
IPv4:
●
●
●
●
Uses 32 bits
Written as four groups of denary
numbers
Example: 254.25.28.77
Limitation: Fewer available addresses
IAdvantages of IPv6 over IPv4
●
●
●
Removes the risk of IP address
collisions
Has built-in authentication checks
Allows more efficient packet routing
● Uses 128 bits
● Written as eight groups of
hexadecimal digits separated by
colons (:)
● Example:
A8FB:7A88:FFF0:0FFF:3D21:208
5:66FB:F0FA
● Designed to support the growth of
the internet (more devices and
data traffic)
Internet Protocol (IP) address - IPv4 and IPv6
MAC Address Explained
What is a Router?
Routers route data packets between different networks (e.g., LAN to WAN).
They convert data from one protocol/format to another so networks can
communicate.
Typically connected with:
●
●
Internet cable
Multiple cables to LAN devices (computers, etc.)
Broadband routers sit behind a firewall.
Firewall protects computers on the network.
Main function:
●
Transmit internet and communication protocols between networks
How Routers Work?
● Routers inspect data packets sent from any connected
computer.
● Devices on the same network share part of an IP address.
● Router sends packet to the correct switch.
● Switch delivers data using the MAC destination address.
● If MAC address does not match:
○ Data is passed to another switch
○ Continues until correct device is found
● Routers can be:
○ Wired
○ Wireless
Savemyexams- flashcards
https://www.savemyexams.com/igcse/computer-science/cie/23/flashcards/3hardware/network-hardware/
Videos to watch
https://craigndave.org/video-categories/cambridge-igcse-0984-0478-3-hardware/
Primary storage RAM & ROM
https://www.youtube.com/watch?v=cuqIg31Jlgw&t=27s
Secondary storage overview
https://www.youtube.com/watch?v=61u6e1onxyE
Common types of storage
https://www.youtube.com/watch?v=DMq_DV1kWuA
Data storage
https://www.youtube.com/watch?v=vC9PMM7KAK0