Hardware vs Software

Hardware is the physical components of a computer that you can touch (e.g. monitor, keyboard, CPU, RAM).

Software is the programs and instructions that tell the hardware what to do. Software is intangible.

Main Components of a Computer

Component	Purpose
CPU (Central Processing Unit)	The "brain" of the computer. Processes instructions and performs calculations.
RAM	Temporary, fast memory that holds currently running programs and data. Volatile (lost when power off).
Motherboard	The main circuit board that connects all components together.
PSU (Power Supply Unit)	Converts mains electricity to the voltages needed by computer components.
GPU (Graphics Processing Unit)	Processes and renders images, video, and animations for the display.

Types of Software

• System software: manages the computer (OS, drivers, utilities)
• Application software: helps users perform tasks (word processor, browser, games)

Input Devices

An input device sends data INTO the computer.

Device	Use
Keyboard	Entering text and commands
Mouse / Trackpad	Pointing, clicking, selecting
Microphone	Recording sound, voice input
Scanner	Digitising paper documents/photos
Webcam / Camera	Capturing images/video
Barcode reader	Reading barcodes (e.g. at supermarket checkout)
Touchscreen	Input by touching the display (also an output device)

Output Devices

An output device presents data FROM the computer to the user.

Device	Use
Monitor / Screen	Displaying visual output
Printer	Producing hard copy on paper
Speakers / Headphones	Producing sound output
Projector	Displaying images on a large surface

Storage Devices

Device	Type	Features
HDD (Hard Disk Drive)	Magnetic	Large capacity, cheap per GB, slower, moving parts (can break)
SSD (Solid State Drive)	Flash memory	Very fast, no moving parts (durable), more expensive per GB
USB flash drive	Flash memory	Portable, small capacity, plug-and-play
Optical disc (CD/DVD/Blu-ray)	Optical	Cheap, portable, limited capacity, slower access
Cloud storage	Remote server	Access anywhere, requires internet, ongoing cost

Why Binary?

Computers use binary (0s and 1s) because their electronic circuits have two states: on (1) and off (0). All data - text, images, sound - must be converted to binary for the computer to process it.

Key Units

Unit	Size
Bit	A single binary digit (0 or 1)
Nibble	4 bits
Byte	8 bits
Kilobyte (KB)	1,024 bytes
Megabyte (MB)	1,024 KB
Gigabyte (GB)	1,024 MB
Terabyte (TB)	1,024 GB

Simple Binary Conversions

Use the place value table to convert between binary and denary:

128	64	32	16	8	4	2	1

Example: Binary 10101100 = 128 + 32 + 8 + 4 = 172

Text is stored using ASCII codes - each character has a number (e.g. A=65, B=66, a=97).

What Is a Network?

A network is two or more devices connected together to share data and resources.

Benefits of Networking

• Share files and resources (printers, internet connection)
• Central backup of data
• Easy communication (email, messaging)
• Central management of software and security

Risks of Networking

• Security risks (hacking, malware can spread across the network)
• If the server fails, all users are affected
• Requires specialist staff to manage
• Initial setup cost

Wired vs Wireless

	Wired	Wireless
Speed	Generally faster and more reliable	Slower, affected by interference/distance
Security	More secure (physical connection needed)	Less secure (signals can be intercepted)
Mobility	Devices must stay near cable points	Users can move freely
Cost	Cabling is expensive to install	Cheaper to set up, fewer cables

Online Threats

• Phishing - fake emails/websites designed to steal personal information
• Malware - malicious software (viruses, worms, trojans, ransomware)
• Identity theft - criminals using stolen personal data to impersonate someone
• Cyberbullying - using technology to harass, threaten, or intimidate others

Staying Safe Online

• Use strong passwords (long, mixed characters, unique for each account)
• Enable two-factor authentication (2FA)
• Keep software and antivirus up to date
• Do not open suspicious emails or attachments
• Check for HTTPS and the padlock icon before entering personal data
• Be careful what personal information you share on social media
• Use privacy settings on social media accounts

Spreadsheet Basics

A spreadsheet organises data in rows and columns of cells. Each cell has a cell reference (e.g. B3).

Feature	Description	Example
Formula	A calculation using cell references	=B2*C2
Function	A built-in calculation	=SUM(B2:B10), =AVERAGE(C2:C20), =MAX(), =MIN(), =COUNT(), =IF()
Absolute reference	A cell reference that does not change when copied (uses $)	=$B$1
Relative reference	A cell reference that adjusts when copied	=B1 (changes to =B2, =B3 etc.)
Charts	Visual representation of data	Bar chart, pie chart, line graph
Sort	Arranging data in order	Alphabetical, numerical, ascending/descending
Filter	Displaying only data that meets certain criteria	Show only Year 10 students

Database Basics

A database is an organised collection of related data. A flat file database stores all data in a single table. A relational database uses multiple linked tables to reduce data redundancy.

• Table: collection of related records
• Record: one row (one entry/item)
• Field: one column (one piece of data about each record)
• Primary key: a unique identifier for each record
• Query: a search of the database to find specific data
• Form: a user-friendly way to enter data
• Report: a formatted printout of data from the database

HTML (HyperText Markup Language)

HTML defines the structure and content of a web page using tags.

<!DOCTYPE html>
<html>
<head>
    <title>My Page</title>
</head>
<body>
    <h1>Welcome</h1>
    <p>This is a paragraph.</p>
    <img src="photo.jpg" alt="A photo">
    <a href="page2.html">Click here</a>
</body>
</html>

CSS (Cascading Style Sheets)

CSS controls the appearance and layout of the web page (colours, fonts, spacing, positioning).

body {
    background-color: #1a1a2e;
    color: white;
    font-family: Arial;
}
h1 {
    color: #e94560;
    text-align: center;
}

Key Difference

HTML = structure/content (WHAT is on the page). CSS = presentation/style (HOW it looks).

Key Principles

Principle	Definition	Example
Decomposition	Breaking a complex problem into smaller, manageable sub-problems	Planning a school event: break into venue, invitations, catering, entertainment
Pattern recognition	Identifying similarities or trends within and between problems	Noticing that sorting a list of names uses the same logic as sorting numbers
Abstraction	Removing unnecessary detail to focus on the important information	A London Underground map abstracts the physical geography to show only stations and connections
Algorithmic thinking	Developing a step-by-step solution (algorithm) to solve the problem	Writing pseudocode or a flowchart for making a cup of tea

Data Protection

The Data Protection Act 2018 controls how personal data is collected, stored, and used. Organisations must:

• Only collect data they need
• Keep it accurate and up to date
• Not keep it longer than necessary
• Keep it secure
• Not share it without consent

Individuals have the right to see what data is held about them (Subject Access Request) and to have inaccurate data corrected.

Copyright

The Copyright, Designs and Patents Act 1988 protects creators' work. You must not copy, download, or share software, music, images, or text without permission. Creative Commons licences allow some sharing with conditions.

Computer Misuse

The Computer Misuse Act 1990 makes it illegal to access computers without permission, hack systems, or create/distribute malware.

Ethical Considerations

• Should companies collect data about users without their full understanding?
• Is it ethical to use AI to make decisions about people (loans, recruitment)?
• Who is responsible when an autonomous vehicle causes an accident?
• Should access to the internet be considered a human right?
• What are the environmental costs of always upgrading to the latest technology?

Pseudocode & Flowcharts

Pseudocode is a way of writing algorithms in structured English that resembles code but is not tied to any programming language. It is used to plan programs before writing actual code.

Flowcharts use standard symbols to represent the steps in an algorithm visually:

Symbol	Name	Purpose
Rounded rectangle	Terminator	Start or End of the algorithm
Rectangle	Process	An instruction or action (e.g. calculate total)
Diamond	Decision	A question with Yes/No paths (selection)
Parallelogram	Input/Output	Getting data in or displaying data out
Arrow	Flow line	Shows direction of flow between steps

Variables & Data Types

A variable is a named storage location in memory that holds a value which can change during the program. A constant is a named value that does not change.

Data Type	Description	Example
Integer	Whole number	42, -7, 0
Real / Float	Decimal number	3.14, -0.5
String	Text (sequence of characters)	"Hello", "GCSE"
Boolean	True or False	TRUE, FALSE
Character	A single letter, digit or symbol	'A', '7', '#'

Sequence, Selection & Iteration

Sequence - instructions are executed one after another in order:

SET price TO 10
SET quantity TO 3
SET total TO price * quantity
OUTPUT total

Selection - the program chooses a path based on a condition (IF...THEN...ELSE):

INPUT age
IF age >= 18 THEN
    OUTPUT "You can vote"
ELSE
    OUTPUT "You are too young to vote"
ENDIF

Iteration - repeating a block of code. There are three types:

FOR loop (count-controlled)

// Print numbers 1 to 5
FOR i FROM 1 TO 5
    OUTPUT i
ENDFOR

WHILE loop (condition-controlled, pre-check)

SET password TO ""
WHILE password != "secret123"
    INPUT password
ENDWHILE
OUTPUT "Access granted"

REPEAT...UNTIL loop (condition-controlled, post-check)

REPEAT
    INPUT number
UNTIL number >= 1 AND number <= 100

Arrays / Lists

An array (or list) is a data structure that stores multiple values of the same type under one name. Each item is accessed using an index (position number, usually starting from 0).

SET names TO ["Alice", "Bob", "Charlie"]
OUTPUT names[0]   // Outputs "Alice"
OUTPUT names[2]   // Outputs "Charlie"

// Loop through all items in the array
FOR i FROM 0 TO 2
    OUTPUT names[i]
ENDFOR

Functions & Procedures

A procedure is a named block of code that performs a task. It does NOT return a value.

A function is a named block of code that performs a task and RETURNS a value.

// Procedure - no return value
PROCEDURE greetUser(name)
    OUTPUT "Hello, " + name
ENDPROCEDURE

// Function - returns a value
FUNCTION calculateArea(length, width)
    RETURN length * width
ENDFUNCTION

SET area TO calculateArea(5, 3)
OUTPUT area   // Outputs 15

Validation & Verification

Validation checks that data entered is reasonable and follows rules (done by the computer). Verification checks that data is entered correctly (done by the user or by double entry).

Validation Check	What It Does	Example
Range check	Data is within a set range	Age must be between 0 and 120
Length check	Data has the correct number of characters	Password must be at least 8 characters
Presence check	Data has been entered (not blank)	Name field must not be empty
Type check	Data is the correct data type	Age must be an integer
Format check	Data matches a required pattern	Postcode must be in the format AA9 9AA

// Validation example: range check
REPEAT
    OUTPUT "Enter a mark (0-100):"
    INPUT mark
    IF mark < 0 OR mark > 100 THEN
        OUTPUT "Invalid. Try again."
    ENDIF
UNTIL mark >= 0 AND mark <= 100

Trace Tables

A trace table is used to track the values of variables as a program executes, line by line. It helps you find logic errors (dry-running).

Example - trace this code:

SET x TO 1
SET total TO 0
WHILE x <= 4
    SET total TO total + x
    SET x TO x + 1
ENDWHILE
OUTPUT total

x	total	x <= 4?
1	0	Yes
2	1	Yes
3	3	Yes
4	6	Yes
5	10	No

Output: 10

Common Algorithms

Linear Search

Checks each item in a list one by one until the target is found or the end is reached. Works on unsorted data.

FUNCTION linearSearch(list, target)
    FOR i FROM 0 TO LENGTH(list) - 1
        IF list[i] == target THEN
            RETURN i        // Found at index i
        ENDIF
    ENDFOR
    RETURN -1           // Not found
ENDFUNCTION

Bubble Sort

Repeatedly steps through the list, compares adjacent items and swaps them if they are in the wrong order. Passes continue until no swaps are needed.

PROCEDURE bubbleSort(list)
    SET n TO LENGTH(list)
    FOR i FROM 0 TO n - 2
        FOR j FROM 0 TO n - i - 2
            IF list[j] > list[j+1] THEN
                // Swap the items
                SET temp TO list[j]
                SET list[j] TO list[j+1]
                SET list[j+1] TO temp
            ENDIF
        ENDFOR
    ENDFOR
ENDPROCEDURE

Flat File vs Relational Databases

	Flat File Database	Relational Database
Structure	All data in a single table	Data split across multiple linked tables
Redundancy	Data is often repeated (redundant)	Data duplication is minimised
Inconsistency	Repeated data can become inconsistent	Changes in one place update everywhere
Complexity	Simple to set up and use	More complex but more powerful
Example	A single CSV file of student data	Separate Students, Courses, and Enrolments tables

Tables, Records, Fields & Keys

• Table - a collection of related data organised in rows and columns
• Record - a single row in a table (one complete entry, e.g. one student)
• Field - a single column in a table (one category of data, e.g. Surname)
• Primary key - a field that uniquely identifies each record (e.g. StudentID). No two records can have the same primary key value.
• Foreign key - a field in one table that links to the primary key in another table. This creates a relationship between the tables.

Data Types in Databases

Data Type	Stores	Example Field
Text / Varchar	Letters, numbers, symbols	Name, Address
Integer	Whole numbers	Age, Quantity
Real / Decimal	Numbers with decimal places	Price, Average
Boolean	Yes/No, True/False	HasPaid, IsActive
Date/Time	Dates and/or times	DateOfBirth, OrderDate

SQL Basics

SQL (Structured Query Language) is used to query and manage data in relational databases.

SELECT ... FROM ... WHERE

-- Select all fields from the Students table
SELECT * FROM Students;

-- Select specific fields
SELECT FirstName, Surname FROM Students;

-- With a condition
SELECT * FROM Students
WHERE YearGroup = 11;

-- Multiple conditions using AND / OR
SELECT FirstName, Surname FROM Students
WHERE YearGroup = 11 AND Gender = 'F';

SELECT * FROM Products
WHERE Price < 10 OR Category = 'Sale';

ORDER BY

-- Sort results alphabetically by Surname
SELECT * FROM Students
ORDER BY Surname ASC;

-- Sort by Price, highest first
SELECT ProductName, Price FROM Products
ORDER BY Price DESC;

Queries, Forms & Reports

• Query - a search of the database using criteria to extract specific data (e.g. "show all students in Year 11 who got above 70%")
• Form - a user-friendly interface for entering, editing and viewing records one at a time (reduces input errors)
• Report - a formatted, printable summary of data from the database, often including totals and groupings

Database Design & Normalisation

Normalisation is the process of organising data to reduce redundancy and improve data integrity. At GCSE level, you should understand:

• Splitting one large table into smaller related tables
• Each table should contain data about ONE entity (e.g. Students, Courses)
• Linking tables using primary and foreign keys
• Removing repeated groups of data

Entity Relationship Diagrams (ERDs)

An ERD shows how tables (entities) are related. The three relationship types are:

Relationship	Meaning	Example
One-to-One (1:1)	Each record in Table A relates to exactly one record in Table B	One student has one student ID card
One-to-Many (1:M)	One record in Table A relates to many records in Table B	One teacher teaches many classes
Many-to-Many (M:M)	Many records in Table A relate to many in Table B (resolved with a linking table)	Students enrol in many courses; each course has many students

The TCP/IP Model

The TCP/IP model is a set of layered protocols that governs how data is transmitted across networks and the internet. It has 4 layers:

Layer	Name	Role	Protocols
4 (Top)	Application	Provides network services to the user (web, email, file transfer)	HTTP, HTTPS, FTP, SMTP, POP3, IMAP
3	Transport	Ensures reliable data delivery; splits data into segments	TCP, UDP
2	Internet	Addresses and routes packets across networks	IP
1 (Bottom)	Network Interface	Handles physical transmission of data on the local network	Ethernet, Wi-Fi

Key Protocols

Protocol	Full Name	Purpose
HTTP	HyperText Transfer Protocol	Transfers web pages (not encrypted)
HTTPS	HTTP Secure	Encrypted version of HTTP (uses SSL/TLS)
FTP	File Transfer Protocol	Uploading/downloading files to/from a server
SMTP	Simple Mail Transfer Protocol	Sending email
POP3	Post Office Protocol v3	Receiving email (downloads and removes from server)
IMAP	Internet Message Access Protocol	Receiving email (keeps email on server, syncs across devices)
TCP	Transmission Control Protocol	Reliable delivery - checks all packets arrive and reassembles them in order
UDP	User Datagram Protocol	Faster but unreliable - no error checking (used for streaming, gaming)
IP	Internet Protocol	Addresses packets and routes them to the correct destination

Packet Switching

When data is sent across a network, it is broken into small packets. Each packet contains:

• Header: source IP, destination IP, packet number, total packets
• Payload: the actual data being transmitted
• Trailer: error-checking data

Packets may take different routes across the network and arrive out of order. The receiving device uses TCP to reassemble them in the correct order. If any packets are missing, they are requested again.

IP Addressing (IPv4)

An IP address is a unique numerical address assigned to every device on a network. IPv4 addresses have 4 groups of numbers separated by dots, e.g. 192.168.1.25

• Each group is a number from 0 to 255 (8 bits = 1 byte per group)
• An IPv4 address is 32 bits (4 bytes) long
• There are about 4.3 billion possible IPv4 addresses

MAC Addresses

A MAC (Media Access Control) address is a unique hardware identifier burned into every network interface card (NIC). It is 48 bits long, written as 6 pairs of hexadecimal digits, e.g. A4:5E:60:B2:F1:C8

• MAC addresses are permanent and set by the manufacturer
• IP addresses can change (assigned by DHCP or manually)
• MAC addresses work at the Network Interface layer; IP addresses work at the Internet layer

DNS (Domain Name System)

The DNS translates human-readable domain names (e.g. www.google.com) into IP addresses (e.g. 142.250.187.206) that computers use to route data.

How DNS works:

1. You type a URL into your browser
2. Your device checks its local DNS cache for the IP address
3. If not found, the request is sent to a DNS server
4. The DNS server returns the corresponding IP address
5. Your browser connects to the web server at that IP address

Client-Server vs Peer-to-Peer

	Client-Server	Peer-to-Peer (P2P)
Structure	Central server provides services to client devices	All devices are equal; each can be client and server
Security	Centrally managed, more secure	Each device manages its own security, less secure
Backup	Centralised backup on server	Each device responsible for its own backup
Cost	Expensive (dedicated server hardware and admin)	Cheaper (no dedicated server needed)
Scalability	Scales well with more clients	Performance drops as more devices join
Example	School network, websites, email	BitTorrent file sharing, small home networks

Symmetric vs Asymmetric Encryption

Encryption is the process of converting plaintext into ciphertext (unreadable form) to protect data. Only someone with the correct key can decrypt it.

	Symmetric Encryption	Asymmetric Encryption
Keys	Same key used to encrypt and decrypt	Two different keys: a public key (to encrypt) and a private key (to decrypt)
Speed	Faster	Slower
Key sharing	Key must be shared securely (risk if intercepted)	Public key can be shared openly; only the private key must be kept secret
Example use	Encrypting files on a hard drive	HTTPS, email encryption, digital signatures

Caesar Cipher

The Caesar cipher is a simple substitution cipher where each letter is shifted by a fixed number of positions. It is a symmetric cipher (same key to encrypt and decrypt).

// Caesar cipher with shift of 3
Plaintext:  HELLO WORLD
Ciphertext: KHOOR ZRUOG

// Each letter shifted 3 places forward:
H -> K,  E -> H,  L -> O,  L -> O,  O -> R
W -> Z,  O -> R,  R -> U,  L -> O,  D -> G

How HTTPS Works (SSL/TLS)

HTTPS uses SSL/TLS (Secure Sockets Layer / Transport Layer Security) to encrypt data between your browser and the web server. The process:

1. Your browser requests a secure connection to the server
2. The server sends its digital certificate (containing its public key)
3. Your browser verifies the certificate is valid (issued by a trusted Certificate Authority)
4. Your browser generates a session key, encrypts it with the server's public key, and sends it
5. Both sides now use the session key (symmetric encryption) for fast, secure communication

Digital Certificates

A digital certificate proves the identity of a website. It is issued by a Certificate Authority (CA) and contains the website's public key, the domain name, the CA's signature, and an expiry date. Your browser checks the certificate to ensure you are connected to the genuine website (not an impersonator).

Hashing vs Encryption

	Hashing	Encryption
Direction	One-way (cannot be reversed)	Two-way (can be decrypted with a key)
Purpose	Verify data integrity, store passwords	Protect data confidentiality during transmission or storage
Output	Fixed-length hash value (digest)	Ciphertext (same length as plaintext)
Example	SHA-256 hash of a password	AES encryption of a file

Authentication Methods

Method	How It Works	Strengths / Weaknesses
Passwords	User enters a secret word/phrase	Easy to implement; can be weak, guessed, or stolen
Biometrics	Uses physical traits (fingerprint, face, iris, voice)	Unique to each person and hard to fake; expensive; cannot be changed if compromised
Two-Factor Authentication (2FA)	Requires two different types of proof (e.g. password + SMS code)	Much more secure; even if password is stolen, attacker needs the second factor

The three authentication factors are: something you know (password, PIN), something you have (phone, smart card), something you are (biometric).

Malware Types

Type	What It Does	How It Spreads
Virus	Attaches to a file; damages/deletes data or corrupts the system	Spread when infected files are opened or shared
Worm	Self-replicates and spreads across networks without user action	Exploits network vulnerabilities automatically
Trojan	Disguises itself as legitimate software; creates a backdoor	Downloaded by the user thinking it is safe software
Ransomware	Encrypts victim's files; demands payment for the decryption key	Phishing emails, malicious downloads
Spyware	Secretly monitors user activity (keystrokes, browsing, passwords)	Bundled with other software, malicious websites

Firewalls

A firewall monitors and controls incoming and outgoing network traffic based on security rules. It acts as a barrier between a trusted internal network and untrusted external networks (the internet).

• Can be hardware (a dedicated device) or software (installed on a computer)
• Blocks unauthorised access while allowing legitimate traffic
• Can filter traffic by IP address, port number, or protocol

Social Engineering

Social engineering tricks people into revealing confidential information or performing actions that compromise security. It targets human weaknesses rather than technical ones.

Attack	Description
Phishing	Fake emails or messages pretending to be from a trusted source (e.g. bank), tricking users into clicking links or revealing passwords
Pharming	Redirecting a legitimate website's traffic to a fake website (by poisoning DNS records) to steal login credentials
Shoulder surfing	Watching someone enter their PIN, password, or other sensitive information
Pretexting	Creating a fabricated scenario (e.g. pretending to be IT support) to trick someone into giving access or information

Binary Arithmetic - Addition

Binary addition follows the same rules as denary but with only 0 and 1:

Rule	Result
0 + 0	0
0 + 1	1
1 + 0	1
1 + 1	10 (write 0, carry 1)
1 + 1 + 1	11 (write 1, carry 1)

Worked Example

  Add 01101010 (106) + 00110101 (53)

    0 1 1 0 1 0 1 0
  + 0 0 1 1 0 1 0 1
  -----------------
    1 0 0 1 1 1 1 1  = 159

  Carries: 0 1 1 0 0 1 1 0

Overflow

Overflow occurs when the result of a binary addition is too large to be stored in the available number of bits. For example, with 8 bits the maximum unsigned value is 255 (11111111). Adding 200 + 100 = 300, which cannot fit in 8 bits - this causes an overflow error.

Hexadecimal

Hexadecimal (hex) is a base-16 number system using digits 0-9 and letters A-F. It is used as a shorthand for binary because it is more human-readable.

Hex	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
Denary	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Binary	0000	0001	0010	0011	0100	0101	0110	0111	1000	1001	1010	1011	1100	1101	1110	1111

Converting Binary to Hex

Split binary into nibbles (groups of 4), convert each to hex:

Binary:  1010 1111
Hex:       A    F    = AF

Binary:  1100 0011
Hex:       C    3    = C3

Uses of Hexadecimal

• Colour codes in web design (e.g. #FF0000 = red, #00FF00 = green)
• MAC addresses (e.g. A4:5E:60:B2:F1:C8)
• Memory addresses
• Error codes

Character Encoding

	ASCII	Unicode
Bits per character	7 bits (extended ASCII = 8 bits)	Up to 32 bits (UTF-8 uses 1-4 bytes)
Characters	128 characters (English letters, digits, symbols)	Over 140,000 characters (all world languages, emoji)
File size	Smaller files	Larger files
Languages	English only	All languages worldwide

Examples: ASCII code for 'A' = 65, 'a' = 97, '0' = 48, space = 32.

Image Representation

Digital images are made up of tiny dots called pixels. Each pixel stores colour information.

• Resolution - the number of pixels in the image (width x height). Higher resolution = more detail but larger file size.
• Colour depth - the number of bits used to represent the colour of each pixel. More bits = more colours available.

Image File Size Calculation

File size (bits) = width (pixels) x height (pixels) x colour depth (bits)

Example:
Image: 800 x 600 pixels, 24-bit colour depth
File size = 800 x 600 x 24 = 11,520,000 bits
         = 11,520,000 / 8 = 1,440,000 bytes
         = 1,440,000 / 1024 = 1,406.25 KB
         = 1,406.25 / 1024 = 1.37 MB

Sound Representation

Sound is analogue. To store it digitally, the sound wave is sampled at regular intervals.

• Sample rate - how many samples are taken per second (measured in Hz). Higher sample rate = better quality. CD quality = 44,100 Hz (44.1 kHz).
• Bit depth - the number of bits used to store each sample. Higher bit depth = more accurate representation of each sample. CD quality = 16 bits.

Sound File Size Calculation

File size (bits) = sample rate (Hz) x bit depth x duration (seconds) x channels

Example: 30 seconds of CD-quality stereo sound
File size = 44,100 x 16 x 30 x 2 = 42,336,000 bits
         = 42,336,000 / 8 = 5,292,000 bytes
         = 5,292,000 / 1024 = 5,168 KB
         = 5,168 / 1024 = 5.05 MB

Data Compression

Compression reduces file size for faster transmission and less storage space.

	Lossy Compression	Lossless Compression
How it works	Permanently removes some data (e.g. sounds humans cannot hear)	Reduces file size without losing any data; original can be perfectly reconstructed
File size	Much smaller files	Smaller than original but larger than lossy
Quality	Some quality loss (usually not noticeable)	No quality loss
Image example	JPEG	PNG
Sound example	MP3	WAV, FLAC
Best for	Photos, music, video (streaming, web)	Text files, medical images, professional audio

Von Neumann Architecture

The Von Neumann architecture is the design used by most modern computers. Its key feature is that both program instructions and data are stored together in main memory.

Component	Full Name	Role
CU	Control Unit	Directs the operation of the CPU; fetches, decodes, and coordinates the execution of instructions
ALU	Arithmetic Logic Unit	Performs calculations (add, subtract, multiply, divide) and logical comparisons (AND, OR, NOT, greater than, equal to)
Accumulator	ACC	A register that temporarily stores the result of calculations performed by the ALU
MAR	Memory Address Register	Holds the address of the memory location currently being read from or written to
MDR	Memory Data Register	Holds the data that has been read from memory or is about to be written to memory
PC	Program Counter	Holds the address of the next instruction to be fetched

The Fetch-Decode-Execute Cycle

The CPU continuously repeats this cycle to process every instruction:

1. FETCH: The address in the PC is copied to the MAR. The instruction at that memory address is fetched and placed in the MDR. The PC is incremented (PC + 1) to point to the next instruction.
2. DECODE: The CU decodes the instruction in the MDR to determine what operation to perform and what data is needed.
3. EXECUTE: The instruction is carried out. This might involve the ALU performing a calculation, data being read/written to memory, or output being produced.

CPU Performance Factors

Factor	How It Affects Performance
Clock speed	Measured in GHz. Higher clock speed = more cycles per second = faster processing. A 3.5 GHz CPU executes 3.5 billion cycles per second.
Number of cores	Each core can process instructions independently. A quad-core CPU can handle 4 tasks simultaneously. More cores = better at multitasking (but software must be designed to use multiple cores).
Cache size	Cache is a small amount of very fast memory inside the CPU. Frequently used data/instructions are stored here. Larger cache = less need to fetch from slower RAM = faster performance.

Embedded Systems

An embedded system is a computer system built into a larger device to perform a specific dedicated function. It is usually not a general-purpose computer.

• Examples: washing machine controller, car engine management, traffic lights, smart thermostat, digital watch
• Characteristics: small, low power, low cost, dedicated to one task, often run continuously, usually no traditional user interface

RAM vs ROM

	RAM (Random Access Memory)	ROM (Read Only Memory)
Volatile?	Yes - contents lost when power is off	No - contents retained without power
Read/Write?	Can be read and written to	Can only be read (not written to in normal use)
Purpose	Stores currently running programs and data	Stores the boot-up instructions (BIOS/firmware)
Speed	Very fast	Slower than RAM
Size	Typically 8-32 GB in modern PCs	Very small (just enough for startup instructions)

Virtual Memory

Virtual memory is a section of the hard drive/SSD that is used as if it were additional RAM when physical RAM is full.

• When RAM is full, the OS moves inactive data from RAM to virtual memory (this is called paging or swapping)
• This allows more programs to run than physical RAM could normally support
• However, the hard drive/SSD is much slower than RAM, so heavy use of virtual memory slows the computer down significantly (known as disk thrashing)