OCR A Level Computer Science Paper 1 Notes (H446)

1.1 - The characteristics of contemporary processors, input, output and storage devices

1.1.1 Structure and function of the processor

CPU components:

ALU (Arithmetic Logic Unit)

Performs arithmetic (add, subtract) and logical (AND, OR, NOT, comparisons) operations on data.

Control Unit (CU)

Coordinates all activities of the CPU: fetches, decodes and executes instructions. Sends control signals to other components.

PC (Program Counter)

Holds the address of the next instruction to be executed.

ACC (Accumulator)

Temporarily stores calculations carried out by the ALU.

MAR (Memory Address Register)

Holds the address of the memory location of the next data or instruction to be fetched or stored.

MDR (Memory Data Register)

Used to temporarily store the data which is read from or written to memory.

CIR (Current Instruction Register)

Holds the instruction currently being decoded and executed.

Buses:

Data bus

Carries the data and instructions being transmitted between the CPU and memory. Bidirectional, so data can travel both ways.

Address bus

Carries the memory address that identifies where data is being read from or written to. Unidirectional, so addresses only travel from the CPU to memory.

Control bus

Carries control signals (such as read/write, clock, and interrupt signals) that manage and coordinate the operations of the CPU. Bidirectional, so signals can travel both ways.

Fetch-Decode-Execute cycle:

Fetch: The PC holds the address of the next instruction. This address is copied to the MAR and sent to main memory via the address bus. The CU sends a read signal via the control bus, and the instruction travels along the data bus into the MDR, which copies it into the CIR. The PC is then incremented by 1.
Decode: The decode unit decodes the instruction held in the CIR, splitting it into the opcode (what operation to perform) and the operand (what data or address to use).
Execute: The CPU carries out the operation specified by the instruction. Results of processing are stored in the ACC.

Factors affecting CPU performance:

Clock speed

Number of cycles per second (GHz). Higher = faster execution.

Limited by heat.

Number of cores

More cores = more instructions processed simultaneously.

Benefits multi-threaded applications.

Cache

Small, fast memory between CPU and RAM.

Reduces memory access time.

Pipelining: while one instruction is being executed, the next is being decoded and the one after is being fetched. Increases throughput but hazards (data dependencies, branching) can cause stalls.

Processor architectures:

Von Neumann

Single shared memory for both instructions and data. Single bus. Simple and cheap. Bottleneck = von Neumann bottleneck (bus limits speed).

Harvard

Separate memory and buses for instructions and data. Faster, as it can fetch instruction and data simultaneously. Used in embedded systems and DSPs.

Contemporary

Modern processor architectures that go beyond the classic Von Neumann model. May incorporate features from both Von Neumann and Harvard designs. The spec lists this as a third category alongside Von Neumann and Harvard.

1.1.2 Types of processor

CISC (Complex Instruction Set Computer)

Has many complex instructions so a single instruction can perform multiple operations. Aims to complete the task in as few lines of assembly as possible.

Advantages:

Compiler must do very little work to translate the high-level language statement into assembly.

Disadvantages:

More complex hardware.
A single instruction usually takes multiple clock cycles.
Slower clock cycles.
Pipelining is more difficult to implement.

RISC (Reduced Instruction Set Computer)

Simple set of instructions designed to execute very quickly. Aims to use simple instructions that will be executed within a single clock cycle.

Advantages:

Cheap.
Each instruction can be completed in a single clock cycle.
As instructions are sequential, pipelining is possible.
Results in lower energy consumption.

Disadvantages:

More instructions needed to do complex tasks.
Larger program size.

GPU (Graphics Processing Unit)

Thousands of smaller, simpler cores optimised for parallel processing. Originally for rendering graphics. Now used beyond graphics, e.g. machine learning, scientific simulation, video processing, physics simulation. Suited to tasks that can be broken into many simultaneous calculations.

GPUs use SIMD (Single Instruction, Multiple Data), where the same instruction is applied to multiple pieces of data at the same time, making them highly efficient for repetitive operations across large datasets.

Multicore and parallel systems

Multiple processor cores on one chip. Tasks divided between cores and run simultaneously. Parallel systems = multiple processors working together.

Benefits: faster for tasks that can be parallelised.

Limitations: some tasks are inherently sequential and cannot benefit from additional cores. Not all software is written to exploit multiple cores, so single-threaded applications see no benefit.

1.1.3 Input, output and storage

Magnetic storage (HDD)

Spinning magnetic platters. Large capacity, low cost per GB. Slower than SSD. Mechanical and susceptible to physical damage. Used for bulk data storage.

Flash storage (SSD, USB)

No moving parts. Fast read/write. More durable and silent. Higher cost per GB. Used in phones, laptops, USB drives.

Optical storage (CD/DVD/Blu-ray)

Laser reads/writes data on reflective disc. Large capacity (Blu-ray up to 128GB). Slower. Used for distribution and archiving.

RAM (Random Access Memory)

Volatile: loses data when power is off. Fast. Stores currently running programs and data. More RAM = more programs run simultaneously without slowdown.

ROM (Read Only Memory)

Non-volatile: retains data without power. Contains BIOS/firmware. Cannot be easily written to. Stores boot instructions.

Virtual storage

Uses part of secondary storage (HDD/SSD) as if it were RAM when RAM is full. Slower than RAM. Allows running programs larger than physical RAM.

Know the trade-offs: speed vs cost vs capacity vs volatility. Exam questions often ask you to justify storage choice for a given scenario.

1.2 - Software and software development

1.2.1 Systems software

Operating system functions: manages hardware resources, provides user interface, manages files, handles security and user accounts, manages processes and memory.

Memory management:

Paging

Memory divided into fixed-size pages. Programs split into pages. Pages stored wherever space exists, not necessarily contiguous.

Segmentation

Memory divided into variable-size segments based on logical divisions of a program (code, stack, data). More flexible than paging.

Virtual memory

Uses secondary storage as extension of RAM. Pages swapped in/out as needed (thrashing = excessive swapping → performance drops).

Interrupts: signal to the CPU that an event needs attention. CPU finishes current instruction, saves state to stack, executes ISR (Interrupt Service Routine), restores state and continues. Types: hardware interrupts (keyboard, mouse, I/O), software interrupts (program calls), timer interrupts.

Scheduling algorithms:

Round robin

Each process gets equal time slice in turn. Fair but higher priority tasks not favoured.

First come first served (FCFS)

Processes run in order of arrival. Simple. Long jobs can block short ones (convoy effect).

Shortest job first (SJF)

Shortest process runs first. Minimises average wait time. Requires knowing job length in advance.

Shortest remaining time (SRT)

Preemptive version of SJF. New shorter job can interrupt current one.

Multi-level feedback queues

Multiple queues with different priorities. Processes can move between queues based on behaviour.

Types of operating system:

Distributed

Manages resources across multiple machines that appear as one system to the user.

Embedded

Built into hardware devices. Minimal, dedicated to specific tasks (washing machine, car ECU).

Multi-tasking

Runs multiple processes simultaneously by rapidly switching between them.

Multi-user

Multiple users share one system simultaneously. Manages resource allocation between users.

Real-time

Guarantees response within a defined time. Used where timing is critical (air traffic control, medical devices). Hard RTOS = missing deadline = failure.

BIOS (Basic Input/Output System): firmware stored in ROM. First code to run on boot. Performs POST (Power-On Self Test), identifies hardware, loads OS bootloader.

Device drivers: software allowing the OS to communicate with hardware. Translates generic OS commands into device-specific instructions.

Virtual machines: software emulation of a computer. Allows one OS to run inside another. Uses: running legacy software, testing, sandboxing, cloud computing (hypervisors manage multiple VMs on one physical machine).

1.2.2 Applications generation

Open source vs closed source

Open source: source code publicly available, free to modify and distribute. Community maintained. Closed source: source code hidden, proprietary. Commercial support available.

Utilities

System tools for maintenance: antivirus, disk defragmenter, backup tools, file compression.

Translators:

Compiler

Translates entire high-level source code to machine code in one go. Creates an executable. Fast execution. Error list produced at end. Examples: C, C++.

Interpreter

Translates and executes source code line by line. No executable produced. Stops at first error. Slower execution. Easier to debug. Examples: Python.

Assembler

Translates assembly language (mnemonics) to machine code. One-to-one translation. Output is object code.

Stages of compilation (4 stages per spec): with each pass, the compiler performs different actions on the source code, preparing it for the next stage.

1. Lexical analysis

The lexer scans the source code letter by letter, identifying lexemes (words/symbols separated by whitespace, operators or special symbols) and converting them into a stream of tokens, typically in the format [tokenclass:token].

Each lexeme is checked against predefined rules and classified as a token: keyword, identifier, operator, separator, literal, datatype, etc.
Whitespace and comments are removed, as they are simply passed over by the lexer (this is a side effect of tokenising, not the main job).
Identifiers (variable and subroutine names) are added to a symbol table, which keeps track of every variable and subroutine used in the program.

2. Syntax analysis

The parser takes the token stream and analyses it against the language's production rules (grammar) to check the code is syntactically valid.

Builds an abstract syntax tree from the tokens, strictly following the syntax diagrams of the language.
Reports syntax errors if any token breaks the rules, stating the exact line and location of the error and possibly suggesting corrections.
The symbol table is updated with extra information about identifiers, e.g. their data type, which is used for semantic checks such as type checking (ensuring operations are applied to compatible types).

3. Code generation

The abstract syntax tree is converted into object code: machine code produced before the final linker step is run.

Each node of the abstract syntax tree is translated into the equivalent low-level (machine code / assembly) instructions.
The object code is then passed to the linker, which combines it with any library code (and a loader places it into memory) to produce the final executable.

4. Code optimisation

Tweaks the generated code so it runs as quickly as possible and uses as little memory as possible. The optimised object code achieves the same effect as the source program, but not necessarily by the same means. Optimisation can considerably increase compilation time.

Redundant instruction elimination: removes unnecessary load/store instructions, e.g. MOV x,R0 then MOV R0,R1 can be replaced with the single instruction MOV x,R1.
Unreachable code removal: deletes code that can never be executed (e.g. a print statement placed after a return).
Flow / control optimisation: removes pointless jumps, e.g. GOTO L1 where L1: GOTO L2 is rewritten as a direct GOTO L2.
Removes subroutines that are never called and variables/constants that are never referenced.

A common exam question is "What happens during the different phases of compilation?" For full marks, name all four stages in order (Lexical, Syntax, Code generation, Optimisation) and describe the key output of each: token stream + symbol table, abstract syntax tree, object code, optimised object code.

Libraries:

A library is a collection of ready-compiled and tested programs (subroutines/functions) that can be called upon when needed. Most programming languages include extensive standard libraries, for example, Python's math module provides functions such as sqrt, factorial, ceil, floor, and constants like pi. Windows programs can call Dynamic Link Libraries (DLLs), shared subroutines for common OS tasks (e.g. a Save As dialog box) that any program can call with the correct parameters.

Benefits of library routines

Quick and easy to integrate into your own code.
Pre-tested, so they are relatively free from errors.
Pre-compiled and typically optimised for fast execution.

Drawbacks of library routines

Adding functionality or making specific tweaks can be difficult or impossible.
Implementation is "black-boxed": you cannot always see or control how it works internally.
You must trust that developers will continue to maintain the library.

Linkers:

The linker combines multiple object code files (produced by compilers) with any required library code to produce a single executable. It resolves all external references by inserting the correct machine addresses into every external call and return instruction, so all modules are correctly linked together.

Static linking

All required library code is copied directly into the final executable at link time. The program is self-contained and does not depend on external library files at runtime. Drawback: larger executable file size; if the library is updated, the program must be recompiled and relinked to benefit.

Dynamic linking

Library code is not included in the executable. Instead, compiled library files (e.g. DLLs) remain separate on the host computer, and the OS links them into memory at runtime when needed. Advantage: smaller executables; multiple programs can share the same library in memory. Drawback: if a dynamic library is updated or removed, the program may fail because it tries to call a subroutine in the wrong way.

Loaders:

The loader is the part of the operating system responsible for loading the executable machine code file into memory so it is ready to run. When dynamic linking is used, the loader is also responsible for locating and loading the required library files into memory at the point they are needed.

Know the full pipeline: source code → compiler → object code → linker (+ libraries) → executable → loader → memory. Be clear on the difference between static linking (library baked in at link time) and dynamic linking (library loaded at runtime by the OS).

1.2.3 Software development methodologies

Waterfall

Linear, sequential. Each phase complete before next begins. Clear documentation. Poor for changing requirements. Good for well-defined projects.

Agile

Iterative sprints. Frequent customer feedback. Adapts to change. Less documentation. Good for unclear or evolving requirements.

Extreme Programming (XP)

Agile variant. Pair programming, test-driven development, frequent releases. Very short iterations.

Spiral model

Iterative + risk management. Each cycle: plan → risk analyse → develop → evaluate. Best for large, high-risk projects.

RAD (Rapid Application Development)

Prototyping + iterative delivery. Heavy user involvement. Fast delivery. Requirements can evolve.

1.2.4 Types of programming language

Programming paradigms:

Procedural / imperative

Step-by-step instructions. Uses procedures/functions. Examples: Python, Pascal, C. Good for straightforward sequential problems.

Object-oriented (OOP)

Models the world as objects. Key concepts: classes, objects, methods, attributes, inheritance, encapsulation, polymorphism.

Declarative

Describes what to do, not how. Examples: SQL, Prolog. Logic-based.

Functional

Based on mathematical functions. No side effects. Examples: Haskell, Erlang.

OOP concepts:

Class

Blueprint/template defining attributes and methods for a type of object.

Object

Instance of a class. Has its own attribute values.

Encapsulation

Bundling attributes and methods together. Hides internal state, which is only accessible via methods (getters/setters).

Inheritance

A subclass inherits attributes and methods of a parent class. Enables code reuse. "is-a" relationship.

Polymorphism

Same method name behaves differently in different classes. Overriding parent methods in subclasses.

Abstraction

Hiding complexity behind simple interfaces. User interacts with simplified interface, not internal workings.

Assembly language and addressing modes:

The spec requires following and writing simple programs using the Little Man Computer (LMC) instruction set:

LMC instruction set

Mnemonic	Operation
INP	Input a value → ACC
OUT	Output ACC
LDA x	Load value at address x → ACC
STA x	Store ACC → address x
ADD x	ACC = ACC + value at x
SUB x	ACC = ACC − value at x
BRA x	Branch always → address x
BRZ x	Branch to x if ACC = 0
BRP x	Branch to x if ACC ≥ 0
HLT	Halt the program
DAT	Define a data value / label a memory location

Addressing modes:

Immediate addressing

Operand IS the actual value. LDA #5 loads the value 5.

Direct addressing

Operand is the memory address containing the value. LDA 50 loads value at address 50.

Indirect addressing

Operand is address that contains the address of the value. Extra memory access needed.

Indexed addressing

Operand + value in index register = effective address. Used for arrays.

1.3 - Exchanging data

1.3.1 Compression, encryption and hashing

Lossy compression

Permanently removes data, so the original cannot be perfectly reconstructed. Smaller files. Used for images (JPEG), audio (MP3), video (MP4). Acceptable quality loss for the use case.

Lossless compression

Original data perfectly reconstructed on decompression. Used for text, executables, ZIP. Smaller reduction in size than lossy.

Run Length Encoding (RLE)

Replaces consecutive repeated values with a count and value. E.g. AAABBBBA → 3A4B1A. Effective for images with large areas of the same colour.

Dictionary coding (LZW)

Builds a dictionary of repeated patterns. Replaces patterns with shorter dictionary references. Effective for text with repeated words/phrases.

Symmetric encryption

Same key used to encrypt and decrypt. Fast. Problem: key must be securely shared. Examples: AES, DES. Used for encrypting large data.

Asymmetric encryption

Public key encrypts, private key decrypts. No need to share private key. Slower. Used for key exchange, digital signatures. Examples: RSA. Used in HTTPS handshake.

Hashing: one-way function that converts data to a fixed-length hash value. Cannot be reversed. Uses: password storage (store hash not password), data integrity checking, hash tables (for fast lookup), digital signatures.

Symmetric vs asymmetric: symmetric is faster but has the key distribution problem. Asymmetric solves key sharing but is slower. In practice, asymmetric is used to exchange a symmetric key, which is then used for bulk encryption (TLS does this).

1.3.2 Databases

Flat file

All data in one table. Simple but leads to data redundancy and update anomalies.

Relational database

Data split into multiple related tables. Linked by keys. Reduces redundancy. More complex to query.

Primary key

Unique identifier for each record in a table. Cannot be null.

Foreign key

Field in one table that is the primary key of another. Creates a link between tables.

Secondary key

Non-unique field used for searching/indexing. Speeds up queries.

Referential integrity

Foreign key values must match an existing primary key value, or be null. Prevents orphaned records.

Normalisation: process of structuring a database to reduce redundancy.

1NF: No repeating groups. Each cell contains one atomic value. Each row is unique.
2NF: In 1NF + no partial dependencies (all non-key attributes depend on the whole primary key, not just part of it). Applies to composite keys.
3NF: In 2NF + no transitive dependencies (non-key attributes depend only on the primary key, not on other non-key attributes).

SQL: the spec requires students to interpret and modify SQL queries:

SELECT column1, column2 FROM table1 WHERE condition ORDER BY column1 ASC INNER JOIN table2 ON table1.id = table2.id;

Also know: INSERT INTO, UPDATE SET, DELETE FROM. The focus is on reading and modifying existing queries rather than writing complex queries from scratch.

Transaction processing: ACID properties:

Atomicity

Transaction is all-or-nothing. If any part fails, the whole transaction is rolled back.

Consistency

Transaction brings database from one valid state to another. All rules/constraints maintained.

Isolation

Concurrent transactions execute as if sequential. Intermediate states not visible to other transactions.

Durability

Once committed, transaction persists even after system failure.

Record locking: prevents two users editing same record simultaneously. Deadlock: two processes each waiting for the other to release a lock; prevented by ordering locks or timeout.

Indexing: creates a separate data structure for fast lookups on a field. Speeds up queries but slows INSERT/UPDATE/DELETE.

1.3.3 Networks

Protocols and standards:

Protocol

A set of rules that define how data is transmitted between devices on a network. Ensures that different devices and software can communicate correctly.

Standard

An agreed specification that ensures compatibility between different hardware and software systems. Without standards, devices from different manufacturers could not interoperate. Examples: IEEE 802.11 (Wi-Fi), Ethernet.

Common application-layer protocols:

HTTP / HTTPS

HyperText Transfer Protocol: transfers web pages between browser and server. HTTPS adds TLS encryption for secure communication.

FTP

File Transfer Protocol: transfers files between a client and server over a network.

SMTP

Simple Mail Transfer Protocol: sends email from a client to a mail server and between mail servers.

POP3

Post Office Protocol 3: downloads emails from a mail server to the client and typically deletes them from the server. Designed for single-device access.

IMAP

Internet Message Access Protocol: reads emails stored on the mail server. Emails remain on the server so multiple devices can access the same inbox.

SSH

Secure Shell: provides an encrypted remote connection to another computer's command line over a network.

TCP/IP stack (4 layers):

Application layer

Protocols: HTTP, HTTPS, FTP, SMTP, POP3, IMAP, SSH, DNS. Provides network services directly to the user.

Transport layer

TCP or UDP. Breaks data into segments. Port numbers identify services (e.g. port 80 = HTTP, port 443 = HTTPS).

Internet/Network layer

IP addressing and routing. Packets routed across networks. IPv4 (32-bit, ~4 billion addresses) and IPv6 (128-bit, vastly larger address space).

Network access/Link layer

Physical transmission. MAC addresses (unique hardware identifier burned into NIC). Ethernet, Wi-Fi. Converts to electrical/optical/radio signals.

TCP (Transmission Control Protocol)

Connection-oriented: three-way handshake establishes connection before data is sent. Guarantees delivery and correct ordering. Error-checked. Used for HTTP, email, file transfer where reliability matters.

UDP (User Datagram Protocol)

Connectionless: data sent without establishing a connection first. No delivery guarantee and no ordering. Faster and lower overhead. Used for live streaming, online gaming, VoIP, DNS lookups where speed matters more than reliability.

POP3 vs IMAP: POP3 downloads and deletes from server (single device), IMAP keeps mail on the server (multi-device). Exam often asks you to justify which to use for a given scenario.

DNS (Domain Name System): translates domain names (e.g. example.com) to IP addresses. Hierarchical: client queries local resolver → root name servers → TLD servers (e.g. .com, .uk) → authoritative name server → returns IP. Results cached to reduce future lookups.

LAN (Local Area Network)

Covers a small geographic area (building/campus). Organisation owns and manages the infrastructure. High speed. Uses Ethernet or Wi-Fi. Example: school or office network.

WAN (Wide Area Network)

Spans large geographic areas (country/worldwide). Uses third-party infrastructure (ISPs, leased lines). The internet is the largest WAN.

Packet switching

Data split into packets, each routed independently and possibly via different paths. Reassembled at destination. Efficient and fault tolerant: if one route fails, packets take another. Used by the internet.

Circuit switching

Dedicated physical path established for the entire communication session. Guaranteed bandwidth. Wasteful (path reserved even when idle). Used in traditional telephone networks.

Network security threats:

Hacking

Unauthorised access to a computer system or network to steal, alter, or destroy data.

Malware / viruses

Malicious programs that attach to files and spread. Includes viruses (attach to files), worms (self-replicating, spread without user action), ransomware (encrypts files and demands payment), trojans (disguised as legitimate software).

Denial of Service (DoS / DDoS)

Floods a server or network with traffic, making it unavailable to legitimate users. DDoS (Distributed DoS) uses a botnet: many compromised machines attacking simultaneously.

Phishing

Fraudulent emails or messages that impersonate a trusted source to trick users into revealing passwords, card numbers or personal information.

Pharming

Corrupts DNS records or the hosts file to redirect users from a legitimate website to a fake one automatically, even if the user types the correct address. More dangerous than phishing as no user deception is required.

Spyware

Secretly monitors user activity, collecting passwords, keystrokes and browsing habits without the user's knowledge, and sends the data to a third party.

SQL injection

Malicious SQL code entered into a web form input that is passed unvalidated to a database query, allowing attackers to read, modify or delete database records.

Phishing requires the user to be deceived into clicking a link. Pharming redirects the user automatically at the DNS/network level; the user does not need to make any mistake.

Network security protection:

Firewall

Monitors and filters incoming and outgoing network traffic based on rules. Blocks unauthorised access. Can be hardware (dedicated device) or software. Packet filtering inspects individual packets; stateful inspection tracks the state of connections.

Proxy server

Intermediate server between client and internet. Hides the client's IP address, caches content for faster access, and can filter or block traffic. Used by organisations to monitor and control web usage.

Anti-virus / anti-malware

Scans files and running processes for known malware signatures and suspicious behaviour. Removes or quarantines threats. Must be kept up to date to recognise new threats.

Encryption (TLS/HTTPS)

Encrypts data in transit so it cannot be read if intercepted. HTTPS uses TLS: asymmetric encryption for the handshake, symmetric encryption for bulk data transfer.

User access levels

Restricts what each user can read, write, or execute. Principle of least privilege: users only have permissions needed for their role, limiting damage from compromised accounts.

Secure passwords / 2FA

Long, complex passwords are harder to crack. Two-factor authentication (2FA) adds a second verification step (e.g. SMS code), so a stolen password alone is insufficient.

Client-server

Central server provides services to clients. Centralised management and security. Server = single point of failure. Easier to back up and control access. Examples: web servers, email servers.

Peer to peer (P2P)

Each node acts as both client and server. Decentralised with no single point of failure. Harder to manage and secure. Examples: BitTorrent, blockchain.

Network hardware:

Router

Connects different networks together (e.g. LAN to the internet). Routes packets between networks using IP addresses. Assigns local IP addresses via DHCP.

Switch

Connects devices within a LAN. Sends data only to the intended device by looking up its MAC address. More efficient than a hub; other devices do not see traffic not meant for them.

Hub

Connects devices in a LAN but broadcasts all data to every device regardless of the intended recipient. Less efficient and less secure than a switch. Largely obsolete.

NIC (Network Interface Card)

Hardware that allows a device to connect to a network. Contains a unique MAC address (Media Access Control) that identifies the device at the link layer.

WAP (Wireless Access Point)

Connects wireless devices to a wired LAN using Wi-Fi. Extends the network's wireless coverage area.

Modem

Converts digital signals from a computer to analogue signals for transmission over phone or cable lines, and vice versa. Required to connect a home/office LAN to the internet via an ISP.

1.3.4 Web technologies

HTML

HyperText Markup Language. Defines structure and content of web pages. Uses tags: <h1>, <p>, <a>, <form>.

CSS

Cascading Style Sheets. Defines presentation/styling: colours, fonts, layout. Separates style from content.

JavaScript

Client-side scripting. Adds interactivity. Runs in the browser. Manipulates the DOM.

Search engine indexing: web crawlers follow links and index page content. Index is a data structure mapping keywords to URLs. Allows fast search queries.

PageRank algorithm: ranks pages by the number and quality of links pointing to them. More links from high-ranked pages = higher rank. Iterative algorithm: rank flows through the link graph.

Client-side processing

Code runs on the user's device (browser). JavaScript. Fast, with no server round trip. Can access local resources. Less secure, as code is visible.

Server-side processing

Code runs on the server. PHP, Python, Node.js. More secure, as code is hidden. Database access. Slower, as it requires a network request.

1.4 - Data types, data structures and algorithms

1.4.1 Data types and number representation

Primitive data types: integer, real/floating-point, character, string, Boolean.

Two's complement (representing negative integers):

To negate: flip all bits, then add 1
MSB has negative place value: e.g. 8-bit: -128 to +127
Addition works normally; overflow detected if carry into and out of MSB differ

Sign and magnitude: MSB = sign bit (0=positive, 1=negative). Simpler but two representations of zero. Less used in practice.

Hexadecimal: base 16, digits 0–9 and A–F. Each hex digit = 4 bits (nibble). Used as shorthand for binary, as it is easier to read. IPv6 addresses, colour codes (#FF5733), memory addresses.

Floating point representation: sign bit + mantissa + exponent. Value = mantissa × 2^exponent.

Normalisation: ensures leading bit of mantissa is always 1 (or 0 for negative in two's complement). Maximises precision for a given number of bits.
More mantissa bits = greater precision. More exponent bits = greater range.
Floating point errors: rounding, representation errors (some decimals can't be exactly represented in binary).

Bitwise operations:

AND mask

Used to check or clear specific bits. Masking with 1 preserves bit; with 0 clears bit.

OR mask

Used to set specific bits. Masking with 1 sets bit; with 0 preserves bit.

XOR mask

Used to toggle specific bits. XOR with 1 flips bit; with 0 preserves bit. Also used in simple encryption.

Logical shift left

Shifts bits left, fills with 0s on right. Equivalent to multiplying by 2 per shift.

Logical shift right

Shifts bits right, fills with 0s on left. Equivalent to integer division by 2 per shift.

Character sets: ASCII (7-bit, 128 characters, basic Latin), Extended ASCII (8-bit, 256), Unicode (up to 32-bit, covers all world languages and symbols). UTF-8 is the most common Unicode encoding.

Binary addition: add column by column right to left. 0+0=0, 0+1=1, 1+1=10 (write 0 carry 1), 1+1+1=11 (write 1 carry 1). Overflow occurs if the result requires more bits than available.

Binary subtraction: use two's complement by negating the number being subtracted, then add. Alternatively, subtract directly: 0−0=0, 1−0=1, 1−1=0, 10−1=1 (borrow from next column).

1.4.2 Data structures

Array

Fixed-size, same data type, indexed. Up to 3D. Fast random access O(1). Slow insertion/deletion.

Record

Collection of related fields of different data types. Like a row in a database.

List

Ordered, variable size, can hold mixed types (in some languages). Dynamic.

Tuple

Immutable ordered sequence. Cannot be changed after creation. Faster than lists.

Stack

LIFO (Last In First Out). Operations: push, pop, peek, isEmpty. Used for: function call stack, undo, expression evaluation, depth-first search.

Queue

FIFO (First In First Out). Operations: enqueue, dequeue, peek, isEmpty. Used for: scheduling, breadth-first search, print spooling. Circular queue avoids wasted space.

Linked list

Nodes containing data + pointer to next node. Dynamic size. O(1) insertion/deletion if position known. O(n) search. No random access.

Binary search tree

Left child < parent < right child. O(log n) search/insert if balanced. O(n) if unbalanced (degenerates to linked list).

Hash table

Key → hash function → index. O(1) average search/insert. Collisions handled by chaining or open addressing. Used for dictionaries, caches, sets.

Graph

Nodes (vertices) connected by edges. Directed (one-way) or undirected. Weighted (edges have costs) or unweighted. Used for networks, maps, social graphs.

Know how to create, traverse, add and remove from each structure. Also know which is best for which scenario, e.g. hash table for fast lookup, queue for scheduling, stack for backtracking.

1.4.3 Boolean algebra

Logic gates: AND, OR, NOT, NAND, NOR, XOR. Know their truth tables and symbols.

Boolean laws for simplification (as listed in spec):

De Morgan's Laws

The NOT of an AND is the OR of the NOTs, and vice versa. Used to convert between NAND/NOR and equivalent AND/OR/NOT expressions.

¬(A ∧ B) ≡ ¬A ∨ ¬B
¬(A ∨ B) ≡ ¬A ∧ ¬B

Distribution

AND distributes over OR, and OR distributes over AND - analogous to multiplication over addition in arithmetic.

A ∧ (B ∨ C) ≡ (A ∧ B) ∨ (A ∧ C)
A ∨ (B ∧ C) ≡ (A ∨ B) ∧ (A ∨ C)

Association

The grouping of operands does not affect the result. Brackets can be rearranged freely when all operators are the same.

A ∧ (B ∧ C) ≡ (A ∧ B) ∧ C
A ∨ (B ∨ C) ≡ (A ∨ B) ∨ C

Commutation

The order of operands does not affect the result. A AND B gives the same output as B AND A.

A ∧ B ≡ B ∧ A
A ∨ B ≡ B ∨ A

Double negation

Negating a value twice returns the original value. Two NOTs cancel each other out.

¬(¬A) ≡ A

Karnaugh maps (K-maps): visual tool for simplifying Boolean expressions. Group 1s in powers of 2 (1, 2, 4, 8). Larger groups = simpler expression. Wrap around edges allowed. Eliminates need for algebraic manipulation.

D-type flip-flop: stores 1 bit. Output Q takes the value of input D on the rising clock edge. Used in registers and memory cells.

Half adder: adds two 1-bit inputs. Sum = A XOR B. Carry = A AND B.

Full adder: adds two 1-bit inputs + carry in. Built from two half adders. Enables multi-bit addition.

1.5 - Legal, moral, cultural and ethical issues

1.5.1 Computing-related legislation

Data Protection Act 1998

Controls how personal data is collected, stored and used. Principles: data must be fairly obtained, used only for stated purpose, accurate, not kept longer than necessary, kept secure. Individuals have right to access their data.

Computer Misuse Act 1990

Three offences: (1) unauthorised access to computer material, (2) unauthorised access with intent to commit further offences, (3) unauthorised modification of computer material (malware, deleting files).

Protects original works (software, music, text, images) from being copied without permission. Software piracy is illegal. Licences define permitted use.

Regulation of Investigatory Powers Act 2000 (RIPA)

Governs interception of communications. Allows government/law enforcement to monitor communications with authorisation. Covers internet and phone surveillance.

1.5.2 Moral and ethical issues

Computers in the workforce

Automation displaces jobs. New jobs created in tech. De-skilling. Digital divide: those without tech skills are left behind.

Automated decision making

Algorithms making decisions (loan approvals, sentencing, hiring). Concerns: bias, lack of transparency, accountability, right to explanation.

Artificial intelligence

Opportunities: healthcare, efficiency, safety. Risks: job displacement, bias, autonomous weapons, loss of human control, existential risk.

Environmental effects

Data centres consume huge energy. E-waste from discarded devices. Carbon footprint of internet. Green computing initiatives.

Censorship and the internet

Governments blocking content. Protecting children vs free speech. The Great Firewall of China. Net neutrality debates.

Monitoring behaviour

Employers monitoring employees. Government surveillance. Smart devices collecting data. Consent and transparency issues.

Analysing personal information

Big data analytics. Targeted advertising. Social credit systems. GDPR protections vs commercial interests.

Piracy and offensive communications

Illegal copying of software/media. Cyberbullying, hate speech, extremism online. Challenges of moderation at scale.