Computer Science (AS & A Level) revision notes

One chapter for each section of the 9618 syllabus, with worked examples. Read a chapter, then practise it with real past-paper questions.

1. 1Information RepresentationHow computers store numbers, text, images and sound in binary; how to convert between bases, add and subtract in two's complement, work out file sizes, and compress files.17 min read
2. 2CommunicationNetworks, topologies, client-server and peer-to-peer, wired and wireless, the hardware of a LAN and the internet, Ethernet, IP addresses, URLs and DNS.16 min read
3. 3HardwareInput, output, memory and storage devices, embedded and control systems, RAM and ROM types, buffers, and logic gates, truth tables and logic circuits.17 min read
4. 4Processor FundamentalsThe Von Neumann model, registers, buses, the fetch-execute cycle and interrupts, assembly language and the instruction set, addressing modes, and bit manipulation.20 min read
5. 5System SoftwareWhy computers need an operating system and what it manages, utility software and program libraries, assemblers, compilers and interpreters, and the features of an IDE.10 min read
6. 6Security, Privacy and Data IntegrityThe difference between security, privacy and integrity; threats from networks and the internet and the measures that reduce them; and validation and verification methods, including check digits, parity and checksums.14 min read
7. 7Ethics and OwnershipProfessional ethics for computing professionals, the impact of acting ethically or not, copyright law, and the types of software licence, including artificial intelligence issues.10 min read
8. 8DatabasesRelational databases, keys and relationships, entity-relationship diagrams and normalisation to third normal form, the DBMS and its tools, and SQL for defining and manipulating data.11 min read
9. 9Algorithm Design and Problem-solvingAbstraction and decomposition, expressing algorithms as structured English, flowcharts and pseudocode, identifier tables, stepwise refinement, trace tables, and logic statements.11 min read
10. 10Data Types and StructuresChoosing and declaring data types, records, one- and two-dimensional arrays with linear search and bubble sort, text files, and the abstract data types stack, queue and linked list.13 min read
11. 11ProgrammingWriting pseudocode with variables, constants, operators and built-in functions; selection and the three loops and when to use each; and procedures, functions, parameters, scope and efficient code.11 min read
12. 12Software DevelopmentProgram development life cycles (waterfall, iterative, RAD), the stages of development, structure charts and state-transition diagrams, types of error, testing methods and test data, and types of maintenance.13 min read
13. 13Data Representation (A Level)User-defined data types (enumerated, pointer, record and set), serial, sequential and direct file organisation and access, hashing, and binary floating-point numbers with normalisation and rounding errors.14 min read
14. 14Communication and Internet TechnologiesWhy protocols are needed, the TCP/IP protocol stack and its four layers, the purpose of HTTP, FTP, POP3, IMAP, SMTP and BitTorrent, and circuit switching compared with packet switching.11 min read
15. 15Hardware and Virtual MachinesRISC and CISC processors, pipelining, the four computer architectures, massively parallel computers and virtual machines; and half and full adders, flip-flops, Boolean algebra and Karnaugh maps.18 min read
16. 16System Software (A Level)How an OS maximises resources and hides the hardware, process states and scheduling, virtual memory with paging and segmentation, interpreters, the stages of compilation, BNF and syntax diagrams, and Reverse Polish Notation.15 min read
17. 17Security (A Level)How symmetric and asymmetric encryption work, public and private keys, SSL/TLS, digital signatures, and digital certificates issued by certificate authorities.9 min read
18. 18Artificial IntelligenceGraphs and shortest-path search, artificial neural networks, machine learning, deep learning and reinforcement learning, back propagation of errors, and regression.10 min read
19. 19Computational Thinking and Problem-solvingLinear and binary search, insertion sort and bubble sort, binary trees and other abstract data types built from each other, comparing algorithms with Big O, and recursion.13 min read
20. 20Further ProgrammingLow-level, imperative, object-oriented and declarative programming paradigms (including classes, inheritance, polymorphism, and facts and rules), plus file processing and exception handling.12 min read