📚 A-Level WJEC Computer Science: Last-Minute Revision Notes | A-Level WJEC 计算机：考前冲刺笔记
As the WJEC A-Level Computer Science exam approaches, a focused review of core topics can boost your confidence and performance. These revision notes cover essential concepts across programming, systems, networking, databases, and software development, tailored to the WJEC specification.
随着 WJEC A-Level 计算机科学考试的临近，有针对性地复习核心主题能够提升信心和成绩。这份冲刺笔记涵盖了编程、系统、网络、数据库和软件开发等重要概念，专为 WJEC 大纲量身打造。
1. Programming Fundamentals and Data Structures | 编程基础与数据结构
Variables store data values and must be declared with appropriate data types: INTEGER, REAL, STRING, BOOLEAN, CHAR. In WJEC pseudocode, you use keywords like DECLARE var AS TYPE.
变量用于存储数据值，必须使用合适的数据类型声明，例如 INTEGER、REAL、STRING、BOOLEAN 和 CHAR。在 WJEC 的伪代码中，使用类似 DECLARE var AS TYPE 的关键字。
Selection is handled by IF…THEN…ELSE…ENDIF, while iteration uses FOR, WHILE, and REPEAT…UNTIL loops. Nested structures are common in exam questions.
选择结构使用 IF…THEN…ELSE…ENDIF 实现，而迭代则使用 FOR、WHILE 和 REPEAT…UNTIL 循环。嵌套结构在考试题目中非常常见。
Arrays hold multiple elements of the same type. One-dimensional array: ARRAY[0:9] OF INTEGER. Two-dimensional arrays are often used for tables or grids. Records allow mixing data types under a single entity.
数组用于存储相同类型的多个元素。一维数组声明为 ARRAY[0:9] OF INTEGER，二维数组常用于表格或网格。记录可以将不同数据类型组合在一个实体下。
File handling in pseudocode includes OPEN, READ, WRITE, and CLOSE operations. You must understand sequential and random access, as well as handling end-of-file checks.
伪代码中的文件操作包括 OPEN、READ、WRITE 和 CLOSE。必须理解顺序访问和随机访问，以及如何检测文件结束标志。
2. Algorithms and Computational Thinking | 算法与计算思维
Computational thinking involves decomposition, pattern recognition, abstraction, and algorithm design. These skills are tested in problem-solving scenarios.
计算思维涉及分解、模式识别、抽象和算法设计。这些技能会在问题解决类题目中进行考查。
Sorting algorithms: bubble sort repeatedly swaps adjacent elements; insertion sort builds the sorted list one item at a time; merge sort uses a divide-and-conquer strategy and has O(n log n) time complexity.
排序算法：冒泡排序反复交换相邻元素；插入排序每次将一个元素插入已排序序列；归并排序采用分治策略，时间复杂度为 O(n log n)。
Searching algorithms: linear search checks each element sequentially (O(n)); binary search requires a sorted array and halves the search space (O(log n)). Binary search is more efficient for large datasets.
搜索算法：线性搜索顺序检查每个元素，时间复杂度 O(n)；二分搜索需要已排序的数组，每次将搜索空间减半，时间复杂度 O(log n)。对于大数据集，二分搜索更高效。
Algorithm efficiency is described using Big O notation: O(1) constant, O(n) linear, O(n²) quadratic. Be able to compare algorithms based on their time and space complexity.
算法效率用大 O 表示法描述：O(1) 常数阶，O(n) 线性阶，O(n²) 平方阶。要能够根据时间和空间复杂度比较算法。
3. Computer Systems and Architecture | 计算机系统与体系结构
The von Neumann architecture features a single shared memory for instructions and data, with a control unit, ALU, and registers. The fetch-decode-execute cycle repeats continuously.
冯·诺依曼体系结构的特点是指令和数据共享同一内存，包含控制单元、ALU 和多个寄存器。取指-译码-执行周期不断重复。
Key registers: PC (program counter), MAR (memory address register), MDR (memory data register), CIR (current instruction register), and ACC (accumulator). Buses: address bus, data bus, and control bus.
关键寄存器：PC（程序计数器）、MAR（内存地址寄存器）、MDR（内存数据寄存器）、CIR（当前指令寄存器）和 ACC（累加器）。总线包括地址总线、数据总线和控制总线。
RISC vs CISC: RISC processors have a small, simple instruction set, usually with fixed-length instructions and more registers. CISC has complex instructions that can do multiple operations, often with variable length.
RISC 与 CISC：RISC 处理器指令集精简且简单，通常采用定长指令和更多寄存器；CISC 拥有复杂指令，可以执行多个操作，指令长度常可变。
Operating systems manage hardware resources, provide a user interface, handle multitasking, and manage memory and peripheral devices through drivers. Interrupts signal the CPU to stop its current task and run an interrupt service routine.
操作系统管理硬件资源，提供用户界面，处理多任务，并通过驱动程序管理内存和外设。中断用于通知 CPU 暂停当前任务并执行中断服务程序。
4. Data Representation | 数据表示
Binary is base-2; hexadecimal is base-16 (0-9, A-F). Denary to binary conversion involves division by 2. Binary addition and overflow must be understood.
二进制是基数为 2 的计数系统；十六进制基数为 16，使用 0-9 和 A-F。十进制转二进制使用除以 2 取余法。必须理解二进制加法和溢出。
Signed integers can be represented using sign-and-magnitude or two’s complement. In two’s complement, the most significant bit is negative; e.g., 8-bit range is -128 to 127.
有符号整数可用原码或补码表示。在补码表示法中，最高位为负权值，例如 8 位补码的范围是 -128 到 127。
Character encoding: ASCII uses 7 or 8 bits for common characters; Unicode supports a vast range of global scripts using variable-length encoding (UTF-8, UTF-16).
字符编码：ASCII 使用 7 或 8 位表示常用字符；Unicode 采用可变长度编码（如 UTF-8、UTF-16），支持全球大量文字字符。
Images are represented as pixels with colour depth and resolution. Sound is sampled at a certain rate and bit depth. Compression methods: lossless (e.g., run-length encoding) and lossy (e.g., JPEG, MP3).
图像由像素组成，涉及颜色深度和分辨率。声音通过采样率和位深度数字化。压缩方法包括无损压缩（如行程长度编码）和有损压缩（如 JPEG、MP3）。
5. Networking and Communication | 网络与通信
LANs cover a small area; WANs span large geographical regions. Topologies: star (central switch/hub), bus (single backbone), mesh (fully or partially connected). Each has advantages in cost, reliability, and scalability.
局域网覆盖较小区域；广域网跨越广阔地理范围。拓扑结构：星型（中心交换机/集线器）、总线型（单一骨干）、网状（全连接或部分连接）。各有成本、可靠性和可扩展性方面的优势。
The TCP/IP stack layers: application, transport, internet, network access. Protocols: HTTP/HTTPS for web, FTP for file transfer, SMTP/POP3/IMAP for email. DNS resolves domain names to IP addresses.
TCP/IP 协议栈层次：应用层、传输层、网际层、网络接入层。常见协议：HTTP/HTTPS 用于网页，FTP 用于文件传输，SMTP/POP3/IMAP 用于电子邮件。DNS 将域名解析为 IP 地址。
Network security involves firewalls (filtering packets), encryption (symmetric and asymmetric), and authentication. Symmetric encryption uses a single shared key; asymmetric uses a public/private key pair.
网络安全涉及防火墙（包过滤）、加密（对称和非对称）以及身份认证。对称加密使用单个共享密钥；非对称加密使用公钥/私钥对。
Client-server and peer-to-peer models differ in resource distribution and management. In client-server, central servers provide services; in P2P, each node acts as both client and server.
客户端-服务器模型和对等网络模型在资源分配和管理上有所不同。客户端-服务器由中央服务器提供服务；P2P 中每个节点既是客户端也是服务器。
6. Databases and SQL | 数据库与 SQL
Relational databases organise data into tables (relations) with rows (records/tuples) and columns (attributes). Each table requires a primary key to uniquely identify rows. Foreign keys create relationships between tables.
关系数据库将数据组织成表（关系），包含行（记录/元组）和列（属性）。每个表需要主键来唯一标识行，外键用于建立表之间的关系。
Entity-Relationship (ER) diagrams model entities, attributes, and relationships (1:1, 1:M, M:M). Normalisation reduces data redundancy up to 3NF by eliminating partial and transitive dependencies.
实体关系图（ER 图）用实体、属性和关系（1:1、1:M、M:M）建模。规范化通过消除部分依赖和传递依赖将数据冗余降低至第三范式（3NF）。
SQL commands: SELECT columns FROM table WHERE condition ORDER BY column; JOIN combines rows from multiple tables; INSERT INTO table VALUES (…); UPDATE table SET …; DELETE FROM table WHERE …
SQL 命令：SELECT 列 FROM 表 WHERE 条件 ORDER BY 列；JOIN 合并多个表的行；INSERT INTO 表 VALUES (…) 插入数据；UPDATE 表 SET … 更新；DELETE FROM 表 WHERE … 删除。
ACID properties ensure reliable transactions: Atomicity, Consistency, Isolation, Durability. These are crucial for maintaining data integrity in multi-user database systems.
ACID 特性确保事务可靠：原子性、一致性、隔离性、持久性。这些特性对于多用户数据库系统中维护数据完整性至关重要。
7. Software Engineering and Project Management | 软件工程与项目管理
The waterfall model follows a linear sequence: requirements analysis, design, implementation, testing, deployment, and maintenance. It suits projects with well-understood requirements.
瀑布模型遵循线性顺序：需求分析、设计、实现、测试、部署和维护。适用于需求明确的项目。
Agile methodologies (e.g., Scrum) use iterative development with sprints, frequent feedback, and adaptive planning. They respond well to changing requirements.
敏捷方法（如 Scrum）采用迭代开发，包含冲刺、频繁反馈和自适应计划，能很好地应对需求变更。
Testing levels: unit testing (individual components), integration testing (combined modules), system testing (whole system), and acceptance testing (user validation). Black-box testing checks functionality without seeing code; white-box tests internal logic.
测试级别：单元测试（单个组件）、集成测试（模块组合）、系统测试（整个系统）和验收测试（用户验证）。黑盒测试在不看代码的情况下验证功能；白盒测试检验内部逻辑。
Version control, documentation, and use of CASE tools are part of professional practice. Trace tables and dry runs help verify algorithm correctness during desk checking.
版本控制、文档编写和 CASE 工具的使用是专业实践的一部分。跟踪表和手工执行（dry run）有助于在桌面检查中验证算法正确性。
8. Legal, Moral, Ethical and Environmental Issues | 法律、道德、伦理和环境问题
The Data Protection Act and GDPR regulate the collection, storage, and processing of personal data. Individuals have rights to access and correct their data. Organisations must obtain consent and ensure security.
《数据保护法》和 GDPR 规范个人数据的收集、存储和处理。个人有权访问和更正自己的数据，组织必须获取同意并确保安全。
The Computer Misuse Act criminalises unauthorised access to computer material, with offences including hacking and malware distribution. Students must understand the implications for cybersecurity.
《计算机滥用法》将未经授权访问计算机材料定为犯罪，包括黑客行为和恶意软件传播。学生需理解其对网络安全的意义。
Copyright and intellectual property protect software, digital content, and creative works. Open source licenses (e.g., GPL, MIT) permit certain uses while retaining some rights.
版权和知识产权保护软件、数字内容和创意作品。开源许可证（如 GPL、MIT）在保留部分权利的同时允许特定用途。
Environmental concerns include e-waste disposal, energy consumption of data centres, and sustainable computing practices. Ethical decision-making frameworks help professionals navigate dilemmas.
环境问题包括电子垃圾处理、数据中心能耗和可持续计算实践。道德决策框架有助于专业人士处理两难问题。
9. Exam Techniques and Common Pitfalls | 考试技巧与常见错误
Read the question carefully and identify command words like ‘describe’, ‘explain’, ‘compare’, or ‘evaluate’. Match your answer’s depth to the mark allocation.
仔细读题，识别指令词，如“描述”、“解释”、“比较”或“评价”。根据分值调整回答的深度。
In pseudocode questions, use consistent indentation and standard keywords. Avoid ambiguous variable names; remember to declare all variables and close control structures properly.
伪代码题中，保持一致的缩进和标准关键字。避免歧义变量名，记得声明所有变量并正确关闭控制结构。
For SQL, watch syntax: SELECT comes before FROM, JOIN follows FROM, and WHERE precedes ORDER BY. Always specify the join condition to avoid Cartesian products.
SQL 要注意语法：SELECT 在 FROM 之前，JOIN 跟在 FROM 之后，WHERE 在 ORDER BY 之前。务必指定连接条件以避免笛卡尔积。
When performing binary calculations, double-check two’s complement conversion, carry bits, and overflow flags. Use small test values to verify logic.
进行二进制计算时，反复检查补码转换、进位位和溢出标志。用较小的测试值验证逻辑。
Manage your time: allocate roughly one minute per mark. Start with the questions you are most confident about, and leave space for longer essay-style answers.
管理好时间：大致按每题每分一分钟分配。先做最有把握的题目，为长篇论述题留出空间。
Practice with past papers under timed conditions. Mark schemes reveal the exact phrasing and key points examiners look for, so study them closely.
在限时条件下练习历年真题。评分方案揭示了考官期望的具体措辞和关键点，务必仔细研究。
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