📚 AS CCEA Computer Science: Core Revision Guide | AS CCEA 计算机：核心知识点梳理
This guide brings together the essential topics for the CCEA AS Level Computer Science specification. It covers the theory examined in Unit AS 2 and the software development principles practiced in Unit AS 1, providing a balanced overview of how modern computer systems are built, programmed and evaluated.
本指南汇集了 CCEA AS 计算机科学课程的核心主题。它涵盖了 AS 2 单元考察的理论知识以及 AS 1 单元实践的软件开发原则，全面概述了现代计算机系统的构建、编程与评估方式。
1. Software Development Life Cycle (SDLC) | 软件开发生命周期
The Systems Development Life Cycle is a staged process for creating software. The traditional waterfall model moves sequentially through feasibility, analysis, design, implementation, testing, deployment and maintenance. Each phase must be signed off before the next begins.
系统开发生命周期是一个分阶段的软件创建过程。传统的瀑布模型依序经过可行性、分析、设计、实施、测试、部署和维护。每个阶段必须批准后方可进入下一阶段。
A feasibility study investigates whether a project is technically possible, financially viable and legally compliant. Analysts produce a report for stakeholders, weighing benefits against costs and risks.
可行性研究调查项目在技术上是否可行、财务上是否有利以及法律上是否合规。分析人员为利益相关者撰写报告，权衡收益与成本及风险。
Requirements analysis uses fact‑finding methods such as interviews, questionnaires and document review. The output is a requirements specification that describes what the system should do, not how it will do it.
需求分析采用访谈、问卷调查和文档审查等事实调查方法。成果是一份需求规格说明书，描述系统应该做什么，而不是如何做。
System design produces detailed artifacts: data flow diagrams, entity‑relationship diagrams, screen mock-ups and database schemas. These documents guide programmers during implementation.
系统设计产出详细的制品：数据流图、实体关系图、界面原型和数据库模式。这些文档在实施阶段指导程序员。
2. Analysis and Design Techniques | 分析与设计技术
Data flow diagrams (DFDs) model how data moves through a system. Processes are shown as circles, external entities as rectangles, data stores as open‑ended rectangles and data flows as arrows. Level 0 gives an overview, while deeper levels decompose processes.
数据流图 (DFD) 对数据在系统中的流动进行建模。过程用圆圈表示，外部实体用矩形，数据存储用开口矩形，数据流用箭头。0 层图给出概览，更深层次则分解过程。
Entity‑relationship diagrams (ER diagrams) represent the structure of a database. Entities become tables, attributes become columns, and relationships are shown with lines and cardinality markers such as 1, M. A many‑to‑many relationship must be resolved by introducing a linking table.
实体关系图 (ER图) 表示数据库的结构。实体成为表，属性成为列，关系用线条和基数标记（如 1, M）表示。多对多关系必须通过引入链接表来解决。
User interface design focuses on layout, consistency and usability. Designers produce wireframes and prototypes, ensuring input controls are intuitive and feedback is provided to the user after every action.
用户界面设计注重布局、一致性和易用性。设计人员制作线框和原型，确保输入控件直观且每次操作后向用户提供反馈。
3. Programming Fundamentals and Algorithms | 编程基础与算法
Variables and data types are the building blocks of any program. Common types include integer, real/float, Boolean, character and string. CCEA pseudocode uses a strongly typed style, requiring variables to be declared before use.
变量和数据类型是任何程序的基本构件。常见类型包括整数、实型/浮点型、布尔型、字符和字符串。CCEA 伪代码采用强类型风格，要求变量在使用前声明。
Control structures govern program flow: sequence, selection (IF…THEN…ELSE, CASE) and iteration (FOR, WHILE, REPEAT…UNTIL). Nesting these structures allows complex logic to be built.
控制结构管理程序流程：顺序、选择（IF…THEN…ELSE, CASE）和迭代（FOR, WHILE, REPEAT…UNTIL）。嵌套这些结构可以构建复杂逻辑。
Standard algorithms include linear search, binary search, bubble sort and insertion sort. A binary search requires a sorted array and repeatedly halves the search space, giving O(log n) efficiency.
标准算法包括线性查找、二分查找、冒泡排序和插入排序。二分查找要求数组已排序，不断将查找空间减半，效率为 O(log n)。
Procedures and functions allow code reuse. A function returns a single value, whereas a procedure performs actions without returning a value. Parameters can be passed by value or by reference.
过程和函数允许代码重用。函数返回单个值，而过程执行操作但不返回值。参数可以按值传递或按引用传递。
4. Testing Strategies and Quality Assurance | 测试策略与质量保证
Testing aims to uncover errors before a system goes live. Key stages include unit testing (individual modules), integration testing (combined modules) and system testing (the complete product). Acceptance testing involves end users confirming the system meets their needs.
测试旨在系统上线前发现错误。关键阶段包括单元测试（单个模块）、集成测试（组合模块）和系统测试（完整产品）。验收测试由最终用户确认系统满足需求。
Test data should cover normal, boundary and erroneous values. For example, if a field accepts 1‑100, normal data might be 50, boundary data 1 and 100, and erroneous data 0 or 101.
测试数据应覆盖正常值、边界值和错误值。例如，若某字段接受 1‑100，正常数据可为 50，边界数据为 1 和 100，错误数据为 0 或 101。
Black‑box testing checks functionality without seeing the internal code. White‑box testing examines the internal logic, branches and paths. Both are needed to achieve thorough validation.
黑盒测试在不查看内部代码的情况下检查功能。白盒测试检查内部逻辑、分支和路径。两者都需要以实现彻底的验证。
5. Computer Architecture: CPU and Memory | 计算机体系结构：CPU 与存储器
The Von Neumann architecture stores both instructions and data in the same memory. The central processing unit (CPU) consists of the control unit (CU), arithmetic logic unit (ALU) and registers such as the program counter (PC), memory address register (MAR) and memory data register (MDR).
冯·诺依曼架构将指令和数据存储在同一内存中。中央处理器 (CPU) 由控制单元 (CU)、算术逻辑单元 (ALU) 以及程序计数器 (PC)、内存地址寄存器 (MAR)、内存数据寄存器 (MDR) 等寄存器组成。
The fetch‑decode‑execute cycle is the heartbeat of a computer. In the fetch step, the PC value is placed in the MAR and the instruction is read into the MDR. Decoding interprets the opcode, then the CU directs execution, often involving the ALU.
取指‑译码‑执行周期是计算机的心跳。在取指步骤，PC 的值放入 MAR，指令读入 MDR。译码阶段解释操作码，然后 CU 指挥执行，通常涉及 ALU。
Factors affecting CPU performance include clock speed (GHz), number of cores and cache size. A higher clock speed executes more cycles per second; multiple cores allow true parallel processing.
影响 CPU 性能的因素包括时钟速度 (GHz)、核心数量和缓存大小。时钟速度越高，每秒执行的周期越多；多核心允许真正的并行处理。
The Little Man Computer (LMC) is an instruction set simulator that helps explain assembly language. Mnemonics such as LDA, STA, ADD, SUB, BRZ and HLT correspond to simple opcodes. Labels represent memory addresses.
小矮人计算机 (LMC) 是一种指令集模拟器，有助于解释汇编语言。助记符如 LDA, STA, ADD, SUB, BRZ 和 HLT 对应简单的操作码。标签代表内存地址。
| Mnemonic | Opcode | Description & 描述 |
|---|---|---|
| LDA | 5 | Load accumulator from address / 从地址加载累加器 |
| STA | 3 | Store accumulator to address / 存储累加器到地址 |
| ADD | 1 | Add value to accumulator / 加到累加器 |
| SUB | 2 | Subtract value from accumulator / 从累加器减去 |
| BRZ | 7 | Branch if zero / 如果为零则跳转 |
| HLT | 0 | Halt execution / 停止执行 |
6. Data Representation and Number Systems | 数据表示与数制
Computers use binary (base‑2). Conversion between binary, denary (base‑10) and hexadecimal (base‑16) is fundamental. Each hex digit represents four bits, making hex a compact notation for binary values.
计算机使用二进制（基数为 2）。二进制、十进制（基数为 10）与十六进制（基数为 16）之间的转换是基础。每个十六进制数字代表四个比特，使十六进制成为二进制值的紧凑记法。
1010₂ = 1×2³ + 0×2² + 1×2¹ + 0×2⁰ = 10₁₀, and A₁₆ = 10₁₀
Negative integers can be represented by sign‑and‑magnitude or two’s complement. In two’s complement, the most significant bit is negative. To find the two’s complement, invert all bits and add 1.
负整数可以用原码或二进制补码表示。在补码表示中，最高有效位为负。求补码时，将所有位取反然后加 1。
Two’s complement of 5 in 8 bits: 00000101 → invert → 11111010 + 1 = 11111011 (for -5)
Character representation uses ASCII (7 or 8 bits, 128 or 256 characters) and Unicode (up to 32 bits, covering all global scripts). Images are stored as bitmaps: resolution (pixels) × colour depth (bits per pixel) determines file size.
字符表示使用 ASCII（7 或 8 位，128 或 256 个字符）和 Unicode（最多 32 位，涵盖全球所有文字）。图像以位图存储：分辨率（像素）× 颜色深度（每像素比特数）决定文件大小。
Image file size ≈ width × height × colour depth (bits)
Sound is sampled at a rate (e.g. 44.1 kHz) with a bit depth (e.g. 16 bits). The sample rate must be at least double the highest frequency to satisfy the Nyquist theorem.
声音以采样率（例如 44.1 kHz）和位深度（例如 16 位）采样。采样率必须至少是最高频率的两倍以满足奈奎斯特定理。
7. Networks and Communication Protocols | 网络与通信协议
Networks can be classified as LAN (Local Area Network) or WAN (Wide Area Network). A LAN connects computers within a limited area using Ethernet or Wi‑Fi, while a WAN spans large geographic distances, often using leased lines or the internet.
网络可分为 LAN（局域网）和 WAN（广域网）。LAN 通过以太网或 Wi‑Fi 连接有限区域内的计算机，WAN 则跨越较大地理距离，通常使用专线或互联网。
Common topologies include star, bus, ring and mesh. In a star network, all devices connect to a central switch; a cable failure affects only one node, but the central device is a single point of failure.
常见拓扑包括星型、总线型、环型和网状。在星型网络中，所有设备连接到一个中央交换机；某根电缆故障只影响一个节点，但中央设备是单点故障。
The TCP/IP protocol stack is organised into layers: application (HTTP, FTP, SMTP), transport (TCP, UDP), internet (IP) and network access. Packet switching breaks data into packets, which may take different routes and are reassembled at the destination.
TCP/IP 协议栈分层组织：应用层（HTTP, FTP, SMTP）、传输层（TCP, UDP）、互联网层（IP）和网络接入层。分组交换将数据分成数据包，它们可能经不同路由并在目的地重新组装。
Firewalls filter traffic based on rules. Encryption scrambles data so that only authorised parties can read it. Symmetric encryption uses one key; asymmetric (public‑key) uses a key pair.
防火墙根据规则过滤流量。加密将数据打乱，使得只有授权方才能读取。对称加密使用一个密钥；非对称（公钥）加密使用密钥对。
8. Database Systems and SQL | 数据库系统与 SQL
A relational database stores data in tables linked by primary and foreign keys. An entity relationship diagram (ERD) models these links. Normalisation (1NF, 2NF, 3NF) reduces redundancy and prevents update anomalies.
关系数据库将数据存储在由主键和外键关联的表中。实体关系图 (ERD) 对这些关联进行建模。规范化（1NF, 2NF, 3NF）减少冗余并防止更新异常。
First normal form (1NF) requires each field to contain atomic values and no repeating groups. Second normal form (2NF) removes partial key dependencies. Third normal form (3NF) eliminates transitive dependencies.
第一范式 (1NF) 要求每个字段包含原子值且无重复组。第二范式 (2NF) 消除部分键依赖。第三范式 (3NF) 消除传递依赖。
Structured Query Language (SQL) is used to manipulate data. The SELECT statement retrieves data; conditions use WHERE, sorting uses ORDER BY. INSERT, UPDATE and DELETE modify records.
结构化查询语言 (SQL) 用于操作数据。SELECT 语句检索数据；条件使用 WHERE，排序使用 ORDER BY。INSERT、UPDATE 和 DELETE 修改记录。
SELECT StudentName, Grade FROM Results WHERE Subject = ‘Computer Science’ ORDER BY Grade DESC;
9. Web Technologies: HTML and CSS | 网页技术：HTML 与 CSS
Hypertext Markup Language (HTML) defines the structure of web pages using elements enclosed in tags like <h1>, <p>, <img>, <a> and <div>. Tags often have attributes, such as src for an image or href for a hyperlink.
超文本标记语言 (HTML) 使用 <h1>、<p>、<img>、<a>、<div> 等标签内的元素定义网页结构。标签通常带有属性，例如图像的 src 或超链接的 href。
Cascading Style Sheets (CSS) control presentation. Styles can be inline, embedded in the <head> or linked from an external file. External stylesheets promote consistency across an entire site.
层叠样式表 (CSS) 控制呈现。样式可以是内联的、嵌入 <head> 中或从外部文件链接。外部样式表促进整个站点的一致性。
Common CSS selectors include element, class (.classname) and id (#idname). Properties like font‑family, color, margin and padding define the look of elements. Responsive design uses media queries to adapt layouts to different screen sizes.
常用 CSS 选择器包括元素、类 (.classname) 和 ID (#idname)。字体、颜色、边距和填充等属性定义元素的外观。响应式设计使用媒体查询让布局适应不同的屏幕尺寸。
10. Ethical, Legal and Environmental Issues | 伦理、法律与环境问题
The Data Protection Act 2018 controls how personal data is collected, stored and processed. Organisations must follow principles such as lawfulness, fairness, transparency and data minimisation. Individuals have the right to access data held about them.
《2018 年数据保护法》规范了个人数据的收集、存储和处理。组织必须遵循合法、公平、透明和数据最小化等原则。个人有权访问关于他们的数据。
The Computer Misuse Act 1990 makes unauthorised access to computer material, intent to commit further offences, and unauthorised modification of data illegal.
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