📚 Complete Computer Science for Cambridge IGCSE & O Level: Essential Revision Notes | 剑桥 IGCSE 与 O Level 计算机科学完全指南：核心复习笔记
The Cambridge IGCSE and O Level Computer Science syllabus challenges students to understand how computer systems work, how to write efficient programs, and how to think computationally. This revision guide, aligned with the popular coursebook ‘Complete Computer Science for Cambridge IGCSE & O Level’ by Alison Page and David Waters, distils the entire syllabus into clear, bilingual explanations of every core topic. Whether you are preparing for Paper 1 (Theory) or Paper 2 (Problem-solving and Programming), these notes will help you consolidate key knowledge, avoid common mistakes, and approach your exams with confidence.
剑桥 IGCSE 和 O Level 计算机科学课程旨在让学生理解计算机系统的工作原理、如何编写高效的程序以及如何进行计算思维。本复习指南与 Alison Page 和 David Waters 编写的畅销教材《Complete Computer Science for Cambridge IGCSE & O Level》相配套，将整个教学大纲浓缩为清晰的双语讲解，覆盖每个核心主题。无论你是在准备试卷一（理论）还是试卷二（问题解决与编程），这些笔记都将帮助你巩固关键知识、避开常见误区，并自信地迎接考试。
1. Data Representation | 数据表示
All data inside a computer is stored as binary digits (bits). A bit can be 0 or 1, and groups of 8 bits form a byte. Understanding binary, denary, and hexadecimal number systems is fundamental, as is knowing how text, sound, and images are encoded.
计算机内所有数据都以二进制数字（比特）的形式存储。一个比特可以是 0 或 1，8 个比特组成一个字节。理解二进制、十进制和十六进制数制是基础，同样重要的还有文本、声音和图像是如何被编码的。
To convert a denary number like 107 to binary, repeatedly divide by 2 and read the remainders backwards: 107 → 1101011₂. For hexadecimal, group binary digits into nibbles (4 bits) and translate each to a hex digit (0–9, A–F).
要将十进制数（比如 107）转换为二进制，可以反复除以 2 并倒序读取余数：107 → 1101011₂。转换为十六进制时，将二进制数字按每 4 位一组分组，再把每组转换成一个十六进制数字（0–9, A–F）。
Text is commonly represented using ASCII (7-bit or 8-bit) or Unicode. ASCII uses one byte per character, while Unicode (e.g. UTF-8) can use up to 4 bytes to cover characters from all writing systems worldwide. Sound is sampled at regular intervals; the sampling rate and bit depth determine the quality and file size.
文本通常用 ASCII（7 位或 8 位）或 Unicode 表示。ASCII 每个字符使用一个字节，而 Unicode（例如 UTF-8）可以使用最多 4 个字节来涵盖全世界所有书写系统的字符。声音是按固定间隔进行采样；采样率和采样精度决定了音质和文件大小。
Images are stored either as bitmaps or vector graphics. A bitmap image is a grid of pixels, each with a colour value; higher resolution and colour depth mean more detail but larger file size. Vector graphics store shapes using mathematical formulas, so they scale without losing quality.
图像既可以位图的形式存储，也可以矢量图的形式存储。位图是一个像素网格，每个像素都有颜色值；分辨率和颜色深度越高，细节越多，但文件也越大。矢量图形使用数学公式存储形状，因此可以在不失真的情况下任意缩放。
2. Hardware and Computer Architecture | 硬件与计算机体系结构
The central processing unit (CPU) executes instructions following the fetch-decode-execute cycle. It consists of the control unit (CU), arithmetic logic unit (ALU), and registers such as the program counter (PC) and accumulator (ACC). The CPU’s speed is affected by clock speed, number of cores, and cache memory.
中央处理器（CPU）按照“取指-译码-执行”的循环来执行指令。它由控制单元（CU）、算术逻辑单元（ALU）以及程序计数器（PC）和累加器（ACC）等寄存器组成。CPU 的速度受到时钟频率、核心数量和缓存大小的影响。
Main memory includes RAM (volatile, stores data and programs currently in use) and ROM (non-volatile, stores firmware like the BIOS). Secondary storage devices such as HDDs, SSDs, and optical discs provide long-term data retention. SSDs are faster and more durable than HDDs because they have no moving parts.
主存储器包括 RAM（易失性，存储当前正在使用的数据和程序）和 ROM（非易失性，存储 BIOS 等固件）。辅助存储设备，如 HDD、SSD 和光盘，可提供长期数据保存。SSD 比 HDD 更快、更耐用，因为它没有移动部件。
Input devices like keyboards, mice, and sensors allow data to enter the computer. Output devices such as monitors, printers, and speakers present processed data. Embedded systems are dedicated computers inside larger devices, such as washing machines or traffic lights, often running on microcontrollers.
键盘、鼠标和传感器等输入设备允许数据进入计算机。显示器、打印机和扬声器等输出设备则呈现处理后的数据。嵌入式系统是内置于洗衣机或交通灯等大型设备中的专用计算机，通常运行在微控制器上。
3. Software and Operating Systems | 软件与操作系统
System software manages hardware and provides a platform for application software. The operating system (OS) is the most important system software; it handles process management, memory management, file management, and user interface (GUI or CLI). Device drivers translate OS commands into instructions that specific hardware can understand.
系统软件管理硬件并为应用软件提供平台。操作系统（OS）是最重要的系统软件，它负责进程管理、内存管理、文件管理以及用户界面（图形界面或命令行界面）。设备驱动程序能将操作系统的命令翻译成特定硬件可以理解的指令。
Application software helps users perform tasks. Examples include word processors, spreadsheets, and database management systems. Utility software maintains the computer, e.g. antivirus, disk defragmenter, and backup tools. Distinguishing between these types is a common exam question.
应用软件帮助用户执行任务，例如文字处理软件、电子表格和数据库管理系统。实用软件用于维护计算机，如杀毒软件、磁盘碎片整理工具和备份工具。区分这些软件的类型是考试中的常见问题。
High-level languages (Python, Java) are easy for humans to read and must be translated into machine code. Translators include compilers (translate whole program at once) and interpreters (translate line by line). Low-level languages like assembly use mnemonics and require an assembler.
高级语言（Python、Java）便于人类阅读，但必须翻译成机器码才能执行。翻译器包括编译器（一次性翻译整个程序）和解释器（逐行翻译）。汇编语言等低级语言使用助记符，而且需要汇编器进行翻译。
4. Networks and the Internet | 网络与互联网
A network connects computers to share resources and data. Two main models are LAN (local area network, e.g. within a school) and WAN (wide area network, e.g. the Internet). Network hardware includes switches, routers, and network interface cards (NICs).
网络将计算机连接起来以共享资源和数据。主要有两种模式：LAN（局域网，例如在学校内部）和 WAN（广域网，例如互联网）。网络硬件包括交换机、路由器和网络接口卡（NIC）。
The Internet is a global WAN built on the TCP/IP protocol suite. Data is broken into packets, which travel independently and are reassembled at the destination. Each device on the Internet has an IP address; domain name system (DNS) converts human-friendly URLs to IP addresses.
互联网是基于 TCP/IP 协议簇构建的全球广域网。数据被分割成数据包，这些包独立传输并在目的地重新组装。互联网上的每个设备都有一个 IP 地址；域名系统（DNS）将人类易记的 URL 转换为 IP 地址。
Common transmission media include copper cables, fibre optic cables, and wireless (Wi-Fi, Bluetooth). Fibre optics use light to transmit data at high speed with minimal interference. Security in networks relies on encryption, firewalls, and secure protocols such as HTTPS and SSL/TLS.
常见的传输介质包括铜缆、光纤和无线（Wi-Fi、蓝牙）。光纤利用光来高速传输数据，且干扰极小。网络安全依赖于加密、防火墙以及 HTTPS 和 SSL/TLS 等安全协议。
5. Algorithm Design and Problem-Solving | 算法设计与问题求解
An algorithm is a step-by-step procedure to solve a problem. Computational thinking involves decomposition, pattern recognition, abstraction, and algorithm design. Flowcharts and pseudocode are the standard tools used in IGCSE to express algorithms before coding.
算法是解决问题的逐步过程。计算思维包括问题分解、模式识别、抽象化和算法设计。流程图和伪代码是 IGCSE 中在编程前表达算法的标准工具。
Important algorithmic constructs are sequence, selection (IF…THEN…ELSE), and iteration (loops: FOR, WHILE, REPEAT…UNTIL). Trace tables are used to step through an algorithm manually, tracking variable values to find logic errors. Example pseudocode for finding the largest number in a list:
重要的算法结构有顺序、选择（IF…THEN…ELSE）和迭代（循环：FOR、WHILE、REPEAT…UNTIL）。跟踪表用于手动逐步执行算法，记录变量值以查找逻辑错误。以下是在列表中寻找最大值的伪代码示例：
SET max TO first number
FOR EACH number IN list
IF number > max THEN
SET max TO number
ENDIF
ENDFOR
OUTPUT max
Standard algorithms you must know include linear search, binary search, bubble sort, and simple validation/verification checks. Binary search requires a sorted list and repeatedly halves the search interval, making it much faster than linear search on large datasets.
你必须掌握的标准算法包括线性搜索、二分搜索、冒泡排序以及简单的验证与校验检查。二分搜索需要一个已排序的列表，并反复将搜索区间减半，这使得它在处理大型数据集时比线性搜索快得多。
6. Programming Concepts | 编程基础
Variables store data that can change during program execution; constants store values that remain fixed. Data types include integer, real (float), char, string, and Boolean. You should be able to declare variables, assign values, and use arithmetic (+, -, *, /, ^) and logical operators (AND, OR, NOT).
变量用来存储程序执行期间可以改变的数据；常量存储的值则保持不变。数据类型包括整数、实数（浮点数）、字符、字符串和布尔型。你应该能够声明变量、赋值，并使用算术运算符（+、-、*、/、^）和逻辑运算符（AND、OR、NOT）。
Selection uses IF statements; CASE (or switch) statements handle multiple branches clearly. Iteration loops include count-controlled (FOR) and condition-controlled (WHILE, REPEAT). Nested loops and nested IF statements are essential for complex decision making and working with multi-dimensional arrays.
选择使用 IF 语句；CASE（或 switch）语句能够清晰地处理多个分支。迭代循环包括计数控制循环（FOR）和条件控制循环（WHILE、REPEAT）。嵌套循环和嵌套 IF 语句对于复杂的决策制定和处理多维数组至关重要。
Arrays store multiple items of the same data type under one identifier. A 1D array is a list; a 2D array forms a table. You must be able to write and trace algorithms that read, write, and search arrays. For example, summing elements of a 1D array:
数组将多个相同数据类型的元素存储在同一个标识符下。一维数组就是一个列表；二维数组则构成一个表格。你必须能够编写和追踪对数组进行读取、写入和搜索的算法。例如，对一个一维数组的元素求和：
SET total TO 0
FOR i ← 0 TO LENGTH(arr)-1
total ← total + arr[i]
ENDFOR
OUTPUT total
Procedures and functions are subprograms that improve code reusability. A function returns a value; a procedure does not. Parameters pass data into subprograms; local and global variables have different scopes. Understanding scope helps prevent unintended side effects.
过程和函数是提高代码复用性的子程序。函数会返回一个值，而过程则不会。参数用于将数据传递给子程序；局部变量和全局变量具有不同的作用域。理解作用域有助于防止意外的副作用。
7. Databases | 数据库
A database is an organised collection of data. The IGCSE syllabus focuses on single-table flat-file databases, but you should also understand the problems of data redundancy and how relational databases solve them. Key fields, primary keys, and foreign keys define relationships.
数据库是经过组织的数据集合。IGCSE 教学大纲侧重于单表平面文件数据库，但你也应该了解数据冗余问题以及关系型数据库是如何解决这些问题的。关键字段、主键和外键用于定义关系。
Tables consist of records (rows) and fields (columns). Each field has a data type (text, number, date/time, Boolean). Validation rules (e.g. range check, presence check) help ensure data integrity. Queries are written to search and filter data using a structured query language, often simplified in the exam.
表由记录（行）和字段（列）组成。每个字段都有一个数据类型（文本、数字、日期/时间、布尔型）。验证规则（例如范围检查、存在性检查）有助于确保数据完整性。查询的编写用于使用结构化查询语言搜索和筛选数据，试卷中通常使用简化版的 SQL。
A typical exam-style query might be: SELECT Name, Age FROM Students WHERE Age > 16 ORDER BY Name ASC. The output is a subset of the table. Be comfortable with using wildcards (*), comparison operators (=, >, <, <>) and logical operators (AND, OR, NOT) in query conditions.
典型的考题式查询可能是：SELECT Name, Age FROM Students WHERE Age > 16 ORDER BY Name ASC。输出结果就是表格的一个子集。要能熟练地在查询条件中使用通配符（*）、比较运算符（=、>、<、<>）和逻辑运算符（AND、OR、NOT）。
8. Logic Gates and Circuits | 逻辑门与电路
Digital logic forms the hardware basis of computer processing. The six fundamental logic gates are NOT, AND, OR, NAND, NOR, and XOR. Each gate produces a specific output for given binary inputs, expressed through truth tables.
数字逻辑构成了计算机处理的硬件基础。六种基本的逻辑门是：NOT、AND、OR、NAND、NOR 和 XOR。每个门针对给定的二进制输入都会产生特定的输出，这可以通过真值表来表达。
| Gate | Symbol (Boolean) | Description |
|---|---|---|
| NOT | Q = NOT A | Outputs the opposite of input |
| AND | Q = A AND B | Output 1 only if both inputs are 1 |
| OR | Q = A OR B | Output 1 if at least one input is 1 |
| NAND | Q = NOT (A AND B) | Opposite of AND |
| NOR | Q = NOT (A OR B) | Opposite of OR |
| XOR | Q = A XOR B | Output 1 if inputs are different |
You must be able to complete truth tables for simple circuits with up to two or three gates. A logic circuit can also be described using a logic expression, and you may be asked to draw the circuit from an expression or vice versa. NAND and NOR gates are universal gates because any other gate can be built using only NAND gates (or only NOR gates).
你必须能够完成包含两到三个门的简单电路的真值表。一个逻辑电路也可以用逻辑表达式来描述，并且可能会要求你根据表达式绘制电路，或者反过来。NAND 和 NOR 门是通用门，因为仅使用 NAND 门（或仅使用 NOR 门）就可以构建出任何其他门电路。
9. Cybersecurity and Ethics | 网络安全与道德
Malware (malicious software) includes viruses, worms, Trojans, spyware, and ransomware. Social engineering attacks like phishing trick users into revealing confidential information. Denial-of-service (DoS) attacks overwhelm a server, making services unavailable. Strong, regularly updated passwords, firewalls, and anti-malware tools are essential defences.
恶意软件包括病毒、蠕虫、特洛伊木马、间谍软件和勒索软件。钓鱼攻击等社会工程手段会诱骗用户泄露机密信息。拒绝服务（DoS）攻击会使服务器不堪重负，导致服务不可用。使用强密码并定期更新、防火墙和反恶意软件工具是必不可少的防御措施。
Encryption protects data by scrambling it so that only authorised parties with the key can read it. Symmetric encryption uses one key; asymmetric uses a public-private key pair. The Computer Misuse Act and Data Protection Act (or GDPR, depending on the region examined) set legal boundaries for data use and computer access.
加密通过打乱数据来保护数据，从而使只有掌握密钥的授权方才能读取。对称加密使用同一把密钥；非对称加密则使用一对公私钥。计算机滥用法案和数据保护法案（或 GDPR，视考试所在地而定）为数据使用和计算机访问设定了法律边界。
Plagiarism and copyright infringement are serious ethical issues in computing. The environmental impact of computing includes e-waste and energy consumption, prompting the design of energy-efficient hardware and responsible recycling. Automated decision-making can introduce bias if the training data is not representative.
剽窃和侵犯版权是计算机领域严重的道德问题。计算对环境的影响包括电子废弃物和能源消耗，这促进了节能硬件的设计和对负责任回收的关注。如果训练数据不具有代表性，自动化决策可能会引入偏见。
10. Exam Strategies and Common Pitfalls | 考试策略与常见失分点
Paper 1 (Theory) rewards precise terminology and structured answers. When defining terms, always give a brief example if possible. For questions on data storage, show your working when converting between number systems. Do not confuse validation (checks data input is sensible) with verification (checks data matches the original source).
试卷一（理论）考查精确的术语和组织良好的答案。在对术语下定义时，如果可能，尽量给出一个简短的例子。对数据存储类题目，在数制转换时要展示计算步骤。不要混淆验证（检查数据输入是否合理）和校验（检查数据是否与原始来源一致）。
Paper 2 (Problem-solving and Programming) requires you to write, read, and debug pseudocode. Pay close attention to loop boundaries (e.g. FOR i ← 1 TO 10 will execute 10 times, not 9). Always initialise counters and totals to zero before adding to them. Use meaningful variable names and indentation to make your logic clear for the examiner.
试卷二（问题解决与编程）要求你编写、阅读和调试伪代码。要特别注意循环边界（例如 FOR i ← 1 TO 10 将执行 10 次，而不是 9 次）。务必在累加前将计数器和总和初始化为零。使用有意义的变量名和缩进，让你的逻辑对考官来说清晰明了。
In algorithm trace questions, update the trace table row by row as each line executes. A single missed value can cascade errors, so double-check each step. When solving programming problems, break the task into smaller parts and consider edge cases (e.g. empty list, first or last element).
在算法追踪题中，要随着每一行的执行逐行更新跟踪表。漏掉一个值就可能导致一连串错误，因此要仔细检查每一步。在解决编程问题时，将任务分解为更小的部分，并考虑边缘情况（例如空列表、首尾元素）。
Time management is crucial. For Paper 2, allocate about 5–10 minutes to plan your algorithm before writing pseudocode. Re-read the question to ensure your solution meets all requirements. Practise past papers under timed conditions; mark schemes reveal exactly where marks are awarded.
时间管理至关重要。对于试卷二，在动手写伪代码之前，先花 5 到 10 分钟规划你的算法。重读题目，确保你的解决方案满足所有要求。在限时条件下练习历年试卷；评分方案会确切地揭示出得分点在哪里。
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