📚 IGCSE WJEC Computer Science: Last-Minute Revision Notes | IGCSE WJEC 计算机：考前冲刺笔记
This revision guide condenses the key topics of the WJEC IGCSE Computer Science specification into essential notes. It is intended for a quick review before the exam, helping you recall critical facts, definitions, and processes. Focus on understanding the concepts, not just memorising, and use this alongside past papers.
这份复习指南将 WJEC IGCSE 计算机科学考试大纲的核心内容浓缩成必背笔记，旨在帮助你在考前快速回顾关键事实、定义与流程。重在理解概念，而非死记硬背，建议结合历年真题一起使用。
1. Data Representation | 数据表示
All data inside a computer is stored in binary (base‑2), using only the digits 0 and 1. Denary (base‑10) numbers must be converted to binary for processing; hexadecimal (base‑ ents exactly 4 binary bits (a nibble). For example, denary 45 = binary 0010 1101 = hex 2D.
计算机内部所有数据都以二进制（基‑2）存储，只使用数字 0 和 1。十进制数必须转换为二进制才能处理；十六进制（基‑16）提供了更紧凑的表示，因为每个十六进制数字恰好代表 4 个二进制位（一个半字节）。例如十进制 45 = 二进制 0010 1101 = 十六进制 2D。
arry 1. An overflow error occurs when the result requires more bits than the available word length (e.g. adding two 8‑bit numbers yields a 9‑ l shifts move bits left (multiply by 2) or right (divide by 2), filling spaces with zeros.
二进制加法遵循规则：0+0=0，1+0=1，1+1=0 进 1，1+1+1=1 进 1。当结果所需的位数超过可用字长时（例如两个 8 位数相加得到 9 位结果无法存储），就会发生溢出错误。逻辑移位将位向左移（乘以 2）或向右移（除以 2），空位填零。
Characters are represented using character sets: ASCII uses 7 bits (128 characters), extended ASCII uses 8 bits (256 characters), while Unicode uses up to 32 bits per character, allowing representation of virtually all world languages and symbols. Choosing the right set matters for storage efficiency.
字符通过字符集表示：ASCII 使用 7 位（128 个字符），扩展 ASCII 使用 8 位（256 个字符），而 Unicode 每个字符最多使用 32 位，能够表示几乎所有世界的语言和符号。选择合适的字符集对存储效率很重要。
ds) × number of channels. Both images and sound can be compressed using lossy (permanently removes data, e.g. JPEG, MP3) or lossless (retains all data, e.g. PNG, FLAC) techniques.
图像由像素构成；图像质量取决于分辨率（像素数）和颜色深度（每像素位数）。数值越高，文件越大。声音通过以设定的采样率和一定的位深度对模拟信号进行数字化；文件大小 = 采样率 × 位深度 × 时长（秒） × 声道数。图像和声音均可使用有损压缩（永久性移除数据，如 JPEG、MP3）或无损压缩（保留所有数据，如 PNG、FLAC）技术。
2. Computer Architecture | 计算机体系结构
The Von Neumann architecture stores both instructions and data in the same memory. Key components of the CPU include the Control Unit (CU), which decodes instructions and controls timing; the Arithmetic Logic Unit (ALU), which performs calculations and logical operations; and a set of registers such as the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR), and Accumulator (ACC).
冯·诺依曼体系结构将指令与数据存放在同一个存储器中。CPU 的关键组件包括：控制单元（CU），负责译码和控制时序；算术逻辑单元（ALU），执行运算和逻辑操作；以及一组寄存器，如程序计数器（PC）、内存地址寄存器（MAR）、内存数据寄存器（MDR）和累加器（ACC）。
The fetch‑decode‑execute cycle repeats continuously: the PC value is copied to MAR, the instruction at that address is fetched from RAM into MDR, the PC is incremented, the CU decodes the instruction, and the ALU (or another unit) executes it. Factors affecting CPU performance are clock speed (cycles per second), number of cores, and cache size.
取指‑译码‑执行周期持续反复：PC 的值被复制到 MAR，该地址处的指令由 RAM 取入 MDR，PC 递增，CU 对指令译码，ALU（或其他单元）执行指令。影响 CPU 性能的因素有时钟速度（每秒周期数）、核心数量和缓存大小。
Embedded systems are dedicated computer systems built into larger devices (e.g. washing machines, car engine control). They typically have a single purpose, low power consumption, and limited memory/storage, often built around a microcontroller.
嵌入式系统是内置在大型设备（如洗衣机、汽车发动机控制）中的专用计算机系统。它们通常有单一用途、低功耗和有限的内存/存储，常围绕微控制器构建。
3. Memory and Storage | 存储器与存储设备
RAM (Random Access Memory) is volatile, holds currently running programs and data, and loses its content when power is off. ROM (Read Only Memory) is non‑volatile, stores firmware and the boot program, and its contents are retained. Cache is a very small, extremely fast memory located close to the CPU that stores frequently used instructions and data to speed up processing.
RAM（随机存取存储器）是易失性的，存放正在运行的程序和数据，断电后内容丢失。ROM（只读存储器）是非易失性的，存储固件和引导程序，内容保留。Cache 是极小而极快的内存，紧邻 CPU，存放频繁使用的指令和数据以加速处理。
Virtual memory uses a portion of the hard drive as an extension of RAM when main memory is full. It allows more applications to run but is significantly slower than physical RAM, and excessive paging (thrashing) can degrade performance. Secondary storage devices include magnetic (HDD), optical (CD/DVD), and solid‑state (SSD, USB flash drive). Compare them by capacity, speed, portability, durability, and cost per GB.
虚拟内存使用硬盘的一部分在主存满时充当 RAM 的扩展。它允许运行更多应用，但远慢于物理 RAM；过多的换页（颠簸）会降低性能。辅助存储设备包括磁介质（HDD）、光介质（CD/DVD）和固态（SSD、USB 闪存盘）。可从容量、速度、便携性、耐用性和每 GB 成本进行比较。
4. Input and Output Devices | 输入输出设备
Input devices convert real‑world data into a form the computer can process. Key examples include keyboard, mouse, touchscreen, microphone, and scanner. Sensors (temperature, light, pressure, moisture, infrared) continuously measure physical quantities and send analogue or digital signals to the processor. Barcode readers scan black‑and‑white lines; QR codes hold more data in a 2D matrix.
输入设备将现实世界的数据转换为计算机可处理的形式。主要例子有键盘、鼠标、触摸屏、麦克风和扫描仪。传感器（温度、光线、压力、湿度、红外）持续测量物理量，并向处理器发送模拟或数字信号。条形码阅读器扫描黑白线条；二维码在二维矩阵中容纳更多数据。
Output devices present processed data to the user. Monitors (LCD, LED) display visual output; inkjet printers spray liquid ink, ideal for high‑quality colour photos; laser printers use toner and are faster for high‑volume text. 3D printers create solid objects by layering material (additive manufacturing) and are used for prototyping and custom parts. Speakers and headphones produce audio output.
输出设备将处理后的数据呈现给用户。显示器（LCD、LED）显示视觉输出；喷墨打印机喷射液态墨水，适合高品质彩色照片；激光打印机使用碳粉，适合大批量文本高速打印。3D 打印机通过逐层材料堆积（增材制造）创建实物，用于原型制作和定制部件。扬声器和耳机产生音频输出。
5. Software | 软件
System software includes the operating system (OS) and utility programs. The OS manages the hardware, provides a user interface (GUI or CLI), handles memory management, multitasking, peripheral management, and file storage. Common OS types are single‑user, multi‑user, and real‑time operating systems (used in safety‑critical embedded devices).
系统软件包括操作系统（OS）和实用程序。操作系统管理硬件，提供用户界面（图形界面或命令行），处理内存管理、多任务、外设管理和文件存储。常见的操作系统类型有单用户、多用户和实时操作系统（用于安全关键型嵌入式设备）。
Utility software performs specific maintenance tasks: antivirus software detects and removes malware, disk defragmenter reorganises fragmented files on magnetic drives to improve access times, backup software creates copies of data to protect against loss, and encryption software scrambles data into an unreadable format without a key.
实用软件执行特定的维护任务：反病毒软件检测并移除恶意软件，磁盘碎片整理程序重新组织磁驱上的碎片文件以缩短访问时间，备份软件创建数据副本以防丢失，加密软件将数据打乱成无密钥不可读的格式。
Application software lets users perform tasks (word processors, spreadsheets, web browsers, games). Software can be proprietary (closed source, paid licences) or open source (source code freely available, can be modified and redistributed). Each licensing model has trade‑offs in cost, support, and customisability.
应用软件让用户执行任务（文字处理、电子表格、网页浏览器、游戏）。软件可以是专有的（闭源，付费许可）或开源的（源代码免费提供，可修改和再分发）。每种许可模式在成本、支持和可定制性上各有利弊。
6. Networks | 网络
A LAN (Local Area Network) covers a small geographical area, typically owned by one organisation, with high bandwidth and low latency. A WAN (Wide Area Network) spans large distances, often using leased telecommunications lines or the internet. Key network hardware includes: network interface card (NIC) – provides a device with a MAC address; switch – connects devices in a LAN and forwards frames intelligently; router – connects different networks and forwards packets based on IP addresses.
LAN（局域网）覆盖较小地理范围，通常由单个组织所有，带宽高延迟低。WAN（广域网）跨越长距离，通常使用租用电信线路或互联网。关键网络硬件包括：网卡（NIC）—— 为设备提供 MAC 地址；交换机 —— 在局域网中连接设备并智能转发数据帧；路由器 —— 连接不同网络，并根据 IP 地址转发数据包。
Common topologies: star – all nodes connect to a central switch; failure of one cable only disconnects that node. Mesh – each node connects to several others, providing high redundancy and reliability. The internet uses the TCP/IP protocol stack (Transmission Control Protocol/Internet Protocol). Other essential protocols: HTTP/HTTPS – web pages, FTP – file transfer, SMTP – sending email, POP3/IMAP – receiving email.
常见拓扑结构：星型 —— 所有节点连接到中央交换机；一条电缆故障只断开该节点。网状 —— 各节点连接多个其他节点，提供高冗余度和可靠性。互联网使用 TCP/IP 协议栈（传输控制协议/互联网协议）。其他重要协议：HTTP/HTTPS —— 网页，FTP —— 文件传输，SMTP —— 发送邮件，POP3/IMAP —— 接收邮件。
DNS (Domain Name System) translates human‑readable domain names (e.g. www.example.com) into IP addresses. The cloud stores data and software on remote servers accessed via the internet, offering convenience, scalability, and cost savings, although it depends on network connectivity and raises security considerations.
DNS（域名系统）将人可读的域名（如 www.example.com）转换为 IP 地址。云将数据和软件存储在通过互联网访问的远程服务器上，提供了便利性、可扩展性和成本节约，但依赖网络连接并涉及安全问题。
7. Cybersecurity | 网络安全
Malware is malicious software: virus – attaches itself to files and spreads; worm – self‑replicates across networks without user action; Trojan – masquerades as legitimate software. Phishing tricks users into revealing sensitive information via fake emails or websites. A brute‑force attack tries every possible password combination until gaining access. A denial‑of‑service (DoS) attack floods a server with traffic to make it unavailable.
恶意软件是恶意程序：病毒 —— 附着于文件并传播；蠕虫 —— 无需用户操作即可通过网络自我复制；特洛伊木马 —— 伪装成合法软件。网络钓鱼通过虚假邮件或网站诱骗用户泄露敏感信息。暴力破解攻击尝试所有可能的密码组合直至获取访问权限。拒绝服务（DoS）攻击用流量淹没服务器使其不可用。
Protection measures include: firewall – monitors and filters incoming/outgoing network traffic based on rules; anti‑malware – scans, detects and removes threats; two‑factor authentication (2FA) – adds a second verification step beyond a password; strong password policies – length, complexity, regular changes; biometrics – fingerprint, retina scans; penetration testing – simulates attacks to find vulnerabilities.
保护措施包括：防火墙 —— 根据规则监控和过滤进出网络流量；反恶意软件 —— 扫描、检测并移除威胁；双因素认证（2FA）—— 在密码之外增加第二步验证；强密码策略 —— 长度、复杂性、定期更换；生物识别 —— 指纹、视网膜扫描；渗透测试 —— 模拟攻击以发现漏洞。
Encryption scrambles data so that only authorised parties with the key can read it. Symmetric encryption uses the same key for encryption and decryption; asymmetric (public key) encryption uses a public key to encrypt and a private key to decrypt, used in secure web transactions (HTTPS). The Caesar cipher is a simple substitution cipher where each letter is shifted by a fixed number, but it is easily broken.
加密将数据打乱，使得只有持有密钥的授权方才能阅读。对称加密使用相同密钥进行加密和解密；非对称（公钥）加密使用公钥加密、私钥解密，用于安全网络交易（HTTPS）。凯撒密码是一种简单的替换密码，每个字母被移固定位数，但很容易破解。
8. Algorithms and Programming | 算法与编程
Computational thinking involves decomposition (breaking a problem into smaller parts), pattern recognition (identifying similarities), abstraction (focusing on key details), and algorithm design (step‑by‑step solution). Flowcharts use standard symbols: oval for start/stop, parallelogram for input/output, rectangle for process, diamond for decision (yes/no). Pseudocode uses structured English: INPUT, OUTPUT, WHILE … ENDWHILE, FOR … NEXT, IF … THEN … ELSE … ENDIF.
计算思维包括分解（将问题拆分为小部分）、模式识别（找出相似性）、抽象（聚焦关键细节）和算法设计（逐步求解）。流程图使用标准符号：椭圆表示开始/结束，平行四边形表示输入/输出，矩形表示处理，菱形表示判断（是/否）。伪代码使用结构化英语：INPUT, OUTPUT, WHILE … ENDWHILE, FOR … NEXT, IF … THEN … ELSE … ENDIF。
Search algorithms: linear search checks each element in sequence – simple but potentially slow, O(n). Binary search works on sorted lists, repeatedly dividing the search interval in half – much faster, O(log n). Sorting algorithms: bubble sort repeatedly steps through the list, comparing and swapping adjacent elements – simple but inefficient, O(n²). Merge sort divides the list into halves, recursively sorts them, and merges the halves – more efficient, O(n log n). Trace tables are used to step through code, recording variable values to identify logic errors.
搜索算法：线性搜索按顺序检查每个元素 —— 简单但可能较慢，O(n)。二分搜索适用于已排序列表，反复将搜索区间减半 —— 快得多，O(log n)。排序算法：冒泡排序反复遍历列表，比较并交换相邻元素 —— 简单但效率低，O(n²)。归并排序将列表分成两半，递归排序后再合并 —— 更高效，O(n log n)。跟踪表用于逐步执行代码，记录变量值以识别逻辑错误。
quence, selection (IF), and iteration (WHILE, FOR). Testing involves normal, boundary, and erroneous data. The development life cycle typically includes analysis, design, coding, testing, and maintenance.
编程基础包括数据类型（整型、实型/浮点型、布尔型、字符型、字符串）、变量（命名的存储位置）、赋值（=）以及控制结构：顺序、选择（IF）和迭代（WHILE、FOR）。测试涉及正常数据、边界数据和错误数据。开发生命周期通常包括分析、设计、编码、测试和维护。
9. Databases and SQL | 数据库与 SQL
A relational database stores data in tables. Each table has records (rows) and fields (columns). A primary key is a field that uniquely identifies each record (e.g. StudentID). A foreign key is a field in another table that links to a primary key, creating relationships between tables. Data integrity and reduced redundancy are key advantages of relational databases.
关系数据库将数据存储在表中。每个表有记录（行）和字段（列）。主键是能唯一标识每条记录的字段（如 StudentID）。外键是另一表中链接到主键的字段，从而建立表之间的关系。数据完整性和减少冗余是关系数据库的主要优点。
or example: SELECT Products.Name, Suppliers.Phone FROM Products INNER JOIN Suppliers ON Products.SupplierID = Suppliers.ID WHERE Products.Price < 10;
结构化查询语言（SQL）用于查询和操作数据库。关键子句：SELECT（选择字段），FROM（指定表），WHERE（按条件筛选行），ORDER BY（按升序 ASC 或降序 DESC 排序），INNER JOIN（基于匹配条件连接两个表的行）。例如： ducts INNER JOIN Suppliers ON Products.SupplierID = Suppliers.ID WHERE Products.Price < 10；
Care must be taken with SQL injection attacks, where malicious SQL statements are inserted into input fields. Using parameterised queries or input validation helps prevent such attacks. When designing databases, data redundancy should be minimised through normalisation, splitting data into separate linked tables to avoid update anomalies.
需要小心 SQL 注入攻击，即将恶意 SQL 语句插入输入字段。使用参数化查询或输入验证有助于防止此类攻击。设计数据库时，应通过规范化将数据拆分到相互关联的多个表中，从而最小化数据冗余，避免更新异常。
10. Ethical, Legal and Environmental Issues | 伦理、法律与环境问题
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