📚 IGCSE CIE Computer Science: Last-Minute Revision Notes | IGCSE CIE 计算机：考前冲刺笔记
This article provides a condensed, exam-focused summary of the core topics for the IGCSE CIE Computer Science syllabus. Use it to quickly refresh your memory before the exam, covering everything from computer architecture and data representation to programming, networks, and databases.
本文为IGCSE CIE计算机科学大纲的核心主题提供精炼、贴近考试的总结，适合在考前快速回顾。内容涵盖计算机体系结构、数据表示、编程、网络和数据库等所有重点领域。
1. Computer System Architecture | 计算机系统架构
Central Processing Unit (CPU) is the brain of the computer. It consists of the Control Unit (CU), Arithmetic Logic Unit (ALU), and registers such as the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR), Current Instruction Register (CIR), and Accumulator (ACC). The CU fetches, decodes, and executes instructions following the fetch-decode-execute cycle.
中央处理器（CPU）是计算机的大脑，由控制单元（CU）、算术逻辑单元（ALU）以及寄存器组成。寄存器包括程序计数器（PC）、内存地址寄存器（MAR）、内存数据寄存器（MDR）、当前指令寄存器（CIR）和累加器（ACC）。控制单元遵循取指-译码-执行周期进行操作。
Clock speed, measured in Hertz, determines how many cycles per second the CPU can execute. Higher clock speed means faster processing. Cache is a small, very fast memory inside the CPU that stores frequently used instructions and data, reducing the need to access slower main memory. Cores allow parallel processing: a dual-core can theoretically double performance.
时钟速度以赫兹为单位，决定了CPU每秒可执行的周期数，时钟速度越高处理速度越快。缓存是CPU内部的一块小型高速内存，用来存放频繁使用的指令和数据，从而减少访问较慢主存的需求。多个内核支持并行处理，双核理论上可以将性能提升一倍。
Embedded systems are special-purpose computers built into devices like microwaves or washing machines. They are designed for a specific function, often with limited power and storage.
嵌入式系统是内置在微波炉或洗衣机等设备中的专用计算机，为特定功能而设计，通常功耗和存储空间有限。
2. Input and Output Devices | 输入与输出设备
Input devices convert physical phenomena into data. Examples: keyboard (text entry), mouse (pointing), touchscreen (direct manipulation), microphone (audio), sensors like temperature, light, pressure, and accelerometer. Output devices present processed data: monitor/screen (visual), printer (hard copy), speakers (audio), actuators (e.g., motor).
输入设备将物理现象转换为数据。例如：键盘（文字输入）、鼠标（指点）、触摸屏（直接操作）、麦克风（音频）以及温度、光线、压力、加速度计等传感器。输出设备呈现处理后的数据：显示器/屏幕（视觉）、打印机（硬拷贝）、扬声器（音频）、执行器（如电机）。
Key concepts for choosing devices: resolution (clarity), speed, accuracy, robustness, and user needs. For example, an OCR scanner needs high accuracy; a barcode scanner needs speed and reliability.
选择设备的关键概念：分辨率、速度、精确度、耐用性和用户需求。例如，OCR扫描仪需要高精确度；条码扫描器需要快速且可靠。
3. Data Representation | 数据表示
Computers use binary (0 and 1). Bits, bytes (8 bits), kilobytes, megabytes, gigabytes, terabytes. Denary (base-10) to binary conversion: repeatedly divide by 2, read remainders backwards. Binary to denary: sum of place values where 1 appears. Hexadecimal (base-16) uses 0-9 and A-F; it is a compact way to represent binary (1 hex digit = 4 bits).
计算机使用二进制（0和1）。位、字节（8位）、千字节、兆字节、千兆字节、太字节。十进制（基数为10）转二进制：反复除以2，反向读取余数。二进制转十进制：将出现1的数位值相加。十六进制（基数为16）使用0-9和A-F，是一种紧凑的二进制表示方式（1个十六进制位=4个二进制位）。
Negative numbers: Two’s complement method. In an 8-bit system, most significant bit indicates sign (0 positive, 1 negative). To negate a number, invert all bits and add 1. Range: -128 to +127 for 8 bits.
负数：二进制补码。在8位系统中，最高位表示符号（0正，1负）。对一个数取负：将所有位取反后加1。8位范围：-128到+127。
Characters: ASCII uses 7 bits (128 characters), Extended ASCII 8 bits (256). Unicode covers all languages and emojis, using up to 32 bits per character. Images: bitmap (pixel grid, each pixel has colour depth) vs vector (mathematical descriptions, scalable without quality loss). Sound: sampling (analogue to digital); sample rate and bit depth affect quality and file size.
字符：ASCII使用7位（128个字符），扩展ASCII 8位（256个字符）。Unicode覆盖所有语言和表情符号，每个字符最多32位。图像：位图（像素网格，每个像素有色彩深度）对比矢量图（数学描述，缩放不失真）。声音：采样（模拟转数字）；采样率和位深度影响质量和文件大小。
4. Memory and Storage | 内存与存储
Primary memory: RAM (Random Access Memory) is volatile, used for running programs and data while computer is on. ROM (Read Only Memory) is non-volatile, stores boot instructions (BIOS). RAM is faster but temporary; ROM is permanent but read-only.
主存储器：RAM（随机存取存储器）是易失性的，用于在计算机开机时保存运行中的程序和数据。ROM（只读存储器）是非易失性的，存储启动指令（BIOS）。RAM速度更快但临时；ROM永久但只读。
Secondary storage: HDD (Hard Disk Drive) uses magnetic platters, high capacity, low cost per byte, but mechanical failure risk. SSD (Solid State Drive) uses flash memory, faster, more durable (no moving parts), but more expensive per byte. Optical media (CD, DVD, Blu-ray). Cloud storage: data stored on remote servers accessed via internet.
辅助存储：HDD（硬盘驱动器）使用磁性盘片，容量高、每字节成本低，但存在机械故障风险。SSD（固态硬盘）使用闪存，速度更快、更耐用（无活动部件），但每字节成本更高。光盘介质（CD、DVD、蓝光）。云存储：数据存储在远程服务器上，通过互联网访问。
5. Networks and the Internet | 网络与互联网
Network types: LAN (Local Area Network, small area like a school), WAN (Wide Area Network, connects LANs, e.g., the Internet). Devices: switch (forwards data in LAN using MAC addresses), router (connects networks, using IP addresses), modem (converts digital to analogue for phone lines).
网络类型：LAN（局域网，小范围如学校）、WAN（广域网，连接多个LAN，如互联网）。设备：交换机（在LAN中使用MAC地址转发数据）、路由器（连接网络，使用IP地址）、调制解调器（将数字信号转换为模拟信号用于电话线）。
IP (Internet Protocol) addresses identify devices; IPv4 (32-bit) and IPv6 (128-bit). MAC addresses are unique hardware identifiers assigned to network interface cards. Domain names (e.g., http://www.example.com) are translated to IP by DNS (Domain Name System).
IP（互联网协议）地址标识设备；IPv4（32位）和IPv6（128位）。MAC地址是分配给网络接口卡的唯一硬件标识符。域名（如 http://www.example.com）由DNS（域名系统）解析为IP地址。
Network topologies: star (central switch, robust but one point of failure), mesh (each device connected to many, internet backbone), bus (single cable, simple but collisions).
网络拓扑：星型（中心交换机，稳健但存在单点故障）、网状（设备之间多点连接，如互联网骨干网）、总线型（单根电缆，简单但易冲突）。
6. Cyber Security | 网络安全
Threats: malware (virus, worm, Trojan), phishing (fake emails to steal data), social engineering (manipulating people), brute force attack (trying all passwords), denial of service (DoS, flooding server).
威胁：恶意软件（病毒、蠕虫、特洛伊木马）、网络钓鱼（伪造邮件窃取数据）、社会工程学（操纵人员）、暴力攻击（尝试所有密码）、拒绝服务攻击（DoS，淹没服务器）。
Protection: firewalls (filter traffic), anti-malware software, encryption (scrambling data, e.g., SSL/TLS), authentication (passwords, biometrics), two-factor authentication, regular updates, data backup, and user education. Encryption types: symmetric (same key) and asymmetric (public/private key).
防护措施：防火墙（过滤流量）、反恶意软件、加密（打乱数据，如SSL/TLS）、身份验证（密码、生物识别）、双重认证、定期更新、数据备份和用户教育。加密类型：对称式（同一密钥）和非对称式（公钥/私钥）。
7. Programming Fundamentals | 编程基础
Three basic constructs: sequence, selection (if…else, switch), iteration (for, while, repeat…until). Data types: integer, real/float, Boolean, character, string. Variables and constants: variable can change, constant fixed.
三种基本结构：顺序、选择（if…else、switch）、迭代（for、while、repeat…until）。数据类型：整数、实数/浮点、布尔、字符、字符串。变量与常量：变量可改变，常量固定。
Arrays: 1D (list) and 2D (table). Index starts at 0. Common algorithms: linear search (check each element) and binary search (sorted list, divide and conquer). Sorting: bubble sort (compare adjacent, swap) and insertion sort (insert into sorted portion).
数组：一维（列表）和二维（表格）。索引从0开始。常用算法：线性查找（逐个检查）和二分查找（有序列表，分而治之）。排序：冒泡排序（比较相邻元素并交换）和插入排序（插入已排序部分）。
Pseudocode and flowcharts: used to design algorithms before coding. Flowchart symbols: oval (start/end), parallelogram (input/output), rectangle (process), diamond (decision).
伪代码与流程图：用于在编程前设计算法。流程图符号：椭圆形（开始/结束）、平行四边形（输入/输出）、矩形（处理）、菱形（判断）。
8. High- and Low-level Languages | 高级语言与低级语言
Low-level: machine code (binary, directly executed by CPU), assembly language (mnemonics like ADD, SUB). Assembly is translated by an assembler. High-level languages (Python, Java, C++) are easier to write, portable, and must be translated by a compiler (whole program at once) or interpreter (line by line).
低级语言：机器码（二进制，由CPU直接执行）、汇编语言（助记符如ADD、SUB）。汇编语言由汇编器翻译。高级语言（如Python、Java、C++）更易编写、可移植，必须由编译器（一次性翻译整个程序）或解释器（逐行翻译）进行转换。
Compilers produce an executable file, faster execution; interpreters run immediately but slower. IDEs (Integrated Development Environments) provide editing, debugging, syntax highlighting. Runtime environments are needed to execute some languages (e.g., Java Virtual Machine).
编译器生成可执行文件，执行速度更快；解释器可立即运行但速度较慢。IDE（集成开发环境）提供编辑、调试、语法高亮等功能。某些语言需要运行时环境（如Java虚拟机）来执行。
9. Algorithm Design and Problem Solving | 算法设计与问题求解
Computational thinking: decomposition (break problem into parts), pattern recognition, abstraction (ignore irrelevant details), algorithm design. Problem solving steps: identify problem, analyse, design solution, implement, test, evaluate.
计算思维：分解（将问题分解成部分）、模式识别、抽象（忽略无关细节）、算法设计。问题解决步骤：识别问题、分析、设计解决方案、实现、测试、评估。
Testing: normal data, boundary data (on the edge of limits), erroneous data (should be rejected). Trace tables help track variable changes step by step. Logical errors produce wrong output; syntax errors prevent translation.
测试：正常数据、边界数据（处于限制边缘）、错误数据（应被拒绝）。跟踪表有助于逐步追踪变量变化。逻辑错误产生错误输出；语法错误阻止翻译执行。
10. Databases | 数据库
Database: structured collection of data. Relational databases use tables linked by primary keys and foreign keys. Primary key: unique identifier for each record. Foreign key: field in one table that references primary key in another, creating a relationship.
数据库：结构化的数据集合。关系型数据库使用由主键和外键关联的表。主键：每条记录的唯一标识符。外键：一个表中的字段，引用另一个表的主键，从而建立关系。
SQL (Structured Query Language) is used to manage data. Basic commands: SELECT fields FROM table WHERE condition ORDER BY field; INSERT INTO table (fields) VALUES (values); UPDATE table SET field=value WHERE condition; DELETE FROM table WHERE condition.
SQL（结构化查询语言）用于管理数据。基本命令：SELECT 字段 FROM 表 WHERE 条件 ORDER BY 字段；INSERT INTO 表 (字段) VALUES (值)；UPDATE 表 SET 字段=值 WHERE 条件；DELETE FROM 表 WHERE 条件。
Flat-file database puts all data in one table, leading to data redundancy and inconsistencies. Relational databases reduce redundancy through normalisation.
平面文件数据库将所有数据放在一张表中，导致数据冗余和不一致。关系型数据库通过规范化来减少冗余。
11. Logic Gates and Circuits | 逻辑门与电路
Logic gates process binary signals. Basic gates: NOT (inverts input), AND (output 1 if both inputs 1), OR (output 1 if at least one input 1), NAND, NOR, XOR. Truth tables show all possible input combinations and outputs. Circuits combine gates to perform operations; e.g., half adder adds two bits, producing sum and carry.
逻辑门处理二进制信号。基本门：NOT（输入取反）、AND（两个输入都为1时输出1）、OR（至少一个输入为1时输出1）、NAND、NOR、XOR。真值表展示所有可能的输入组合及其输出。电路将门组合起来执行操作；例如，半加器将两个位相加，产生和值与进位。
Boolean algebra: expressions can represent logic circuits. Simplification reduces gate count. Notation: A AND B = A.B, A OR B = A+B, NOT A = Ā. The expression A+B.(C+D) can be drawn and simplified.
布尔代数：表达式可以表示逻辑电路。简化可减少门的数量。记法：A AND B = A·B，A OR B = A+B，NOT A = Ā。表达式A+B·(C+D)可以绘制并简化。
12. Operating Systems and System Software | 操作系统与系统软件
Operating system (OS) functions: providing a user interface (GUI/CLI), managing memory (allocating RAM to processes), managing hardware/peripherals through drivers, managing files and user accounts, handling interrupts, and multitasking.
操作系统（OS）的功能：提供用户界面（GUI/CLI）、管理内存（为进程分配RAM）、通过驱动程序管理硬件/外设、管理文件和用户账户、处理中断以及多任务处理。
Utility software: antivirus, backup, disk defragmentation (reorganises files on HDD to improve speed), compression tools (reduce file size for storage/transmission).
实用工具软件：杀毒、备份、磁盘碎片整理（重组HDD上的文件以提高速度）、压缩工具（减小文件体积便于存储/传输）。
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