📚 IGCSE Edexcel Computer Science: Last-Minute Revision Notes | IGCSE Edexcel 计算机科学：考前冲刺笔记
This last-minute revision guide covers the most essential topics for the IGCSE Edexcel Computer Science exam. Use these concise notes to reinforce key concepts, recall definitions, and sharpen your problem-solving approach. Each section pairs critical explanations in English and Chinese to help you revise efficiently and confidently.
这份考前冲刺笔记涵盖IGCSE Edexcel计算机科学考试中最核心的主题。用这些简明笔记巩固关键概念、回忆定义、磨练解题思路。每个小节都提供中英对照的要点讲解，帮你高效自信地复习。
1. Algorithms and Flowcharts | 算法与流程图
An algorithm is a step-by-step sequence of instructions designed to perform a specific task. It must be unambiguous, finite, and produce a correct output for all valid inputs.
算法是一步一步的指令序列，旨在完成特定任务。它必须无歧义、有穷，并且对所有有效输入都能产生正确输出。
Flowcharts use standard symbols to represent algorithms visually: ovals for Start/End, parallelograms for Input/Output, rectangles for Processes, and diamonds for Decisions.
流程图使用标准符号直观表示算法：椭圆形表示开始/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。
Pseudocode is a text-based way to express algorithms without strict language syntax. Edexcel uses a specific pseudocode style including keywords like SET, IF...THEN...ELSE...ENDIF, and WHILE...DO...ENDWHILE.
伪代码是一种基于文本的表达算法的方式，无需严格的语言语法。Edexcel使用特定的伪代码风格，包括SET、IF...THEN...ELSE...ENDIF和WHILE...DO...ENDWHILE等关键词。
Common algorithmic structures include sequence (instructions executed in order), selection (IF/ELSE), and iteration (FOR, WHILE loops). Trace tables help track variable values step by step during testing.
常见的算法结构包括顺序（按顺序执行指令）、选择（IF/ELSE）和迭代（FOR、WHILE循环）。追踪表帮助在测试过程中逐步跟踪变量值。
2. Programming Fundamentals | 编程基础
Variables store data values and must be declared with a meaningful identifier. Data types include integer (whole numbers), real/float (decimal), Boolean (TRUE/FALSE), character, and string.
变量存储数据值，必须用有意义的标识符声明。数据类型包括整数（整数）、实数/浮点（小数）、布尔（TRUE/FALSE）、字符和字符串。
IV (integer division). Relational operators: =, <>, <, >, <=, >= are used for comparisons.
算术运算符：+、-、*、/、MOD（余数）、DIV（整除）。关系运算符：=、<>、<、>、<=、>=用于比较。
String manipulation is often tested: LENGTH(string), SUBSTRING(string, start, length), and concatenation using & or +. Case conversion and character extraction are key skills.
字符串操作常考：LENGTH(string)、SUBSTRING(string, start, length)以及用&或+拼接。大小写转换和字符提取是关键技能。
Arrays (1D and 2D) store multiple values under one name. Indexing usually starts from 0. For example, A[3] accesses the fourth element. Nested loops handle 2D arrays.
数组（一维和二维）用一个名称存储多个值。索引通常从0开始。例如A[3]访问第四个元素。嵌套循环处理二维数组。
Functions and procedures are subprograms. A function returns a value; a procedure does not. Parameters can be passed by value or by reference.
函数和过程是子程序。函数返回一个值；过程不返回值。参数可以按值或按引用传递。
3. Data Representation | 数据表示
All data in a computer is represented in binary (0s and 1s). A bit is a single binary digit; a nibble is 4 bits; a byte is 8 bits. Larger units: kilobyte (kB), megabyte (MB), gigabyte (GB).
计算机中所有数据都用二进制（0和1）表示。一个比特是单个二进制位；半字节4位；字节8位。更大单位：千字节(kB)、兆字节(MB)、吉字节(GB)。
Denary to binary conversion: repeatedly divide by 2, read remainders backward. Binary to denary: sum of place values where bit is 1 (e.g., 128, 64, 32, 16, 8, 4, 2, 1).
十进制转二进制：重复除以2，反向读取余数。二进制转十进制：将位为1的位值相加（例如128, 64, 32, 16, 8, 4, 2, 1）。
Hexadecimal (base 16) uses 0-9 and A-F. It is a shorter way to represent binary, as one hex digit equals 4 bits. Convert by grouping binary in nibbles.
十六进制（基16）使用0-9和A-F。它是表示二进制的一种更简短方式，一个十六进制数字等于4个比特。通过将二进制分成半字节组来转换。
erflow occurs when the result exceeds the allocated number of bits, causing a carry out of the most significant bit.
二进制加法遵循简单规则：0+0=0，0+1=1，1+1=10（进位1）。当结果超出分配的位数时发生溢出，导致最高有效位产生进位。
Two's complement represents negative integers. To find the negative of a number, invert all bits and add 1. The most significant bit indicates sign (1 for negative).
二进制补码表示负整数。求一个数的负数，将所有位反转并加1。最高有效位表示符号（1为负）。
Character sets: ASCII uses 7 bits (128 characters), extended ASCII 8 bits. Unicode uses up to 32 bits to represent a vast range of international symbols and emojis.
字符集：ASCII用7位（128个字符），扩展ASCII用8位。Unicode使用最多32位，表示大量国际符号和表情。
Resolution is the number of pixels (width×height). File size = resolution × colour depth.
图像：位图图像由像素组成，每个像素被赋予颜色值。颜色深度是每像素比特数；深度越高颜色越多。分辨率是像素数量（宽×高）。文件大小 = 分辨率 × 颜色深度。
th determine quality. File size = sample rate × bit depth × duration (seconds) × channels.
声音通过间隔采样；采样率（Hz）和位深度决定质量。文件大小 = 采样率 × 位深度 × 时长（秒）× 声道数。
4. Computer Systems Architecture | 计算机系统体系结构
The Von Neumann architecture stores both program instructions and data in the same memory. The CPU fetches, decodes, and executes instructions in a continuous cycle.
冯·诺依曼体系结构将程序指令和数据存储在同一内存中。CPU在一个连续循环中取指、译码和执行指令。
Key CPU components: Control Unit (CU) directs operations, Arithmetic Logic Unit (ALU) performs calculations and logic, registers (PC, MAR, MDR, ACC) hold temporary data and addresses.
CPU关键部件：控制单元(CU)指挥操作，算术逻辑单元(ALU)执行运算和逻辑，寄存器（PC, MAR, MDR, ACC）存放临时数据和地址。
Buses are communication pathways: address bus carries memory addresses (unidirectional), data bus carries data (bidirectional), control bus sends timing and control signals.
总线是通信路径：地址总线传输内存地址（单向），数据总线传输数据（双向），控制总线发送时序和控制信号。
Clock speed (GHz) measures how many cycles per second. Cache memory is high-speed memory inside the CPU. Number of cores enables parallel processing. All affect performance.
时钟速度（GHz）衡量每秒周期数。缓存是CPU内部的高速内存。内核数使并行处理成为可能。这些都影响性能。
Embedded systems are dedicated computers within larger devices (e.g., microwave, washing machine). They are typically low-power, reliable, and run a single program.
嵌入式系统是大型设备内的专用计算机（如微波炉、洗衣机）。它们通常功耗低、可靠性高，运行单一程序。
5. Memory and Storage | 存储器与存储
RAM (Random Access Memory) is volatile, stores data and programs currently in use. More RAM allows more tasks to run simultaneously without slowing down.
RAM（随机存取存储器）是易失性的，存储当前使用的数据和程序。更多RAM允许同时运行更多任务而不会减速。
ROM (Read Only Memory) is non-volatile and stores the boot program (BIOS). Its contents are retained when power is off and cannot be easily modified.
ROM（只读存储器）是非易失性的，存储引导程序(BIOS)。断电后内容保留，不容易修改。
Secondary storage is non-volatile: magnetic (HDD), solid state (SSD), and optical (CD/DVD). SSDs are faster and more durable but costlier per GB than HDDs.
辅助存储是非易失性的：磁性(HDD)、固态(SSD)和光学(CD/DVD)。SSD更快更耐用，但每GB成本高于HDD。
Cloud storage stores data on remote servers accessed via the internet. Advantages: accessibility, backup, and collaboration; risks include security and reliance on internet.
云存储将数据存储在通过互联网访问的远程服务器上。优点：可访问性、备份和协作；风险包括安全性和依赖互联网。
Virtual memory uses part of the hard drive as extra RAM when physical RAM is full. It prevents crashes but is much slower than actual RAM.
虚拟内存使用硬盘的一部分作为额外RAM，当物理RAM已满时。它防止崩溃但比实际RAM慢得多。
6. Networks and Communication | 网络与通信
A network is two or more connected computers sharing resources. LAN (Local Area Network) spans a small area like a building; WAN (Wide Area Network) covers large geographical areas.
网络是两台或多台连接并共享资源的计算机。LAN（局域网）覆盖小区域如一栋建筑；WAN（广域网）覆盖大地理区域。
Topologies: star (all nodes connected to a central switch), mesh (each node connected to many others), bus (single backbone cable). Star is common due to ease of fault finding.
拓扑：星型（所有节点连接到中心交换机），网状（每个节点与许多其他节点相连），总线型（单根主干电缆）。星型常见，因为易于查错。
Network protocols are rules for communication: TCP/IP for internet, HTTP/HTTPS for web, FTP for file transfer, SMTP/IMAP for email. HTTPS encrypts data.
网络协议是通信规则：互联网用TCP/IP，网页用HTTP/HTTPS，文件传输用FTP，电子邮件用SMTP/IMAP。HTTPS加密数据。
IP addresses identify devices; IPv4 (32-bit) and IPv6 (128-bit). MAC addresses are unique hardware identifiers. DNS translates domain names to IP addresses.
IP地址标识设备；IPv4(32位)和IPv6(128位)。MAC地址是唯一硬件标识符。DNS将域名转换为IP地址。
Packet switching breaks data into packets that travel independently, each with source/destination address, sequence number, and error check. Routers forward packets.
分组交换将数据拆分成独立传输的数据包，每个包包含源/目标地址、序列号和差错校验。路由器转发数据包。
7. Cybersecurity and Threats | 网络安全与威胁
Malware includes viruses (attach to files, spread), worms (self-replicate), Trojan horses (disguised as legitimate), ransomware (locks files), spyware (steals data).
恶意软件包括病毒（附着文件传播）、蠕虫（自我复制）、特洛伊木马（伪装为合法软件）、勒索软件（锁定文件）、间谍软件（窃取数据）。
Social engineering tricks users into revealing information, e.g., phishing emails that mimic trusted sources, pretexting, baiting. Often the weakest link is the human.
社会工程学诱骗用户泄露信息，如仿冒可信来源的网络钓鱼邮件、托辞、利诱。通常最薄弱的环节是人。
Denial of Service (DoS) attacks flood a server with traffic to make it unavailable. A firewall monitors incoming/outgoing network traffic based on security rules.
拒绝服务(DoS)攻击用流量淹没服务器使其不可用。防火墙根据安全规则监控进出网络流量。
Encryption scrambles data using a key, making it unreadable without decryption. Symmetric (one key) and asymmetric (public/private key pair) encryption are used.
加密使用密钥扰乱数据，没有解密就无法读取。使用对称加密（一个密钥）和非对称加密（公钥/私钥对）。
Staying safe: strong passwords, two-factor authentication, regular software updates, backups, and cautious use of removable media. Penetration testing identifies weaknesses.
保持安全：强密码、双因素认证、定期软件更新、备份，以及谨慎使用可移动介质。渗透测试识别弱点。
8. Software and Operating Systems | 软件与操作系统
System software includes the operating system (OS), utilities, and drivers. Application software performs user-oriented tasks like word processing, spreadsheets, browsers.
系统软件包括操作系统(OS)、实用工具和驱动程序。应用软件执行面向用户的任务，如文字处理、电子表格、浏览器。
The OS manages hardware, provides a user interface, handles multitasking, memory management, file management, and security. Examples: Windows, macOS, Linux.
操作系统管理硬件，提供用户界面，处理多任务、内存管理、文件管理和安全。例子：Windows、macOS、Linux。
Utility software maintains the computer: disk defragmenter, backup, compression, antivirus, disk cleanup, file manager. They help optimise performance and protect data.
实用工具软件维护计算机：磁盘碎片整理、备份、压缩、防病毒、磁盘清理、文件管理器。它们有助于优化性能和保护数据。
Open source software allows users to view, modify, and distribute the source code. Proprietary software restricts access to the source code and requires a licence.
开源软件允许用户查看、修改和分发源代码。专有软件限制对源代码的访问并需要许可证。
High-level languages are closer to human language (Python, Java). They must be translated by a compiler (translates whole program at once) or interpreter (line by line).
高级语言更接近人类语言（Python、Java）。它们必须由编译器（一次性翻译整个程序）或解释器（逐行翻译）翻译。
9. Boolean Logic and Logic Gates | 布尔逻辑与逻辑门
Logic gates are electronic circuits that produce an output based on Boolean inputs. The basic gates: AND, OR, NOT. Derived gates: NAND, NOR, XOR.
逻辑门是根据布尔输入产生输出的电子电路。基本门：与、或、非。派生门：与非、或非、异或。
Truth tables show all possible input combinations and the resulting output. A NOT gate inverts the input; an AND gate outputs 1 only if all inputs are 1; an OR gate outputs 1 if at least one input is 1.
真值表显示所有可能的输入组合及输出结果。非门反转输入；与门仅在所有输入为1时输出1；或门在至少一个输入为1时输出1。
| A | B | AND | OR | XOR |
| 0 | 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 1 | 1 |
| 1 | 1 | 1 | 1 | 0 |
Logic circuits combine multiple gates to produce a desired output. Draw gate symbols correctly: AND (flat back), OR (curved back), NOT (triangle with bubble).
逻辑电路组合多个门以产生期望输出。正确绘制门符号：与门（平背）、或门（弧形背）、非门（带小圆的三角形）。
Boolean algebra simplifies expressions: e.g., De Morgan' aws: NOT (A AND B) = (NOT A) OR (NOT B). Use these laws to minimise gates and reduce cost.
布尔代数简化表达式：如德摩根定律：非(A与B) = (非A) 或 (非B)。使用这些定律最小化门数量，降低成本。
10. Ethics, Legal and Environmental Issues | 伦理、法律与环境问题
The digital divide refers to the gap between those who have easy access to modern ICT and those who do not, often due to income, location, or education.
数字鸿沟指能轻松访问现代信息通信技术的人与无法访问的人之间的差距，通常因收入、地理位置或教育程度造成。
Data Protection Act (UK) governs the use of personal data – it must be collected fairly, used for specified purposes, kept secure, and not kept longer than needed. GDPR extends these rights in Europe.
数据保护法（英国）约束个人数据的使用——必须公平收集、用于特定目的、保持安全，且保存时间不超过必要。GDPR在欧洲扩展了这些权利。
Computer Misuse Act makes unauthorised access to computer material, unauthorised modification, and creation of malware illegal. Ethical hacking is performed with permission.
计算机滥用法案将未经授权访问计算机材料、未经授权的修改以及制造恶意软件定为非法。道德黑客攻击是在许可下进行的。
Environmental concerns: e-waste from discarded electronics contains toxic materials; manufacture and server farms use significant energy. Recycling and energy-efficient hardware are solutions.
环境关切：废弃电子产品产生的电子垃圾含有毒物质；制造和服务器农场消耗大量能源。回收和节能硬件是解决方案。
Automation and AI can replace human jobs but also create new roles. Stakeholders must consider the ethical impact of decisions made by algorithms (bias, accountability).
自动化和人工智能可取代人类工作，但也创造新职位。利益相关者必须考虑算法决策的伦理影响（偏见、问责）。
Copyright and creativity: software and digital content are protected by copyright law. Plagiarism and software piracy are unethical and illegal.
版权与创意：软件和数字内容受版权法保护。剽窃和软件盗版是不道德且非法的。
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