📚 IGCSE OCR Computer Science: Last-Minute Revision Notes | IGCSE OCR 计算机：考前冲刺笔记
This article condenses every core topic from the IGCSE OCR Computer Science specification into bite-sized, bilingual revision notes. Use it to refresh your memory on system architecture, data representation, networks, algorithms, and programming concepts right before the exam.
本文将 IGCSE OCR 计算机科学考纲中的每个核心主题浓缩为简短的双语复习笔记。你可以用它快速回顾系统架构、数据表示、网络、算法和编程概念，非常适合考前冲刺使用。
1. System Architecture | 系统架构
The Central Processing Unit (CPU) executes program instructions using the fetch-decode-execute cycle. Its main components are the Control Unit (CU) that directs data flow, the Arithmetic Logic Unit (ALU) for calculations and logic, and registers such as the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR), and Accumulator (ACC).
中央处理器 (CPU) 通过取指‑解码‑执行循环来运行程序指令。它的主要组件包括控制数据流向的控制单元 (CU)、负责计算与逻辑运算的算术逻辑单元 (ALU)，以及程序计数器 (PC)、存储器地址寄存器 (MAR)、存储器数据寄存器 (MDR) 和累加器 (ACC) 等寄存器。
Von Neumann architecture stores both data and instructions in the same memory unit, with a single bus system. Its bottleneck is the limited data transfer rate between CPU and memory. In contrast, Harvard architecture uses separate memories and buses for data and instructions, allowing parallel access but being more expensive.
冯·诺依曼架构将数据和指令存放在同一个存储器中，使用单一总线系统。它的瓶颈在于 CPU 与存储器之间有限的数据传输速率。相比之下，哈佛架构为数据和指令使用独立的存储器和总线，可以并行存取但成本更高。
CPU performance is determined by clock speed (GHz, cycles per second), number of cores (parallel processing ability), and cache size (high‑speed on‑die memory reducing average access time). An embedded system is a dedicated computer inside a larger device, often with limited resources and real‑time constraints, e.g. a washing machine controller.
CPU 性能取决于时钟速度（GHz，每秒周期数）、核心数量（并行处理能力）和缓存大小（减少平均访问时间的高速片上内存）。嵌入式系统是位于较大设备内部的专用计算机，通常资源有限且有实时性要求，例如洗衣机的控制器。
2. Memory and Storage | 存储器与存储设备
Primary storage directly accessible by the CPU includes RAM (Random Access Memory) and ROM (Read Only Memory). RAM is volatile, holds currently running programs and data, and loses content when power is off. ROM is non‑volatile, stores the BIOS/bootloader, and retains data permanently.
CPU 可以直接访问的主存储器包括 RAM（随机存取存储器）和 ROM（只读存储器）。RAM 是易失性的，存放正在运行的程序和数据，断电后内容会丢失。ROM 是非易失性的，存储 BIOS/引导程序，永久保留数据。
Virtual memory uses part of the hard disk as an extension of RAM when physical RAM is full. This allows larger programs to run but reduces performance because disk access is much slower than RAM.
当物理 RAM 已满时，虚拟内存将一部分硬盘空间用作 RAM 的扩展。这样可以运行更大的程序，但会降低性能，因为磁盘访问比 RAM 慢得多。
Secondary storage devices are non‑volatile and store data long‑term. Common types include magnetic HDDs (high capacity, moving parts), solid‑state SSDs (faster, no moving parts, more expensive per GB), and optical discs (CD, DVD, Blu‑ray). Selection depends on capacity, speed, portability, durability, and cost.
二级存储设备是非易失性的，用于长期存放数据。常见类型包括磁性硬盘 HDD（容量大、有机械运动部件）、固态硬盘 SSD（速度更快、无运动部件、每 GB 更贵）和光盘（CD、DVD、蓝光）。选择依据为容量、速度、便携性、耐用性和成本。
3. Networks | 网络
A LAN (Local Area Network) covers a small geographical area, uses privately owned infrastructure, and offers high data transfer rates. A WAN (Wide Area Network) spans large distances, often using leased telecommunications lines or satellite links, with slower speeds compared to LANs.
局域网 (LAN) 覆盖较小的地理区域，使用私有基础设施，提供高数据传输速率。广域网 (WAN) 跨越较大距离，通常租用电信线路或卫星链路，速度比局域网慢。
Client‑server networks have a central server managing resources and security, while clients request services. Peer‑to‑peer networks allow each device to share resources directly without a central server, making them easy to set up but less secure and harder to manage.
客户端‑服务器网络由中央服务器管理资源和安全，客户端请求服务。对等网络 (peer‑to‑peer) 允许每台设备直接共享资源，无需中央服务器，设置简单但安全性较差且不易管理。
Star topology connects all nodes to a central switch; a single cable failure affects only one device, but the central switch is a single point of failure. Mesh topology connects every node to many others, providing high redundancy and reliability, but requires more cabling and is expensive.
星型拓扑将所有节点连接到中央交换机；单条电缆故障只影响一台设备，但中央交换机是单点故障。网状拓扑将每个节点与多个其他节点相连，冗余性和可靠性高，但需要更多布线且成本较高。
Protocols are sets of rules for communication. TCP/IP is the fundamental suite for the Internet; HTTP/HTTPS governs web page transfer; FTP transfers files; SMTP, POP and IMAP handle email. Layering (application, transport, internet, link) divides networking tasks, simplifying design and troubleshooting.
协议是通信的规则集合。TCP/IP 是互联网的基础协议族；HTTP/HTTPS 管理网页传输；FTP 传输文件；SMTP、POP 和 IMAP 处理电子邮件。分层模型（应用层、传输层、互联网层、链路层）将网络任务分解，简化了设计和故障诊断。
4. Network Security | 网络安全
Malware (viruses, worms, trojans, ransomware) can steal, corrupt or encrypt data. Phishing tricks users into revealing credentials via fake emails. SQL injection inserts malicious SQL statements into web form inputs to manipulate databases. Brute‑force attacks guess passwords repeatedly. Denial of Service (DoS) floods a server with traffic to make it unavailable.
恶意软件（病毒、蠕虫、特洛伊木马、勒索软件）可以窃取、损坏或加密数据。网络钓鱼通过伪造电子邮件诱骗用户泄露凭据。SQL 注入将恶意 SQL 语句插入网页表单以操控数据库。暴力攻击反复猜测密码。拒绝服务 (DoS) 攻击通过大量流量使服务器不可用。
Penetration testing simulates attacks to identify vulnerabilities before malicious hackers can exploit them. Prevention methods include anti‑malware software, firewalls that filter traffic based on rules, strong passwords and two‑factor authentication, encryption of data at rest and in transit, and regular software updates.
渗透测试模拟攻击以在恶意黑客利用之前发现漏洞。防御措施包括反恶意软件、基于规则过滤流量的防火墙、强密码与双因素认证、对静态和传输中的数据加密，以及定期软件更新。
5. System Software | 系统软件
The operating system (OS) manages hardware, provides a user interface, handles multitasking, memory management, peripheral management, and security (user accounts, file permissions). Utility software maintains the computer: defragmentation reorganises fragmented files on HDDs; backup creates data copies; compression reduces file size; encryption encodes data to protect confidentiality.
操作系统 (OS) 管理硬件、提供用户界面、处理多任务、内存管理、外设管理和安全性（用户账户、文件权限）。实用程序维护计算机：碎片整理重组 HDD 上零散的文件；备份创建数据副本；压缩减小文件体积；加密对数据编码以保护机密性。
6. Ethical, Legal, Cultural and Environmental Issues | 道德、法律、文化与环境问题
The Data Protection Act mandates that personal data be processed fairly, lawfully, and kept secure. The Computer Misuse Act criminalises unauthorised access, modification, and the creation of malware. Copyright and patents protect intellectual property; open‑source software allows users to view, modify, and share code, while proprietary software restricts access.
《数据保护法》要求个人数据必须公平、合法地处理并保持安全。《计算机滥用法》将未经授权访问、修改以及创建恶意软件定为刑事犯罪。版权和专利权保护知识产权；开源软件允许用户查看、修改和共享代码，而专有软件限制访问。
The digital divide is the gap between those who have access to modern ICT and those who do not, affected by economic, geographical, and educational factors. E‑waste (discarded electronic devices) contains toxic materials; responsible recycling and green manufacturing aim to reduce environmental harm.
数字鸿沟是指能够获取现代信息通信技术的人与不能获取的人之间的差距，受经济、地理和教育因素影响。电子垃圾（废弃电子设备）含有有毒物质；负责任回收和绿色制造旨在减少环境危害。
7. Algorithms | 算法
Computational thinking breaks a problem down via decomposition (splitting into smaller parts), pattern recognition, abstraction (ignoring unnecessary detail), and algorithm design. Pseudocode uses plain‑language constructs to describe algorithms; flowcharts use standard symbols (ovals for start/end, rectangles for processes, diamonds for decisions) connected by arrows.
计算思维通过分解（拆分为较小部分）、模式识别、抽象（忽略不必要细节）和算法设计来解决问题。伪代码使用接近自然语言的构造描述算法；流程图使用标准符号（椭圆表示开始/结束、矩形表示处理、菱形表示判断）并用箭头连接。
Binary search works on sorted lists by repeatedly dividing the search interval in half, giving O(log n) efficiency. Linear search checks each element sequentially, O(n). Bubble sort compares adjacent elements and swaps them, O(n²); merge sort divides the list and merges sorted halves, O(n log n); insertion sort builds the sorted list one element at a time.
二分查找要求在有序列表中通过反复将查找区间一分为二来工作，效率为 O(log n)。线性查找顺序检查每个元素，效率为 O(n)。冒泡排序比较相邻元素并交换，时间复杂度 O(n²)；归并排序将列表分割并合并有序子列表，O(n log n)；插入排序逐个插入元素构建有序序列。
8. Programming Concepts | 编程概念
Variables hold data values and have a data type (integer, real, boolean, char, string). Sequence, selection (if, else, switch/case) and iteration (for, while, do‑while) are the three basic control structures. Nested if statements allow multi‑level decisions; iteration loops repeat code until a condition is met.
变量保存数据值并拥有数据类型（整型、实型、布尔型、字符、字符串）。顺序、选择（if、else、switch/case）和迭代（for、while、do‑while）是三种基本控制结构。嵌套 if 语句支持多层次判断；循环迭代重复代码直至满足条件。
Arrays store multiple values of the same type under one identifier, accessed via index. Two‑dimensional arrays are useful for tables. Procedures are named blocks of code that perform tasks without returning a value; functions return a value. Parameters allow values to be passed into subprograms.
数组在同一个标识符下存储相同类型的多个值，通过索引访问。二维数组适用于表格。过程是有名称的代码块，执行任务但不返回值；函数会返回一个值。参数允许将值传入子程序。
9. Data Representation | 数据表示
Binary uses only 0 and 1 (base 2). Hexadecimal (base 16, digits 0‑9 and A‑F) is a compact way to represent binary, because one hex digit matches exactly four bits. Binary addition follows column rules: 0+0=0, 0+1=1, 1+1=0 carry 1. Overflow occurs when a result exceeds the available bit width.
二进制仅使用 0 和 1（基数为 2）。十六进制（基数为 16，数字 0‑9 和 A‑F）是表示二进制的简洁方式，因为一个十六进制数字正好对应四个二进制位。二进制加法遵循竖式规则：0+0=0，0+1=1，1+1=0 进 1。当结果超出可用位宽时发生溢出。
Characters are encoded using standard sets: ASCII uses 7 bits (128 characters), extended ASCII 8 bits; Unicode supports worldwide scripts with up to 32 bits per character. Images are represented as bitmaps of pixels, each pixel assigned a binary colour value. Colour depth is the number of bits per pixel; higher depth gives more colours but larger files.
字符使用标准集编码：ASCII 使用 7 位（128 个字符），扩展 ASCII 8 位；Unicode 支持全球文字，每个字符最多 32 位。图像以像素位图表示，每个像素分配一个二进制颜色值。颜色深度是每个像素的比特数；深度越高颜色越丰富但文件越大。
Sound is sampled at regular intervals; sample rate (kHz) and bit depth determine quality and file size. File size formulas are essential:
Image file size (bits) = width × height × colour depth
Sound file size (bits) = sample rate × bit depth × duration (s) × channels
声音以固定间隔采样；采样率 (kHz) 和位深度决定质量与文件大小。文件大小计算公式至关重要：图像文件大小 (位) = 宽度 × 高度 × 颜色深度；声音文件大小 (位) = 采样率 × 位深度 × 时长 (秒) × 声道数。
Lossy compression (JPEG, MP3) permanently removes non‑essential data to drastically shrink file size; lossless compression (PNG, ZIP) reduces file size without losing any information, allowing exact reconstruction.
有损压缩（JPEG、MP3）永久删除非必要数据以大幅缩小文件；无损压缩（PNG、ZIP）在不丢失任何信息的前提下减小文件，能精确还原。
10. Logic Gates and Boolean Logic | 逻辑门与布尔逻辑
Logic gates are the building blocks of digital circuits. The AND gate outputs 1 only when all inputs are 1; OR outputs 1 when at least one input is 1; NOT inverts the input. Truth tables list all input combinations and corresponding outputs.
逻辑门是数字电路的构建块。与门 (AND) 仅在所有输入为 1 时输出 1；或门 (OR) 在至少一个输入为 1 时输出 1；非门 (NOT) 将输入取反。真值表列出所有输入组合及对应输出。
Boolean expressions can represent logic circuits. For example, Q = (A AND B) OR (NOT C). Logic circuits are drawn using standard gate symbols; combining gates creates complex decision‑making hardware.
布尔表达式可以表示逻辑电路，例如 Q = (A AND B) OR (NOT C)。逻辑电路使用标准门符号绘制；组合多个门可创建复杂决策硬件。
11. Testing and Debugging | 测试与调试
Syntax errors occur when code breaks the language’s rules and are caught by the interpreter/compiler. Logic errors produce unexpected results because the algorithm’s logic is flawed but the code runs without crashing. Test data should include normal (typical expected) values, boundary (edges of valid range) values, and erroneous (invalid) data to ensure robust software.
语法错误在代码违反语言规则时发生，会被解释器/编译器捕捉。逻辑错误产生非预期结果，因为算法逻辑有缺陷但代码能够运行而不崩溃。测试数据应包含正常（典型预期）值、边界（有效范围边缘）值和异常（无效）数据，以确保软件的健壮性。
Trace tables help debug algorithms by stepping through code line by line, recording the changing values of variables. They are crucial for identifying where a program’s behaviour deviates from the intended algorithm.
跟踪表通过逐行执行代码并记录变量值的变化来帮助调试算法。它们对于发现程序行为偏离预期算法的位置至关重要。
12. Exam Tips and Final Review | 考前技巧与最后回顾
Read each question carefully, noting command words (state, describe, explain, compare). When calculating file sizes, show all working and convert units correctly (8 bits = 1 byte, 1024 bytes = 1 KB, etc.). For programming questions, annotate code or write pseudocode before coding if using a high‑level language.
仔细阅读每个问题，注意指令词（陈述、描述、解释、比较）。在计算文件大小时，展示所有步骤并正确转换单位（8 比特 = 1 字节，1024 字节 = 1 KB 等等）。对于编程题，如果使用高级语言，先标注代码注释或编写伪代码。
Allocate time per mark; about 1 minute per mark. Practice tracing logic gates and truth tables, binary‑hexadecimal conversions, and writing simple algorithms in pseudocode. Check your answers for missing units, incomplete explanations, and reversed logic conditions.
按分值分配时间：约每 1 分分配 1 分钟。练习逻辑门和真值表的追踪、二进制与十六进制的转换，以及用伪代码编写简单算法。检查答案是否有遗漏单位、解释不完整或逻辑条件写反的问题。
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