📚 AS CIE Computer Science: Core Concepts Review | AS CIE 计算机：核心知识点梳理
This article provides a structured overview of the most important topics in the AS CIE Computer Science syllabus. It summarises key theory and practical concepts to help students consolidate their understanding and prepare effectively for examinations.
本文为 AS CIE 计算机科学大纲中最核心的主题提供了结构化的梳理。文章总结了重要的理论和实践概念，帮助学生巩固知识，高效备考。
1. Data Representation | 数据表示
All data in a computer is stored using binary digits (bits). Understanding number systems and encoding schemes is fundamental to knowing how computers interpret text, numbers, and instructions.
计算机中的所有数据都以二进制位存储。理解进制系统和编码方案，是弄清计算机如何解释文本、数字和指令的基础。
Denary (base-10), binary (base-2), and hexadecimal (base-16) can be converted into one another. Grouping binary digits into nibbles simplifies translation to hex: for example, 1100 0111₂ = C7₁₆.
十进制（基数为10）、二进制（基数为2）和十六进制（基数为16）可以相互转换。将二进制位每四位一组，可简化与十六进制的对应：例如 1100 0111₂ = C7₁₆。
Text is encoded using character sets. ASCII uses 7 or 8 bits per character, while Unicode (e.g. UTF-8) supports a vast range of symbols from different languages.
文本通过字符集编码。ASCII 使用 7 或 8 位表示一个字符，而 Unicode（如 UTF-8）可支持各种语言的庞大符号集。
Negative integers can be represented using sign and magnitude or two’s complement. In two’s complement, the most significant bit denotes the sign, and the value of an n-bit number can be found by inverting bits and adding 1.
负整数可以用原码（符号与数值）或二进制补码表示。补码中最高位为符号位，求一个 n 位数值时，可对各位取反再加 1。
Real numbers are stored in floating-point form: a mantissa holds the significant digits and an exponent scales the number. Normalisation ensures maximum precision by adjusting the mantissa so that its first two bits are different.
实数以浮点形式存储：尾数包含有效数字，指数决定数值的大小。规范化通过调整尾数使前两位不同，从而保证最大精度。
Value = Mantissa × 2Exponent
2. Data Compression | 数据压缩
Compression reduces file size for storage or transmission. Lossless compression allows exact reconstruction of the original data, while lossy compression permanently removes some detail to achieve higher compression ratios.
压缩可减少存储或传输的文件大小。无损压缩允许原样重建数据，而有损压缩则永久性舍弃部分细节以换取更高的压缩率。
Run-length encoding (RLE) is a simple lossless method that replaces sequences of identical values with a count and the value. It works well on data with many repeated elements, such as simple graphics.
行程长度编码（RLE）是一种简单的无损方法，用一个计数和该值替代连续相同的序列。它适用于重复元素较多的数据，如简单图形。
Huffman coding is a more advanced lossless technique that assigns shorter binary codes to more frequent symbols, building an optimal prefix-free tree.
哈夫曼编码是一种更高级的无损技术，它为出现频率更高的符号分配更短的二进制码，构建一棵最优无前缀树。
Lossy compression is commonly used for images (JPEG), audio (MP3) and video (MPEG). These formats exploit limitations in human perception to discard inaudible or invisible data.
有损压缩常用于图像（JPEG）、音频（MP3）和视频（MPEG）。这些格式利用人类感知的局限，丢弃那些听不见或不可见的数据。
3. Computer Architecture | 计算机体系结构
The von Neumann architecture stores both programs and data in the same memory. The central processing unit (CPU) fetches, decodes and executes instructions in a repeating cycle.
冯·诺依曼架构将程序和数据存储在同一内存中。中央处理器（CPU）通过重复的取指、解码和执行周期来运行指令。
Key CPU components include the arithmetic logic unit (ALU) for calculations, the control unit (CU) for directing operations, and registers such as the program counter (PC), memory address register (MAR), memory data register (MDR), current instruction register (CIR) and accumulator (ACC).
CPU 的关键部件包括进行运算的算术逻辑单元（ALU）、指挥操作的控制单元（CU），以及程序计数器（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）等寄存器。
The fetch-decode-execute cycle moves instructions from memory into the CPU. The PC holds the address of the next instruction; after fetching, the instruction is placed in the CIR and decoded, then the CU and ALU carry out the required operation.
取指-解码-执行周期将指令从内存送入 CPU。PC 保存下一条指令的地址；取指后，指令放入 CIR 并解码，随后由 CU 和 ALU 执行所需操作。
CPU performance is influenced by clock speed (cycles per second), number of cores (parallel execution), and cache size (faster access to frequently used data).
CPU 性能受时钟频率（每秒周期数）、核心数（并行执行）和缓存大小（更快访问常用数据）的影响。
4. Operating Systems | 操作系统
An operating system (OS) manages hardware resources and provides a platform for software. Functions include memory management, process scheduling, file management, and handling interrupts.
操作系统（OS）管理硬件资源并为软件提供平台。其功能包括内存管理、进程调度、文件管理以及中断处理。
Interrupts are signals from hardware or software that cause the CPU to suspend its current task, save its state, and execute an interrupt service routine (ISR). After handling the interrupt, the previous task resumes.
中断是由硬件或软件发出的信号，使 CPU 挂起当前任务、保存状态并执行中断服务例程（ISR）。处理中断后，之前的任务恢复执行。
Multitasking is achieved by rapid context switching, where the OS allocates CPU time slices to multiple processes. Memory management uses paging or segmentation to allocate and protect memory spaces.
多任务通过快速上下文切换实现，操作系统为多个进程分配 CPU 时间片。内存管理则使用分页或分段技术来分配并保护内存空间。
The OS also provides a user interface (CLI or GUI) and manages security through user authentication and access rights.
操作系统还提供用户界面（命令行界面或图形用户界面），并通过用户认证和访问权限管理安全性。
5. Networking Fundamentals | 网络基础
Networks connect computers to share resources and data. A local area network (LAN) covers a small geographical area, while a wide area network (WAN) connects LANs over large distances.
网络将计算机连接起来以共享资源和数据。局域网（LAN）覆盖较小的地理范围，而广域网（WAN）则远距离连接多个局域网。
Common network topologies include star (central switch with dedicated links), bus (single shared cable) and mesh (multiple redundant paths). The star topology is widely used in modern Ethernet LANs due to its fault tolerance and ease of troubleshooting.
常见网络拓扑包括星型（中心交换机，专用链路）、总线型（单根共享电缆）和网状（多条冗余路径）。星型拓扑因容错性强且便于排错，广泛用于现代以太局域网。
Network hardware includes switches (forward frames within a LAN using MAC addresses), routers (forward packets between networks using IP addresses), and network interface cards (NICs).
网络硬件包括交换机（使用 MAC 地址在局域网内转发帧）、路由器（使用 IP 地址在网络间转发数据包）以及网络接口卡（NIC）。
Protocols define the rules for communication. Key application-layer protocols include HTTP/HTTPS for web, FTP for file transfer, SMTP for sending emails, and POP3/IMAP for receiving emails.
协议定义了通信规则。关键的应用层协议包括用于网页的 HTTP/HTTPS、用于文件传输的 FTP、用于发送电子邮件的 SMTP，以及用于接收邮件的 POP3/IMAP。
6. Internet and Communication | 互联网与通信
The Internet is built on the TCP/IP protocol suite. TCP is connection-oriented and ensures reliable, ordered delivery of data, while IP handles addressing and routing packets across networks.
互联网建立在 TCP/IP 协议族之上。TCP 面向连接，确保数据可靠有序地交付；IP 则负责跨网络的寻址和数据包路由。
The four layers of the TCP/IP model are Application, Transport, Internet, and Network Access. Each layer encapsulates data with its own header, building a stack that enables interoperability between different hardware and software.
TCP/IP 模型的四层包括应用层、传输层、互联网层和网络接入层。每一层用各自的头部封装数据，形成一个可让不同硬件和软件互操作的协议栈。
Data is split into packets containing source and destination IP addresses, sequence numbers, and error-checking fields. Routers use these addresses to forward packets along the best available path.
数据被分割为数据包，每个包包含源 IP 地址、目的 IP 地址、序号和检错字段。路由器根据这些地址沿最佳可用路径转发数据包。
The Domain Name System (DNS) translates human-readable domain names (e.g. aleveler.com) into IP addresses. Distributed name servers handle queries to resolve names efficiently across the Internet.
域名系统（DNS）将人类可读的域名（如 aleveler.com）转换为 IP 地址。分布式的域名服务器处理查询，在互联网上高效地解析名称。
Network security includes firewalls to filter traffic, SSL/TLS to encrypt data during transmission, and authentication mechanisms to verify users.
网络安全包括用于过滤流量的防火墙、用于传输中加密数据的 SSL/TLS，以及用于验证用户的身份认证机制。
7. Database Fundamentals | 数据库基础
A database is an organised collection of data managed by a Database Management System (DBMS). Relational databases store information in tables, where each row is a record and each column is an attribute.
数据库是由数据库管理系统（DBMS）管理的有组织的数据集合。关系数据库将信息存储在表中，每一行为一条记录，每一列为一个属性。
Primary keys uniquely identify each record in a table; foreign keys create links between tables to reduce data redundancy and enforce referential integrity.
主键唯一标识表中的每条记录；外键在表之间创建关联，从而减少数据冗余并确保引用完整性。
Structured Query Language (SQL) is used to define, query, and manipulate data. Basic commands include SELECT … FROM … WHERE …, INSERT INTO, UPDATE, and DELETE.
结构化查询语言（SQL）用于定义、查询和操作数据。基本命令包括 SELECT … FROM … WHERE …、INSERT INTO、UPDATE 和 DELETE。
Normalisation reduces data redundancy by organising data into related tables. First Normal Form (1NF) requires atomic values, Second Normal Form (2NF) removes partial dependencies, and Third Normal Form (3NF) eliminates transitive dependencies.
规范化通过将数据组织到相关表中来减少冗余。第一范式（1NF）要求原子值，第二范式（2NF）消除部分依赖，第三范式（3NF）消除传递依赖。
8. Algorithm Design | 算法设计
An algorithm is a step-by-step sequence of instructions to solve a problem. Pseudocode and flowcharts are used to express algorithms in a language-independent manner.
算法是解决问题的一步步指令序列。伪代码和流程图用于以独立于编程语言的方式表示算法。
Three basic control structures form all algorithms: sequence (executing steps in order), selection (IF…THEN…ELSE…ENDIF), and iteration (WHILE…DO…ENDWHILE or FOR…NEXT).
三种基本控制结构构成所有算法：顺序结构（依次执行步骤）、选择结构（IF…THEN…ELSE…ENDIF）和迭代结构（WHILE…DO…ENDWHILE 或 FOR…NEXT）。
Searching algorithms include linear search, which checks each element in turn, and binary search, which repeatedly divides a sorted list in half. Binary search is much faster on large, ordered datasets, with a worst-case complexity of O(log₂ n).
搜索算法包括逐一检查每个元素的线性搜索和不断对半分割有序列表的二分搜索。二分搜索在大型有序数据集上快得多，最坏情况复杂度为 O(log₂ n)。
Sorting algorithms such as bubble sort and insertion sort are commonly examined. Bubble sort compares adjacent elements and swaps them if they are out of order, while insertion sort builds the sorted list one element at a time by inserting each new item into its correct position.
冒泡排序和插入排序等排序算法是常见考点。冒泡排序比较相邻元素并在顺序错误时交换，插入排序则通过将每个新元素插入正确位置，逐个构建有序列表。
Basic data structures include arrays (fixed-size, indexed), records (custom grouping of fields), and files. Stacks (LIFO) and queues (FIFO) are often used in algorithm implementation.
基本数据结构包括数组（定长、索引）、记录（自定义字段组合）和文件。栈（后进先出）和队列（先进先出）常用于算法实现。
9. Software Development | 软件开发
The software development life cycle (SDLC) typically includes stages: analysis (gathering requirements), design (architecture, interfaces, algorithms), development (coding), testing (verification and validation), implementation (installation), and maintenance (updates and fixes).
软件开发生命周期通常包括以下阶段：分析（收集需求）、设计（架构、界面、算法）、开发（编码）、测试（验证与确认）、实施（安装）和维护（更新与修复）。
Testing methods include black-box testing, which examines functionality without viewing internal code, and white-box testing, which involves examining the logic and paths within the program.
测试方法包括黑盒测试（不查看内部代码，只检查功能）和白盒测试（检查程序内部的逻辑和路径）。
During analysis, data flow diagrams (DFDs) and entity-relationship diagrams (ERDs) help model information. Flowcharts and pseudocode are refined in the design phase to produce clear specifications for developers.
在分析阶段，数据流图和实体关系图有助于信息建模。设计阶段则细化流程图和伪代码，为开发人员提供清晰的规格说明。
Development often follows models such as the waterfall model (linear stages) or agile methodologies, where iterative feedback drives rapid, incremental delivery.
开发常遵循瀑布模型（线性阶段）或敏捷方法，通过迭代反馈推动快速、增量的交付。
10. Security and Ethics | 安全与伦理
Computer security involves protecting systems from threats such as malware, phishing, denial-of-service attacks, and unauthorised access. Common defences include firewalls, encryption, antimalware software, and user access controls.
计算机安全涉及保护系统免受恶意软件、网络钓鱼、拒绝服务攻击和未授权访问等威胁。常见防御手段包括防火墙、加密、反恶意软件和用户访问控制。
Encryption transforms plaintext into ciphertext using an algorithm and a key. Symmetric encryption uses the same key for both encryption and decryption, while asymmetric encryption uses a key pair (public and private).
加密使用算法和密钥将明文转换为密文。对称加密使用同一密钥进行加解密，而非对称加密则使用一对密钥（公钥和私钥）。
Ethical issues include the digital divide, data privacy, intellectual property, and the impact of automation on employment. Computer professionals should follow codes of conduct that emphasise honesty, competence, and respect for privacy.
伦理问题包括数字鸿沟、数据隐私、知识产权以及自动化对就业的影响。计算机专业人员应遵守强调诚实、能力和尊重隐私的行为准则。
Data protection laws regulate how personal information can be collected, stored, and shared. Organisations must obtain consent, keep data accurate, and protect it against loss or disclosure.
数据保护法规范了如何收集、存储和共享个人信息。组织必须获得许可、保持数据准确，并防止数据丢失或泄露。
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