📚 AS Edexcel Computer Science: Core Knowledge Review | 核心知识点梳理
This article provides a comprehensive review of the essential concepts covered in the AS Level Edexcel Computer Science specification. It is designed to help students consolidate their understanding of core topics, from data representation and computer architecture to networking, databases, and programming. Each section presents key ideas in both English and Chinese, ensuring clarity for bilingual learners.
本文全面梳理了 AS Level Edexcel 计算机科学课程的核心概念，旨在帮助学生巩固对核心主题的理解，涵盖数据表示、计算机体系结构、网络、数据库和编程等领域。每个小节以英文和中文呈现关键思想，确保双语学习者清晰掌握。
1. Data Representation | 数据表示
In computer systems, all data is stored and processed in binary (base-2). Understanding how numbers, text, images and sound are represented is fundamental. Binary digits (bits) are grouped into bytes (8 bits), and larger units such as kilobytes (KB), megabytes (MB) and gigabytes (GB) follow powers of 2: 1 KB = 2¹⁰ bytes = 1024 bytes.
在计算机系统中，所有数据都以二进制（基数为2）存储和处理。理解数字、文本、图像和声音的表示方法是基础。二进制位（比特）组成字节（8位），更大单位如千字节（KB）、兆字节（MB）和吉字节（GB）使用2的幂次：1 KB = 2¹⁰ 字节 = 1024字节。
Decimal integers can be converted to binary by successive division by 2, recording remainders. Hexadecimal (base-16) provides a compact way to represent binary values, with each hex digit corresponding to 4 bits (a nibble). For example, the binary number 1101 1010₂ equals DA₁₆.
十进制整数可通过连续除以2取余数转换为二进制。十六进制（基数为16）提供了表示二进制值的紧凑方式，每个十六进制数字对应4位（半字节）。例如，二进制数 1101 1010₂ 等于 DA₁₆。
Characters are represented using standardised encoding systems. ASCII uses 7 bits to represent 128 characters, while extended ASCII uses 8 bits for 256 characters. Unicode supports a far wider range of symbols and languages, using variable-length encoding (e.g., UTF-8).
字符使用标准化编码系统表示。ASCII使用7位表示128个字符，扩展ASCII使用8位表示256个字符。Unicode支持更广泛的符号和语言，使用变长编码（如UTF-8）。
Images are typically stored as bitmaps, where each pixel’s colour is defined by a binary number. Colour depth (bits per pixel) determines the number of available colours. Higher resolution (more pixels) and greater colour depth increase file size. Sound is digitised by sampling the analogue signal at regular intervals; sampling rate and bit depth affect sound quality and file size.
图像通常以位图形式存储，每个像素的颜色由一个二进制数定义。色深（每像素位数）决定了可用颜色数量。更高的分辨率（更多像素）和更大的色深会增加文件大小。声音通过以固定间隔对模拟信号采样来数字化；采样率和位深度影响声音质量和文件大小。
2. Computer Systems and Architecture | 计算机系统与体系结构
The Von Neumann architecture is the foundation of most modern computers. It comprises a central processing unit (CPU), memory, input/output devices, and a system bus that connects them. The stored-program concept means both data and instructions are held in the same memory.
冯·诺依曼体系结构是大多数现代计算机的基础。它包含中央处理器（CPU）、存储器、输入/输出设备以及连接它们的系统总线。存储程序概念意味着数据和指令存放在同一存储器中。
The CPU consists of the control unit (CU), arithmetic logic unit (ALU), and a set of registers. Key registers include the program counter (PC), memory address register (MAR), memory data register (MDR), current instruction register (CIR), and accumulator (ACC). The fetch-decode-execute cycle describes how instructions are processed: the CPU fetches an instruction from memory, decodes it, and then executes it, repeating indefinitely.
CPU由控制单元（CU）、算术逻辑单元（ALU）和一组寄存器组成。关键寄存器包括程序计数器（PC）、存储器地址寄存器（MAR）、存储器数据寄存器（MDR）、当前指令寄存器（CIR）和累加器（ACC）。取指-译码-执行周期描述了指令处理过程：CPU从内存中取出指令，译码，然后执行，如此无限循环。
The performance of a CPU is influenced by clock speed (measured in Hertz), number of cores, and cache memory. Pipelining and parallel processing can improve throughput. Memory hierarchy consists of registers, cache, RAM, and secondary storage, with speed decreasing and capacity increasing from registers to secondary storage.
CPU性能受时钟速度（以赫兹计）、内核数量和高速缓存影响。流水线和并行处理可提高吞吐量。存储器层次结构包括寄存器、高速缓存、RAM和辅助存储器，从寄存器到辅助存储器，速度降低、容量增大。
3. System Software and Operating Systems | 系统软件与操作系统
System software manages hardware and provides services for application software. The operating system (OS) is the most important type, performing functions such as memory management, process scheduling, file management, providing a user interface, and handling input/output operations.
系统软件管理硬件并为应用软件提供服务。操作系统（OS）是最重要的类型，执行内存管理、进程调度、文件管理、提供用户界面和处理输入/输出操作等功能。
Utility programs perform specific maintenance tasks, including disk defragmentation, backup, antivirus scanning, and compression. Language translators convert high-level source code into machine code: compilers translate the whole program at once, while interpreters translate line by line. Assemblers convert assembly language into machine code.
实用程序执行特定的维护任务，包括磁盘碎片整理、备份、防病毒扫描和压缩。语言翻译器将高级源代码转换为机器码：编译器一次性翻译整个程序，解释器逐行翻译。汇编器将汇编语言转换为机器码。
4. Networks and Communication | 网络与通信
A computer network allows devices to share resources and communicate. Networks can be classified by geographical scope: LAN (Local Area Network) covers a small area, WAN (Wide Area Network) spans large distances, and PAN (Personal Area Network) connects personal devices. Topologies such as star, bus, ring, and mesh define physical or logical layout.
计算机网络允许设备共享资源和通信。网络可按地理范围分类：LAN（局域网）覆盖小区域，WAN（广域网）跨越长距离，PAN（个域网）连接个人设备。拓扑结构如星型、总线、环型和网状定义了物理或逻辑布局。
Protocols are sets of rules governing data transmission. The TCP/IP stack comprises four layers: application, transport, internet, and network interface. Key protocols include HTTP/HTTPS, FTP, SMTP, POP3, and DNS. IP addresses (IPv4 and IPv6) uniquely identify devices on a network, while MAC addresses identify network interface hardware. Domain Name System (DNS) translates domain names into IP addresses.
协议是管理数据传输的规则集。TCP/IP协议栈包含四层：应用层、传输层、互联网层和网络接口层。关键协议包括HTTP/HTTPS、FTP、SMTP、POP3和DNS。IP地址（IPv4和IPv6）唯一标识网络上的设备，MAC地址标识网络接口硬件。域名系统（DNS）将域名转换为IP地址。
5. Databases and SQL | 数据库与SQL
A relational database organises data into tables (relations) consisting of rows (records) and columns (attributes). Each table has a primary key that uniquely identifies each record. Foreign keys link tables by referencing primary keys in other tables, enabling complex queries across multiple tables while reducing data redundancy (normalisation).
关系数据库将数据组织成由行（记录）和列（属性）组成的表（关系）。每个表都有一个唯一标识每条记录的主键。外键通过引用其他表中的主键来连接表，从而实现跨多个表的复杂查询，同时减少数据冗余（规范化）。
Structured Query Language (SQL) is used to create, query, and modify databases. The SELECT statement retrieves data: SELECT * FROM students WHERE grade > 70;. INSERT adds new records, UPDATE modifies existing ones, and DELETE removes records. JOIN clauses combine rows from multiple tables based on a related column.
结构化查询语言（SQL）用于创建、查询和修改数据库。SELECT语句检索数据：SELECT * FROM students WHERE grade > 70；。INSERT添加新记录，UPDATE修改现有记录，DELETE删除记录。JOIN子句基于相关列组合多个表的行。
6. Problem Solving and Algorithms | 问题解决与算法
Computational thinking involves decomposition, pattern recognition, abstraction, and algorithm design. An algorithm is a step-by-step procedure to solve a problem, expressed using pseudocode or flowcharts. Key algorithm constructs are sequence, selection (IF-THEN-ELSE), and iteration (FOR, WHILE, REPEAT loops).
计算思维包括分解、模式识别、抽象和算法设计。算法是解决问题的逐步步骤，可用伪代码或流程图表示。关键算法结构是顺序、选择（IF-THEN-ELSE）和迭代（FOR、WHILE、REPEAT 循环）。
Standard searching algorithms include linear search (checking each item) and binary search (repeatedly dividing a sorted list). Sorting algorithms include bubble sort, insertion sort, and merge sort. Efficiency is compared using time complexity, often expressed in Big O notation (e.g., O(n), O(log n), O(n²)).
标准搜索算法包括线性搜索（检查每一项）和二分搜索（反复将有序列表一分为二）。排序算法包括冒泡排序、插入排序和归并排序。效率通过时间复杂度比较，通常用大O记号表示（如 O(n)、O(log n)、O(n²)）。
7. Programming Concepts | 编程概念
Programs are written using high-level languages (e.g., Python, Java). Variables store data of different data types: integer, real (float), Boolean, character, and string. Type casting converts between types. Arrays and lists hold multiple values, and two-dimensional arrays represent grids or tables.
程序使用高级语言（如Python、Java）编写。变量存储不同数据类型的数据：整型、实数（浮点）、布尔型、字符和字符串。类型转换在不同类型之间转换。数组和列表保存多个值，二维数组表示网格或表格。
Subprograms (procedures and functions) break code into reusable modules. Functions return a value, while procedures perform actions without returning a value. Parameters can be passed by value or by reference. Understanding scope of variables (local vs. global) is essential for debugging.
子程序（过程和函数）将代码分解成可重用模块。函数返回值，过程执行操作而不返回值。参数可通过值或引用传递。理解变量作用域（局部与全局）对于调试至关重要。
8. Information Security, Ethics and Law | 信息安全、伦理与法律
Network security threats include malware (viruses, worms, trojans), phishing, denial-of-service attacks, and SQL injection. Defensive measures involve firewalls,
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