📚 Year 12 Edexcel Computer Science: Core Knowledge Review | Year 12 Edexcel 计算机：核心知识点梳理
Year 12 Edexcel Computer Science builds the foundation for advanced study by covering the essential principles of how computers work, how data is represented, the fundamentals of programming and algorithms, and the wider impact of digital technology. This revision guide walks through the core topics you must master for the AS Principles of Computer Science paper, clarifying key concepts in both English and Chinese to support bilingual learners.
Year 12 Edexcel 计算机科学课程为高阶学习打下基础，涵盖计算机工作原理、数据表示、编程与算法基础，以及数字技术的广泛影响。本复习指南梳理了 AS 计算机科学理论试卷必须掌握的核心主题，用中英双语澄清关键概念，帮助双语学习者巩固知识。
1. Data Representation | 数据表示
All data inside a computer is stored and processed as binary. Numbers, characters, images, and sound are all encoded using sequences of bits. Understanding how conversion, arithmetic, and different data types work is essential.
计算机内的所有数据都以二进制形式存储和处理。数字、字符、图像和声音都使用位序列进行编码。理解数制转换、二进制运算和不同数据类型的表示方式是基础。
imal. Hexadecimal (base 16) is used as a compact way to represent binary, where each hex digit corresponds to 4 bits (a nibble).
无符号二进制表示正整数。例如 1101₂ = (1 × 2³) + (1 × 2²) + (0 × 2¹) + (1 × 2⁰) = 十进制 13。十六进制（基数为 16）用于紧凑地表示二进制，每个十六进制数字对应 4 位（一个半字节）。
For signed integers, Edexcel expects you to understand two’s complement. The most significant bit acts as the sign bit (weight –128 in an 8‑bit system). Negative numbers are formed by inverting bits of the positive magnitude and adding 1. Arithmetic can be performed directly, and overflow is detected when the carry into the sign bit differs from the carry out.
对于有符号整数，Edexcel 要求掌握二进制补码。最高有效位充当符号位（8 位系统中权重 –128）。负数通过将正数绝对值按位取反再加 1 得到。可以直接进行算术运算，当符号位的进位与出位不同时则发生溢出。
ced: a number is stored as mantissa × 2^exponent. The mantissa holds the significant digits, the exponent scales the value. Normalisation maximises precision.
二进制小数使用二进制小数点。例如 101.101₂ = 十进制 5.625。定点表示范围有限，因此引入浮点：一个数以 尾数 × 2^指数 的形式存储。尾数保存有效数字，指数缩放数值。规格化可以最大化精度。
Characters are encoded using standards such as ASCII (7-bit, 128 characters) and Unicode (supports many languages). For images, bitmaps store individual pixel colours, while vector graphics use mathematical primitives. Sound is sampled at a given frequency and bit depth; higher sample rate and bit depth improve quality but increase file size.
字符使用 ASCII（7 位，128 个字符）和 Unicode（支持多种语言）等标准进行编码。位图图像存储每个像素的颜色，而矢量图使用数学图元。声音按给定频率和位深度采样；更高的采样率和位深度可以提高质量，但会增加文件大小。
2. Computer Architecture and the CPU | 计算机体系结构与 CPU
The Von Neumann architecture underpins most modern computers. It stores both instructions and data in the same memory, connected to the CPU via buses. The main components of the CPU are the Control Unit (CU), the Arithmetic Logic Unit (ALU), and a set of registers.
冯·诺依曼架构是大多数现代计算机的基础。它把指令和数据存储在同一个存储器中，通过总线与 CPU 相连。CPU 的主要组件包括控制单元（CU）、算术逻辑单元（ALU）和一组寄存器。
The Fetch‑Decode‑Execute cycle repeatedly carries out program instructions. The Program Counter (PC) holds the address of the next instruction; it is copied to the Memory Address Register (MAR), and the instruction is fetched into the Memory Data Register (MDR) before the CU decodes and executes it, often using the Accumulator (ACC) for arithmetic results.
取指‑译码‑执行周期反复执行程序指令。程序计数器（PC）存放下一条指令的地址；该地址被复制到存储器地址寄存器（MAR），指令取入存储器数据寄存器（MDR），然后控制单元译码并执行，执行过程中常用累加器（ACC）存放算术结果。
Key factors affecting CPU performance include clock speed (cycles per second), number of cores, and cache memory. Increasing clock speed allows more cycles per second, multiple cores enable true parallel execution, and larger cache reduces the need to access slower main memory.
影响 CPU 性能的关键因素包括时钟速度（每秒周期数）、核心数量和高速缓存。提高时钟速度可增加每秒执行周期数，多核可以实现真正的并行处理，更大的缓存能减少访问速度较慢的主存的次数。
Pipelining is a technique that allows the processor to begin fetching the next instruction before the current one has finished executing, thereby improving throughput. However, branch instructions can cause pipeline stalls.
流水线技术允许处理器在当前指令执行完成前就开始取下一条指令，从而提高吞吐量。然而，分支指令可能导致流水线停顿。
3. Memory and Storage | 存储器与存储
Random Access Memory (RAM) is volatile, high‑speed memory used to hold the operating system, applications, and data currently in use. Read‑Only Memory (ROM) is non‑volatile and typically contains the boot program (BIOS).
随机存取存储器（RAM）是易失性的高速存储器，用于存放正在使用的操作系统、应用程序和数据。只读存储器（ROM）是非易失性的，通常包含引导程序（BIOS）。
Virtual memory uses a portion of secondary storage (e.g. HDD/SSD) as an extension of RAM, allowing the computer to run larger programs or multiple programs simultaneously. However, thrashing can occur if the system continuously swaps pages between RAM and disk, severely reducing performance.
虚拟内存将辅助存储器（如硬盘/固态硬盘）的一部分用作 RAM 的扩展，使计算机能够运行较大的程序或多个程序。但是，如果系统不断在 RAM 和磁盘之间交换页面，就会发生抖动，严重降低性能。
Secondary storage technologies are compared by capacity, speed, portability, durability, and cost. Magnetic hard disks store data on spinning platters; solid‑state drives use NAND flash memory with no moving parts, offering faster access times. Optical discs (CD, DVD) and cloud storage provide alternatives for backup and distribution.
辅助存储技术根据容量、速度、可携带性、耐久性和成本进行比较。磁性硬盘将数据存储在旋转的盘片上；固态硬盘使用无移动部件的 NAND 闪存，访问速度更快。光盘（CD、DVD）和云存储为备份和分发提供了其他选择。
4. Networks and Protocols | 网络与协议
A network connects computing devices so they can share resources and data. LANs (Local Area Networks) cover a small geographical area, while WANs (Wide Area Networks) connect LANs over long distances. Topologies such as star, bus, and mesh describe the physical or logical layout of connections.
网络连接计算设备，使它们能够共享资源和数据。局域网（LAN）覆盖较小的地理区域，广域网（WAN）则跨长距离连接局域网。星型、总线和网状等拓扑结构描述了连接的物理或逻辑布局。
The TCP/IP stack is a set of protocols that governs data transmission over the internet. It is typically divided into four layers: Application (HTTP, FTP, SMTP, POP3), Transport (TCP/UDP), Internet (IP), and Network Access. TCP provides reliable, ordered delivery using ports and acknowledgements.
TCP/IP 协议栈是一组控制互联网数据传输的协议。它通常分为四层：应用层（HTTP、FTP、SMTP、POP3）、传输层（TCP/UDP）、网络层（IP）和网络接入层。TCP 通过端口和确认提供可靠、有序的传输。
Hardware such as routers forwards data packets between networks using IP addresses, while switches connect devices within a LAN using MAC addresses. A gateway connects different networks that may use different protocols. DNS translates human‑readable domain names into IP addresses.
路由器等硬件使用 IP 地址在网络间转发数据包，而交换机使用 MAC 地址在局域网内连接设备。网关连接可能使用不同协议的不同网络。DNS 将人类可读的域名转换为 IP 地址。
Network security measures include firewalls (filter traffic based on rules), encryption (protecting confidentiality), MAC address filtering, and using VPNs to create secure tunnels across public networks.
网络安全措施包括防火墙（按规则过滤流量）、加密（保护机密性）、MAC 地址过滤以及使用 VPN 在公共网络上创建安全隧道。
5. System Software and Operating Systems | 系统软件与操作系统
The operating system (OS) is a fundamental piece of software that manages hardware resources and provides services for application programs. Its core functions include memory management (allocating RAM to processes), processor scheduling (deciding which process runs next), file management, and providing a user interface.
操作系统（OS）是管理硬件资源并为应用程序提供服务的基础软件。其核心功能包括内存管理（为进程分配 RAM）、处理器调度（决定下一个运行哪个进程）、文件管理以及提供用户界面。
Utility software helps maintain, analyse, and optimise the computer. Common examples are disk defragmenters (reorganising fragmented files on HDDs), backup software, compression tools, and antivirus programs. Encrypting utilities protect data at rest.
实用软件有助于维护、分析和优化计算机。常见的例子有磁盘碎片整理程序（重组 HDD 上的碎片文件）、备份软件、压缩工具和防病毒程序。加密实用程序可以保护静止数据。
Programs written in high‑level languages must be translated into machine code. A compiler translates the entire source code into an executable file before execution, while an interpreter translates and executes line by line. An assembler converts low‑level assembly language into machine code. Edexcel also covers the concept of intermediate code and Just‑in‑Time (JIT) compilation.
用高级语言编写的程序必须翻译成机器码。编译器在执行前将整个源代码翻译成可执行文件，而解释器逐行翻译并执行。汇编器将低级汇编语言转换为机器码。Edexcel 还涵盖中间代码和即时编译（JIT）的概念。
6. Programming Fundamentals | 编程基础
Programs are built from fundamental constructs: sequence, selection, and iteration. Variables store data that can change during execution; constants hold values that do not change. Data types include integer, real (floating‑point), char, string, and Boolean.
程序由基本结构构建：顺序、选择和迭代。变量存储执行期间可更改的数据；常量保存不变的值。数据类型包括整数、实数（浮点）、字符、字符串和布尔型。
Selection is implemented with if, else, and switch (or case) statements that allow a program to take different paths based on conditions. Iteration uses for loops (definite iteration), while loops, and do‑while loops (indefinite iteration). Nested structures combine these constructs to solve complex problems.
选择通过 if、else 和 switch（或 case）语句实现，允许程序根据条件采取不同路径。迭代使用 for 循环（确定次数迭代）、while 循环和 do‑while 循环（不确定次数迭代）。嵌套结构组合这些构造成分以解决复杂问题。
Subroutines (procedures and functions) promote modularity and reuse. Procedures perform actions; functions return a value. Parameters can be passed by value or by reference. Local variables are accessible only inside the subroutine, while global variables are visible throughout the program, though they can cause side effects.
子程序（过程和函数）促进模块化和重用。过程执行操作；函数返回一个值。参数可以按值传递或按引用传递。局部变量只能在子程序内部访问，全局变量在整个程序中可见，但可能导致副作用。
7. Algorithms and Computational Thinking | 算法与计算思维
Computational thinking involves decomposition (breaking a problem into smaller parts), pattern recognition, abstraction (focusing on relevant details), and algorithmic design. These skills form the basis of systematic problem solving in computer science.
计算思维包括分解（将问题分解为更小的部分）、模式识别、抽象（专注于相关细节）和算法设计。这些技能构成了计算机科学中系统化问题解决的基础。
Algorithms can be expressed using pseudocode or flowcharts. Edexcel pseudocode uses a clear, examinable syntax for assignment (←), selection, and loops. Flowcharts use standard symbols: ovals for start/stop, parallelograms for input/output, diamonds for decisions.
算法可以用伪代码或流程图表示。Edexcel 伪代码使用清晰、可考试的语法，如赋值（←）、选择和循环。流程图使用标准符号：椭圆表示开始/停止，平行四边形表示输入/输出，菱形表示判断。
Searching algorithms locate a target in a dataset. Linear search checks each element sequentially, with O(n) time complexity. Binary search works on a sorted list by repeatedly halving the search interval, achieving O(log n). However, it requires the data to be sorted first.
搜索算法在数据集中定位目标。线性搜索按顺序检查每个元素，时间复杂度为 O(n)。二分搜索通过反复将搜索区间减半在已排序列表中工作，达到 O(log n)。但它要求数据事先已排序。
Sorting algorithms arrange data in order. Bubble sort repeatedly swaps adjacent out‑of‑order elements; it is O(n²) in the worst case but can detect an already sorted list early. Merge sort uses a divide‑and‑conquer approach, splitting the list and merging sorted halves, providing O(n log n) performance. Edexcel expects you to trace and compare simple algorithms by efficiency and space usage.
排序算法按顺序排列数据。冒泡排序重复交换相邻的逆序元素；最坏情况为 O(n²)，但能提前检测已排序列表。归并排序使用分治法，拆分列表并合并已排序的子部分，性能为 O(n log n)。Edexcel 期望学生能够追踪并根据效率和空间使用比较简单算法。
8. Data Structures | 数据结构
Data structures organise data so that it can be accessed and modified efficiently. The one‑dimensional array stores a list of elements of the same type, accessed via an index. Two‑dimensional arrays form a grid, used for tables or matrices. Records group related data of different types under one name (e.g. a Student record with name, age, grade).
数据结构组织数据，使数据能够被高效地访问和修改。一维数组存储相同类型元素的列表，通过索引访问。二维数组形成网格，用于表格或矩阵。记录将不同类型相关的数据组合在一个名称下（例如一个包含姓名、年龄、成绩的学生记录）。
A stack is a LIFO (Last In, First Out) structure. Common operations are push (add to top) and pop (remove from top). Stacks are used for function call returns, undo features, and expression evaluation. A queue is FIFO (First In, First Out) with enqueue and dequeue operations. Queues are vital for printer spooling and process scheduling in an OS.
栈是一种 LIFO（后进先出）结构。常见操作有 push（压入栈顶）和 pop（弹出栈顶）。栈用于函数调用返回、撤销功能和表达式求值。队列是 FIFO（先进先出），具有入队和出队操作。队列对于操作系统中的打印假脱机和进程调度至关重要。
9. Ethical, Legal and Environmental Issues | 伦理、法律与环境问题
The widespread use of computers raises ethical concerns around privacy, surveillance, and the digital divide. Personal data must be handled responsibly, and professionals must follow codes of conduct. Automated decision‑making and AI can lead to bias and accountability challenges.
计算机的广泛使用引发了围绕隐私、监视和数字鸿沟的伦理关切。个人数据必须被负责任地处理，专业人员必须遵守行为准则。自动化决策和人工智能可能导致偏见和问责挑战。
Key UK legislation includes the Data Protection Act 2018 (incorporating GDPR), which regulates the collection and processing of personal data. The Computer Misuse Act 1990 makes unauthorised access, modification, or creation of malware illegal. The Copyright, Designs and Patents Act 1988 protects intellectual property, including software and digital content.
英国关键立法包括《2018 年数据保护法》（纳入 GDPR），它规范了个人数据的收集和处理。《1990 年计算机滥用法》将未经授权的访问、修改或创建恶意软件列为非法。《1988 年版权、设计和专利法》保护知识产权，包括软件和数字内容。
Environmental considerations include the energy consumption of data centres, e‑waste from discarded devices, and the use of scarce materials in manufacturing. Strategies such as virtualisation, recycling, and designing for longevity help reduce the carbon footprint of computing.
环境方面的考虑包括数据中心的能源消耗、废弃设备产生的电子垃圾以及制造过程中稀有材料的使用。虚拟化、回收和长寿命设计等策略有助于减少计算的碳足迹。
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