📚 Year 11 WJEC Computer Science: Core Knowledge Review | Year 11 WJEC 计算机：核心知识点梳理
This comprehensive revision guide covers the essential topics for the WJEC GCSE Computer Science Year 11 course. It is designed to help you consolidate your understanding of computer systems, data representation, networks, cybersecurity, algorithms, programming, and the wider ethical issues. Each section presents key concepts in a clear, exam-focused way.
这份综合复习指南涵盖了 WJEC GCSE 计算机科学 Year 11 课程的核心主题。它旨在帮助你巩固对计算机系统、数据表示、网络、网络安全、算法、编程及更广泛的伦理问题的理解。每个部分均以清晰、紧扣考试的方式呈现关键概念。
1. Number Systems: Binary and Hexadecimal | 数字系统：二进制与十六进制
Computers use binary (base‑2) because they rely on two‑state switches. The smallest unit is a bit, and 8 bits form a byte. Denary numbers are converted to binary by successive division by 2, recording remainders.
计算机使用二进制（基数为2），因为它们依赖两态开关。最小的单位是位，8 位构成一个字节。十进制数转换为二进制通过不断除以 2 并记录余数来实现。
Hexadecimal (base‑16) provides a more compact way to represent binary values. One hex digit corresponds to 4 bits (a nibble). It uses digits 0–9 and letters A–F.
十六进制（基数为16）提供了一种更紧凑的表示二进制值的方式。一个十六进制数字对应 4 个比特（半字节）。它使用数字 0–9 和字母 A–F。
use 1101₂ = D, 1010₂ = A. To convert decimal 200 to hex: 200 ÷ 16 = 12 remainder 8 → C8₁₆.
转换示例：1101 1010₂ → DA₁₆，因为 1101₂ = D，1010₂ = A。将十进制 200 转换为十六进制：200 ÷ 16 = 12 余 8 → C8₁₆。
2³ + 2² + 2¹ + 2⁰ = 8+4+2+1, used for binary weighting
2³ + 2² + 2¹ + 2⁰ = 8+4+2+1，用于二进制加权
2. Data Representation: Images, Sound, Compression | 数据表示：图像、声音、压缩
be represented: 1 bit → 2 colours, 8 bits → 256 colours. Resolution is the total number of pixels (width × height); higher resolution gives more detail but larger file size.
位图图像由像素组成。每个像素的颜色存储为二进制数。颜色深度（每像素位数）决定可表示的颜色数量：1 位 → 2 种颜色，8 位 → 256 种颜色。分辨率是总像素数（宽 × 高）；分辨率越高细节越多，但文件也越大。
Sound is sampled at regular intervals. The sample rate (e.g. 44.1 kHz) and bit depth (e.g. 16 bits) affect sound quality and file size. Higher sample rate captures higher frequencies; greater bit depth gives a wider dynamic range.
声音按固定间隔采样。采样率（如 44.1 kHz）和位深度（如 16 位）影响音质和文件大小。更高的采样率能捕获更高频率；更大的位深度提供更宽的动态范围。
Lossless compression (e.g. run-length encoding, Huffman) preserves all original data, while lossy compression (e.g. JPEG, MP3) permanently removes some data to reduce file size, exploiting human perception limits.
无损压缩（如游程编码、哈夫曼编码）保留所有原始数据，而有损压缩（如 JPEG、MP3）永久删除部分数据以减小文件大小，利用人类感知的局限性。
3. Boolean Logic and Logic Gates | 布尔逻辑与逻辑门
Boolean algebra deals with TRUE/FALSE values. Basic operators are AND, OR, NOT. Truth tables list all input combinations and their outputs.
布尔代数处理 TRUE/FALSE 值。基本运算符为 AND、OR、NOT。真值表列出所有输入组合及其输出。
| A | B | A AND B | A OR B |
| 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 |
Logic gates are electronic circuits that perform Boolean operations. The NOT gate inverts its input. An AND gate outputs 1 only when all inputs are 1. An OR gate outputs 1 if at least one input is 1. Combinations of gates can create adders and memory circuits.
逻辑门是执行布尔运算的电子电路。非门将输入取反。与门仅当所有输入为 1 时输出 1。或门只要至少一个输入为 1 就输出 1。门电路组合可构成加法器和存储电路。
XOR gate: Q = A ⊕ B, true when inputs differ
异或门：Q = A ⊕ B，当输入不同时为真
4. CPU Architecture and Fetch-Decode-Execute | CPU 架构与取指-解码-执行
The Central Processing Unit (CPU) is the brain of the computer. It contains the Control Unit (CU) which directs operations, the Arithmetic Logic Unit (ALU) which performs 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) 是计算机的大脑。它包含指挥操作的控制单元 (CU)、执行计算和逻辑的算术逻辑单元 (ALU) 以及寄存器，如程序计数器 (PC)、内存地址寄存器 (MAR)、内存数据寄存器 (MDR) 和累加器 (ACC)。
The fetch-decode-execute cycle repeats endlessly: fetch – next instruction is fetched from memory (PC → MAR, then MDR receives instruction); decode – the CU interprets the instruction; execute – arts carry out the operation. The PC is then incremented to point to the next instruction.
取指-解码-执行周期不断重复：取指——从内存取出下一条指令（PC → MAR，然后 MDR 接收指令）；解码——CU 解释该指令；执行——ALU 或其他部件执行操作。然后 PC 递增，指向下一条指令。
Clock speed (measured in GHz) determines how many cycles per second the CPU can perform. More cores allow multiple instructions to be processed simultaneously.
时钟速度（以 GHz 为单位）决定 CPU 每秒可执行多少周期。更多核心允许同时处理多条指令。
5. Memory and Storage | 内存与存储
RAM (Random Access Memory) is volatile, fast, and used to store the operating system, programs, and data currently in use. ROM (Read Only Memory) is non-volatile and holds the boot instructions (BIOS/UEFI).
RAM（随机存取存储器）是易失性的、快速的，用于存储当前使用的操作系统、程序和数据。ROM（只读存储器）是非易失性的，保存引导指令 (BIOS/UEFI)。
Secondary storage is non-volatile and retains data when power is off. Magnetic storage (HDD) uses spinning platters; optical storage (CD, DVD) uses lasers; solid-state storage (SSD, USB flash) uses flash memory with no moving parts, making it faster and more durable but often more expensive per GB.
辅助存储器是非易失性的，断电后仍可保留数据。磁性存储器 (HDD) 使用旋转盘片；光学存储器 (CD、DVD) 使用激光；固态存储器 (SSD、USB 闪存) 使用无活动部件的闪存，速度更快、更耐用，但每 GB 通常更贵。
Virtual memory uses a portion of the hard drive/SSD as an extension of RAM when RAM is full, but this is slower. Cache memory is very fast, small-capacity memory located close to the CPU to store frequently used instructions.
虚拟内存使用硬盘/SSD 的一部分作为 RAM 的扩展（当 RAM 满时），但这会变慢。高速缓存是位于 CPU 附近、速度极快、容量小的存储器，用于存放频繁使用的指令。
6. Networks: LAN, WAN, Topologies | 网络：局域网、广域网、拓扑结构
A LAN (Local Area Network) covers a small geographical area, often within one building, and usually uses Ethernet or Wi‑Fi. A WAN (Wide Area Network) spans a large area, connecting LANs across cities or countries, often using leased telecommunications lines or the Internet.
LAN（局域网）覆盖小范围地理区域，通常在一栋建筑内，常用以太网或 Wi‑Fi。WAN（广域网）跨越广阔区域，连接城市或国家间的局域网，通常使用租用电信线路或互联网。
Network topologies describe how devices are connected. In a star topology, each device connects to a central switch or hub; failure of one cable only affects that device, but if the central device fails, the whole network goes down. In a mesh topology, every device (or most) connects to several others, providing high redundancy but requiring more cabling.
网络拓扑描述设备的连接方式。在星型拓扑中，每台设备连接到中央交换机或集线器；一条电缆故障仅影响该设备，但中央设备故障则整个网络瘫痪。在网状拓扑中，每台设备（或大多数）相互连接，提供高冗余，但需要更多布线。
Client‑server and peer‑to‑peer are two network models. In client‑server, a central server provides resources to clients; in peer‑to‑peer, all computers can share resources directly without a central server.
客户端-服务器和对等网络是两种网络模型。客户端-服务器中，中央服务器向客户端提供资源；对等网络中，所有计算机可直接共享资源，无需中央服务器。
7. Network Protocols and the TCP/IP Model | 网络协议与 TCP/IP 模型
Protocols are rules that allow devices to communicate. The TCP/IP stack is a four‑layer model: Application, Transport, Internet (Network), and Link (Network Access). Each layer has specific protocols.
协议是允许设备通信的规则。TCP/IP 栈是一个四层模型：应用层、传输层、互联网层（网络层）和链路层（网络接入层）。每层有特定协议。
| Layer | Example Protocols | Purpose |
| Application | HTTP, HTTPS, FTP, SMTP, IMAP | Provides network services to applications |
| Transport | TCP, UDP | End‑to‑end communication, error checking |
| Internet | IP | Addressing and routing packets |
| Link | Ethernet, Wi‑Fi | Transmission of data over physical media |
HTTP/HTTPS are used for web pages; HTTPS adds encryption via SSL/TLS. FTP transfers files. SMTP sends emails, IMAP receives emails. TCP guarantees delivery by establishing a connection and acknowledging packets, while UDP is faster but connectionless.
HTTP/HTTPS 用于网页；HTTPS 通过 SSL/TLS 增加加密。FTP 传输文件。SMTP 发送邮件，IMAP 接收邮件。TCP 通过建立连接和确认数据包保证可靠交付，而 UDP 更快但是无连接的。
8. Cybersecurity: Threats and Defences | 网络安全：威胁与防御
Common threats include malware (viruses, worms, trojans), phishing, denial‑of‑service (DoS) attacks, SQL injection, and social engineering. Malware can steal, encrypt, or delete data. Phishing tricks users into revealing personal information via fake emails or websites.
常见威胁包括恶意软件（病毒、蠕虫、木马）、网络钓鱼、拒绝服务 (DoS) 攻击、SQL 注入和社会工程学。恶意软件可能窃取、加密或删除数据。网络钓鱼通过虚假邮件或网站诱骗用户泄露个人信息。
Defence methods: firewalls filter incoming and outgoing traffic based on rules. Encryption scrambles data so only authorised parties can read it. Penetration testing simulates attacks to identify weaknesses. Strong passwords, two‑factor authentication, and regular software updates are essential.
防御方法：防火墙基于规则过滤进出流量。加密扰乱数据，只有授权方才能读取。渗透测试模拟攻击以发现弱点。强密码、双因素认证和定期软件更新至关重要。
Anti‑malware software detects and removes malicious programs. Access rights limit what users can do. Physical security (locks, CCTV) also protects hardware. Organisations should have an acceptable use policy and train users to recognise threats.
反恶意软件检测并删除恶意程序。访问权限限制用户操作。物理安全（门锁、CCTV）也保护硬件。组织应制定可接受使用政策，并培训用户识别威胁。
9. Ethical, Legal, and Environmental Issues | 伦理、法律与环境问题
Computing raises ethical issues around privacy, surveillance, and the digital divide. Personal data must be collected and processed fairly. The Data Protection Act 2018 (UK GDPR) gives individuals rights over their data, including the right to access and erasure.
计算技术引发了关于隐私、监控和数字鸿沟的伦理问题。个人数据必须公平地收集和处理。《2018 年数据保护法》（英国 GDPR）赋予个人对其数据的权利，包括访问权和删除权。
Copyright law protects software, music, and other digital content. The Computer Misuse Act 1990 makes unauthorised access to computer material illegal, covering hacking and malware distribution.
版权法保护软件、音乐和其他数字内容。《1990 年计算机滥用法》将未经授权访问计算机资料定为非法，涵盖黑客行为和恶意软件传播。
Environmental concerns include the energy consumption of data centres and the disposal of e‑waste. Manufacturers are encouraged to use recyclable materials and to design energy‑efficient devices. The WEEE directive promotes recycling of electronic waste.
环境问题包括数据中心的能源消耗和电子垃圾的处置。鼓励制造商使用可回收材料并设计节能设备。WEEE 指令促进电子废弃物的回收。
10. Algorithms: Searching and Sorting | 算法：搜索与排序
A linear search inspects each element in turn until the target is found or the list ends. It works on unsorted lists but is inefficient for large datasets, with a worst‑case time of O(n).
线性搜索逐个检查元素，直到找到目标或列表结束。它适用于未排序列表，但对于大数据集效率低下，最坏情况时间复杂度为 O(n)。
Binary search requires a sorted list. It repeatedly divides the search interval in half, comparing the middle value to the target. This reduces the number of comparisons dramatically, with a time complexity of O(log n).
二分搜索需要一个已排序的列表。它反复将搜索区间减半，将中间值与目标进行比较。这显著减少了比较次数，时间复杂度为 O(log n)。
Bubble sort repeatedly steps through the list, swapping adjacent items if they are in the wrong order. It is simple but slow, O(n²). Merge sort divides the list into halves, recursively sorts them, then merges the sorted halves. It is more efficient, O(n log n), and is a divide‑and‑conquer algorithm.
冒泡排序反复遍历列表，若相邻元素顺序错误则交换它们。它简单但慢，O(n²)。归并排序将列表分成两半，递归排序，然后合并已排序的两半。它更高效，O(n log n)，是一种分治算法。
11. Programming Fundamentals and Control Structures | 编程基础与控制结构
eger, real (float), string, and Boolean. Assignment uses the = operator (e.g., score = 0).
变量存储程序执行中可变的数据；常量保存固定不变的值。数据类型包括整数、实数（浮点数）、字符串和布尔型。赋值使用 = 运算符（如 score = 0）。
Sequence means instructions are executed in order. Selection (if‑else, switch/case) allows different paths based on a condition. Iteration repeats a block of code: count‑controlled (for) loops run a set number of times; condition‑controlled (while, repeat‑until) loops continue while a condition is true/until it becomes true.
顺序意味着按顺序执行指令。选择（if-else、switch/case）根据条件选择不同分支。迭代重复代码块：计数控制（for）循环运行固定次数；条件控制（while、repeat‑until）循环在条件为真时继续/直到条件为真时停止。
Programs often use arrays (lists) to store multiple values under one name. A 1D array is like a list; a 2D array resembles a table. Basic operators: +, –, *, /, MOD (remainder), DIV (integer division).
程序常使用数组（列表）在一个名称下存储多个值。一维数组类似于列表；二维数组类似于表格。基本运算符：+、–、*、/、MOD（取余）、DIV（整除）。
12. Testing and Software Development Lifecycle | 测试与软件开发生命周期
Testing ensures a program works as intended. Test data should include normal (typical values), boundary (edge of acceptable range), and erroneous (invalid) data. Boundary test values are crucial – for example, if a permit is for ages 13–19, test with 12, 13, 19, 20.
测试确保程序按预期工作。测试数据应包括正常数据（典型值）、边界数据（可接受范围的边缘）和错误数据（无效值）。边界测试值至关重要——例如，若许可年龄为 13–19，则用 12、13、19、20 测试。
Types of testing: iterative testing during development, final/terminal testing when the whole program is complete. A syntax error involves breaking language rules (e.g., missing colon); a logic error produces unexpected results (e.g., using < instead of >).
测试类型：开发过程中的迭代测试，整个程序完成后的最终/终点测试。语法错误涉及违反语言规则（如缺少冒号）；逻辑错误产生意外结果（如误用 < 代替 >）。
The software development lifecycle often follows stages: analysis (gathering requirements), design (creating algorithms, flowcharts, pseudocode), implementation (writing and compiling code), testing (verifying and validation), and evaluation (reflecting on success against criteria). This structured approach helps produce reliable, maintainable software.
软件开发生命周期通常遵循以下阶段：分析（收集需求）、设计（创建算法、流程图、伪代码）、实施（编写和编译代码）、测试（验证与确认）以及评估（对照标准反思成功情况）。这种结构化方法有助于生产可靠、可维护的
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