📚 GCSE Computer Science: Last-Minute Revision Notes | GCSE 计算机：考前冲刺笔记
These last-minute revision notes cover the essential topics for GCSE Computer Science. Each key point is presented in English followed immediately by its Chinese translation to help you review efficiently and strengthen your understanding before the exam. Focus on the core concepts, common pitfalls, and exam technique.
这些考前冲刺笔记涵盖了 GCSE 计算机科学的核心主题。每个关键点先用英文呈现，紧接着提供中文翻译，帮助你高效复习并在考前加深理解。重点关注核心概念、常见易错点和考试技巧。
1. Data Representation | 数据表示
Computers use binary (base-2) because their circuits have two states: on (1) and off (0). All data, including text, images and sound, is stored as sequences of bits.
计算机使用二进制（基数为2），因为电路有两种状态：开（1）和关（0）。所有数据，包括文本、图像和声音，都以比特序列的形式存储。
The smallest unit is a bit. A group of 4 bits is a nibble, 8 bits form a byte. Units increase by powers of 2: 1 kilobyte (KB) = 1024 bytes, 1 megabyte (MB) = 1024 KB, 1 gigabyte (GB) = 1024 MB, 1 terabyte (TB) = 1024 GB.
最小单位是位（bit）。4 位组成半字节（nibble），8 位构成一个字节（byte）。单位以 2 的幂次递增：1 千字节（KB）= 1024 字节，1 兆字节（MB）= 1024 KB，1 吉字节（GB）= 1024 MB，1 太字节（TB）= 1024 GB。
Place values in binary are powers of 2. For example, in an 8-bit number the columns are 128, 64, 32, 16, 8, 4, 2, 1. To convert binary to denary, multiply each bit by its place value and sum the results.
1011₂ = 1×2³ + 0×2² + 1×2¹ + 1×2⁰ = 8 + 0 + 2 + 1 = 11₁₀
二进制的位权是 2 的幂。例如，在 8 位数中，位权从右到左依次为 128、64、32、16、8、4、2、1。将二进制转为十进制时，将每位乘以位权并求和。
1011₂ = 1×2³ + 0×2² + 1×2¹ + 1×2⁰ = 8 + 0 + 2 + 1 = 11₁₀
Hexadecimal (base-16) uses digits 0–9 and letters A–F. It is a compact way to represent binary, as one hex digit stands for four bits. Two hex digits represent a byte.
十六进制（基数为16）使用数字 0–9 和字母 A–F。它是二进制的简洁表示法，因为一位十六进制对应四位二进制。两个十六进制位正好是一个字节。
Overflow occurs when the result of a binary addition exceeds the available number of bits, e.g. adding two 8-bit numbers and getting a 9-bit result. Processors usually have an overflow flag to detect this.
当二进制加法的结果超过可用位数时，会发生溢出，例如两个 8 位数相加得到 9 位结果。处理器通常有溢出标志来检测这种情况。
2. Logic Gates & Boolean Algebra | 逻辑门与布尔代数
The basic logic gates are AND, OR and NOT. The AND gate gives output 1 only when all inputs are 1. The OR gate gives output 1 when at least one input is 1. NOT inverts a single input.
基本逻辑门包括与门（AND）、或门（OR）和非门（NOT）。与门仅在所有输入均为 1 时输出 1。或门在至少一个输入为 1 时输出 1。非门将单个输入取反。
Truth tables show all possible input combinations and the resulting output. For a two-input AND gate: 0 0 → 0, 0 1 → 0, 1 0 → 0, 1 1 → 1. For OR: 0 0 → 0, 0 1 → 1, 1 0 → 1, 1 1 → 1.
真值表列出所有可能的输入组合及对应输出。双输入与门：0 0 → 0，0 1 → 0，1 0 → 0，1 1 → 1。或门：0 0 → 0，0 1 → 1，1 0 → 1，1 1 → 1。
Boolean expressions can be written in algebraic form: A AND B is written as A.B or A∧B; A OR B as A+B or A∨B; NOT A as Ā or ¬A. When simplifying circuits, use identities like A+0=A, A.1=A, A+A=A, A.A=A.
布尔表达式可以用代数形式书写：A 与 B 写为 A·B 或 A∧B；A 或 B 写为 A+B 或 A∨B；非 A 写为 Ā 或 ¬A。简化电路时，可使用恒等式，如 A+0=A、A·1=A、A+A=A、A·A=A。
3. Computer Architecture | 计算机系统结构
The von Neumann architecture consists of a central processing unit (CPU), memory unit, input/output devices and a system bus. Both program instructions and data are stored in the same memory.
冯·诺依曼体系结构包括中央处理器（CPU）、存储单元、输入/输出设备和系统总线。程序指令和数据都存储在同一内存中。
Inside the CPU, the control unit (CU) decodes instructions and directs operations. The arithmetic logic unit (ALU) performs calculations and logical comparisons. Registers are small, fast storage locations: program counter (PC), memory address register (MAR), memory data register (MDR), accumulator (ACC).
CPU 内部，控制单元（CU）负责译码并指挥操作。算术逻辑单元（ALU）执行计算和逻辑比较。寄存器是小型高速存储位置：程序计数器（PC）、存储地址寄存器（MAR）、存储数据寄存器（MDR）、累加器（ACC）。
The fetch-decode-execute cycle repeats endlessly. Fetch: address from PC → MAR, instruction → MDR, PC incremented. Decode: CU interprets the instruction. Execute: the ALU, registers or memory carry out the operation.
取指-译码-执行循环不断重复。取指：将 PC 中的地址送入 MAR，指令送入 MDR，PC 递增。译码：CU 解析指令。执行：由 ALU、寄存器或存储器完成操作。
4. Memory & Storage | 内存与存储
Primary memory includes RAM and ROM. RAM is volatile (loses content when power off), used to hold the operating system, applications and data currently in use. ROM is non-volatile, contains the boot program (BIOS) and cannot be altered easily.
主存包含 RAM 和 ROM。RAM 是易失性的（断电后内容丢失），用于存储正在运行的操作系统、应用程序和数据。ROM 是非易失性的，存放引导程序（BIOS），不易被修改。
Virtual memory uses a portion of the hard disk/SSD as if it were RAM, allowing the computer to run larger programs. When RAM is full, data is swapped to disk, which slows performance (disk thrashing).
虚拟内存将硬盘或固态硬盘的一部分当作 RAM 使用，允许计算机运行更大的程序。当 RAM 满时，数据被交换到磁盘，这会降低性能（磁盘颠簸）。
Secondary storage is non-volatile, used for long-term saving. Magnetic storage (HDD) uses spinning platters; optical storage (CD, DVD) uses lasers; solid state (SSD, USB flash) uses NAND flash memory. SSDs are faster, more durable and use less power than HDDs, but cost more per gigabyte.
辅助存储是非易失性的，用于长期保存数据。磁存储（HDD）使用旋转盘片；光存储（CD、DVD）使用激光；固态存储（SSD、USB 闪存）使用 NAND 闪存。SSD 比 HDD 更快、更耐用且功耗更低，但每 GB 成本更高。
5. Networking Basics | 网络基础
A network connects two or more devices to share resources. LANs (Local Area Networks) cover a small geographical area, usually within one building. WANs (Wide Area Networks) connect LANs over a large area, e.g. the internet.
网络连接两台或多台设备以共享资源。局域网（LAN）覆盖小地理范围，通常在一栋建筑内。广域网（WAN）在广阔区域内连接多个局域网，例如互联网。
Common topologies: star (all devices connect to a central switch; if one cable fails, the rest remain connected), mesh (each device connects to many others, offering high redundancy), bus (all share one backbone; cheap but prone to collisions).
常见拓扑结构：星形（所有设备连接到中心交换机；一条电缆故障不影响其他连接）、网状（每台设备与多台相连，冗余度高）、总线型（所有设备共享一条主干；成本低但易发生冲突）。
Protocols are rules governing data transmission. TCP/IP is the fundamental suite: TCP ensures reliable delivery, IP handles addressing and routing. HTTP/HTTPS transfers web pages, FTP transfers files, SMTP sends email, POP3/IMAP retrieve email.
协议是管理数据传输的规则。TCP/IP 是基础协议套件：TCP 确保可靠传输，IP 处理寻址和路由。HTTP/HTTPS 传输网页，FTP 传输文件，SMTP 发送邮件，POP3/IMAP 接收邮件。
6. Cybersecurity | 网络安全
Malware includes viruses (attach to files, spread when run), worms (self-replicate across networks), trojans (disguised as legitimate software), ransomware (encrypts files and demands payment). Spyware secretly monitors user activity.
恶意软件包括病毒（附在文件上，运行时传播）、蠕虫（在网络上自我复制）、特洛伊木马（伪装成合法软件）、勒索软件（加密文件并要求付款）。间谍软件秘密监视用户活动。
Social engineering tricks people into revealing confidential information, e.g. phishing emails that appear to be from a bank. SQL injection exploits poorly secured databases by inserting malicious SQL code. Denial-of-Service (DoS) attacks flood a server with traffic to make it unavailable.
社会工程学诱骗人们泄露机密信息，例如看似来自银行的钓鱼邮件。SQL 注入通过插入恶意 SQL 代码攻击安全防护薄弱的数据库。拒绝服务（DoS）攻击通过大量流量淹没服务器使其不可用。
Defences include firewalls (monitor incoming/outgoing traffic), encryption (scrambles data so only authorised parties can read it), two-factor authentication (2FA) using password + code, regular software updates, and strong password policies.
防御措施包括防火墙（监控进出流量）、加密（扰乱数据，仅授权方可读取）、双因素认证（2FA，密码+验证码）、定期软件更新以及强密码策略。
7. Algorithmic Thinking | 算法思维
An algorithm is a step-by-step procedure to solve a problem. Key concepts: sequence, selection (if-else), iteration (loops). Pseudocode uses structured English to describe algorithms; flowcharts use shapes (oval for start/end, rectangle for process, diamond for decision).
算法是解决问题的分步过程。关键概念：顺序、选择（if-else）、迭代（循环）。伪代码使用结构化英文描述算法；流程图使用各种形状（椭圆表示开始/结束，矩形表示处理，菱形表示判断）。
Searching algorithms: linear search checks each element sequentially, works on unsorted data, time complexity O(n). Binary search requires a sorted list; repeatedly divides the search interval in half, O(log n), much faster for large datasets.
搜索算法：线性搜索按顺序检查每个元素，适用于未排序数据，时间复杂度 O(n)。二分搜索需要已排序列表；每次将搜索区间减半，O(log n)，对于大型数据集快得多。
Sorting algorithms: bubble sort compares adjacent items and swaps if out of order; passes repeat until no swaps. Insertion sort builds a sorted list one item at a time by inserting each into its correct position. Bubble sort is simple but inefficient (O(n²)), insertion sort is also O(n²) but often faster in practice.
排序算法：冒泡排序比较相邻项并在顺序错误时交换；重复多趟直到无交换发生。插入排序通过将每个元素插入正确位置来逐步构建有序列表。冒泡排序简单但效率低（O(n²)），插入排序也是 O(n²)，但实践中通常更快。
8. Programming Concepts | 编程概念
Variables are named memory locations that store values which can change. Constants are fixed values. Data types include integer, real/float, Boolean (true/false), character and string.
变量是命名的内存区域，存储的值可以改变。常量是固定值。数据类型包括整型、实型/浮点型、布尔型（真/假）、字符型和字符串型。
Selection uses if, else if and else to control program flow. Example: if score >= 90 then grade = ‘A*’. Iteration: for loops (count-controlled, known number of repetitions), while loops (condition-controlled, repeats while condition is true), do-until loops (exits when condition becomes true).
选择结构使用 if、else if 和 else 控制程序流程。例如：if score >= 90 then grade = ‘A*’。迭代：for 循环（计数控制，重复次数已知）、while 循环（条件控制，当条件为真时重复）、do-until 循环（直到条件成立时退出）。
Subprograms break code into reusable blocks: functions return a value, procedures do not. Parameters pass data into subprograms. Arrays store multiple items of the same data type under one name, accessed via index (starting from 0).
子程序将代码分解为可复用的模块：函数返回值，过程不返回值。参数向子程序传递数据。数组使用一个名称存储多个相同数据类型的元素，通过索引访问（通常从0开始）。
9. Legal & Ethical Issues | 法律与伦理
The Data Protection Act governs how personal data is collected, used and stored. Principles: data must be used fairly and lawfully, kept for no longer than necessary, accurate, and held securely. Individuals have the right to access their data.
数据保护法规定了个人数据的收集、使用和存储方式。原则包括：数据必须公平合法地使用，保存时间不得超过必要，保持准确，并安全保存。个人有权访问自己的数据。
The Computer Misuse Act criminalises unauthorised access to computer material, unauthorised access with intent to commit further offences, and unauthorised modification of data (e.g. spreading viruses).
计算机滥用法将未经授权访问计算机材料、意图实施进一步犯罪的未经授权访问以及未经授权修改数据（如传播病毒）定为刑事犯罪。
Copyright and intellectual property laws protect software, music, images and text. Unauthorised copying, sharing or using cracked software (piracy) is illegal. Open source software allows users to view, modify and share the source code under specific licences.
版权和知识产权法保护软件、音乐、图像和文本。未经授权的复制、分享或使用破解软件（盗版）是违法的。开源软件允许用户根据特定许可证查看、修改和分享源代码。
Ethical concerns include the digital divide (inequality in access to technology), AI bias, facial recognition privacy, and environmental impacts of e-waste. Responsible computing involves considering the wider consequences of technology.
伦理问题包括数字鸿沟（技术获取的不平等）、人工智能偏见、人脸识别隐私以及电子废物对环境的影响。负责任的计算机实践需要考量技术带来的更广泛后果。
10. Exam Tips | 考试技巧
Read the question carefully, underlining command words such as “describe”, “explain”, “compare”. “Describe” asks for what happens, “explain” needs reasons or cause-and-effect, “compare” requires similarities and differences.
仔细读题，将指令词如“描述”、“解释”、“比较”划出来。“描述”要求说明发生了什么，“解释”需要给出原因或因果关系，“比较”要求指出相似点和不同点。
In programming questions, show tracing steps if asked. Write the value of variables at each stage. For algorithm questions, present a clear flowchart or pseudocode with correct indentation.
在编程题中，如需跟踪变量请展示步骤。写出各阶段变量的值。对于算法题，给出清晰的流程图或缩进正确的伪代码。
Manage your time: allocate roughly 1 minute per mark. Don’t spend too long on a difficult multiple-choice question; mark it and return later. Leave 5–10 minutes to review your answers, especially the spelling of technical terms.
管理好时间：大致按每分分配 1 分钟。不要在难题上花太多时间；先标记，稍后再做。留出 5-10 分钟检查答案，尤其注意技术术语的拼写。
Show all working for binary/hex conversions and storage calculations. Even if the final answer is wrong, you can earn method marks. Use the correct units and clearly label your answer.
在二进制/十六进制转换和存储计算题中，要展示所有步骤。即使最终答案错误，仍可获得步骤分。使用正确的单位并清晰标注答案。
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