📚 IGCSE Computer Science: Last-Minute Cram Notes | IGCSE 计算机考前冲刺笔记
These concise, exam-focused revision notes summarise the entire IGCSE Computer Science syllabus. Each section pairs an English explanation with its Chinese equivalent to strengthen bilingual understanding and ensure you can recall key facts, definitions, algorithms and concepts under time pressure. Use these notes for active recall, flashcard creation or as a quick reference before your final papers.
这套精炼的考前冲刺笔记涵盖了IGCSE计算机科学课程的全部核心内容。每一部分都采用英文与中文配对讲解，帮助你在紧张的备考时间内快速回顾关键定义、算法和概念。可用来进行主动回忆、制作闪卡或作为考前最后翻阅的速查手册。
1. Binary & Hexadecimal Fundamentals | 二进制与十六进制基础
All digital computers use the binary number system (base 2) because transistors can represent only two states: off (0) and on (1). A single binary digit is called a bit; eight bits form a byte. Larger groupings include a nibble (4 bits), kilobyte (1024 bytes) and megabyte (1024 KB).
所有数字计算机都使用二进制（基数为2），因为晶体管只有两种状态：关（0）和开（1）。一个二进制位称为比特（bit），八个比特组成一个字节（byte）。更大的单位包括半字节（nibble，4位）、千字节（1024字节）和兆字节（1024 KB）。
To convert from binary to denary (decimal), write down the binary number and assign place values as powers of 2 from right to left: 128, 64, 32, 16, 8, 4, 2, 1. Add the place values wherever a 1 appears. Example: 01001010₂ = 0+64+0+0+8+0+2+0 = 74.
将二进制转换为十进制时，从右向左依次标注2的幂次位权：128, 64, 32, 16, 8, 4, 2, 1。将出现1的位权相加即可。例如：01001010₂ = 0+64+0+0+8+0+2+0 = 74。
To convert denary to binary, repeatedly divide the number by 2 and record the remainders, reading from bottom to top. For 74: 74÷2=37 r0, 37÷2=18 r1, 18÷2=9 r0, 9÷2=4 r1, 4÷2=2 r0, 2÷2=1 r0, 1÷2=0 r1 → 1001010₂ (add leading zero to make 8 bits: 01001010₂).
十进制转二进制采用连续除以2取余数，然后从下往上读取。以74为例：74÷2=37余0, 37÷2=18余1, 18÷2=9余0, 9÷2=4余1, 4÷2=2余0, 2÷2=1余0, 1÷2=0余1 → 1001010₂（补前导零得8位：01001010₂）。
Hexadecimal (base 16) uses digits 0–9 followed by A (10), B (11), C (12), D (13), E (14), F (15). Hex is used as a shorter, more human-readable representation of binary. One hex digit represents exactly four bits (a nibble). For example, 1011 1100₂ can be grouped as 1011 (B) and 1100 (C) → BC₁₆.
十六进制（基数为16）使用0–9和A(10)、B(11)、C(12)、D(13)、E(14)、F(15)。十六进制更短且便于人类阅读，每个十六进制位正好代表四位二进制（一个半字节）。例如，1011 1100₂ 分组为 1011 (B) 和 1100 (C) → BC₁₆。
Conversion between hex and denary: treat each hex digit’s value multiplied by 16ⁿ where n is the position index from the right, starting at 0. Example: 2F₁₆ = (2×16¹) + (15×16⁰) = 32 + 15 = 47.
十六进制与十进制之间的转换：每位十六进制数乘以16ⁿ（n为从右侧开始计数的位置指数，起始为0）。例如：2F₁₆ = (2×16¹) + (15×16⁰) = 32 + 15 = 47。
2. Binary Arithmetic & Logical Shifts | 二进制算术与逻辑移位
Binary addition follows these rules: 0+0=0, 0+1=1, 1+0=1, 1+1=0 carry 1 to the next column. When adding two 8‑bit numbers, if a carry occurs from the most significant bit (MSB), an overflow error occurs because the result cannot fit in 8 bits.
二进制加法规则：0+0=0, 0+1=1, 1+0=1, 1+1=0并向高位进1。当两个8位二进制数相加时，如果最高位（MSB）发生进位，就会产生溢出错误，因为结果无法用8位表示。
Left logical shift moves all bits one place to the left, filling the vacated LSB with 0. Each left shift multiplies the original denary value by 2. For example, shifting 00001101 (13) left once gives 00011010 (26). If bits are shifted out of the 8‑bit register, overflow may occur.
逻辑左移将所有位向左移动一位，空出的最低位（LSB）补0。每左移一位相当于原十进制值乘以2。例如，将00001101 (13) 左移一位得到00011010 (26)。如果有位移出8位寄存器，可能发生溢出。
Right logical shift moves bits one place to the right, filling the vacated MSB with 0. Each right shift divides the denary value by 2, ignoring the remainder (integer division). For example, 00011010 (26) right‑shifted gives 00001101 (13).
逻辑右移将所有位向右移动一位，空出的最高位补0。每右移一位相当于原十进制值除以2并忽略余数（整数除法）。例如，00011010 (26)右移得到00001101 (13)。
Arithmetic shifts preserve the sign bit for signed binary numbers (using two’s complement). In an arithmetic right shift, the MSB (the sign) is replicated to keep the number negative or positive.
算术移位用于带符号的二进制数（补码表示）。算术右移时，最高位（符号位）被复制以保持数的正负属性不变。
3. Representing Data: Text, Sound & Images | 数据表示：文本、声音与图像
Text is represented using character sets such as ASCII and Unicode. Standard ASCII uses 7 bits, giving 128 characters; extended ASCII uses 8 bits (256 characters). Unicode, especially UTF‑8, uses up to 4 bytes per character and can encode virtually all writing systems.
文本使用字符集表示，如ASCII和Unicode。标准ASCII用7位表示128个字符；扩展ASCII用8位表示256个字符。Unicode（尤其是UTF‑8）每个字符最多用4个字节，能编码几乎所有的书写系统。
Sound is analogue in nature and must be converted to digital by sampling. The sound wave is measured at regular intervals (sample rate, in Hz) and each sample is given a digital value (bit depth). Higher sample rate and higher bit depth give better quality but produce larger files.
声音本质上是模拟信号，必须通过采样转换为数字信号。声波按固定时间间隔（采样率，单位Hz）测量，每个采样点赋予一个数字值（采样精度/位深度）。采样率越高、位深度越大，音质越好，但生成的文件也越大。
File size for an uncompressed sound file = sample rate (Hz) × bit depth × duration (s) × number of channels. For a 10‑second stereo recording at 44.1 kHz with 16 bits: 44100 × 16 × 10 × 2 = 14,112,000 bits ≈ 1.68 MB.
未压缩声音文件大小 = 采样率（Hz）× 位深度 × 时长（秒）× 声道数。一段10秒长、44.1 kHz、16位的立体声录音：44100 × 16 × 10 × 2 = 14,112,000 位 ≈ 1.68 MB。
Images are made of pixels. In a bitmap image, each pixel’s colour is stored. Colour depth (bits per pixel) determines how many distinct colours can be represented: 2ⁿ colours, where n is the colour depth. An 8‑bit image has 256 colours; 24‑bit (true colour) has about 16.7 million colours.
图像由像素构成。在位图中，每个像素的颜色被存储。颜色深度（每像素位数）决定了可表示的颜色数量：2ⁿ 种颜色（n为颜色深度）。8位图像有256种颜色；24位（真彩色）约有1670万种颜色。
Image file size (uncompressed) = width (px) × height (px) × colour depth (bits). Resolution is the total number of pixels; higher resolution gives sharper images but larger file size.
未压缩图像文件大小 = 宽度（像素）× 高度（像素）× 颜色深度（位）。分辨率就是总像素数；分辨率越高图像越清晰，但文件也越大。
4. Data Compression & Storage | 数据压缩与存储
Lossless compression reduces file size without losing any data; the original file can be perfectly reconstructed. Examples: run‑length encoding (RLE), Huffman coding, ZIP files. Lossy compression permanently removes some data, exploiting limitations of human perception to make the loss acceptable. Used for JPEG (images), MP3 (audio), MPEG (video).
无损压缩在减小文件大小的同时不丢失任何数据，原始文件可被完整复原。例如：行程长度编码（RLE）、哈夫曼编码、ZIP文件。有损压缩永久性删除部分数据，利用人类感知的局限性使失真能被接受。常用于JPEG（图像）、MP3（音频）、MPEG（视频）。
Run‑length encoding replaces consecutive identical data values with a count and the value. For example, ‘AAAAABBBCC’ becomes 5A3B2C. It works well on simple images with large blocks of the same colour.
行程长度编码将连续重复的数据值替换为一个计数值和该值本身。例如，“AAAAABBBCC” 变成 5A3B2C。该方法非常适用于有大片相同颜色的简单图像。
Storage devices are categorised as primary memory (RAM, ROM) and secondary storage (magnetic, optical, solid state). RAM is volatile, fast, and stores data currently in use. ROM is non‑volatile and holds the firmware / boot instructions. Secondary storage provides non‑volatile, long‑term data retention. Hard disk drives (HDD) are magnetic; SSDs use flash memory and are faster, more durable, but costlier per GB. Optical discs (CD, DVD, Blu‑ray) use lasers.
存储设备分为主存储器（RAM、ROM）和辅助存储器（磁、光、固态）。RAM易失、速度快，存放正在使用的数据；ROM非易失，存放固件/启动指令。辅助存储器提供非易失的长期数据保存。硬盘（HDD）是磁介质；SSD使用闪存，速度更快、更耐用，但每GB成本更高。光盘（CD、DVD、蓝光）利用激光读写。
5. CPU Architecture & the Fetch‑Decode‑Execute Cycle | CPU体系结构与取指译码执行周期
The Central Processing Unit (CPU) consists of the Control Unit (CU), Arithmetic Logic Unit (ALU), registers, and buses. The CU directs operations and sends control signals. The ALU performs arithmetic (+, −, ×, ÷) and logical (AND, OR, NOT) operations.
中央处理器（CPU）由控制单元（CU）、算术逻辑单元（ALU）、寄存器和总线组成。控制单元指挥操作并发出控制信号；算术逻辑单元执行算术（+、−、×、÷）和逻辑（AND、OR、NOT）运算。
Key registers include the Program Counter (PC) — holds the address of the next instruction; Memory Address Register (MAR) — holds the address to be read/written; Memory Data Register (MDR) — holds the data being transferred; Current Instruction Register (CIR) — holds the instruction being decoded; Accumulator (ACC) — stores results of ALU operations.
关键寄存器有：程序计数器（PC）——存放下一条指令的地址；存储器地址寄存器（MAR）——存放要读/写的地址；存储器数据寄存器（MDR）——存放正在传输的数据；当前指令寄存器（CIR）——存放正在译码的指令；累加器（ACC）——存放ALU运算结果。
The fetch‑decode‑execute cycle: Fetch — address from PC is copied to MAR, read signal sent, instruction fetched into MDR then copied to CIR, PC incremented. Decode — CU interprets the opcode and operand. Execute — ALU or other unit performs the instruction, result stored in ACC or memory. Repeat.
取指‑译码‑执行周期：取指——将PC中的地址复制到MAR，发出读信号，指令取入MDR后复制到CIR，PC递增。译码——控制单元解析操作码与操作数。执行——ALU或其他部件执行指令，结果存入ACC或存储器。如此循环。
Factors affecting CPU performance: clock speed (GHz), number of cores, cache size and level. Higher clock speed means more cycles per second; multiple cores allow true parallel processing; larger cache reduces the need to access slower main memory.
影响CPU性能的因素：时钟频率（GHz）、核心数量、高速缓存大小与层级。时钟频率越高，每秒执行周期越多；多核心支持真正的并行处理；更大的缓存可减少访问慢速主存的次数。
6. Input, Output & Storage Devices | 输入、输出及存储设备
Input devices allow data to enter the computer. Common ones: keyboard, mouse, touchscreen, microphone, sensors (temperature, light, pressure, motion), barcode reader, QR scanner, digital camera. When describing a device, identify its purpose, how it works, and its typical use case.
输入设备将数据送入计算机。常见的有：键盘、鼠标、触摸屏、麦克风、传感器（温度、光、压力、运动）、条形码阅读器、QR码扫描器、数码相机。描述设备时需说明用途、工作原理及典型应用场景。
Output devices communicate information to the user. Examples: monitor (LCD/LED), printer (inkjet, laser, 3D), speakers, headphones, actuators (motors, buzzers, lights). An actuator converts electrical signals into physical movement or action.
输出设备向用户传递信息。例如：显示器（LCD/LED）、打印机（喷墨、激光、3D）、音箱、耳机、执行器（电机、蜂鸣器、指示灯）。执行器将电信号转化为物理运动或动作。
Secondary storage selection depends on capacity, speed, portability, durability and cost. For example, a USB flash drive is portable and solid‑state, making it ideal for transferring files; an HDD offers large capacity at low cost for bulk backup; an SSD improves boot times and application loading speed.
辅助存储设备的选择取决于容量、速度、便携性、耐用性和成本。例如，U盘便携且为固态，适合传输文件；HDD以低成本提供大容量，适合批量备份；SSD可加快启动和程序加载速度。
7. Networks, Topologies & Protocols | 网络、拓扑与协议
A network is two or more computers connected to share resources (files, printers, Internet). Types: LAN (small area, e.g., school), WAN (large area, e.g., Internet). Client‑server networks have a central server providing services; peer‑to‑peer networks share resources equally without a central server.
网络是指两台或多台计算机连接以共享资源（文件、打印机、互联网）。类型：LAN（小范围，如校园）、WAN（大范围，如互联网）。客户端‑服务器网络由中央服务器提供服务；对等网络则无中心服务器，各计算机平等共享资源。
Common network topologies: Star — all devices connect to a central switch; if one cable fails, only that device is affected, but the switch is a single point of failure. Bus — all devices share a single backbone cable; cheap but collisions increase with more devices. Ring — data travels in one direction around the ring; a break disables the whole network. Mesh — each device connects to many others; highly redundant and fault‑tolerant but expensive.
常见网络拓扑：星型——所有设备连接到中央交换机；一条线缆故障仅影响该设备，但交换机是单点故障源。总线型——所有设备共享一根主干线缆；成本低但设备增多时冲突增加。环型——数据沿环单向传输；一处断开则全网瘫痪。网状——每台设备与多台相连；冗余度高、容错性好，但成本高昂。
Network hardware: switch (forwards frames based on MAC addresses), router (forwards packets between networks), modem (converts between digital and analogue signals), NIC (Network Interface Card). Wi‑Fi uses radio waves; Ethernet uses twisted‑pair or fibre‑optic cables.
网络硬件：交换机（根据MAC地址转发帧）、路由器（在网络间转发分组）、调制解调器（数字与模拟信号转换）、网卡（NIC）。Wi‑Fi使用无线电波；以太网使用双绞线或光纤。
8. Protocols & the TCP/IP Model | 协议与TCP/IP模型
Protocols are rules governing data transmission. TCP/IP is the foundational suite. The four layers: Application (HTTP, HTTPS, FTP, SMTP), Transport (TCP, UDP), Internet (IP), Link (Ethernet, Wi‑Fi). Each layer adds its own header to the data.
协议是管理数据传输的规则。TCP/IP是基础协议族。四层模型：应用层（HTTP、HTTPS、FTP、SMTP）、传输层（TCP、UDP）、网络层（IP）、链路层（以太网、Wi‑Fi）。每层都会为数据添加自己的头部。
HTTP (Hypertext Transfer Protocol) transfers web pages; HTTPS adds encryption via SSL/TLS. FTP transfers files. SMTP sends emails; POP3/IMAP retrieve emails. TCP provides reliable, ordered delivery with error checking; UDP is faster but connectionless, used for streaming.
HTTP传输网页；HTTPS通过SSL/TLS加密。FTP传输文件。SMTP发送邮件；POP3/IMAP接收邮件。TCP提供可靠、有序、带校验的传输；UDP更快但无连接，用于流媒体。
IP is responsible for logical addressing and routing. Packets contain source and destination IP addresses. Routers examine the destination IP and forward the packet along the best path.
IP负责逻辑寻址与路由。数据包包含源和目标IP地址。路由器检查目标IP，沿最佳路径转发数据包。
9. Cybersecurity Threats & Prevention | 网络安全威胁与防护
Malware includes viruses (attach to files, need human interaction), worms (self‑replicate across networks), Trojan horses (disguised as legitimate software), spyware (monitors user activity), ransomware (encrypts files and demands payment). Phishing uses fake emails/websites to steal credentials; social engineering manipulates users into revealing information.
恶意软件包括：病毒（附着于文件，需人为触发）、蠕虫（跨网络自我复制）、特洛伊木马（伪装成合法软件）、间谍软件（监视用户活动）、勒索软件（加密文件并索要赎金）。网络钓鱼利用虚假邮件/网站窃取凭证；社会工程学通过操控用户泄露信息。
Denial‑of‑Service (DoS) attacks flood a server with traffic to make it unavailable. SQL injection inserts malicious SQL code into input fields to manipulate a database. Brute‑force attacks try many passwords systematically.
拒绝服务攻击（DoS）用大量流量瘫痪服务器。SQL注入通过输入字段插入恶意SQL代码，操控数据库。暴力攻击系统地尝试众多密码。
Prevention measures: firewalls (filter incoming/outgoing traffic), encryption (scramble data), two‑factor authentication, automatic updates, user access levels, anti‑malware software, backup and recovery plans, staff training to recognise phishing.
防护措施：防火墙（过滤进出流量）、加密（打乱数据）、双因素认证、自动更新、用户访问级别、反恶意软件、备份与恢复计划、员工识别钓鱼培训。
10. Logic Gates & Boolean Algebra | 逻辑门与布尔代数
Logic gates are the building blocks of digital circuits. The six basic gates: NOT, AND, OR, NAND, NOR, XOR. Their behaviours are defined by truth tables. Each gate can be represented by a Boolean expression.
逻辑门是数字电路的构建基石。六种基本门：非门（NOT）、与门（AND）、或门（OR）、与非门（NAND）、或非门（NOR）、异或门（XOR）。其行为由真值表定义，每种门可用布尔表达式表示。
Truth tables summary:
| A | B | AND | OR | NAND | NOR | XOR |
|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 |
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