📚 GCSE CIE Computer Science: Last-Minute Revision Notes | GCSE CIE 计算机科学：考前冲刺笔记
This set of revision notes covers the key concepts for the CIE IGCSE Computer Science examination. It is designed for last-minute revision, summarising essential facts in a clear, bilingual format.
本套复习笔记涵盖了 CIE IGCSE 计算机科学考试的核心概念。专为考前冲刺设计，以清晰的双语格式总结必备知识。
1. Data Representation | 数据表示
Computers use binary (base-2) with digits 0 and 1. A group of 8 bits forms a byte. Hexadecimal (base-16) uses 0–9 and A–F, where each hex digit represents 4 bits. To convert binary to hex, group bits in fours from the right.
计算机使用二进制（基 2），数字为 0 和 1。8 位一组构成一个字节。十六进制（基 16）使用 0–9 和 A–F，每个十六进制数字代表 4 位。二进制转十六进制时，从右向左每 4 位一组。
Overflow occurs when the result of a calculation exceeds the number of bits available for storage. For example, adding two 8-bit numbers may produce a 9-bit result, causing an overflow error.
当计算结果超出可用的存储位数时，会发生溢出。例如，两个 8 位二进制数相加可能产生 9 位结果，导致溢出错误。
Characters are represented using ASCII (7-bit, 128 characters) or Unicode (up to 32-bit, covering many languages and symbols). Unicode is more widely used today.
字符通过 ASCII（7 位，128 个字符）或 Unicode（最多 32 位，涵盖多种语言和符号）来表示。如今 Unicode 使用更为广泛。
Images are stored as bitmaps using pixels. Resolution is the number of pixels (e.g. 1920×1080). Colour depth is bits per pixel; a 24-bit colour depth allows 2²⁴ ≈ 16.7 million colours. Higher resolution and colour depth increase file size.
图像以位图形式用像素存储。分辨率是像素数量（如 1920×1080）。颜色深度是每像素的位数；24 位颜色深度支持 2²⁴ ≈ 1670 万种颜色。分辨率和颜色深度越高，文件大小越大。
Sound is digitised through sampling: sampling rate (Hz) and bit depth affect quality and file size. A typical CD-quality sample uses 44.1 kHz and 16 bits per sample. File size = sampling rate × bit depth × duration × channels.
声音通过采样数字化：采样率（赫兹）和位深度影响质量与文件大小。典型 CD 音质使用 44.1 kHz 和每样本 16 位。文件大小 = 采样率 × 位深度 × 时长 × 声道数。
2. Logic Gates & Circuits | 逻辑门与电路
Logic gates process binary inputs. The fundamental gates are NOT, AND, OR. Derived gates include NAND (NOT AND) and NOR (NOT OR). Truth tables define output for every input combination.
逻辑门处理二进制输入。基本门包括非门、与门、或门。派生门有与非门（非与）和或非门（非或）。真值表定义了每种输入组合下的输出。
| A | B | A AND B | A OR B | A NAND B | A NOR B |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 | 1 | 0 |
| 1 | 1 | 1 | 1 | 0 | 0 |
The NOT gate simply inverts the input: output = ¬A. Logic expressions can be written as Boolean equations, e.g. Q = (A AND B) OR C. Logic circuits can be created to solve real-world problems, such as a car warning light system.
非门仅将输入取反：输出 = ¬A。逻辑表达式可以用布尔方程表示，例如 Q = (A AND B) OR C。可以用逻辑电路解决现实问题，比如汽车警示灯系统。
3. Computer Architecture | 计算机体系结构
The Von Neumann architecture stores both program instructions and data in the same main memory. Key components: Control Unit (CU), Arithmetic Logic Unit (ALU), Registers, and System Bus.
冯·诺依曼体系结构将程序指令和数据存储在同一主存储器中。关键组件：控制单元（CU）、算术逻辑单元（ALU）、寄存器和系统总线。
Registers include 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 just read or to be written; Current Instruction Register (CIR) – holds the instruction being executed; Accumulator (ACC) – temporarily stores results from the ALU.
寄存器包括：程序计数器（PC）——存放下一条指令的地址；内存地址寄存器（MAR）——存放要读/写的地址；内存数据寄存器（MDR）——存放刚读出或将要写入的数据；当前指令寄存器（CIR）——存放正在执行的指令；累加器（ACC）——临时存放 ALU 的运算结果。
The fetch-execute cycle: Fetch (MAR ← PC, MDR ← memory, CIR ← MDR, PC incremented) → Decode (CU interprets) → Execute (ALU performs operation, result may go to ACC or memory). The cycle repeats billions of times per second.
取指-执行周期：取指（MAR ← PC，MDR ← 内存，CIR ← MDR，PC 递增）→ 译码（CU 解释）→ 执行（ALU 执行操作，结果可能送往 ACC 或内存）。该周期每秒重复数十亿次。
Factors affecting CPU performance: clock speed (GHz), number of cores, and cache size. Higher clock speed means more FDE cycles per second; multiple cores allow parallel processing; larger cache reduces need to access slower main memory.
影响 CPU 性能的因素：时钟速度（GHz）、核心数量和缓存大小。时钟速度越快，每秒 FDE 周期越多；多核允许并行处理；更大的缓存减少访问较慢主存的需求。
4. Memory & Storage | 存储器
Primary memory: RAM (Random Access Memory) is volatile, stores data/instructions currently in use. ROM (Read Only Memory) is non-volatile, typically holds firmware/BIOS.
主存储器：RAM（随机存取存储器）是易失性的，存储当前使用的数据/指令。ROM（只读存储器）是非易失性的，通常存放固件/BIOS。
Virtual memory uses part of secondary storage (e.g. HDD/SSD) as an extension of RAM when RAM is full. It allows larger programs to run but is slower than RAM; excessive paging causes thrashing, degrading performance.
虚拟内存将部分辅助存储器（如 HDD/SSD）用作 RAM 的扩展，当 RAM 满时使用。这允许运行更大的程序，但速度比 RAM 慢；过度页面交换会导致系统抖动，降低性能。
Secondary storage types: Magnetic (HDD) – large capacity, mechanical, slower; Solid State (SSD) – fast, durable, no moving parts, more expensive per GB; Optical (CD/DVD/Blu-ray) – portable but small capacity. Cloud storage provides remote access via the internet.
辅助存储类型：磁性（HDD）——容量大、机械式、较慢；固态（SSD）——快速、耐用、无移动部件、每 GB 更贵；光学（CD/DVD/蓝光）——便携但容量小。云存储通过互联网提供远程访问。
5. Input & Output Devices | 输入输出设备
Input devices: keyboard, mouse, microphone, touchscreen (resistive and capacitive), 2D scanner (uses CCD sensor to capture image), 3D scanner (uses lasers to capture shape), barcode reader, QR code scanner, sensors (temperature, pressure, light, motion).
输入设备：键盘、鼠标、麦克风、触摸屏（电阻式与电容式）、2D 扫描仪（使用 CCD 传感器捕获图像）、3D 扫描仪（使用激光捕获形状）、条码阅读器、二维码扫描器、传感器（温度、压力、光线、运动）。
Output devices: monitor, printer (inkjet, laser, 3D), speakers, actuators (motors, buzzers, lights). Actuators convert digital signals into physical movement or action.
输出设备：显示器、打印机（喷墨、激光、3D 打印）、扬声器、致动器（电机、蜂鸣器、灯光）。致动器将数字信号转换为物理动作或运动。
Capacitive touchscreens detect the electrical properties of a finger, allowing multi-touch; resistive touchscreens use pressure, work with any stylus but do not support multi-touch. Both are output as well as input devices.
电容式触摸屏检测手指的电特性，支持多点触控；电阻式触摸屏使用压力，可用任何触笔但不支持多点触控。两者兼具输入与输出功能。
6. System Software & Operating System | 系统软件与操作系统
Software is divided into application software (word processor, spreadsheet, game) and system software (operating system, utility programs, language translators).
软件分为应用软件（文字处理、电子表格、游戏）和系统软件（操作系统、实用程序、语言翻译程序）。
Operating system functions: providing a user interface (GUI, CLI), memory management, multitasking, managing peripherals (drivers), handling interrupts, providing security (user accounts, access rights), and file management.
操作系统功能：提供用户界面（GUI、CLI）、内存管理、多任务处理、管理外设（驱动程序）、处理中断、提供安全（用户账户、访问权限）以及文件管理。
Interrupts are signals from hardware or software that cause the CPU to suspend the current task, run an interrupt service routine (ISR), and then resume. This enables real-time responses.
中断是来自硬件或软件的信号，使 CPU 暂停当前任务，运行中断服务程序（ISR），然后恢复。这实现了实时响应。
Language translators: Assembler converts assembly code into machine code; Compiler translates entire high-level source code into machine code before execution; Interpreter translates and executes line by line, slower but useful for debugging.
语言翻译程序：汇编器将汇编代码转换为机器码；编译器在执行前将整个高级源代码翻译为机器码；解释器逐行翻译并执行，较慢但有助于调试。
7. Networks & The Internet | 网络与互联网
LAN (Local Area Network) spans a small geographic area, often using Ethernet/Wi-Fi. WAN (Wide Area Network) spans large areas, often using leased lines or VPNs. The Internet is a global WAN.
LAN（局域网）覆盖小范围地理区域，常使用以太网/Wi-Fi。WAN（广域网）覆盖大范围，常使用租用专线或 VPN。互联网是一个全球性的 WAN。
Client-server model: a server provides resources/services (files, web pages) and clients request them. Peer-to-peer (P2P): each device acts as both client and server, sharing resources directly.
客户-服务器模型：服务器提供资源/服务（文件、网页），客户请求这些服务。对等（P2P）模型：每个设备同时充当客户和服务器，直接共享资源。
Network hardware: NIC (Network Interface Card), switch (connects devices in a LAN, forwards frames based on MAC addresses), router (connects networks, forwards packets using IP addresses), wireless access point (WAP), and transmission media (UTP cable, fibre optic).
网络硬件：网卡（NIC）、交换机（在 LAN 中连接设备，基于 MAC 地址转发帧）、路由器（连接网络，使用 IP 地址转发数据包）、无线接入点（WAP）以及传输介质（UTP 线缆，光纤）。
Protocols are rules for communication: HTTP/HTTPS (web), FTP (file transfer), SMTP (send email), POP3/IMAP (receive email). TCP/IP is a layered protocol suite: Application, Transport (TCP, UDP), Internet (IP), Network Access (Ethernet, Wi-Fi).
协议是通信规则：HTTP/HTTPS（网页）、FTP（文件传输）、SMTP（发送邮件）、POP3/IMAP（接收邮件）。TCP/IP 是分层协议集：应用层、传输层（TCP、UDP）、网际层（IP）、网络接入层（以太网、Wi-Fi）。
IP addresses (IPv4 32-bit, e.g. 192.168.1.1) identify devices on a network; IPv6 uses 128-bit for more addresses. MAC addresses are 48-bit unique identifiers burned into NICs.
IP 地址（IPv4 32 位，如 192.168.1.1）标识网络上的设备；IPv6 使用 128 位提供更多地址。MAC 地址是 48 位唯一标识，烧录在网卡中。
8. Cybersecurity Threats & Prevention | 网络安全威胁与防护
Malware: virus (attaches to files, spreads when executed), worm (self-replicates across networks), trojan (disguised as legitimate software), ransomware (encrypts files and demands payment), spyware (monitors user activity).
恶意软件：病毒（附着于文件，执行时传播）、蠕虫（在网络中自我复制）、木马（伪装为合法软件）、勒索软件（加密文件索要赎金）、间谍软件（监控用户活动）。
Social engineering: phishing (fraudulent emails/websites to steal credentials), pharming (redirecting a website’s traffic to a fake site). Brute-force attacks try all possible passwords; SQL injection inserts malicious SQL code into input fields to manipulate databases. DDoS attacks flood a server with traffic to make it unavailable.Published by TutorHao | GCSE Computer Science Revision Series | aleveler.com
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