📚 Core Knowledge Review for Year 12 Cambridge Computer Science | Year 12 剑桥计算机核心知识点梳理
This revision guide covers the essential topics from the Cambridge AS Level Computer Science syllabus (9618), designed for Year 12 students. It presents key concepts in both English and Chinese to aid understanding and retention. Use it to consolidate your knowledge before exams or as a quick reference.
本复习指南涵盖了剑桥 AS 计算机科学课程 (9618) 的核心内容，专为 Year 12 学生设计。每个关键概念均以中英文对照呈现，有助于理解和记忆。可用于考前巩固或快速查阅。
1. Data Representation | 数据表示
All data inside a computer is stored and processed in binary, using only the two digits 0 and 1. Each binary digit is called a bit, and a group of 8 bits forms a byte.
计算机内部的所有数据都以二进制形式存储和处理，仅使用 0 和 1 两个数字。一个二进制位称为 1 bit，8 个 bit 构成 1 byte。
Denary (base‑10) numbers can be converted to binary through repeated division by 2, recording the remainders in reverse order. To convert between binary and hexadecimal, each group of four bits maps directly to a single hex digit (0‑9, A‑F).
十进制数可以通过反复除以 2 并记录余数再逆序排列转换为二进制。二进制与十六进制之间转换时，每 4 个二进制位对应一个十六进制数字 (0‑9, A‑F)。
Negative integers are often represented using two’s complement. In an 8‑bit system, the most significant bit (MSB) acts as a sign bit, and the range is from –128 to +127. To find the two’s complement of a binary number, invert all bits and add 1.
负整数常采用二进制补码表示。在 8 位系统中，最高位为符号位，表示范围是 –128 到 +127。求一个二进制数的补码时，需将所有位取反后加 1。
Characters are encoded using standardised mappings such as ASCII (7 or 8 bits per character) and Unicode (variable width, supporting international scripts). Unicode enables consistent representation across all languages and platforms.
字符通过标准化方案（如 ASCII，每字符 7 或 8 位）和 Unicode（变长编码，支持全球文字）进行编码。Unicode 确保了所有语言和平台上的统一表示。
Images are made of pixels, each assigned a binary colour value. The colour depth (bits per pixel) and resolution (pixel dimensions) determine image quality and file size. A higher colour depth gives more colours but increases storage requirements.
图像由像素组成，每个像素分配一个二进制颜色值。颜色深度（每像素位数）和分辨率（像素尺寸）决定了图像质量和文件大小。颜色深度越高，颜色越丰富，但存储空间需求更大。
(e.g., 16 bits per sample) influence sound fidelity. The bit rate = sampling rate × sampling resolution × number of channels.
声音通过对模拟信号的幅度进行等间隔采样而数字化。采样率（如 44.1 kHz）和采样分辨率（如每样本 16 位）影响声音保真度。比特率 = 采样率 × 采样分辨率 × 声道数。
Data compression reduces file size for storage or transmission. Lossy compression (e.g., JPEG, MP3) discards some detail permanently, while lossless compression (e.g., ZIP, PNG) allows exact original reconstruction.
数据压缩通过减少文件大小以利于存储或传输。有损压缩（如 JPEG、MP3）永久丢弃部分细节，而无损压缩（如 ZIP、PNG）可以完全还原原始数据。
2. Communication and Internet Technologies | 通信与互联网技术
A Local Area Network (LAN) connects computers over a small geographical area using wired or wireless links, whereas a Wide Area Network (WAN) spans large distances and often uses leased telecommunications lines.
局域网 (LAN) 通过有线或无线连接覆盖小范围区域，而广域网 (WAN) 跨越远距离，常租用电信线路。
Networks use topologies such as star, mesh, and bus. In a star topology, all nodes connect to a central switch; failure of one cable does not affect the rest. A mesh topology provides multiple redundant paths, increasing reliability but cost.
网络采用星型、网状、总线等拓扑结构。星型拓扑中所有节点连接到中央交换机，单条线路故障不会影响全网。网状拓扑提供多条冗余路径，可靠性高但成本大。
Key networking hardware includes routers (forward packets between networks using IP addresses), switches (connect devices within a LAN and use MAC addresses to forward frames), and wireless access points (WAPs).
关键网络硬件包括路由器（依据 IP 地址在网络间转发数据包）、交换机（在局域网内基于 MAC 地址转发帧）和无线接入点 (WAP)。
Every device on a network has a Media Access Control (MAC) address, a unique identifier burned into its network interface card. An IP address is a logical address assigned dynamically or statically, enabling routing across the internet.
网络上的每个设备都有唯一的媒体访问控制 (MAC) 地址，固化在网卡中。IP 地址是动态或静态分配的逻辑地址，支持互联网路由。
The Domain Name System (DNS) translates human‑readable domain names (e.g., www.example.com) into IP addresses. The client‑server model defines how clients request services and servers respond, e.g., a web server delivering HTML pages over HTTP.
域名系统 (DNS) 将人类可读的域名 (如 www.example.com) 转换为 IP 地址。客户端-服务器模型定义了客户端请求服务、服务器响应的方式，例如 web 服务器通过 HTTP 协议发送 HTML 页面。
Protocols such as TCP/IP manage reliable data transmission. HTTPS adds encryption via Transport Layer Security (TLS). Email protocols include SMTP for sending and POP3/IMAP for retrieving mail.
TCP/IP 等协议管理可靠的数据传输。HTTPS 通过传输层安全 (TLS) 增加加密。电子邮件协议包括发送用的 SMTP 和接收用的 POP3/IMAP。
A firewall monitors and controls incoming and outgoing traffic based on predetermined security rules, protecting the network from unauthorised access.
防火墙根据预设的安全规则监控并控制进出流量，保护网络免受未授权访问。
3. Hardware and Logic Circuits | 硬件与逻辑电路
The central processing unit (CPU) consists of the Control Unit (CU), which decodes instructions and directs data flow, and the Arithmetic Logic Unit (ALU), which performs arithmetic and logical operations.
中央处理器 (CPU) 由控制单元 (CU) 和算术逻辑单元 (ALU) 组成。CU 负责译码指令并控制数据流，ALU 执行算术和逻辑运算。
Dedicated registers include the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR), Current Instruction Register (CIR), and Accumulator (ACC). They temporarily hold addresses, data, and instructions during the fetch‑decode‑execute cycle.
专用寄存器包括程序计数器 (PC)、内存地址寄存器 (MAR)、内存数据寄存器 (MDR)、当前指令寄存器 (CIR) 和累加器 (ACC)。它们在取指-译码-执行周期中暂时保存地址、数据和指令。
The Von Neumann architecture stores both data and instructions in the same memory, using a single shared bus. Performance is influenced by clock speed, number of processor cores, and cache memory size.
冯·诺依曼结构将数据和指令存放在同一内存中，共用一组总线。处理器性能受时钟频率、核心数量和高速缓存大小的影响。
Primary memory is divided into RAM (volatile, read‑write, holds currently running programs and data) and ROM (non‑volatile, holds firmware such as the BIOS). Cache memory is high‑speed SRAM that stores frequently used data to speed up access.
主存分为 RAM（易失、可读写，存放正在运行的程序和数据）和 ROM（非易失，存储固件如 BIOS）。高速缓存是高速静态随机存取存储器，存放常用数据以加快访问。
Logic gates are the building blocks of digital circuits. The basic gates are AND, OR, NOT, NAND, NOR, and XOR. A truth table lists all possible input combinations and their corresponding outputs. Below is the truth table for an AND gate:
逻辑门是数字电路的构建模块，基本门包括 AND、OR、NOT、NAND、NOR 和 XOR。真值表列出所有可能的输入组合及其对应的输出。下面是 AND 门的真值表：
| Input A | Input B | Output (A AND B) |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
ng Boolean algebra, expressions can be simplified, e.g., A · (A + B) = A. Karnaugh maps (K‑maps) are also used for visual simplification of circuits with 2 to 4 variables.
利用布尔代数可以简化表达式，例如 A · (A + B) = A。卡诺图 (K-map) 也用于对 2 至 4 个变量的电路进行可视化化简。
4. Processor Fundamentals and Assembly | 处理器基础与汇编
Machine code consists of binary instructions that the CPU can execute directly. Assembly language uses mnemonics (e.g., LDR, STR, ADD) to represent these instructions, making programming more human‑readable.
机器码由 CPU 可直接执行的二进制指令组成。汇编语言使用助记符（如 LDR、STR、ADD）表示这些指令，使程序更易于人类阅读。
An instruction typically contains an opcode (the operation to perform) and one or more operands (the data or addresses involved). Addressing modes specify how the CPU locates the operand: immediate (the operand is a constant), direct (the operand address is given), indirect (the address in the instruction points to a location that holds the actual address), and indexed (the address is formed by adding an index register to a base address).
一条指令通常包含操作码（要执行的操作）和一个或多个操作数（涉及的数据或地址）。寻址模式指定 CPU 如何定位操作数：立即寻址（操作数是常数）、直接寻址（给出操作数地址）、间接寻址（指令中的地址指向存放实际地址的位置）以及变址寻址（地址由变址寄存器与基地址相加得到）。
During execution, the processor uses the fetch‑decode‑execute cycle. A simplified representation: MAR ← [PC]; PC ← PC + 1; MDR ← [Memory] addressed by MAR; CIR ← MDR; decode and execute the instruction in CIR.
执行过程中，处理器遵循取指-译码-执行周期。简化表示为：MAR ← [PC]；PC ← PC + 1；MDR ← 由 MAR 寻址的内存内容；CIR ← MDR；译码并执行 CIR 中的指令。
Tracing a short assembly program involves tracking the contents of registers and memory locations after each instruction, which helps in understanding how high‑level constructs are implemented at low level.
追踪一段简短的汇编程序需要跟踪每条指令执行后寄存器和内存位置的内容变化，这有助于理解高级语言结构在底层的实现方式。
5. System Software | 系统软件
The operating system (OS) acts as an interface between hardware and user applications. Its key functions include memory management, process scheduling, file management, handling input/output, and providing security and user interfaces.
操作系统 (OS) 是硬件与用户应用程序之间的接口，主要功能包括内存管理、进程调度、文件管理、输入/输出处理，以及提供安全性和用户界面。
Interrupts are signals sent by hardware or software to the processor, causing it to temporarily suspend the current task and execute an Interrupt Service Routine (ISR). After the ISR completes, the processor resumes the original task.
中断是由硬件或软件发送给处理器的信号，使其暂停当前任务并执行中断服务程序 (ISR)。ISR 完成后，处理器恢复原任务。
Process scheduling algorithms decide which process gets CPU time. Common algorithms include First Come First Served (FCFS), Shortest Job First (SJF), and Round Robin. These aim to maximise throughput, minimise response time, and ensure fairness.
进程调度算法决定哪个进程获得 CPU 时间。常见算法包括先来先服务 (FCFS)、短作业优先 (SJF) 和轮转调度 (Round Robin)，目标是最大化吞吐量、最小化响应时间并保证公平性。
Translation software converts source code into machine code. A compiler translates the entire high‑level program at once, generating an executable file. An interpreter translates and executes line‑by‑line. An assembler translates assembly language into machine code.
翻译软件将源代码转换为机器码。编译器一次翻译整个高级语言程序，生成可执行文件；解释器逐行翻译并执行；汇编器将汇编语言翻译为机器码。
6. Security, Ethics and Legal Issues | 安全、道德与法律问题
Malware is malicious software designed to damage, disrupt, or gain unauthorised access. Types include viruses (attach to files, spread through human action), worms (self‑replicate across networks), Trojans (disguise as legitimate software), spyware (monitors user activity), and ransomware (encrypts data and demands payment).
恶意软件是旨在破坏、扰乱或未授权访问的软件。类型包括病毒（附着于文件，通过人为操作传播）、蠕虫（通过网络自我复制）、木马（伪装成合法软件）、间谍软件（监视用户活动）和勒索软件（加密数据并索要赎金）。
Social engineering attacks exploit human psychology, e.g., phishing emails trick users into revealing passwords. Denial of Service (DoS) attacks flood a server with traffic, making it unavailable to legitimate users.
社交工程攻击利用人的心理，例如网络钓鱼邮件欺骗用户泄露密码。拒绝服务 (DoS) 攻击通过大量流量淹没服务器，使其无法为合法用户提供服务。
Defence mechanisms include installing antivirus and anti‑malware software, enabling firewalls, applying regular patches, using strong passwords and two‑
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