📚 Year 12 Cambridge Computer Science: Key Terms Memory Guide | 剑桥AS计算机：核心术语速记指南
Mastering the terminology of Cambridge International AS & A Level Computer Science is essential for success in both theory papers and practical problem-solving. This guide presents a structured overview of the most important terms you need to know, complete with concise definitions, exam-focused context, and memory tips. Each term is paired with its Chinese equivalent to support bilingual learners at aleveler.com.
掌握剑桥国际AS与A Level计算机科学中的术语，对理论考试和实际问题解决至关重要。本指南系统梳理了必备核心术语，配有简明定义、考试要点和记忆窍门，并提供中英对照，助力双语学习者快速巩固。
1. Data Units & Binary Representation | 数据单位与二进制表示
Bit – The smallest unit of data in computing, holding a value of 0 or 1. The word ‘bit’ is a contraction of ‘binary digit’. Think of a light switch: off (0) and on (1).
比特 – 计算中最小的数据单位，存储0或1。’bit’是’binary digit’的缩写。可以联想电灯开关：关（0）与开（1）。
Byte – A group of 8 bits, treated as a single unit. The byte is the fundamental addressable unit in most computer architectures. A single character of text typically occupies one byte in ASCII.
字节 – 由8个比特组成的组，作为一个整体处理。字节是大多数计算机体系结构中最基本的可寻址单元。在ASCII编码中，一个字符通常占用一个字节。
Nibble – A group of 4 bits, equivalent to half a byte. Nibbles are convenient for representing a single hexadecimal digit (0–F). Remember: ‘nibble’ is a small bite.
半字节 – 4个比特的组，等于半个字节。半字节便于表示一个十六进制数字（0–F）。记忆：nibble（小口）源于bite（字节）的一半。
Most Significant Bit (MSB) – The bit with the highest place value in a binary number, usually the leftmost bit. In signed binary representations, the MSB often indicates the sign.
最高有效位 – 二进制数中权值最高的位，通常是最左边的一位。在有符号二进制表示中，最高有效位往往表示符号。
2. Number Bases and Conversion | 数制与转换
Denary (Decimal) – The base-10 number system, using digits 0–9. It is the system humans use in daily life. Each position represents a power of 10.
十进制 – 基数为10的数字系统，使用数字0–9。这是我们日常使用的系统，每位权值为10的幂。
Binary – The base-2 number system, using only 0 and 1. Each position represents a power of 2. All instructions and data inside a computer are ultimately encoded in binary.
二进制 – 基数为2的数字系统，仅使用0和1。每位权值为2的幂。计算机内部所有的指令与数据最终都以二进制编码。
Hexadecimal – The base-16 system, using digits 0–9 and letters A–F (where A=10, F=15). One hex digit corresponds exactly to one nibble, making it a compact way to represent long binary strings. For example, 1101 0010₂ = D2₁₆.
十六进制 – 基数为16的系统，使用0–9和A–F（A=10, F=15）。一个十六进制数字恰好对应一个半字节，是长二进制串的简洁表示。例如，1101 0010₂ = D2₁₆。
Place Value – The value of a digit based on its position in a number. In binary, moving left doubles the value; in hex each left shift multiplies by 16. Understanding place value is crucial for conversions.
位权 – 数字中根据位置决定的数值。二进制中左移一位数值加倍；十六进制中左移一位乘以16。掌握位权是进制转换的关键。
3. Two’s Complement & Overflow | 补码与溢出
Two’s Complement – The standard method for representing signed integers in binary. To find the two’s complement of a binary number, invert all bits and add 1. It allows subtraction to be performed using addition circuitry.
二进制补码 – 表示有符号整数的标准方法。求一个二进制数的补码：将所有位取反后加1。这样就可以用加法电路完成减法运算。
Sign-magnitude – An alternative signed representation where the MSB is the sign bit (0 for positive, 1 for negative) and the remaining bits indicate magnitude. Rarely used in modern CPUs due to two zeros and complex arithmetic.
原码表示 – 一种有符号数表示法，最高位为符号位（0正1负），其余位表示数值大小。因存在正负零且运算复杂，现代CPU很少采用。
Overflow – An error that occurs when the result of an arithmetic operation exceeds the available number of bits. In two’s complement, overflow is detected when the carry into the MSB differs from the carry out of the MSB.
溢出 – 当算术运算的结果超出可用位数时发生的错误。在补码运算中，如果进位入最高位与进位出最高位不同，则发生溢出。
Binary Coded Decimal (BCD) – A representation where each denary digit is encoded using its 4-bit binary equivalent. For example, 25 becomes 0010 0101. BCD avoids recurring fraction errors in decimal arithmetic but is space-inefficient.
二-十进制编码(BCD) – 将每个十进制位用其4位二进制等价形式编码。例如，25表示为0010 0101。BCD避免了十进制算术中的循环小数误差，但浪费存储空间。
4. Character Encoding | 字符编码
ASCII – American Standard Code for Information Interchange. A 7-bit character set representing 128 characters, including control codes and printable English characters. Extended ASCII uses 8 bits for an additional 128 symbols.
ASCII – 美国信息交换标准代码。7位字符集，表示128个字符，包括控制字符和可打印英文字符。扩展ASCII使用8位，额外增加128个符号。
Unicode – A universal character encoding standard designed to represent text from all writing systems. It assigns a unique code point to each character. UTF-8, UTF-16 and UTF-32 are different encoding forms that map code points to bit sequences.
Unicode – 通用字符编码标准，旨在涵盖全世界所有书写系统的文字。每个字符分配唯一的码点。UTF-8、UTF-16和UTF-32是不同编码形式，将码点映射为比特序列。
Code Point – A numerical value that identifies a specific character in Unicode. For example, U+0041 is ‘A’. Do not confuse code points with the final byte sequence determined by the encoding.
码点 – Unicode中标识特定字符的数值。例如，U+0041代表’A’。注意不要把码点与由具体编码决定的最终字节序列混淆。
Character Set vs Encoding – A character set defines the repertoire of characters and their integer assignments; encoding specifies how those integers are stored as bytes. For instance, Unicode is a character set; UTF-8 is an encoding.
字符集与编码 – 字符集定义字符的集合及其对应的数字编号；编码则规定如何将这些数字存储为字节。例如，Unicode是字符集，UTF-8是编码方式。
5. Image and Sound Representation | 图像与声音表示
Pixel – The smallest addressable element of a bitmap image. Each pixel holds a colour value. Image resolution is measured in pixels (e.g., 1920×1080). Higher resolution gives finer detail but increases file size.
像素 – 位图图像中最小的可寻址元素。每个像素存储一个颜色值。图像分辨率以像素衡量（如1920×1080）。分辨率越高细节越精细，但文件体积也越大。
Colour Depth – The number of bits used to represent the colour of a single pixel. A depth of 24 bits (8 per RGB channel) allows 2²⁴ ≈ 16.7 million colours. Higher colour depth produces smoother gradients.
色深 – 用于表示单个像素颜色的位数。24位色深（每RGB通道8位）可产生约1670万种颜色。色深越高，色彩过渡越平滑。
Sampling Rate – In sound recording, the number of samples taken per second from an analogue signal, expressed in hertz (Hz). According to the Nyquist theorem, the sampling rate must be at least twice the highest frequency to avoid aliasing.
采样率 – 在声音录制中，每秒从模拟信号采集的样本数量，单位为赫兹（Hz）。根据奈奎斯特定理，采样率必须至少为最高频率的两倍，才能避免混叠失真。
Bit Rate – The number of bits processed per unit of time, usually kilobits per second (kbps). For audio, bit rate = sampling rate × sample resolution × number of channels. It directly influences sound quality and file size.
比特率 – 单位时间内处理的比特数，通常以千比特/秒（kbps）计。音频比特率 = 采样率 × 样本分辨率 × 声道数。比特率直接影响音质和文件大小。
6. Networking Fundamentals | 网络基础
LAN (Local Area Network) – A network covering a small geographical area, such as a school or office building. LANs typically use Ethernet or Wi-Fi and offer high data transfer rates with low latency.
局域网(LAN) – 覆盖较小地理范围的网络，如学校或办公楼。局域网通常使用以太网或Wi-Fi技术，具有高传输速率和低延迟。
WAN (Wide Area Network) – A network spanning large geographical distances, often connecting multiple LANs. The internet is the largest WAN. WANs rely on routers and leased telecommunication lines.
广域网(WAN) – 跨越较大地理范围的网络，通常连接多个局域网。互联网是最大的广域网，广域网依赖路由器与租用的电信线路。
Client-Server Model – A network architecture where clients request services and servers provide them. The server is a powerful, always-on machine; clients are typically user devices. Email, web and file sharing often use this model.
客户端-服务器模型 – 一种网络架构，客户端请求服务，服务器提供服务。服务器通常是高性能且持续运行的机器；客户端一般为用户设备。电子邮件、网页和文件共享常采用此模型。
Peer-to-Peer (P2P) – A decentralised network model where each node acts as both client and server, sharing resources directly. P2P is common in file sharing and blockchain technologies.
对等网络(P2P) – 去中心化的网络模型，每个节点同时作为客户端和服务器，直接共享资源。P2P常用于文件共享和区块链技术。
7. Network Devices and Topologies | 网络设备与拓扑
Router – A device that forwards data packets between different networks, using IP addresses to determine the best path. Routers work at the network layer and are essential for connecting a LAN to the internet.
路由器 – 在不同网络之间转发数据包的设备，根据IP地址决定最佳路径。路由器工作在网络层，是将局域网接入互联网的关键设备。
Switch – A network device that connects devices within a LAN and forwards frames based on MAC addresses. Unlike a hub, a switch sends data only to the intended recipient, reducing collisions.
交换机 – 连接局域网内设备的网络设备，根据MAC地址转发帧。与集线器不同，交换机只向目标设备发送数据，减少碰撞。
Star Topology – A network layout where all nodes connect to a central switch or hub. If one cable fails, only that node is affected, but the central device is a single point of failure.
星形拓扑 – 所有节点均连接到中央交换机或集线器的网络布局。某条线缆损坏时仅影响对应节点，但中央设备是单点故障源。
Mesh Topology – Each device is connected to several others, providing redundant paths. Full mesh connects every node to every other node; partial mesh connects only some. Used for high reliability, as in WAN backbones.
网格拓扑 – 每台设备与多台设备相连，提供冗余路径。全网状拓扑将每个节点均互连；部分网状只连接部分节点。用于广域网骨干等对可靠性要求高的场景。
8. Internet Protocols and the TCP/IP Stack | 互联网协议与TCP/IP栈
Protocol – A set of rules governing data communication. Protocols define message format, order, and expected actions. Examples include HTTP, FTP, and TCP.
协议 – 管理数据通信的一组规则。协议定义了消息格式、顺序及预期操作。例如HTTP、FTP和TCP。
TCP (Transmission Control Protocol) – A connection-oriented protocol at the transport layer that ensures reliable, ordered delivery of data. It retransmits lost packets and uses acknowledgements.
TCP（传输控制协议） – 传输层的面向连接协议，确保数据的可靠、有序传输。它重传丢失的数据包并使用确认机制。
IP Address – A unique numerical label assigned to each device on a network. IPv4 uses 32-bit addresses (e.g., 192.168.1.1); IPv6 uses 128 bits to accommodate more devices.
IP地址 – 分配给网络上每台设备的唯一数字标识。IPv4使用32位地址（如192.168.1.1）；IPv6使用128位地址以容纳更多设备。
DNS (Domain Name System) – Acts as the internet’s phonebook, translating human-friendly domain names (e.g., aleveler.com) into IP addresses. DNS servers handle these translations hierarchically.
DNS（域名系统） – 相当于互联网的电话簿，将人类友好的域名（如aleveler.com）转换为IP地址。DNS服务器分层进行解析。
9. Logic Gates and Boolean Algebra | 逻辑门与布尔代数
Logic Gate – An elementary building block of digital circuits that performs a Boolean function on one or more binary inputs to produce a single binary output. Basic gates: AND, OR, NOT.
逻辑门 – 数字电路的基本构建块，对二进制输入执行布尔函数并产生一个二进制输出。基本门包括AND（与）、OR（或）、NOT（非）。
Truth Table – A table showing all possible input combinations and the corresponding output for a logic circuit or expression. It is a exhaustive way to define Boolean behaviour.
真值表 – 展示逻辑电路或表达式所有可能输入组合及其相应输出的表格，是定义布尔行为的一种穷举方式。
Boolean Expression – An algebraic expression formed using Boolean variables, constants (0, 1), and operators (AND, OR, NOT). For example, Q = (A ∧ B) ∨ ¬C.
布尔表达式 – 由布尔变量、常量（0,1）和运算符（与、或、非）构成的代数表达式。例如，Q = (A ∧ B) ∨ ¬C。
NAND and NOR Gates – Universal gates from which any other gate can be constructed. A NAND gate outputs 0 only when all inputs are 1; NOR outputs 1 only when all inputs are 0.
与非门和或非门 – 通用门，可构成任何其他门。与非门仅在输入全为1时输出0；或非门仅在输入全为0时输出1。
10. Processor Architecture | 处理器架构
CPU (Central Processing Unit) – The brain of the computer that executes instructions. It contains the ALU, control unit, and registers. Clock speed, measured in gigahertz (GHz), indicates how many cycles per second the CPU can perform.
中央处理器(CPU) – 计算机的”大脑”，负责执行指令。它包含算术逻辑单元、控制单元和寄存器。时钟频率以吉赫兹（GHz）为单位，表示CPU每秒可完成的周期数。
Fetch-Decode-Execute Cycle – The fundamental sequence by which a processor runs a program. Fetch: load the next instruction from memory; Decode: interpret the instruction; Execute: perform the required operation.
取指-译码-执行周期 – 处理器运行程序的基本步骤。取指：从内存加载下条指令；译码：解释指令；执行：完成所需操作。
Program Counter (PC) – A special register that holds the memory address of the next instruction to be fetched. After fetch, the PC is incremented to point to the subsequent instruction unless a jump occurs.
程序计数器(PC) – 存放待取指的下一条指令内存地址的专用寄存器。取指后PC会递增指向后续指令，除非发生跳转。
MAR and MDR – Memory Address Register holds the address of the memory location to be accessed; Memory Data Register (or MBR) holds the data read from or to be written to memory. These are vital interface registers between CPU and main memory.
MAR与MDR – 存储器地址寄存器存放待访问的内存地址；存储器数据寄存器存放已读或待写的数据。它们是CPU与主存之间的关键接口寄存器。
11. Memory and Storage | 存储器
RAM (Random Access Memory) – Volatile, fast memory that holds data and instructions currently in use. Data is lost when power is switched off. More RAM allows more programs to run simultaneously.
随机存取存储器(RAM) – 易失性快速存储，存放当前使用的数据和指令。断电后数据消失。更大的RAM允许同时运行更多程序。
ROM (Read-Only Memory) – Non-volatile memory that stores firmware and boot instructions. Data persists without power and is typically written once, though variants like EEPROM can be updated.
只读存储器(ROM) – 非易失性存储器，存放固件和引导指令。断电后数据不丢失，通常仅写入一次，但EEPROM等可擦写变体允许更新。
Virtual Memory – A technique that uses part of the secondary storage (e.g., HDD or SSD) as if it were RAM. It allows running programs larger than physical memory, but excessive paging causes thrashing and slows performance.
虚拟内存 – 将部分辅存（如HDD或SSD）当作RAM使用的技术。它允许运行比物理内存更大的程序，但频繁的页面置换会导致”抖动”，降低性能。
SSD vs HDD – Solid State Drives use flash memory with no moving parts, offering faster access, lower
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