📚 AS CAIE Computer Science: Core Concepts Review | AS CAIE 计算机：核心知识点梳理
Mastering AS-level Computer Science under the CAIE syllabus means building a rock-solid understanding of fundamental concepts, from binary arithmetic to database design and ethical computing. This article systematically sorts through the core topics that repeatedly appear in exam questions, providing bilingual explanations to help you grasp both the technical depth and the precise Chinese terminology needed for confident revision.
掌握 CAIE 大纲下的 AS 计算机科学，意味着你需要牢固掌握从二进制运算到数据库设计和计算伦理等一系列基本概念。本文系统梳理了考卷中反复出现的核心主题，并以中英双语解释，帮助你既理解技术深度，也掌握准确的中文术语，从而充满信心地复习备考。
1. Information Representation | 信息表示
The foundation of data representation lies in number systems: binary (base 2), denary (base 10), and hexadecimal (base 16). You must be able to convert between these bases, handle fractional binary numbers, and perform binary addition. A clear example is converting 101.01₂ to 5.25 in denary.
数据表示的基础在于数制：二进制（基数为 2）、十进制（基数为 10）和十六进制（基数为 16）。你必须能够相互转换这些进制，处理二进制小数，并执行二进制加法。例如，将 101.01₂ 转换为十进制的 5.25。
Negative integers are represented with two’s complement. The most significant bit indicates sign, and the value is found by inverting all bits and adding 1. For an 8-bit representation, the range is -128 to +127. Subtraction is performed by adding the two’s complement of the subtrahend.
负整数用补码表示。最高位表示符号，数值通过将所有位取反后加 1 得到。8 位补码的范围为 -128 至 +127。减法通过加上减数的补码来完成。
Real numbers use floating-point notation consisting of a mantissa and an exponent, expressed as ±M × 2ᴱ. Increasing bits for the mantissa improves precision, while expanding the exponent widens the range. The binary point is shifted according to the exponent value.
实数采用浮点记数法，由尾数和指数组成，形式为 ±M × 2ᴱ。增加尾数的位数可提高精度，增加指数的位数可扩大表示范围。小数点根据指数值移位。
Characters are encoded with character sets: ASCII uses 7 bits (128 characters), while Unicode uses variable-length encoding to cover global scripts. Binary-Coded Decimal (BCD) encodes each denary digit in 4 bits, frequently used in applications where exact decimal arithmetic is critical.
字符通过字符集编码：ASCII 使用 7 位（128 个字符），Unicode 采用可变长度编码以涵盖全球文字。二进制编码十进制 (BCD) 用 4 位表示每位十进制数字，常用于对精确十进制算术要求严格的应用。
Images are stored as bitmaps, where each pixel’s colour is determined by the colour depth (e.g., 24 bits for true colour) and the image resolution defines the total number of pixels. Sound is digitised by sampling the analogue signal at a sampling rate (e.g., 44.1 kHz) and quantising each sample with a given bit depth (e.g., 16 bits).
图像以位图形式存储，每个像素的颜色由颜色深度决定（如 24 位真彩色），图像分辨率则定义像素总数。声音通过以采样率（如 44.1 kHz）对模拟信号进行采样，并用给定位深（如 16 位）量化每个样本而数字化。
Data compression reduces file size. Lossless methods (run-length encoding, Huffman coding) allow exact original reconstruction; lossy techniques (JPEG, MP3) permanently discard some data, trading quality for higher compression ratios. Exam questions often expect you to describe their applications and trade-offs.
数据压缩可减小文件大小。无损方法（游程编码、霍夫曼编码）能够精确重建原始数据；有损技术（JPEG、MP3）则永久性舍弃部分数据，以质量为代价换取更高的压缩比。考题常要求描述它们的应用与权衡。
2. Communication and Internet Technologies | 通信与互联网技术
Data transmission can be serial (bits sent one after another on a single wire) or parallel (multiple bits simultaneously). Serial is preferred for long distances due to fewer synchronisation issues. Transmission also varies between simplex, half-duplex, and full-duplex modes.
数据传输可以是串行（在一根线上逐位发送）或并行（多比特同时发送）。串行由于同步问题少，适合长距离传输。传输模式还分为单工、半双工和全双工。
Error detection methods include parity checks (even/odd parity bit added), checksums (sum of data bytes), and Cyclic Redundancy Checks (CRC), which uses polynomial division for greater reliability. These help maintain data integrity during transfer.
错误检测方法包括奇偶校验（添加偶/奇校验位）、校验和（数据字节之和）和循环冗余校验 (CRC)，后者利用多项式除法提供更高可靠性。它们有助于在传输过程中维护数据完整性。
Networking basics: a LAN connects computers within a small area using topologies like star (central switch) or bus. Ethernet uses CSMA/CD to manage collisions. WANs connect geographically dispersed LANs. The Internet relies on TCP/IP protocol suite, where IP handles addressing and routing, while TCP ensures reliable delivery.
网络基础：局域网 (LAN) 连接小范围计算机，采用星型（中央交换机）或总线型等拓扑结构。以太网使用 CSMA/CD 管理冲突。广域网 (WAN) 连接地理上分散的局域网。互联网依赖于 TCP/IP 协议族，其中 IP 处理寻址和路由，TCP 保证可靠传输。
IP addresses (IPv4: 32-bit dotted decimal; IPv6: 128-bit hex) uniquely identify devices. Domain Name System (DNS) translates human-readable names to IP addresses. A URL identifies a web resource, and HTTP/HTTPS is the protocol for web transfer, with HTTPS using SSL/TLS encryption.
IP 地址（IPv4: 32 位点分十进制；IPv6: 128 位十六进制）唯一标识设备。域名系统 (DNS) 将人类可读的名称转换为 IP 地址。URL 识别网络资源，HTTP/HTTPS 是网页传输协议，HTTPS 使用 SSL/TLS 加密。
Basic web technologies include HTML for structuring content and CSS for presentation. Understanding inline, internal, and external stylesheets, as well as simple selectors, is part of the AS syllabus.
基本的网络技术包括用于内容结构的 HTML 和用于表现的 CSS。理解内联样式、内部样式表和外部样式表，以及简单的选择器，是 AS 大纲的一部分。
3. Computer Hardware and Architecture | 计算机硬件与体系结构
Von Neumann architecture forms the backbone of most computers. It features a Control Unit (CU), Arithmetic Logic Unit (ALU), and registers: Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR), Current Instruction Register (CIR), Accumulator (ACC), and Index Register (IX). A single set of buses (address, data, control) connects all components.
冯·诺依曼架构是大多数计算机的主干。它包括控制单元 (CU)、算术逻辑单元 (ALU) 以及寄存器：程序计数器 (PC)、存储器地址寄存器 (MAR)、存储器数据寄存器 (MDR)、当前指令寄存器 (CIR)、累加器 (ACC) 和变址寄存器 (IX)。一组总线（地址总线、数据总线、控制总线）连接所有部件。
The fetch-execute cycle continuously fetches an instruction from memory (address in PC), decodes it, and executes. PC is incremented unless a jump occurs. Performance is influenced by clock speed, number of processor cores, and cache memory (L1/L2) that provides faster data access than RAM.
取指—执行周期不断从存储器中取出指令（地址在 PC 中）、译码并执行。除非发生跳转，否则 PC 递增。性能受时钟速度、处理器核数以及缓存（L1/L2）的影响，缓存提供比 RAM 更快的数据访问。
The memory hierarchy ranges from fast, small registers and cache, through main memory (RAM), to slower secondary storage (HDD/SSD). Embedded systems are specialised microprocessors dedicated to a single task, often with real-time constraints, and found in devices like washing machines and car engine controllers.
存储器层次结构从快速、小容量的寄存器和缓存，到主存 (RAM)，再到速度较慢的辅助存储器 (HDD/SSD)。嵌入式系统是专用于单一任务的专用微处理器，通常具有实时限制，常见于洗衣机和汽车引擎控制器等设备中。
4. Processor Instruction Set and Assembly Language | 处理器指令集与汇编语言
An instruction consists of an opcode (operation to perform) and operands (data or addresses). Addressing modes include immediate (the operand itself), direct (absolute address), indirect (address held in a register), indexed (base + offset), and relative (offset relative to PC). These modes offer flexibility in accessing data.
一条指令由操作码（要执行的操作）和操作数（数据或地址）组成。寻址模式包括立即寻址（操作数本身）、直接寻址（绝对地址）、间接寻址（地址存于寄存器）、变址寻址（基址 + 偏移量）和相对寻址（相对于 PC 的偏移）。这些模式为访问数据提供了灵活性。
Assembly language uses mnemonics like LDD (load), STO (store), ADD, SUB, CMP, JMP, and JPE. A two-pass assembler translates source code into machine code: the first pass builds a symbol table (labels and addresses), the second pass generates binary code using the symbol table.
汇编语言使用助记符，如 LDD（加载）、STO（存储）、ADD、SUB、CMP、JMP 和 JPE。两遍汇编器将源代码翻译成机器码：第一遍建立符号表（标签和地址），第二遍利用符号表生成二进制代码。
Understanding the link between high-level language constructs and assembly is crucial. For instance, a simple loop may use a counter in IX register, compare, and branch back. Questions often ask to trace or write short assembly programs.
理解高级语言结构与汇编之间的联系至关重要。例如，一个简单的循环可能使用 IX 寄存器作为计数器，进行比较并跳转回去。考题常要求追踪或编写简短汇编程序。
5. System Software | 系统软件
An operating system manages hardware, provides a user interface, handles multitasking (scheduling processes), manages memory (allocation, paging), and ensures security through user accounts. Interrupts signal the processor to pause current tasks and run an Interrupt Service Routine (ISR), enabling responsive I/O handling.
操作系统管理硬件，提供用户接口，处理多任务（进程调度），管理存储器（分配、分页），并通过用户账户确保安全。中断信号通知处理器暂停当前任务并运行中断服务程序 (ISR)，从而实现响应式输入/输出处理。
Translators convert source code: a compiler translates the entire program into machine code ahead of time (producing an executable), while an interpreter translates and executes line by line. Compilers yield faster execution but longer development cycles; interpreters aid debugging and portability.
翻译器转换源代码：编译器将整个程序预先转换为机器码（生成可执行文件），而解释器逐行翻译并执行。编译器执行速度更快但开发周期长；解释器便于调试和可移植。
Assemblers translate assembly language to machine code. A linker combines multiple object files into a single executable, and a loader places the program into memory for execution. Library files provide reusable routines to be linked at compile or run time.
汇编器将汇编语言翻译为机器码。链接器将多个目标文件合并成一个可执行文件，加载器将程序放入内存以便执行。库文件提供可重用的例程，可在编译时或运行时链接。
6. Data Security, Privacy and Integrity | 数据安全、隐私与完整性
Threats include malware (viruses, worms, ransomware), phishing, and Denial of Service (DoS) attacks. Encryption protects data: symmetric encryption (e.g., AES) uses the same key for encryption and decryption; asymmetric encryption (e.g., RSA) uses a public/private key pair.
威胁包括恶意软件（病毒、蠕虫、勒索软件）、网络钓鱼和拒绝服务 (DoS) 攻击。加密保护数据：对称加密（如 AES）使用相同密钥加解密；非对称加密（如 RSA）使用公钥/私钥对。
Digital signatures use the sender’s private key to encrypt a hash of the message, allowing recipients to verify authenticity and integrity with the sender’s public key. Digital certificates, issued by Certificate Authorities, bind public keys to identities. SSL/TLS secures web communications by encrypting data in transit.
数字签名使用发送者的私钥加密消息哈希值，使接收者能够用发送者的公钥验证真实性和完整性。数字证书由证书颁发机构签发，将公钥与身份绑定。SSL/TLS 通过加密传输中的数据来保护网络通信。
Data integrity is maintained through validation (checking input meets rules, e.g., range check, type check) and verification (ensuring data matches original, e.g., double entry). Backup procedures and disaster recovery plans safeguard against data loss. Regular checksums or hashes detect accidental alterations.
数据完整性通过验证（检查输入符合规则，如范围检查、类型检查）和核实（确保数据与原件一致，如双重录入）来维护。备份程序和灾难恢复计划可防止数据丢失。定期校验和或哈希能检测意外更改。
7. Ethics and Intellectual Property | 伦理与知识产权
Ethical considerations in computing cover privacy (respecting personal data), intellectual property rights, and acceptable use. Data protection laws (e.g., GDPR) regulate collection and processing of personal information. Software can be proprietary (closed source, paid license), open source, freeware, or shareware, each with distinct licensing models.
计算机伦理考量包括隐私（尊重个人数据）、知识产权和可接受使用。数据保护法（如 GDPR）规范个人信息的收集和处理。软件可以是专有（闭源、付费许可证）、开源、免费软件或共享软件，各有不同的许可证模式。
Artificial intelligence raises ethical issues such as algorithmic bias, accountability for decisions, and impact on employment. AS questions may ask you to discuss the responsible development and use of AI systems, alongside the importance of maintaining transparency and fairness.
人工智能引发了诸如算法偏见、决策问责和对就业的影响等伦理问题。AS 考题可能要求你讨论 AI 系统的负责任开发和使用，以及保持透明度和公平性的重要性。
8. Database Systems and SQL | 数据库系统与 SQL
A relational database organises data into
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