📚 Year 13 CIE Computer Science: Core Knowledge Review | Year 13 CIE 计算机：核心知识点梳理
The Year 13 CIE Computer Science syllabus delves into advanced theory and practical programming, bridging the gap between how computers work at the hardware level and high‑level problem solving. Students explore data representation, communication protocols, processor architecture, system software, security, abstract data types, object‑oriented programming, databases and digital logic. Mastering these interconnected topics is essential for success in A‑Level papers and for building a strong foundation in computer science.
Year 13 CIE 计算机科学课程深入探讨高级理论和实践编程，在硬件底层工作原理与高层问题解决之间架起桥梁。学生将学习数据表示、通信协议、处理器架构、系统软件、安全、抽象数据类型、面向对象编程、数据库和数字逻辑。掌握这些相互关联的主题对于在 A‑Level 考试中取得成功以及打下坚实的计算机科学基础至关重要。
1. Data Representation | 数据表示
Real numbers are stored in floating‑point format with three components: sign, mantissa, and exponent. The mantissa holds the significant digits, and the exponent scales the value by a power of two. Normalisation shifts the binary point to eliminate leading zeros in the mantissa, maximising precision for a given bit width.
实数以浮点数格式存储，包含三个部分：符号、尾数和指数。尾数保存有效数字，指数以 2 的幂对数值进行缩放。规格化将二进制小数点移位，消除尾数中的前导零，从而在给定位宽下实现最高精度。
Value = (–1)S × 1.M × 2E–Bias
Negative integers use two’s complement representation: the most significant bit carries a negative place value (–2n–1). Addition and subtraction operate identically irrespective of sign, simplifying ALU design. Overflow occurs when the result falls outside the representable range.
负整数使用二进制补码表示：最高有效位携带负的权值（–2n–1）。无论符号如何，加法和减法操作均相同，简化了 ALU 设计。当结果超出可表示范围时会发生溢出。
Characters are encoded with Unicode, which extends ASCII to support worldwide scripts. UTF‑8 uses variable‑length encoding of 1 to 4 bytes per character, maintaining backward compatibility with ASCII. Images are stored as bitmaps (pixels with colour depth and resolution) or as vectors; sound is captured by sampling the signal amplitude at regular intervals and quantising each sample.
字符使用 Unicode 编码，它扩展了 ASCII 以支持全球书写系统。UTF‑8 每个字符使用 1 至 4 字节的可变长度编码，并保持对 ASCII 的向后兼容。图像以位图（带有颜色深度和分辨率的像素）或矢量形式存储；声音通过定期对信号幅度进行采样并量化每个样本来捕获。
2. Communication and Internet Technologies | 通信与互联网技术
The TCP/IP protocol suite is organised into four layers: application, transport, internet, and network access. The application layer handles high‑level protocols (HTTP, FTP, SMTP, DNS); the transport layer ensures reliable (TCP) or best‑effort (UDP) delivery; the internet layer routes packets using IP addresses; and the network access layer manages the physical transmission. Encapsulation adds headers at each layer.
TCP/IP 协议族划分为四层：应用层、传输层、互联网层和网络接入层。应用层处理高层协议（HTTP、FTP、SMTP、DNS）；传输层确保可靠（TCP）或尽力而为（UDP）的交付；互联网层使用 IP 地址路由数据包；网络接入层管理物理传输。封装在每一层添加头部。
IPv4 addresses are 32‑bit and have been exhausted globally; IPv6 uses 128‑bit addresses offering enormous address space. Subnetting allows a large network to be divided into smaller sub‑networks by applying a subnet mask, which determines the network and host portions. Routers use the subnet mask to forward packets correctly.
IPv4 地址为 32 位，已全球耗尽；IPv6 使用 128 位地址，地址空间极大。子网划分通过应用子网掩码将大网络分割成更小的子网，子网掩码决定网络部分和主机部分。路由器使用子网掩码正确转发数据包。
Client‑server architecture hosts resources on a central server, while clients request services. Peer‑to‑peer networks let each node act as both client and server, offering resilience but raising security concerns. Cloud computing delivers on‑demand services—IaaS, PaaS, SaaS—over the internet.
客户端‑服务器架构在中央服务器上托管资源，客户端请求服务。对等网络让每个节点同时充当客户端和服务器，具有弹性但也带来安全顾虑。云计算通过互联网按需交付服务——IaaS、PaaS、SaaS。
3. Processor and Assembly Language | 处理器与汇编语言
The CPU consists of the control unit, arithmetic logic unit (ALU), and a set of dedicated registers: program counter (PC), memory address register (MAR), memory data register (MDR), current instruction register (CIR), and accumulator (ACC). The system bus transfers addresses, data, and control signals between components.
CPU 由控制单元、算术逻辑单元 (ALU) 和一组专用寄存器组成：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)。系统总线在组件之间传输地址、数据和控制信号。
The fetch‑decode‑execute cycle repeats continuously: the PC supplies the address to MAR, the instruction is fetched into MDR and copied to CIR, the control unit decodes the opcode, and the ALU executes the operation, using other registers as needed. Interrupts can suspend the cycle to handle high‑priority events.
取指‑译码‑执行循环不断重复：PC 将地址送至 MAR，指令被获取到 MDR 并复制到 CIR，控制单元译码操作码，ALU 使用其他寄存器执行操作。中断可暂停该循环以处理高优先级事件。
Assembly language provides mnemonics like LDR (load register), STR (store register), ADD, SUB, CMP, and B (branch). Labels mark memory locations; directives define constants. Addressing modes include immediate (LDR R1, #5), direct (LDR R1, 100), and indirect (LDR R1, [R2]).
汇编语言提供助记符，例如 LDR（加载寄存器）、STR（存储寄存器）、ADD、SUB、CMP 和 B（分支）。标签标记内存位置；伪指令定义常量。寻址模式包括立即寻址 (LDR R1, #5)、直接寻址 (LDR R1, 100) 和间接寻址 (LDR R1, [R2])。
4. System Software and Language Translators | 系统软件与语言翻译器
The operating system manages hardware resources and provides services for programs. Key functions include process scheduling (round‑robin, priority queues), memory management (paging, segmentation), file system organisation, and input/output control. It also offers a user interface and enforces security policies.
操作系统管理硬件资源并为程序提供服务。关键功能包括进程调度（轮转、优先级队列）、内存管理（分页、分段）、文件系统组织以及输入/输出控制。它还提供用户界面并执行安全策略。
A compiler translates high‑level source code into machine code in a single pass, generating an executable file. An interpreter reads and executes source code line‑by‑line, making debugging easier but slower at runtime. An assembler converts assembly language into object code. The linker combines multiple object modules and library routines into one executable; the loader then puts the program into memory for execution.
编译器一次性将高级源代码翻译为机器码，生成可执行文件。解释器逐行读取并执行源代码，便于调试但运行时较慢。汇编器将汇编语言转换为目标代码。链接器将多个目标模块和库例程合并为一个可执行文件；加载器随后将程序放入内存以供执行。
Lexical analysis tokenises the source code, syntax analysis builds a parse tree, semantic analysis checks for type consistency, and code generation produces the target code. Intermediate representations allow optimisations across the compilation stages.
词法分析将源代码标记化，语法分析构建解析树，语义分析检查类型一致性，代码生成产生目标代码。中间表示允许在编译各阶段进行优化。
5. Security and Encryption | 安全与加密
Symmetric encryption uses a single key for both encryption and decryption (e.g. AES, DES). It is fast but requires secure key distribution. Asymmetric encryption uses a public‑key pair (e.g. RSA): the public key encrypts and the private key decrypts. Hybrid schemes exchange a symmetric session key using asymmetric encryption for efficiency.
对称加密使用同一密钥进行加解密（如 AES、DES）。其速度快，但需要安全的密钥分发。非对称加密使用公钥‑私钥对（如 RSA）：公钥加密，私钥解密。混合方案利用非对称加密交换对称会话密钥以提高效率。
A digital signature is created by hashing a message and encrypting the hash with the sender’s private key. The recipient decrypts the hash with the sender’s public key and compares it with a freshly computed hash, verifying both integrity and origin. Digital certificates bind a public key to an identity via a trusted certificate authority (CA).
数字签名通过对消息进行散列并用发送者的私钥加密该散列值生成。接收者用发送者公钥解密散列，并与刚计算的散列值比较，从而验证完整性和来源。数字证书通过受信任的证书颁发机构 (CA) 将公钥与身份绑定。
Firewalls filter incoming and outgoing traffic based on predefined rules; packet‑filtering and stateful inspection are common. Proxy servers act as intermediaries, enhancing security and caching content. Malware includes viruses, worms, trojan horses, and ransomware; anti‑malware tools rely on signature databases and heuristic analysis.
防火墙根据预定义规则过滤入站和出站流量；数据包过滤和状态检测是常见类型。代理服务器充当中介，增强安全性并缓存内容。恶意软件包括病毒、蠕虫、特洛伊木马和勒索软件；反恶意软件工具依赖于特征数据库和启发式分析。
6. Abstract Data Types and Algorithms | 抽象数据类型与算法
A stack is a Last‑In‑First‑Out (LIFO) structure with operations push and pop. A queue is First‑In‑First‑Out (FIFO) with enqueue and dequeue. Both can be implemented statically using arrays and pointers, or dynamically with linked lists, where each node holds data and a pointer to the next node.
栈是一种后进先出 (LIFO) 结构，包含 push 和 pop 操作。队列是先进先出 (FIFO)，包含 enqueue 和 dequeue。两者均可使用数组和指针静态实现，或使用链表动态实现，链表中每个节点包含数据和指向下一节点的指针。
Binary trees support traversal orders: pre‑order (root, left, right), in‑order (left, root, right), and post‑order (left, right, root). In a binary search tree (BST), for any node, left subtree values are smaller and right subtree values are larger, enabling O(log n) search on average. Recursive algorithms reduce a problem to smaller instances; divide‑and‑conquer splits, solves, and merges (e.g. merge sort, binary search).
二叉树支持遍历顺序：前序（根、左、右）、中序（左、根、右）和后序（左、右、根）。在二叉搜索树 (BST) 中，对于任意节点，左子树的值较小，右子树的值较大，平均可实现 O(log n) 的搜索。递归算法将问题缩减为更小实例；分治法拆分、解决并合并（如归并排序、二分查找）。
Complexity is expressed using Big O notation: O(1) constant, O(log n) logarithmic, O(n) linear, O(n²) quadratic. Choosing the right ADT and algorithm based on time and space trade‑offs is a core skill. Dynamic data structures like linked lists allow efficient insertion and deletion, whereas arrays offer direct access.
复杂度使用大 O 符号表示：O(1) 常数，O(log n) 对数，O(n) 线性，O(n²) 平方。根据时间和空间权衡选择合适的 ADT 和算法是一项核心技能。链表等动态数据结构允许高效插入和删除，而数组提供直接访问。
7. Object‑Oriented Programming | 面向对象编程
A class defines a blueprint: attributes (data) and methods (functions). An object is an instance of a class. Encapsulation bundles data with the methods that operate on it and restricts direct access by marking attributes as private, exposing a controlled public interface.
类定义了蓝图：属性（数据）和方法（函数）。对象是类的实例。封装将数据与操作数据的方法捆绑在一起，并限制直接访问，通过将属性标记为私有，暴露受控的公共接口。
Inheritance models an “is‑a” relationship: a subclass inherits the properties and methods of a superclass, allowing code reuse and extension. Overriding provides a new implementation of an inherited method in the subclass. Polymorphism enables objects of different subclasses to be treated uniformly through a common superclass reference, and method calls are resolved at runtime (dynamic binding).
继承建模 “is‑a” 关系：子类继承超类的属性和方法，实现代码重用和扩展。重写（覆盖）在子类中提供继承方法的新实现。多态性允许通过共同的超类引用统一处理不同子类的对象，方法调用在运行时解析（动态绑定）。
UML class diagrams represent classes as rectangles divided into name, attributes, and methods. Associations show relationships with multiplicity notation (e.g. 1..*, 0..1); aggregation and composition represent whole‑part relationships. Abstract classes and interfaces
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