📚 A-Level OCR Computer Science: Last-Minute Revision Notes | A-Level OCR 计算机科学：考前冲刺笔记
Welcome to your last-minute revision guide for OCR A-Level Computer Science (H446). This curated set of notes condenses the most important concepts across the specification, helping you quickly refresh your memory before the exam. Use it alongside past papers and mark schemes for maximum impact.
欢迎阅读 OCR A-Level 计算机科学 (H446) 考前冲刺指南。这份精选的笔记浓缩了大纲中最重要的概念，帮助你在考前快速回顾。请结合历年真题和评分方案使用，以达到最佳效果。
1. Processor Fundamentals | 处理器基础
The CPU operates on the von Neumann architecture, where both data and instructions are stored in the same memory. The processor repeatedly fetches, decodes and executes instructions, controlled by a system clock.
CPU 基于冯·诺依曼架构运行，数据和指令存储在同一内存中。处理器在系统时钟的控制下不断重复取指、解码和执行指令。
Key registers involved in the fetch-decode-execute cycle are summarised below.
取指-解码-执行周期中涉及的关键寄存器总结如下。
| Register | Function | 中文说明 |
|---|---|---|
| PC | Program Counter: holds address of the next instruction | 程序计数器：存放下一条指令的地址 |
| MAR | Memory Address Register: holds address of memory location to be read/written | 内存地址寄存器：存放要读写的内存地址 |
| MDR | Memory Data Register: holds data just read from or about to be written to memory | 内存数据寄存器：存放刚从内存读取或即将写入的数据 |
| CIR | Current Instruction Register: stores the instruction currently being decoded/executed | 当前指令寄存器：存储正在被解码/执行的指令 |
| ACC | Accumulator: holds intermediate results from the ALU | 累加器：保存 ALU 的中间结果 |
In the fetch stage, MAR ← PC, MDR ← memory[MAR], CIR ← MDR and PC is incremented. During decode, the control unit interprets the instruction in CIR. In the execute stage, the ALU may perform an arithmetic or logic operation, or data may be read/written via the MDR.
在取指阶段，MAR ← PC，MDR ← 内存[MAR]，CIR ← MDR 且 PC 递增。解码阶段，控制单元解释 CIR 中的指令。执行阶段，ALU 可能执行算术或逻辑运算，或通过 MDR 读写数据。
Factors affecting CPU performance: Clock speed (Hz), number of cores, and cache size/levels. A higher clock speed means more cycles per second; multiple cores allow parallel execution; larger cache reduces the average memory access time.
影响 CPU 性能的因素：时钟速度 (Hz)、核心数、缓存大小/层级。更高的时钟速度意味着每秒更多周期；多个核心允许并行执行；更大的缓存可减少平均内存访问时间。
2. Memory and Storage | 存储与存储器
RAM is volatile memory that holds currently executing programs and data. SRAM is faster and more expensive, used for cache; DRAM is slower but denser, used for main memory. ROM is non-volatile and stores firmware such as the bootstrap loader.
RAM 是易失性存储器，存放当前运行的程序和数据。SRAM 速度快、成本高，用于缓存；DRAM 较慢但密度大，用于主存。ROM 是非易失性存储器，存储固件，如引导加载程序。
Virtual memory uses a section of the hard disk as an extension of RAM when main memory is full. Pages are swapped between RAM and disk, which can cause disk thrashing if swapping is excessive.
虚拟内存当主存已满时，使用硬盘的一部分作为 RAM 的扩展。页在 RAM 和磁盘之间交换，若交换过度会引起磁盘抖动。
Secondary storage is non-volatile and includes magnetic (HDD), optical (CD/DVD/Blu-ray) and solid-state (SSD, USB flash). SSDs have no moving parts, faster access times and lower power consumption but are more expensive per GB than HDDs. Cloud storage offers remote data access with benefits of scalability and collaboration, but relies on internet connectivity and raises security concerns.
二级存储器是非易失性的，包括磁（HDD）、光（CD/DVD/Blu-ray）和固态（SSD、USB 闪存）。SSD 无移动部件，访问速度快、功耗低，但每 GB 成本比 HDD 高。云存储提供远程数据访问，具有可扩展性和协作优势，但依赖网络连接并引发安全问题。
3. System Software | 系统软件
An operating system (OS) manages hardware resources and provides services such as memory management (paging, segmentation), processor scheduling (round-robin, priority-based), input/output control, and a user interface. Interrupts are signals that cause the CPU to suspend its current task, save its state, and execute an interrupt service routine (ISR).
操作系统 (OS) 管理硬件资源并提供服务，如内存管理（分页、分段）、处理器调度（轮转、基于优先级）、输入/输出控制和用户界面。中断是信号，使 CPU 挂起当前任务、保存状态并执行中断服务程序 (ISR)。
Translators convert source code into machine code. A compiler translates the whole program at once into an executable file; an interpreter translates and executes line-by-line; an assembler converts assembly language into machine code. Compilers produce faster final code; interpreters are useful for debugging.
翻译器将源代码转换成机器码。编译器一次性将整个程序翻译成可执行文件；解释器逐行翻译并执行；汇编器将汇编语言转换为机器码。编译器生成的最终代码运行更快；解释器便于调试。
4. Data Representation | 数据表示
Binary is the base-2 number system used by computers. Hexadecimal (base-16) provides a more compact representation—each hex digit corresponds to a 4-bit nibble. To convert binary to hex, group bits in fours: 1101 1010₂ = DA₁₆.
二进制是计算机使用的基数为 2 的数字系统。十六进制（基数为 16）提供更紧凑的表示——每个十六进制数字对应一个 4 位半字节。将二进制转换为十六进制时，按四位分组：1101 1010₂ = DA₁₆。
Negative integers are often stored using two’s complement. For an n-bit number, the most significant bit represents −2ⁿ⁻¹. Two’s complement makes addition of positive and negative values straightforward.
负整数通常使用补码存储。对于 n 位数字，最高位表示 −2ⁿ⁻¹。补码使正负数的加法运算变得简单。
Floating-point representation uses a mantissa and an exponent: value = mantissa × 2ᵉˣᵖᵒⁿᵉⁿᵗ. The mantissa holds the significant digits; the exponent scales the number, allowing a wide range of values at the cost of precision.
浮点表示使用尾数和指数：值 = 尾数 × 2ᵉˣᵖᵒⁿᵉⁿᵗ。尾数保存有效数字；指数对数值进行缩放，从而在损失精度的代价下扩展表示范围。
Text is encoded using character sets. ASCII uses 7 bits (0–127); extended ASCII uses 8 bits. Unicode (UTF-8, UTF-16) encodes a much wider range of characters, supporting international languages.
文本使用字符集编码。ASCII 使用 7 位（0–127）；扩展 ASCII 使用 8 位。Unicode（UTF-8、UTF-16）编码的字符范围更广，支持国际化语言。
Images can be bitmaps (grid of pixels, each with a colour depth) or vector graphics (mathematical descriptions). Bitmap file size (bits) = width × height × colour depth. Sound is digitised by sampling at a rate; sample rate × bit depth × duration × channels gives the file size. Lossy compression (e.g. JPEG, MP3) discards some data; lossless compression (e.g. PNG, ZIP) preserves all original data.
图像可以是位图（像素网格，每个像素具有颜色深度）或矢量图形（数学描述）。位图文件大小（位）= 宽度 × 高度 × 颜色深度。声音通过采样数字化；采样率 × 位深度 × 时长 × 声道数得出文件大小。有损压缩（如 JPEG、MP3）会舍弃部分数据；无损压缩（如 PNG、ZIP）保留所有原始数据。
5. Networks and the Internet | 网络与互联网
The TCP/IP protocol stack consists of four layers: Application (HTTP, FTP, SMTP), Transport (TCP, UDP), Internet (IP), and Network Interface (Ethernet, Wi-Fi). TCP provides reliable, connection-oriented delivery with error checking and acknowledgements; UDP is connectionless and faster, used for streaming.
TCP/IP 协议栈分为四层：应用层（HTTP、FTP、SMTP）、传输层（TCP、UDP）、互联网层（IP）和网络接口层（以太网、Wi-Fi）。TCP 提供可靠的、面向连接的传输，带有错误校验和确认；UDP 是无连接的，速度更快，用于流媒体。
IP addresses uniquely identify devices on a network; packets are routed using these addresses. DNS (Domain Name System) translates human-readable domain names into IP addresses. Packet switching splits data into packets that travel independently and are reassembled at the destination.
IP 地址唯一标识网络上的设备；数据包使用这些地址进行路由。DNS（域名系统）将人类可读的域名翻译为 IP 地址。分组交换将数据拆分为独立传送的数据包，在目的地重新组装。
Client-server and peer-to-peer are two fundamental network models. In client-server, clients request services from central servers; in P2P, each node can act as both client and server, sharing resources directly.
客户端-服务器和对等网络是两种基本的网络模型。在客户端-服务器模型中，客户端向中央服务器请求服务；在 P2P 中，每个节点可以同时充当客户端和服务器，直接共享资源。
6. Cybersecurity | 网络安全
Malware includes viruses (self-replicating, attach to files), worms (spread independently across networks), Trojans (disguised as legitimate software), spyware (secretly gathers information), and ransomware (encrypts data and demands payment).
恶意软件包括病毒（自我复制并附着于文件）、蠕虫（在网络上独立传播）、特洛伊木马（伪装成合法软件）、间谍软件（秘密收集信息）和勒索软件（加密数据并索要赎金）。
Social engineering tricks users into revealing confidential information, e.g. phishing emails. SQL injection exploits poorly validated input to manipulate database queries. Denial-of-service (DoS) attacks flood a system with traffic to disrupt services.
社会工程学骗术诱使用户泄露机密信息，例如钓鱼邮件。SQL 注入利用未验证的输入操纵数据库查询。拒绝服务 (DoS) 攻击以大量流量淹没系统，从而中断服务。
Defences: Firewalls filter incoming/outgoing traffic based on rules (packet filtering, stateful inspection). Encryption scrambles data—symmetric uses one shared key, asymmetric uses a public/private key pair. Digital signatures and certificates verify identity and data integrity.
防御措施：防火墙根据规则（包过滤、状态检测）过滤进出流量。加密对数据加扰——对称加密使用一个共享密钥，非对称加密使用公钥/私钥对。数字签名和证书用于验证身份和数据完整性。
7. Algorithms and Computational Thinking | 算法与计算思维
Linear search checks each element in turn, O(n) worst-case. Binary search requires a sorted list and repeatedly halves the search interval, O(log₂ n). Bubble sort repeatedly swaps adjacent out-of-order elements, O(n²) in worst case. Merge sort recursively divides and then merges sorted sublists, O(n log₂ n) consistently.
线性搜索依次检查每个元素，最坏情况 O(n)。二分搜索要求列表已排序，反复将搜索区间减半，O(log₂ n)。冒泡排序反复交换相邻的乱序元素，最坏情况 O(n²)。归并排序递归地将子列表分割再合并，时间复杂度稳定 O(n log₂ n)。
Big O notation describes the upper bound of an algorithm’s time or space complexity, ignoring constants and less significant terms. Abstraction removes unnecessary detail; decomposition breaks a problem into manageable parts. Backtracking explores possible solutions and abandons paths that fail, used in maze solving and puzzles.
大 O 记法描述算法时间或空间复杂度的上界，忽略常数和次要项。抽象去除不必要的细节；分解将问题拆分为可管理的部分。回溯法探索可能的解，并在路径失败时放弃，用于迷宫求解和谜题。
8. Object-Oriented Programming | 面向对象编程
A class is a blueprint defining attributes and methods; an object is an instance of a class. Encapsulation bundles data with methods and restricts direct access using private/protected attributes. Inheritance allows a subclass to derive properties and behaviours from a superclass, promoting code reuse.
类是定义属性和方法的蓝图；对象是类的实例。封装将数据与方法捆绑，并通过私有/保护属性限制直接访问。继承允许子类从超类派生属性和行为，促进代码重用。
Polymorphism enables objects of different classes to be treated as objects of a common superclass, with the correct method called at runtime. Association describes a relationship where objects use each other; aggregation is a ‘has-a’ relationship where a container holds references to components that can exist independently.
多态允许将不同类的对象当作共同超类的对象来处理，在运行时调用正确的方法。关联描述对象之间互相使用的关系；聚合是一种“拥有”关系，其中容器持有可独立存在的组件引用。
9. Legal, Moral and Ethical Issues | 法律、道德与伦理问题
The Data Protection Act 2018 (and UK GDPR) governs how personal data must be processed—fairly, lawfully, and for specified purposes. The Computer Misuse Act 1990 criminalises unauthorised access, modification, and the creation of malware. Copyright and patents protect intellectual property, including software code and algorithms.
2018 年数据保护法（及英国 GDPR）规定了必须公平、合法并为特定目的处理个人数据。1990 年计算机滥用法将未经授权的访问、修改和创建恶意软件定为犯罪。版权和专利保护知识产权，包括软件代码和算法。
Ethical considerations include the digital divide, accessibility, environmental impact of e-waste and energy use, and bias in AI systems. Computer scientists must balance innovation with responsibility to society.
伦理考量包括数字鸿沟、可访问性、电子废弃物和能耗的环境影响，以及人工智能系统中的偏见。计算机科学家必须在创新与对社会的责任之间取得平衡。
10. Databases and SQL | 数据库与 SQL
A relational database stores data in tables (relations) with rows (records) and columns (fields). A primary key uniquely identifies each record; a foreign key creates a link between tables referring to a primary key in another table. Referential integrity ensures foreign key values always match an existing primary key.
关系数据库将数据存储在表（关系）中，内含行（记录）和列（字段）。主键唯一标识每条记录；外键通过引用另一表的主键在表之间建立链接。参照完整性确保外键值始终与现有主键匹配。
Entity-relationship diagrams model data using entities, attributes and relationships (one-to-one, one-to-many
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