📚 IB and OCR Computer Science: Last-Minute Revision Notes | IB 及 OCR 计算机考前冲刺笔记
This revision guide covers the core topics examined in both IB Computer Science and OCR A-Level Computer Science. Each section presents key facts concisely, pairing English explanations with Chinese translations to support bilingual learners. Use these notes to consolidate your understanding and boost your exam readiness.
本冲刺笔记涵盖 IB 计算机科学和 OCR A-Level 计算机科学的核心考点。每个小节简明扼要地呈现关键事实，中英双语对照，帮助双语学习者巩固理解、提升考试准备度。
1. Computer System Fundamentals | 计算机系统基础
A computer system integrates hardware (physical components) and software (programs and data) to process input, produce output, and store information.
计算机系统集成了硬件（物理组件）和软件（程序与数据），用以处理输入、产生输出并存储信息。
Hardware includes the central processing unit (CPU), memory, storage devices, input devices (e.g. keyboard, sensor), and output devices (e.g. monitor, speaker).
硬件包括中央处理器（CPU）、内存、存储设备、输入设备（如键盘、传感器）和输出设备（如显示器、扬声器）。
Software is divided into system software (operating systems, utilities, device drivers) and application software (word processors, browsers, games).
软件分为系统软件（操作系统、实用程序、设备驱动程序）和应用软件（文字处理器、浏览器、游戏）。
Embedded systems are specialised computers built into larger devices (e.g. microwaves, cars) with dedicated functions and real‑time constraints.
嵌入式系统是内置于大型设备（如微波炉、汽车）中的专用计算机，具有专属功能和实时约束。
2. Data Representation | 数据表示
Computers store all data as binary (base‑2). Denary (base‑10) and hexadecimal (base‑16) are used for human readability and compact notation.
计算机将所有数据存储为二进制（基数为2）。十进制（基数为10）和十六进制（基数为16）用于人类可读和紧凑表示。
Hexadecimal numbers use digits 0–9 and letters A–F, where each hex digit represents 4 bits (a nibble). Conversion between binary and hex is straightforward by grouping bits.
十六进制数字使用 0–9 和字母 A–F，每个十六进制位表示 4 个二进制位（一个半字节）。通过分组可以轻松进行二进制与十六进制的转换。
Signed integers are often represented in two’s complement. The most significant bit indicates sign (0 positive, 1 negative), and negation is performed by inverting all bits and adding 1.
有符号整数通常用二进制补码表示。最高有效位表示符号（0 正，1 负），取反可通过反转所有位再加 1 实现。
Characters are encoded using standards such as ASCII (7‑bit, 128 characters) and Unicode (variable length, covering global scripts and emojis). Unicode’s UTF‑8 is backward‑compatible with ASCII.
字符使用 ASCII（7 位，128 个字符）和 Unicode（变长，涵盖全球文字和表情）等标准进行编码。Unicode 的 UTF‑8 向后兼容 ASCII。
Images are represented as a grid of pixels; colour depth determines the number of bits per pixel. Higher resolution and colour depth increase file size. Sound is digitised by sampling the analogue signal at a given sample rate and bit depth.
图像表示为像素网格；颜色深度决定每个像素的位数。分辨率和颜色深度越高，文件越大。声音通过对模拟信号以给定采样率和位深进行采样来数字化。
Lossy compression (e.g. JPEG, MP3) discards some data to reduce file size, whereas lossless compression (e.g. PNG, ZIP) preserves all original data and uses redundancy removal.
有损压缩（如 JPEG、MP3）丢弃部分数据以减小文件大小，而无损压缩（如 PNG、ZIP）保留所有原始数据并利用冗余消除。
3. Computer Architecture | 计算机体系结构
The Von Neumann architecture stores both instructions and data in the same memory. The CPU contains the Control Unit (CU), Arithmetic Logic Unit (ALU), and registers.
冯·诺依曼架构将指令和数据存储在同一内存中。CPU 包含控制单元（CU）、算术逻辑单元（ALU）和寄存器。
Key registers include the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR), Current Instruction Register (CIR), and Accumulator (ACC).
关键寄存器包括程序计数器（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）和累加器（ACC）。
Buses transmit information: address bus carries memory addresses (unidirectional), data bus carries actual data (bidirectional), and control bus sends control signals.
总线传输信息：地址总线传送内存地址（单向），数据总线传送实际数据（双向），控制总线发送控制信号。
The fetch‑decode‑execute cycle repeats: fetch instruction from memory (address in PC), decode in CU, and execute (ALU or load/store). PC is incremented after fetch.
取指‑译码‑执行周期不断重复：从内存取指令（地址在 PC 中），由 CU 译码，并执行（ALU 或加载/存储）。取指后 PC 递增。
Processor performance is influenced by clock speed, number of cores, and cache size. Pipelining improves throughput by overlapping instruction stages.
处理器性能受时钟速度、核心数量和缓存大小影响。流水线通过重叠指令阶段来提高吞吐量。
4. Operating Systems & Resource Management | 操作系统与资源管理
An operating system (OS) manages hardware resources, provides a user interface, and enables application execution. Core functions include memory management, process scheduling, file management, and input/output control.
操作系统（OS）管理硬件资源、提供用户界面并支持应用程序执行。核心功能包括内存管理、进程调度、文件管理和输入/输出控制。
Virtual memory uses disk space as an extension of RAM, allowing larger programs to run by swapping pages between RAM and disk (paging). However, excessive swapping causes disk thrashing.
虚拟内存使用磁盘空间作为 RAM 的扩展，通过在 RAM 和磁盘之间交换页面（分页）来运行更大的程序。但过度交换会导致磁盘抖动。
Process scheduling strategies (e.g. round‑robin, priority‑based, shortest job first) aim to maximise CPU utilisation and fairness. Multi‑tasking is supported by context switching.
进程调度策略（如轮转、优先级、短作业优先）旨在最大化 CPU 利用率和公平性。多任务通过上下文切换实现。
File systems organise data on storage using directories, file allocation tables (FAT) or inode structures. Access rights and file permissions protect data.
文件系统使用目录、文件分配表（FAT）或 inode 结构组织存储上的数据。访问权限和文件权限保护数据。
5. Networks & Data Transmission | 网络与数据传输
LANs cover a small geographical area and often use Ethernet or Wi‑Fi; WANs span large distances and rely on routers and public infrastructure.
局域网覆盖较小地理区域，常使用以太网或 Wi‑Fi；广域网跨越长距离，依赖路由器和公共基础设施。
Network topologies: star (central switch/hub, easy fault isolation), mesh (fully/partially connected, high redundancy), bus (single backbone, low cost).
网络拓扑：星型（中央交换机/集线器，易于隔离故障）、网状（全/部分连接，高冗余）、总线（单一骨干，成本低）。
The TCP/IP model layers: Application (HTTP, FTP, SMTP), Transport (TCP, UDP), Internet (IP), and Network Access (Ethernet, Wi‑Fi). TCP provides reliable, connection‑oriented delivery; UDP is connectionless and faster.
TCP/IP 模型层次：应用层（HTTP、FTP、SMTP）、传输层（TCP、UDP）、互联网层（IP）和网络接入层（以太网、Wi‑Fi）。TCP 提供可靠、面向连接的传输；UDP 无连接、速度更快。
Packet switching breaks data into packets, each routed independently, leading to efficient use of bandwidth. Routers forward packets based on destination IP addresses.
分组交换将数据分解为数据包，每个独立路由，从而高效利用带宽。路由器根据目标 IP 地址转发数据包。
Network security measures include firewalls (filter traffic), encryption (e.g. SSL/TLS), MAC address filtering, and virtual private networks (VPNs).
网络安全措施包括防火墙（过滤流量）、加密（如 SSL/TLS）、MAC 地址过滤和虚拟专用网络（VPN）。
6. Computational Thinking & Problem Solving | 计算思维与问题解决
Computational thinking involves decomposition (breaking problems into smaller parts), pattern recognition, abstraction (removing unnecessary detail), and algorithmic design.
计算思维包括分解（将问题拆分为更小部分）、模式识别、抽象（移除无关细节）和算法设计。
Trace tables manually step through algorithms to track variable values and identify logic errors. Dry‑running code helps verify correctness without execution.
跟踪表手动逐步执行算法，追踪变量值并识别逻辑错误。纸上运行代码有助于在不执行的情况下验证正确性。
Debugging techniques include adding print statements, using breakpoints, and step‑by‑step execution. Understanding syntax, runtime, and logic errors is essential.
调试技术包括添加打印语句、使用断点和逐步执行。理解语法错误、运行时错误和逻辑错误至关重要。
In problem solving, developers produce IPO (Input‑Process‑Output) charts, flowcharts, or pseudocode to plan solutions before coding.
在问题解决中，开发人员制作 IPO（输入‑处理‑输出）图表、流程图或伪代码，以便在编码前规划解决方案。
7. Algorithms & Efficiency | 算法与效率
Linear search examines each element sequentially; binary search requires sorted data and repeatedly halves the search interval. Binary search is more efficient for large datasets.
线性搜索逐个检查元素；二分搜索需要有序数据并反复将搜索区间减半。二分搜索在大数据集上效率更高。
Sorting algorithms: bubble sort repeatedly swaps adjacent out‑of‑order elements; insertion sort builds the sorted portion incrementally; merge sort uses divide‑and‑conquer with O(n log n) time.
排序算法：冒泡排序反复交换相邻乱序元素；插入排序逐步构建已排序部分；归并排序采用分治法，时间复杂度为 O(n log n)。
Common complexities: O(1) constant, O(log n) logarithmic, O(n) linear, O(n log n) log‑linear, O(n²) quadratic, O(2ⁿ) exponential.
常见复杂度：O(1) 常数级、O(log n) 对数级、O(n) 线性级、O(n log n) 对数线性级、O(n²) 平方级、O(2ⁿ) 指数级。
Dijkstra's algorithm finds the shortest path from a start node to all others in a weighted graph using priority queues and greedy selection.
Dijkstra 算法利用优先队列和贪心选择，找出加权图中从起始节点到所有其他节点的最短路径。
Recursion occurs when a function calls itself with a base case to terminate. Recursive solutions often mirror mathematical induction but may cause stack overflow if not limited.
递归指函数调用自身并通过基准情形终止。递归解法常反映数学归纳思想，但若无限制可能导致栈溢出。
8. Programming Concepts | 编程概念
Variables store data of specific types (integer, real/float, Boolean, character, string). Constants hold values that cannot be changed during execution.
变量存储特定类型的数据（整型、浮点型、布尔型、字符型、字符串）。常量保存执行期间不可更改的值。
Control structures: sequence (step‑by‑step), selection (if‑else, switch/case), and iteration (for, while, do‑while loops). These form the basis of structured programming.
控制结构：顺序（逐步执行）、选择（if‑else、switch/case）和迭代（for、while、do‑while 循环）。这些构成结构化编程的基础。
Subroutines include functions (return a value) and procedures (do not return a value). Parameters can be passed by value (copy) or by reference (original memory address).
子程序包括函数（返回值）和过程（无返回值）。参数可按值传递（副本）或按引用传递（原始内存地址）。
Scope determines where a variable is accessible: local variables exist within a subroutine; global variables exist throughout the program.
作用域决定变量可访问的位置：局部变量存在于子程序内；全局变量存在于整个程序中。
File handling operations include opening, reading, writing, and closing files. Exception handling (try‑catch) manages runtime errors gracefully.
文件处理操作包括打开、读取、写入和关闭文件。异常处理（try‑catch）可优雅地管理运行时错误。
9. Abstract Data Structures | 抽象数据结构
A stack is a Last‑In‑First‑Out (LIFO) structure with push (add) and pop (remove). It is used in recursion, undo mechanisms, and expression evaluation.
栈是一种后进先出（LIFO）结构，有压入（push）和弹出（pop）操作。用于递归、撤销机制和表达式求值。
A queue is a First‑In‑First‑Out (FIFO) structure with enqueue (add to rear) and dequeue (remove from front). Priority queues retrieve elements based on priority.
队列是一种先进先出（FIFO）结构，有入队（添加到后端）和出队（从前端移除）。优先队列根据优先级检索元素。
Linked lists consist of nodes containing data and a pointer to the next node. They allow efficient insertion/deletion but no random access without traversing.
链表由包含数据和指向下一个节点的指针的节点组成。它们支持高效插入/删除，但需遍历才能实现随机访问。
Binary trees have at most two children per node. A binary search tree (BST) keeps left subtree values < node < right subtree values, enabling O(log n) search on average.
二叉树每个节点最多有两个子节点。二叉搜索树（BST）保持左子树值 < 节点 < 右子树值，平均可实现 O(log n) 的搜索效率。
Graphs model relationships with vertices (nodes) and edges. They can be represented by adjacency matrices or adjacency lists. Traversal algorithms include depth‑first and breadth‑first search.
图使用顶点（节点）和边来建模关系。图可以用邻接矩阵或邻接表表示。遍历算法包括深度优先搜索和广度优先搜索。
10. Databases & SQL | 数据库与 SQL
A relational database stores data in tables (relations). Each table has rows (records/tuples) and columns (attributes). Relationships are established via keys.
关系数据库以表（关系）存储数据。每个表有行（记录/元组）和列（属性）。通过键建立关系。
Primary keys uniquely identify each record; foreign keys reference primary keys in another table to enforce referential integrity.
主键唯一标识每条记录；外键引用另一表的主键以强制参照完整性。
SQL (Structured Query Language) is used to query and manipulate data. Common clauses: SELECT ... FROM ... WHERE ... ORDER BY ... GROUP BY ... HAVING ...
SQL（结构化查询语言）用于查询和操作数据。常用子句：SELECT ... FROM ... WHERE ... ORDER BY ... GROUP BY ... HAVING ...
JOIN operations combine rows from two or more tables based on a related column: INNER JOIN returns matching rows; LEFT JOIN returns all left rows plus matches.
JOIN 操作根据相关列合并两个或多个表的行：INNER JOIN 返回匹配行；LEFT JOIN 返回左表所有行及匹配行。
Normalisation reduces data redundancy. 1NF eliminates repeating groups; 2NF removes partial dependencies; 3NF removes transitive dependencies.
规范化减少数据冗余。第一范式消除重复组；第二范式消除部分依赖；第三范式消除传递依赖。
Data definition commands: CREATE TABLE, ALTER TABLE, DROP TABLE. Data manipulation commands: INSERT INTO, UPDATE ... SET, DELETE FROM.
数据定义命令：CREATE TABLE、ALTER TABLE、DROP TABLE。数据操作命令：INSERT INTO、UPDATE ... SET、DELETE FROM。
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