📚 Computer Science for IB and CIE: Core Concepts | IB 与 CIE 计算机科学核心概念
This revision guide distils the core ideas of the IB Diploma Programme Computer Science course, with a sharp focus on the command terms, algorithms, and system thinking also valued by CIE-style assessments. You will find paired English-Chinese explanations so that both language tracks reinforce the same technical concept.
本复习指南提炼 IB 文凭课程计算机科学的核心思想，同时强调 CIE 风格评估同样看重的指令词、算法和系统思维。每段英文与中文配对，帮助双语学习者巩固同一技术概念。
1. Systems Fundamentals | 系统基础
A computer system is not just hardware. It is a combination of hardware, software, data, people, and processes working together to achieve a defined purpose.
计算机系统不只是硬件。它是由硬件、软件、数据、人员和流程共同作用以实现特定目标的组合。
When planning a new system, analyse inputs, outputs, storage, processing, and feedback. You should also consider environmental, social, and ethical impacts before implementation.
规划新系统时，要分析输入、输出、存储、处理和反馈。实施前还应考虑环境、社会和伦理影响。
Use a table to compare ‘old system’ and ‘new system’ requirements. This helps stakeholders see the need for change and clarifies the scope of the project.
用表格比较 “旧系统” 和 “新系统” 的需求，这有助于利益相关者理解变革的必要性并明确项目范围。
The system life cycle includes analysis, design, development, testing, implementation, and evaluation. Each stage must be documented for accountability and maintenance.
系统生命周期包括分析、设计、开发、测试、实施和评估。每个阶段都必须记录，以便问责和维护。
2. Computer Organization | 计算机组成
The CPU fetches, decodes, and executes instructions in a cycle. Registers such as the program counter (PC), memory address register (MAR), and memory data register (MDR) manage this process.
CPU 按取指、译码、执行的周期工作。程序计数器 PC、存储器地址寄存器 MAR 和存储器数据寄存器 MDR 等寄存器管理这一过程。
Cache memory sits between the CPU and RAM. It is smaller and faster, holding frequently used instructions to reduce the average access time.
高速缓存位于 CPU 和 RAM 之间。它更小、更快，保存频繁使用的指令以降低平均访问时间。
The von Neumann architecture stores both data and instructions in the same memory, unlike Harvard architecture which uses separate stores. Most modern desktop computers follow the von Neumann model.
冯·诺依曼体系结构将数据和指令存储在同一存储器中，而哈佛体系结构使用分开的存储器。大多数现代台式计算机遵循冯·诺依曼模型。
Secondary storage can be magnetic, optical, or solid state. Solid-state drives have no moving parts, so they are faster and more durable but often more expensive per gigabyte.
辅助存储器可分为磁性、光学或固态。固态硬盘没有移动部件，因此速度更快、更耐用，但每 GB 成本通常更高。
3. Data Representation | 数据表示
All data in a computer is stored as binary bits. Integers can be represented in unsigned, sign-magnitude, or two’s complement form. Two’s complement is the most common for signed arithmetic because it creates a single representation for zero.
计算机中的所有数据都以二进制位存储。整数可用无符号、原码或二进制补码表示。补码是带符号运算中最常用的形式，因为它使零只有一种表示。
To convert a negative number to two’s complement, invert all bits and add 1. For an 8-bit value, −3 is represented as below.
将负数转换为补码：将所有位取反再加 1。8 位时，−3 的补码表示如下。
−3 (8-bit two’s complement) = 11111101
Real numbers are represented using IEEE 754 floating-point format with sign, exponent, and mantissa. This can introduce rounding errors, so never compare floats for exact equality.
实数使用 IEEE 754 浮点格式表示，包含符号、指数和尾数。这可能引入舍入误差，因此不要对浮点数做精确相等比较。
| Decimal | Binary (8-bit) | Hexadecimal |
| 0 | 00000000 | 00 |
| 15 | 00001111 | 0F |
| 255 | 11111111 | FF |
Hexadecimal is often used as a shorter, more readable way to display binary values, because each hex digit maps directly to four bits.
十六进制通常被用作一种更短、更易读的二进制表示方式，因为每个十六进制数字直接对应四个二进制位。
4. Networks and Communication | 网络与通信
The OSI model has seven layers: physical, data link, network, transport, session, presentation, and application. The TCP/IP model simplifies this into four practical layers used on the Internet.
OSI 模型有七层：物理层、数据链路层、网络层、传输层、会话层、表示层和应用层。TCP/IP 模型将其简化为互联网使用的四个实用层。
Protocols are agreed rules. HTTP and HTTPS carry web traffic, FTP transfers files, SMTP sends email, and TCP ensures reliable packet delivery through acknowledgements.
协议是约定的规则。HTTP 和 HTTPS 承载网页流量，FTP 传输文件，SMTP 发送邮件，TCP 通过确认机制保证数据包可靠交付。
A router forwards packets between networks based on IP addresses, while a switch forwards frames within a local network based on MAC addresses.
路由器根据 IP 地址在网络间转发数据包，交换机根据 MAC 地址在局域网内转发数据帧。
Wireless networks use standards such as Wi-Fi and Bluetooth. Security concerns include eavesdropping, unauthorised access, and weak encryption, so protocols like WPA3 are recommended.
无线网络使用 Wi-Fi 和蓝牙等标准。安全隐患包括窃听、未授权访问和弱加密，因此建议使用 WPA3 等协议。
5. Computational Thinking | 计算思维
Computational thinking involves decomposition, pattern recognition, abstraction, and algorithm design. These skills help solve problems that are too complex for one-step solutions.
计算思维包括分解、模式识别、抽象和算法设计。这些技能帮助解决过于复杂、无法一步完成的问题。
Abstraction means hiding unnecessary detail. For example, a map of a city ignores buildings but keeps roads and landmarks needed for navigation.
抽象意味着隐藏不必要的细节。例如，城市地图忽略建筑物，但保留导航所需的道路和地标。
In exam questions, identify the input, process, output, and constraints before writing an algorithm. This prevents over-complicated solutions and wasted time.
在考试题中，先识别输入、处理、输出和约束条件，再编写算法。这可以避免解决方案过于复杂并浪费时间。
Decomposition breaks a large system into smaller modules. Each module can be coded, tested, and debugged independently before integration.
分解将大系统拆
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