📚 GCSE CCEA Computer Science: Last-Minute Revision Notes | GCSE CCEA 计算机：考前冲刺笔记
As your CCEA GCSE Computer Science exam approaches, these concise revision notes cover the most critical topics. Use them to boost your confidence and reinforce key concepts during the final stretch.
随着 CCEA GCSE 计算机科学考试临近，这些精炼的冲刺笔记涵盖最核心的主题。用它们在最后关头增强自信，巩固关键概念。
1. Data Representation | 数据表示
All digital data is stored as binary digits (bits). A single bit hold a value of either 0 or 1, and eight bits together make up one byte.
所有数字数据都以二进制位（比特）存储。一个比特保存 0 或 1，八个比特组成一个字节（byte）。
Numbers are represented in binary (base-2), denary (base-10) and hexadecimal (base-16). Hexadecimal uses digits 0–9 and letters A–F, making long binary strings easier to read.
数值使用二进制（基数为 2）、十进制（基数为 10）和十六进制（基数为 16）表示。十六进制采用 0–9 和 A–F，使冗长的二进制串更易读。
To convert from denary to binary, repeatedly divide by 2 and write the remainders in reverse order. For example, 13 in denary is 1101 in binary (13 ÷ 2 = 6 r1, 6 ÷ 2 = 3 r0, 3 ÷ 2 = 1 r1, 1 ÷ 2 = 0 r1).
将十进制转为二进制：反复除以 2，将余数由下往上排列。例如十进制 13 转为二进制 1101（13 ÷ 2 = 6 余1；6 ÷ 2 = 3 余0；3 ÷ 2 = 1 余1；1 ÷ 2 = 0 余1）。
| Unit | Value | Power of 2 |
|---|---|---|
| 1 bit | 0 or 1 | – |
| 1 nibble | 4 bits | – |
| 1 byte | 8 bits | – |
| 1 kibibyte (KiB) | 1,024 bytes | 2¹⁰ |
| 1 mebibyte (MiB) | 1,048,576 bytes | 2²⁰ |
Text is encoded using character sets like ASCII (7-bit) or Unicode. Unicode can represent characters from all major writing systems, including emojis.
文本通过字符集编码，如 ASCII（7位）或 Unicode。Unicode 可表示所有主要书写系统的字符，包括表情符号。
Images are stored as a grid of pixels, each given a binary value representing its colour. Higher colour depth (bits per pixel) gives more colours but larger file sizes.
图像以像素网格存储，每个像素用二进制值表示颜色。色彩位深（每像素位数）越高，颜色越多，但文件体积越大。
Sound is digitised by sampling the analog wave at regular intervals. Sample rate is measured in hertz (Hz) and bit depth defines the accuracy of each sample. Higher values improve quality at the cost of file size.
声音通过定期对模拟波形采样来数字化。采样频率以赫兹（Hz）衡量，采样位数定义每次采样的精度。数值越高，音质越好，文件也越大。
Compression reduces file sizes. Lossless compression (e.g., run-length encoding, zip) preserves all original data; lossy compression (e.g., JPEG, MP3) permanently removes less noticeable detail.
压缩减少文件大小。无损压缩（如行程编码、ZIP）保留所有原始数据；有损压缩（如 JPEG、MP3）永久去除不易察觉的细节。
2. Computer Hardware and the CPU | 计算机硬件与中央处理器
The Central Processing Unit (CPU) executes instructions following the fetch-decode-execute cycle. Its speed is driven by a clock, measured in gigahertz (GHz).
中央处理器（CPU）按照“取指-解码-执行”周期运行指令。其速度由时钟驱动，以吉赫（GHz）为单位。
The CPU contains the Arithmetic Logic Unit (ALU) for calculations and logic, the Control Unit (CU) for coordinating operations, and registers for temporary, ultra-fast storage.
CPU 包含执行运算与逻辑的算术逻辑单元（ALU）、协调操作的控制单元（CU）以及用作超高速暂存空间的寄存器。
Key registers: the Program Counter (PC) holds the address of the next instruction, the Memory Address Register (MAR) stores the address being read/written, and the Memory Data Register (MDR) holds the actual data.
关键寄存器：程序计数器（PC）保存下一条指令的地址，内存地址寄存器（MAR）存放正在读/写的地址，内存数据寄存器（MDR）保存实际数据。
Factors affecting CPU performance include clock speed, number of cores (parallel processing), and cache size. More cache memory reduces the need to fetch data from slower RAM.
影响 CPU 性能的因素包括时钟频率、核心数（并行处理）和缓存大小。更大的缓存可减少对较慢 RAM 的读取次数。
Embedded systems are specialised computers built into larger devices (e.g., microwave ovens, cars). They are optimised for a dedicated function, often with low power consumption.
嵌入式系统是集成在大型设备中的专用计算机（如微波炉、汽车）。它们针对特定功能优化，通常功耗极低。
3. Memory and Storage | 内存与存储器
Primary memory includes RAM (Random Access Memory) and ROM (Read-Only Memory). RAM is volatile and holds the operating system, programs and data in current use. ROM is non-volatile and stores the boot sequence (BIOS).
主存储器包括 RAM（随机存取存储器）和 ROM（只读存储器）。RAM 易失，保存当前运行的操作系统、程序和资料；ROM 非易失，存储启动程序（BIOS）。
Virtual memory uses part of the hard disk as an extension of RAM when physical RAM is full. This lets the computer run larger programs, but disk access is much slower.
虚拟内存将硬盘的一部分用作 RAM 的扩展，当物理 RAM 不足时启用。这允许运行更大的程序，但磁盘存取速度慢得多。
Secondary storage is non-volatile and holds data permanently. Magnetic storage (HDD) uses spinning platters, solid-state storage (SSD, USB flash) uses flash memory with no moving parts, and optical discs (CD, DVD) use lasers.
辅助存储器非易失，永久保存数据。磁性存储（HDD）使用旋转盘片，固态存储（SSD、U盘）使用无活动部件的闪存，光盘（CD、DVD）依赖激光。
SSDs are faster, lighter and more durable than HDDs, but often more expensive per gigabyte. Optical media have low capacity but offer portability for software distribution.
固态硬盘比机械硬盘更快、更轻、更耐用，但每 GB 成本通常更高。光盘容量较小，但在软件分发上具有便携优势。
Storage capacity units based on powers of 2: 1 KiB = 2¹⁰ bytes, 1 MiB = 2²⁰ B, 1 GiB = 2³⁰ B, 1 TiB = 2⁴⁰ B. Manufacturers often use decimal (1 KB = 10³ B) for marketing.
基于 2 的幂的存储容量单位：1 KiB = 2¹⁰ B，1 MiB = 2²⁰ B，1 GiB = 2³⁰ B，1 TiB = 2⁴⁰ B。厂商宣传时常使用十进制（1 KB = 10³ B）。
4. Software: Operating Systems and Utilities | 软件：操作系统与实用工具
The operating system (OS) provides a user interface (GUI or CLI), manages hardware resources (memory, processes, peripherals), handles file management and ensures security through user accounts.
操作系统（OS）提供用户界面（图形界面或命令行）、管理硬件资源（内存、进程、外设）、处里文件管理与通过用户帐户保障安全。
Multitasking allows several programs to run seemingly simultaneously by rapidly switching between processes. The OS allocates CPU time slices to each process.
多任务处理通过快速切换进程，使多个程序看上去同时运行。操作系统为每个进程分配 CPU 时间片。
Utility software performs maintenance tasks: antivirus detects malware, disk defragmentation rearranges files on HDDs to improve speed, and backup utilities create copies of data.
实用工具软件执行维护任务：防病毒检测恶意软件，磁盘碎片整理重新排列 HDD 上的文件以提升速度，备份工具创建数据副本。
Encryption software scrambles data so only authorised users with the correct key can read it. Compression utilities reduce file sizes for storage or transfer.
加密软件对数据加扰，只有持有正确密钥的授权用户才能读取。压缩工具缩减文件体积以便存储或传输。
5. Networks and the Internet | 网络与互联网
A network connects two or more devices to share resources and communicate. Local Area Networks (LANs) cover a small area like a school; Wide Area Networks (WANs) connect LANs over a large geographic area.
网络连接两台或更多设备，实现资源共享与通信。局域网（LAN）覆盖如学校之类的小范围；广域网（WAN）将多个 LAN 连接在广阔地理区域。
Key hardware includes: Network Interface Card (NIC), switch (connects devices within a LAN and directs data only to target), router (forwards data between different networks), and modem (converts digital signals for telephone/cable lines).
重点硬件：网络接口卡（NIC）、交换机（在局域网内连接设备并定向数据至目标）、路由器（在不同网络间转发数据）、调制解调器（将数字信号转换为适合电话/有线线路的形式）。
Transmission media can be wired (Ethernet cables: twisted pair, fibre optic) or wireless (Wi‑Fi, Bluetooth). Fibre optics use light signals for very high speed and low interference.
传输介质可以是有线（以太网线：双绞线、光纤）或无线（Wi‑Fi、蓝牙）。光纤利用光信号，提供极高速度和低干扰。
The Internet is a global WAN. It uses protocols like TCP/IP (Transmission Control Protocol / Internet Protocol). IP routes packets via addresses, while TCP ensures reliable, ordered delivery.
互联网是一个全球广域网。使用 TCP/IP（传输控制协议/网际协议）等协议。IP 根据地址路由数据包，TCP 确保可靠、有序的交付。
A client–server model has central servers providing services (web pages, email) to multiple client devices. A peer‑to‑peer (P2P) network connects devices directly, sharing files without a central server.
客户端-服务器模型由中央服务器向多台客户端提供服物（网页、邮件）。点对点（P2P）网络直接连接设备，无需中心服务器即可共享文件。
6. Network Security | 网络安全
Malware (malicious software) includes viruses (attach to files), worms (self‑replicate across networks), trojans (disguised as legitimate software) and ransomware (encrypts files demanding payment).
恶意软件包括病毒（附着于文件）、蠕虫（在网络中自我复制）、木马（伪装为合法软件）和勒索软件（加密文件索要赎金）。
Social engineering attacks like phishing trick users into revealing passwords or personal data by mimicking trustworthy sources. Shoulder surfing and blagging are other human-based attacks.
社会工程攻击如钓鱼，通过模仿可信来源诱骗用户泄露密码或个人信息。肩膀窥探和冒充诈骗也是基于人性的攻击。
Network protection measures: firewalls filter incoming/outgoing traffic, anti‑malware software detects and removes threats, and encryption secures data during transmission (e.g., HTTPS).
网络防护措施：防火墙过滤出入流量，反恶意软件检测并清除威胁，加密保护传输中的数据（如 HTTPS）。
Authentication methods prove identity. Passwords should be strong (mix of characters). Two‑factor authentication (2FA) adds a second layer, such as a code sent to a mobile phone.
身份验证方法用于证明身份。密码应足够强壮（混合字符）。双因素认证（2FA）增加第二层保护，例如发送至手机的验证码。
7. Database Management | 数据库管理
A database is an organised collection of data. Relational databases use tables (relations) with rows (records) and columns (fields). Each table has a primary key to uniquely identify each record.
数据库是经过组织的数据集合。关系型数据库使用表（关系），包含行（记录）和列（字段）。每个表有主键，唯一标识每条记录。
Foreign keys link tables together, enforcing referential integrity. Queries written in Structured Query Language (SQL) extract specific data using SELECT, FROM, WHERE clauses.
外键将表联接在一起，强制执行参照完整性。使用结构化查询语言（SQL）通过 SELECT、FROM、WHERE 子句提取特定数据。
Data types help ensure consistency: INTEGER, VARCHAR (text), BOOLEAN, DATE, REAL (floating-point). Validation rules restrict input (e.g., a range check on age).
数据类型帮助保持一致性：INTEGER（整数）、VARCHAR（文本）、BOOLEAN（布尔）、DATE（日期）、REAL（浮点数）。验证规则约束输入（如年龄的范围检查）。
A data dictionary stores metadata about the database structure, including field names, data types and validation rules. It acts as a blueprint for developers.
数据字典存储数据库结构的元数据，包括字段名、数据类型和验证规则。它充当开发者的蓝图。
8. Algorithms and Problem Solving | 算法与问题求解
An algorithm is a step‑by‑step procedure to solve a problem. It can be expressed using pseudocode, flowcharts or code. Key constructs: sequence, selection (IF/ELSE), and iteration (loops).
算法是解决问题的分步流程。可用伪代码、流程图或代码表达。核心结构为顺序、选择（IF/ELSE）与迭代（循环）。
A linear search checks each item in a list one by one; simple but slow for large datasets. A binary search requires a sorted list, repeatedly dividing the search interval in half, giving O(log n) efficiency.
线性搜索逐一检查列表中的每一项，简单但大数据集下较慢。二分搜索要求列表已排序，反复将搜索区间折半，效率为 O(log n)。
Bubble sort repeatedly compares adjacent items and swaps them if out of order. It is simple to implement but inefficient for large lists (O(n²)). Merge sort is a divide‑and‑conquer algorithm with O(n log n) performance.
冒泡排序反复比较相邻项，若次序不对则交换。实现简单但对于大数据集效率低（O(n²)）。归并排序是分治算法，性能为 O(n log n)。
Standard algorithm methods: count occurrences, find maximum/minimum, calculate an average. Flowchart symbols include oval (start/stop), rectangle (process), diamond (decision), and parallelogram (input/output).
标准算法任务：统计出现次数、查找最大值/最小值、计算平均值。流程图符号包括椭圆（开始/结束）、矩形（处理）、菱形（判断）和平行四边形（输入/输出）。
9. Programming Concepts | 编程概念
Variables store data that can change during program execution. Constants hold values that never change. The scope of a variable (local/global) determines where it can be accessed.
变量存储程序执行期间可改变的数据。常量保存不会更改的值。变量的作用域（局部/全局）决定可访问它的范围。
Data types include integer, float (real numbers), string, boolean (True/False) and char (single character). Casting converts between types, e.g., int(“5”) → 5.
数据类型包含整型、浮点型（实型）、字符串、布尔型（True/False）和字符型（单字符）。类型转换在类型间转换，例如 int(“5”) → 5。
Conditional statements: IF…ELIF…ELSE allows branching. Loops: FOR (count‑controlled, with a known number of repeats) and WHILE (condition‑controlled, repeats while a condition is true).
条件语句：IF…ELIF…ELSE 实现分支。循环：FOR（计数控制，重复次数已知）和 WHILE（条件控制，条件为真时持续循环）。
An array is a collection of elements of the same data type, accessed via an index. A 2D array is like a grid with rows and columns.
数组是同一数据类型的元素集合，通过索引访问。二维数组类似于带有行与列的网格。
Subroutines (procedures and functions) break programs into reusable blocks. Functions return a value; procedures perform actions without returning a value.
子程序（过程和函数）将程序拆分为可重用模块。函数返回值；过程执行操作，不返回值。
File handling: open a file in read (‘r’), write (‘w’) or append (‘a’) mode. Always close the file after use to avoid data corruption.
文件处理：以读取（’r’）、写入（’w’）或追加（’a’）模式打开文件。使用后务必关闭文件，防止数据损坏。
10. Ethical, Legal, and Environmental Issues | 伦理、法律与环境议题
Legislation: The Data Protection Act 2018 sets rules for collecting and processing personal data. It requires companies to keep data accurate, secure and used only for specified purposes.
立法：《2018 年数据保护法》规定了收集和处理个人数据的规则，要求公司确保数据准确、安全，且仅用于指定目的。
The Computer Misuse Act 1990 criminalises unauthorised access to computer material, hacking, and creating or spreading malware. GDPR strengthens data rights across Europe and beyond.
《1990 年计算机滥用法》将未经授权访问计算机资料、黑客入侵与制播恶意软件定为犯罪。GDPR 加强了欧洲及更广范围的数据权利。
The Copyright, Designs and Patents Act protects intellectual property. Software, music and images cannot be copied or distributed without permission.
《版权、设计与专利法》保护知识产权。软件、音乐和图像未经许可不得复制或分发。
Environmental concerns: manufacturing devices uses finite resources and energy. E‑waste contains toxic materials. Reducing energy consumption, recycling and virtualisation help minimise impact.
环境问题：制造设备消耗有限资源和能源。电子垃圾含有毒物质。降低能耗、回收与虚拟化有助于减小影响。
Ethical dilemmas arise around data collection, surveillance, and algorithmic bias. Computer scientists should follow professional codes of conduct that prioritise privacy, honesty and public good.
伦理困境涉及数据收集、监视与算法偏见。计算机科学家应遵循重视隐私、诚实与公共利益的职业行为准则。
11. Binary and Logical Operations | 二进制与逻辑运算
Binary addition follows simple rules: 0 + 0 = 0, 0 + 1 = 1, 1 + 0 = 1, 1 + 1 = 0 carry 1, and 1 + 1 + 1 (if carry‑in) = 1 carry 1. Resulting overflow occurs when the sum exceeds the available bits.
二进制加法遵循简单规则：0 + 0 = 0，0 + 1 = 1，1 + 0 = 1，1 + 1 = 0 进位1，以及 1+1+1（有进位时）= 1 进位1。当和超出可用位数便发生溢出。
Logical operators: AND (both true → true), OR (at least one true → true), NOT (inverts true/false). These are used in truth tables and database queries.
逻辑运算符：AND（两者为真则为真），OR（至少一个为真则为真），NOT（取反）。这些在真值表和数据库查询中使用。
Logic gates correspond to these operators: AND gate, OR gate, NOT gate. Combining them forms logic circuits. Boolean expressions can be simplified, e.g., A AND (A OR B) = A (absorption law).
逻辑门对应这些运算：与门、或门、非门。组合它们形成逻辑电路。布尔表达式可以化简，例如 A AND (A OR B) = A（吸收律）。
Binary shifts multiply or divide by powers of 2. Left shift << multiplies (e.g., 0011 << 1 → 0110, which is 6 in denary). Right shift >> divides, discarding the least significant bits if doing integer division.
二进制位移以 2 的幂乘除。左移 << 做乘法（如 0011 << 1 → 0110，十进制 6）。右移 >>
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