📚 GCSE OCR Computer Science: Last-Minute Revision Notes | GCSE OCR 计算机科学：考前冲刺笔记
Welcome to your last-minute revision guide for GCSE OCR Computer Science. This article condenses the entire specification into bite-sized, bilingual notes so you can quickly review key concepts. Whether you’re preparing for Paper 1 (Computer Systems) or Paper 2 (Computational Thinking, Algorithms and Programming), these notes will help reinforce your knowledge right before the exam.
欢迎阅读 GCSE OCR 计算机科学考前冲刺指南。本文浓缩了整个考纲内容，以简练的双语笔记形式，助你快速回顾关键概念。无论你是在准备 Paper 1（计算机系统）还是 Paper 2（计算思维、算法与编程），这些笔记都能帮你考前巩固知识。
1. Systems Architecture | 系统架构
The CPU is the central processing unit of a computer. It follows the Von Neumann architecture, where both data and instructions are stored in the same memory. Key components include the ALU (Arithmetic Logic Unit) for calculations, the CU (Control Unit) for directing operations, and registers such as the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR) and Accumulator.
CPU 是计算机的中央处理器。它遵循冯·诺依曼架构，数据和指令存储在同一个内存中。关键部件包括用于计算的 ALU（算术逻辑单元）、用于指挥操作的 CU（控制单元）以及寄存器，如程序计数器 (PC)、内存地址寄存器 (MAR)、内存数据寄存器 (MDR) 和累加器。
The fetch-decode-execute cycle describes how the CPU processes instructions. Fetch: the instruction at the address held in the PC is copied into the MDR, then the PC is incremented. Decode: the CU interprets the instruction. Execute: the ALU performs the required operation, and the result is stored in the Accumulator or memory. This cycle repeats billions of times per second.
取指-解码-执行周期描述了 CPU 如何处理指令。取指：PC 中地址处的指令被复制到 MDR，然后 PC 递增。解码：CU 解释指令。执行：ALU 执行所需操作，结果存入累加器或内存。这个循环每秒重复数十亿次。
CPU performance is affected by clock speed (GHz), number of cores and cache size. Higher clock speed means more cycles per second. Multiple cores allow parallel processing. Larger cache provides faster access to frequently used data.
CPU 性能受时钟频率（GHz）、核心数量和缓存大小的影响。时钟频率越高，每秒周期数越多。多核允许并行处理。更大的缓存可以更快地访问常用数据。
2. Memory and Storage | 内存与存储
Primary memory consists of RAM (volatile, holds currently running programs and data) and ROM (non-volatile, stores boot instructions such as the BIOS). Virtual memory uses part of the hard disk as temporary RAM when physical RAM is full, but it is slower.
主存储器包括 RAM（易失性，存放当前运行的程序和数据）和 ROM（非易失性，存储启动指令如 BIOS）。当物理 RAM 已满时，虚拟内存会使用硬盘的一部分作为临时 RAM，但速度较慢。
Secondary storage is non-volatile and includes magnetic (HDD), optical (CD/DVD) and solid-state (SSD) devices. SSDs are faster and more durable but more expensive per GB. HDDs are cheaper for high capacity but are slower and more fragile. Optical media have low capacity but are portable.
辅助存储器是非易失性的，包括磁性存储器 (HDD)、光学存储器 (CD/DVD) 和固态存储器 (SSD)。SSD 速度更快、更耐用，但每 GB 成本更高。HDD 容量大且便宜，但速度较慢且更易损坏。光学介质容量小，但便于携带。
| Unit | Equal to |
|---|---|
| 1 bit | 0 or 1 |
| 1 nibble | 4 bits |
| 1 byte | 8 bits |
| 1 kilobyte (KB) | 1000 bytes |
| 1 megabyte (MB) | 1000 KB |
| 1 gigabyte (GB) | 1000 MB |
| 1 terabyte (TB) | 1000 GB |
| 1 petabyte (PB) | 1000 TB |
You must be able to convert between bit, byte, kilobyte, megabyte, gigabyte and terabyte. Note that exam boards sometimes use the 1000 definition.
你必须能够在比特、字节、千字节、兆字节、吉字节和太字节之间进行转换。请注意，考试局有时使用 1000 的进制定义。
3. Data Representation and Storage | 数据表示与存储
Computers use binary (base 2). Denary (base 10) numbers can be converted to binary by successive division by 2. Hexadecimal (base 16) uses digits 0-9 and letters A-F to represent 4-bit nibbles. For example, 1101₂ = D₁₆ and 1010 1111₂ = AF₁₆.
计算机使用二进制（基数为 2）。十进制（基数为 10）的数可以通过连续除以 2 转换为二进制。十六进制（基数为 16）使用数字 0-9 和字母 A-F 来表示 4 位半字节。例如，1101₂ = D₁₆，1010 1111₂ = AF₁₆。
Characters are represented using character sets. ASCII uses 7 bits to represent 128 characters. Unicode extends this to cover thousands of characters from different languages and emojis, using up to 32 bits per character. Unicode requires more storage but enables global communication.
字符使用字符集表示。ASCII 使用 7 位来表示 128 个字符。Unicode 扩展了这一范围，可涵盖数千种不同语言的字符和表情符号，每个字符最多使用 32 位。Unicode 需要更多存储空间，但能实现全球通用。
Images are represented as bitmaps made of pixels. Each pixel is assigned a binary code according to its colour depth (bits per pixel). Higher resolution (more pixels) and greater colour depth improve quality but increase file size. Metadata stores properties like width, height and colour depth.
图像以位图形式表示，由像素组成。每个像素根据其颜色深度（每像素位数）分配一个二进制代码。分辨率越高（像素越多）且颜色深度越大，图像质量越好，但文件尺寸也越大。元数据存储宽度、高度和颜色深度等属性。
Sound is digitised by sampling the analogue wave at regular intervals. Sampling rate (Hz) determines how many samples per second; bit depth determines the number of bits per sample. Higher sample rate and bit depth give better quality but larger files.
声音通过定期采样模拟波形进行数字化。采样率 (Hz) 决定每秒的采样数量；位深度决定每个采样的位数。采样率和位深度越高，音质越好，但文件越大。
Compression reduces file size. Lossless compression (e.g. run-length encoding, Huffman coding) preserves all original data and allows exact reconstruction. Lossy compression (e.g. JPEG, MP3) permanently removes less perceptible data to achieve smaller files, but some quality is lost.
压缩能减小文件大小。无损压缩（如游程编码、霍夫曼编码）保留所有原始数据，可完全重建。有损压缩（如 JPEG、MP3）永久去除不易察觉的数据以实现更小文件，但质量会有所损失。
4. Computer Networks and Topologies | 计算机网络与拓扑
A LAN (Local Area Network) covers a small geographical area and usually has its own infrastructure. A WAN (Wide Area Network) connects networks across larger distances using external infrastructure, such as the internet. Factors affecting network performance include bandwidth, number of users, transmission media (wired vs. wireless) and error rate.
LAN（局域网）覆盖较小的地理区域，通常拥有自己的基础设施。WAN（广域网）使用外部基础设施（如互联网）连接跨区域的网络。影响网络性能的因素包括带宽、用户数量、传输介质（有线与无线）和错误率。
Network hardware includes NIC (Network Interface Card), which connects a device to a network; a switch, which forwards data to the correct device using MAC addresses; and a router, which directs data between networks using IP addresses. Wireless access points enable Wi-Fi connectivity.
网络硬件包括 NIC（网络接口卡），用于将设备连接至网络；交换机，利用 MAC 地址将数据转发到正确设备；路由器，利用 IP 地址在网络之间引导数据。无线接入点则提供 Wi-Fi 连接。
Star topology has all devices connected to a central switch or hub. If one cable fails, only that device is affected, but if the central node fails, the whole network goes down. Mesh topology connects every device to several others, providing excellent redundancy but requiring more cabling and complexity.
星型拓扑将所有设备连接到中央交换机或集线器。若一条电缆出现故障，只有该设备受影响；但若中央节点故障，整个网络瘫痪。网状拓扑使每台设备与多台其他设备相连，提供了出色的冗余能力，但需要更多布线和更高的复杂性。
5. Network Protocols and Layers | 网络协议与分层
Protocols are rules for communication. Key protocols include TCP/IP (Transmission Control Protocol/Internet Protocol) for reliable data transmission, HTTP/HTTPS for web pages, FTP for file transfer, SMTP for sending emails, and POP3/IMAP for receiving emails. HTTPS encrypts data using SSL/TLS.
协议是通信规则。关键协议包括用于可靠数据传输的 TCP/IP（传输控制协议/互联网协议）、网页浏览的 HTTP/HTTPS、文件传输的 FTP、发送电子邮件的 SMTP 以及接收邮件的 POP3/IMAP。HTTPS 使用 SSL/TLS 对数据加密。
The TCP/IP stack is a layered model: Application layer (provides network services to applications, e.g. HTTP, SMTP), Transport layer (splits data into packets and manages reliable delivery, e.g. TCP), Network/Internet layer (addresses and routes packets, e.g. IP), and Link layer (physical connection and MAC addressing). Each layer only communicates with the layer above or below.
TCP/IP 栈是一个分层模型：应用层（为应用程序提供网络服务，如 HTTP、SMTP）、传输层（将数据拆分为数据包并管理可靠传输，如 TCP）、网络/互联网层（寻址和路由数据包，如 IP），以及链路层（物理连接与 MAC 寻址）。每一层只与相邻的上下层通信。
6. Network Security Threats and Prevention | 网络安全威胁与防护
Threats include malware (viruses, worms, trojans, ransomware) which can damage or steal data; phishing (fraudulent emails or websites to gain personal information); social engineering (tricking people into revealing passwords); brute-force attacks (repeatedly trying passwords); and Denial of Service (DoS) attacks that overwhelm a server with traffic.
威胁包括恶意软件（病毒、蠕虫、特洛伊木马、勒索软件），可损坏或窃取数据；网络钓鱼（欺诈性的电子邮件或网站以获取个人信息）；社会工程（诱骗人们泄露密码）；暴力破解攻击（反复尝试密码）；以及拒绝服务 (DoS) 攻击，用流量淹没服务器。
Prevention methods: Firewalls monitor and filter incoming/outgoing traffic; anti-malware software detects and removes malicious programs; encryption scrambles data so only authorised parties can read it; strong password policies enforce complexity; user access levels restrict permissions; penetration testing identifies vulnerabilities.
防护方法：防火墙监控和过滤进出流量；反恶意软件检测并清除恶意程序；加密扰乱数据，使只有授权方才能读取；强密码策略强制复杂性；用户访问级别限制权限；渗透测试识别漏洞。
7. Systems Software and Utilities | 系统软件与实用工具
The operating system (OS) provides a user interface, manages memory and multitasking, handles peripheral devices through drivers, manages files, and enforces security. It acts as an intermediary between hardware and application software.
操作系统 (OS) 提供用户界面、管理内存和多任务、通过驱动程序管理外设、管理文件，并实施安全策略。它充当硬件和应用软件之间的中介。
Utility software performs maintenance tasks. Disk defragmentation reorganises fragmented files on HDD to speed up access (not needed for SSDs). Backup utilities create copies of data. Encryption software secures files. Compression software reduces file size. Disk cleanup removes temporary files.
实用工具软件执行维护任务。磁盘碎片整理重新组织 HDD 上的碎片文件，加快访问速度（SSD 不需要）。备份工具创建数据副本。加密软件保护文件安全。压缩软件减小文件尺寸。磁盘清理可移除临时文件。
8. Ethical, Legal, Cultural and Environmental Issues | 伦理、法律、文化与环境问题
Key legislation: the Data Protection Act 2018 (controls how personal data is collected, stored and used), the Computer Misuse Act 1990 (makes unauthorised access, modification and creation of malware illegal), and Copyright, Designs and Patents Act (protects intellectual property). Open source software allows users to view and modify source code; proprietary software restricts this.
关键法规：《2018 年数据保护法》（管控个人数据的收集、存储和使用）、《1990 年计算机滥用法》（将未经授权访问、修改及制造恶意软件定为非法）以及《版权、设计与专利法》（保护知识产权）。开源软件允许用户查看和修改源代码；专有软件则限制这些行为。
Cultural divide: the digital divide is the gap between those who have access to technology and the internet and those who do not, often due to economic or geographical factors. Technology also raises ethical issues around surveillance, privacy and automation replacing jobs.
文化鸿沟：数字鸿沟是指能够访问技术和互联网的人与无法访问的人之间的差距，常因经济或地理因素造成。技术还引发了监控、隐私以及自动化取代工作等伦理问题。
Environmental impact: manufacturing and disposing of devices generates e-waste and uses energy. Data centres consume huge amounts of electricity. Recycling and energy-efficient designs help reduce the carbon footprint.
环境影响：制造和处置设备会产生电子垃圾并消耗能源。数据中心耗电量巨大。回收利用和节能设计有助于减少碳足迹。
9. Computational Thinking, Searching and Sorting | 计算思维、搜索与排序
Computational thinking involves decomposition (breaking a problem into smaller parts), pattern recognition, abstraction (focusing on important details and ignoring irrelevant ones), and algorithm design (creating step-by-step solutions). These skills are applied to write efficient programs.
计算思维包括分解（将问题拆解为更小的部分）、模式识别、抽象（关注重要细节，忽略无关信息）以及算法设计（创建逐步解决方案）。这些技能用来编写高效的程序。
Searching algorithms: Linear search goes through each element sequentially; O(n) time. Binary search works on sorted lists by repeatedly dividing the search interval in half; O(log n) but requires a sorted list. Be able to trace both with pseudocode and trace tables.
搜索算法：线性搜索逐个顺序查看元素，时间复杂度 O(n)。二分搜索在有序列表中重复将搜索区间减半，时间复杂度 O(log n)，但要求列表已排序。你需要能用伪代码和跟踪表追溯这两个算法。
Sorting algorithms: Bubble sort repeatedly steps through the list, swapping adjacent items if they are in the wrong order; simple but O(n²). Merge sort uses divide-and-conquer: splits list into halves, recursively sorts and then merges; O(n log n). Insertion sort builds a sorted portion by taking each item and placing it in the correct position; O(n²) but efficient for small or nearly sorted data.
排序算法：冒泡排序反复遍历列表，若相邻项顺序错误则交换；简单但复杂度 O(n²)。归并排序使用分治法：将列表分成两半，递归排序后合并；复杂度 O(n log n)。插入排序通过取出每个项目并将其插入已排序部分的正确位置来构建有序序列；复杂度 O(n²)，但对于小规模或接近有序的数据很高效。
10. Programming Fundamentals | 编程基础
Data types include integer, real (float), Boolean, character and string. Constants are fixed values, variables can change. Use meaningful identifiers. Assign values with = (in pseudocode). Input: statement like input name. Output: print name.
数据类型包括整数、实数（浮点数）、布尔型、字符和字符串。常量是固定值，变量可以改变。使用有意义的标识符。用 = 赋值（在伪代码中）。输入：如 input name。输出：print name。
Arithmetic operators: +, -, *, /, MOD (remainder), DIV (integer division). Comparison operators: ==, !=, <, >, <=, >=. Logical operators: AND, OR, NOT. String concatenation uses +.
算术运算符：+、-、*、/、MOD（取余）、DIV（整数除法）。比较运算符：==、!=、<、>、<=、>=。逻辑运算符：AND、OR、NOT。字符串拼接使用 +。
Selection: if...else...elseif and switch case statements allow choice. Iteration: for loops (count-controlled), while loops (condition-controlled, may never execute), do...while loops (executes at least once). Arrays store multiple values of the same type; one-dimensional (list) and two-dimensional (table). Subroutines: procedures (perform actions, no return value) and functions (return a value).
选择结构：if...else...elseif 和 switch case 语句实现分支。迭代：for 循环（计数控制）、while 循环（条件控制，可能从不执行）、do...while 循环（至少执行一次）。数组存储多个
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