📚 Year 11 OCR Computer Science: Core Concepts Revision | Year 11 OCR 计算机：核心知识点梳理
This article reviews the core content of Year 11 OCR Computer Science (specification J277), covering systems architecture, memory and storage, data representation, networks, security, systems software, ethical and legal issues, algorithms, programming, robust program design, Boolean logic, and programming languages. Each section highlights the definitions, comparisons and calculations that examiners reward most often, so that you can revise efficiently before mocks and final assessments.
本文系统梳理 Year 11 OCR 计算机科学（考纲 J277）的核心内容，涵盖系统架构、内存与存储、数据表示、网络、网络安全、系统软件、伦理与法律问题、算法、编程、健壮程序设计、布尔逻辑以及编程语言。每一节都突出考官最常给分的定义、对比与计算，帮助你在模拟考试与最终评估前高效复习。
1. Systems Architecture | 系统架构
memory (RAM), and the CPU repeatedly moves them in and out. The three internal parts you must be able to describe are the Control Unit (CU), which directs the flow of data and coordinates the other components; the Arithmetic Logic Unit (ALU), which carries out arithmetic (+, −, ×, ÷) and logic (AND, OR, NOT, comparisons) operations; and cache, a small amount of very fast memory that holds frequently used instructions close to the CPU.
CPU 是负责取指、译码和执行指令的部件。在冯·诺依曼架构中，数据与指令共同存放在主存（RAM）中，CPU 不断在两者之间搬运数据。你必须能描述的三个内部部件是：控制单元（CU），负责指挥数据流向并协调其他部件；算术逻辑单元（ALU），执行算术运算（+、−、×、÷）与逻辑运算（AND、OR、NOT、比较）；以及缓存，容量很小但速度极快，用于存放 CPU 经常使用的指令。
| Register | 寄存器 | Role | 作用 |
| PC (Program Counter) | Holds the address of the next instruction to be fetched | 保存下一条待取指令的地址 |
| MAR (Memory Address Register) | Holds the address of the data or instruction being read or written | 保存当前读取或写入的数据／指令地址 |
| MDR (Memory Data Register) | Holds the actual data or instruction transferred to or from memory | 保存与内存之间传输的实际数据或指令 |
| ACC (Accumulator) | Stores the result of calculations performed by the ALU | 存放 ALU 计算得出的结果 |
The fetch-decode-execute cycle follows a fixed order: the address in the PC is copied to the MAR; the instruction at that address is fetched into the MDR and then sent to the CU; the PC is incremented so it points to the next instruction; the CU decodes the instruction and executes it, writing results back to registers or memory. CPU performance depends on clock speed (cycles per second, measured in GHz), the number of cores (more cores allow parallel processing) and cache size. Embedded systems, such as those in washing machines, traffic lights and digital watches, are computers built into a larger device to perform a dedicated function.
取指—译码—执行周期遵循固定顺序：PC 中的地址被复制到 MAR；该地址处的指令被取入 MDR，再送往 CU；PC 自增以指向下一条指令；CU 对指令译码并执行，将结果写回寄存器或内存。CPU 的性能取决于时钟频率（每秒周期数，单位为 GHz）、核心数量（核心越多越能并行处理）和缓存容量。嵌入式系统，如洗衣机、交通信号灯和电子表中的计算机，是嵌入更大设备中、执行专门功能的计算机。
2. Memory and Storage | 内存与存储
RAM is volatile main memory that stores the operating system, programs in use and open data; its contents are lost when power is removed. ROM is non-volatile and read-only, and holds the bootstrap program (BIOS) needed to start the computer and load the operating system. Virtual memory uses part of secondary storage as an extension of RAM when RAM is full, and is divided into fixed-size pages; excessive paging causes disk thrashing, which slows the machine dramatically.
RAM 是易失性主存，存放操作系统、正在运行的程序和打开的数据，断电后内容丢失。ROM 是非易失性、只读的，存放启动计算机并加载操作系统所需的引导程序（BIOS）。虚拟内存是在 RAM 占满时，用部分辅助存储充当 RAM 的扩展，并被划分为固定大小的页；频繁换页会导致磁盘抖动，使机器明显变慢。
Secondary storage must be compared by capacity, speed, durability, portability, cost and reliability. Magnetic storage (HDD) is cheap and high capacity but slow and easily damaged by shock; optical storage (CD, DVD, Blu-ray) is portable and inexpensive but limited in capacity and easily scratched; solid state storage (SSD, USB flash) is fast, silent, shock-resistant and uses less power, but is more expensive per gigabyte and has a limited number of write cycles.
辅助存储需要按容量、速度、耐用性、便携性、成本和可靠性进行比较。磁存储（机械硬盘）便宜、容量大，但速度慢且怕震动；光存储（CD、DVD、蓝光）便携且便宜，但容量有限、易划伤；固态存储（SSD、U 盘）速度快、无噪音、抗震动、功耗低，但单位容量价格更高，且写入次数有限。
3. Data Representation | 数据表示
mple, 13 in denary is 0000 1101 in binary and 0D in hexadecimal; 0110 1011 + 0001 1101 = 1000 1000 carries no overflow, while adding two large 8-bit values such as 1111 1111 + 0000 0001 produces an overflow because the result no longer fits in 8 bits. A left shift by one place doubles a binary value; a right shift halves it. Character sets matter too: ASCII uses 7 bits and represents 128 characters, whereas Unicode uses more bits and supports the characters of most languages plus emoji.
所有数据都以二进制存储。你必须熟练掌握十进制、二进制与十六进制之间的转换，以及两个 8 位二进制数的加法。例如十进制 13 的二进制是 0000 1101，十六进制是 0D；0110 1011 + 0001 1101 = 1000 1000 不产生溢出，而两个较大的 8 位数相加如 1111 1111 + 0000 0001 会产生溢出，因为结果已无法用 8 位表示。二进制左移一位等于数值翻倍，右移一位等于减半。字符集同样重要：ASCII 使用 7 位、可表示 128 个字符；Unicode 使用更多位，支持世界大多数语言的字符及表情符号。
Image file size (bits) = width × height × colour depth
Sound file size (bits) = sample rate × bit depth × duration (seconds)
An image is a grid of pixels; a higher resolution (more pixels) and a larger colour depth (more bits per pixel, so more colours) improve quality but increase file size. Sound is sampled at regular intervals: a higher sample rate and bit depth give a more faithful recording but a larger file. Compression reduces file size. Lossless compression (for example RLE or FLAC) restores the original data exactly, whereas lossy compression (JPEG, MP3) permanently discards detail that the human eye or ear is unlikely to notice.
图像由像素网格组成；分辨率越高（像素越多）、色深越大（每像素位数越多，颜色越多），画质越好但文件越大。声音按固定间隔采样：采样率与位深越高，录音越真实但文件越大。压缩可以减小文件体积。无损压缩（如 RLE、FLAC）可精确还原原始数据；有损压缩（JPEG、MP3）会永久丢弃人眼或人耳难以察觉的细节。
4. Computer Networks, Connections and Protocols | 计算机网络、连接与协议
A LAN covers a small geographical area such as a school and uses its own hardware; a WAN covers a large area and typically uses infrastructure owned by third parties, with the internet being the largest WAN. Network performance depends on bandwidth, the number of simultaneous users, the type of transmission media, signal interference, and latency. A client-server network provides centralised management, backup and security but is expensive to set up; a peer-to-peer network is cheap and easy for a few devices but offers weaker security and no central control.
局域网（LAN）覆盖学校等小范围区域并使用自有硬件；广域网（WAN）覆盖大范围区域，通常使用第三方基础设施，互联网即最大的 WAN。网络性能取决于带宽、同时在线用户数量、传输介质类型、信号干扰以及延迟。客户端—服务器网络便于集中管理、备份和安全防护，但搭建成本高；对等网络（P2P）用于少量设备时便宜且容易，但安全性较弱且没有集中控制。
Hardware includes the Network Interface Controller (NIC), switch, router, wireless access point and hub. Common protocols include TCP/IP (data transmission and addressing), HTTP/HTTPS (web pages, with HTTPS encrypted), FTP (file transfer), SMTP (sending email), IMAP (storing and retrieving email on a server), Wi-Fi (wireless LAN standards) and Ethernet (wired LAN standards). A star topology connects every device to a central switch, is easy to add to and suffers little collision, but fails if the switch fails. A mesh topology gives many routes between devices, so it is reliable and fast but expensive to install and maintain.
网络硬件包括网卡（NIC）、交换机、路由器、无线接入点和集线器。常见协议包括：TCP/IP（数据传输与寻址）、HTTP/HTTPS（网页，HTTPS 加密）、FTP（文件传输）、SMTP（发送邮件）、IMAP（在服务器上存储与读取邮件）、Wi-Fi（无线局域网标准）以及以太网（有线局域网标准）。星型拓扑将每台设备连接到中央交换机，易于扩展、冲突少，但交换机故障会导致整个网络瘫痪。网状拓扑在不同设备间提供多条路径，因此可靠且快速，但安装与维护成本高。
5. Network Security | 网络安全
Threats fall into two broad groups. Malware includes viruses (attach to files and spread when the file is executed), worms (self-replicating across networks), trojans (disguised as legitimate software), ransomware (encrypts files and demands payment), spyware and adware. Social engineering manipulates people rather than machines, and includes phishing emails, blagging (inventing a believable story), shouldering (watching someone enter a password) and pharming (redirecting users to fake websites). Other attacks include brute force password attacks, denial of service attacks that flood a server with requests, SQL injection that enters malicious SQL through user input, man-in-the-middle interception and zero-day exploits of unknown vulnerabilities.
威胁大致分为两类。恶意软件包括：病毒（附着于文件，文件运行即传播）、蠕虫（可自行在网络中复制传播）、木马（伪装成合法软件）、勒索软件（加密文件并索要赎金）、间谍软件与广告软件。社会工程攻击针对的是人而非机器，包括网络钓鱼邮件、诱骗（编造可信说辞）、窥视（偷看他人输入密码）和域名欺骗（将用户重定向到假网站）。其他攻击还包括暴力破解密码、以大量请求淹没服务器的拒绝服务攻击、通过用户输入注入恶意 SQL 的 SQL 注入、中间人拦截，以及利用未知漏洞的零日攻击。
Protection combines technical and human measures: a firewall filters incoming and outgoing traffic; anti-malware software detects and removes harmful code; strong passwords and multi-factor authentication make access harder; encryption protects data in transit and at rest; penetration testing finds weaknesses before attackers do; user access levels limit what each account can change; automatic backups allow recovery after ransomware; and clear network policies plus staff training reduce social engineering success.
防护需要技术与人为措施结合：防火墙过滤进出流量；反恶意软件检测并清除有害代码；强密码与多因素认证提高访问门槛；加密保护传输中和存储中的数据；渗透测试在攻击者之前发现漏洞；用户访问权限限制各账户可修改的范围；自动备份使勒索软件攻击后可恢复；清晰的网络政策和员工培训可降低社会工程攻击的成功率。
6. Systems Software | 系统软件
Systems software manages the hardware and provides a platform for applications. The operating system performs five main functions: the user interface (graphical or command line), memory management (allocating RAM and using virtual memory), multitasking (sharing CPU time between processes), peripheral management (communicating with devices through drivers) and user management (accounts, passwords and access rights), plus file management. Utility software, by contrast, performs a single maintenance job: backup, defragmentation, compression, encryption, disk formatting and antivirus scanning.
系统软件管理硬件并为应用程序提供平台。操作系统承担五项主要功能：用户界面（图形界面或命令行）、内存管理（分配 RAM 并使用虚拟内存）、多任务处理（在进程之间分配 CPU 时间）、外围设备管理（通过驱动程序与设备通信）和用户管理（账户、密码与访问权限），此外还包括文件管理。相比之下，工具软件只完成单一的维护任务：备份、碎片整理、压缩、加密、磁盘格式化和杀毒扫描。
Open source software has source code that anyone can view, modify and redistribute; it is usually free, benefits from community support and can be adapted, but documentation and support may be limited. Proprietary software keeps its source code secret and requires a licence; it often offers professional support, regular updates and tested reliability, but users cannot modify it and costs can be high. Interrupts are signals that temporarily stop the CPU, for example when a key is pressed, so the processor can service the request before resuming.
开源软件的源代码任何人都可以查看、修改和再发布；它通常免费，有社区支持且可自行改造，但文档与技术支持可能有限。专有软件源代码保密并需购买许可；它通常提供专业支持、定期更新和经过验证的可靠性，但用户无法修改，成本也可能较高。中断是使 CPU 暂停当前任务的信号，例如按键被按下时，处理器会先响应该请求，再恢复原任务。
7. Ethical, Legal, Cultural and Environmental Impacts | 伦理、法律、文化与环境影响
Technology affects privacy, accuracy, accessibility, censorship and surveillance. Personal data is increasingly collected by apps and devices, raising questions about who owns it and who can access it; algorithms can also reflect the bias of the people who write them. Key UK legislation includes the Data Protection Act 2018 (personal data must be used lawfully, kept secure and used only for the stated purpose), the Computer Misuse Act 1990 (illegal to access or modify data without permission), the Copyright, Designs and Patents Act 1988 (protects creative work and software) and the Freedom of Information Act 2000 (allows public access to data held by public bodies).
技术影响隐私、准确性、可及性、审查与监控。应用程序与设备收集的个人数据越来越多，由此引发数据归属与访问权限的问题；算法也可能反映编写者的偏见。英国主要立法包括：《2018 年数据保护法》（个人数据必须合法使用、安全保存且仅用于声明目的）、《1990 年计算机滥用法》（未经许可访问或修改数据即属违法）、《1988 年版权、外观设计与专利法》（保护创作成果与软件）以及《2000 年信息自由法》（公众可获取公共机构持有的数据）。
Cultural and environmental effects include the digital divide, where unequal access to devices, connectivity and skills puts some groups at a disadvantage; changes in employment caused by automation and artificial intelligence; and the environmental cost of manufacturing and disposing of hardware, which creates e-waste and consumes substantial energy in data centres. Responsible practice includes recycling equipment, extending device lifetimes, using energy-efficient settings and designing accessible products.
文化与环境影响包括数字鸿沟——设备、网络与技能获取的不平等使部分群体处于劣势；自动化与人工智能带来的就业结构变化；以及硬件制造与废弃造成的环境代价，产生电子垃圾并让数据中心消耗大量能源。负责任的做法包括回收设备、延长使用寿命、使用节能设置以及设计无障碍产品。
8. Computational Thinking and Algorithms | 计算思维与算法
Computational thinking has four strands: decomposition (breaking a problem into smaller sub-problems), pattern recognition (spotting similarities between problems), abstraction (removing unnecessary detail and keeping what matters) and algorithmic thinking (designing a step-by-step solution). Algorithms can be expressed as flowcharts, using standard symbols such as oval terminators, parallelogram inputs and outputs, rectangle processes and diamond decisions, or as recognised pseudocode.
计算思维包含四个方面：分解（把问题拆成更小的子问题）、模式识别（发现不同问题之间的相似性）、抽象（去除无关细节、保留关键信息）和算法思维（设计分步解决方案）。算法可以用流程图表示，使用椭圆表示起止、平行四边形表示输入输出、矩形表示处理、菱形表示判断等标准符号，也可以用标准伪代码表示。
A linear search checks each item in turn and works on any list, but becomes slow as the list grows; a binary search is much faster on a sorted list because it repeatedly ignores half the remaining data, but it requires the list to be ordered first. Bubble sort repeatedly compares and swaps adjacent items, is easy to code but inefficient on large data sets; merge sort splits the list, sorts the parts and merges them, and is more efficient for large lists though harder to implement; insertion sort builds a sorted sublist one item at a time and is efficient for small or nearly sorted lists.
线性查找依次检查每一项，适用于任何列表，但列表变大时速度变慢；二分查找在有序列表上快得多，因为它每次都排除剩余数据的一半，但必须先对列表排序。冒泡排序反复比较并交换相邻元素，易编写但在大数据集上效率低；归并排序将列表拆分、分别排序后再合并，对大型列表更高效但实现较难；插入排序逐个把元素构建进已排序的子列表，对小规模或接近有序的列表效率较高。
9. Programming Fundamentals | 编程基础
You must know the basic data types: integer (whole numbers), real or float (decimals), Boolean (True or False), character (a single symbol) and string (text). Variables hold values that can change during a program; constants hold values that cannot change while the program runs, which makes code easier to read and prevents accidental edits. Casting converts one data type into another, such as converting a string input into an integer before doing arithmetic.
你必须掌握基本数据类型：整数（integer，整数值）、实数或浮点（real / float，小数）、布尔（Boolean，真或假）、字符（character，单个符号）和字符串（string，文本）。变量保存程序运行中可以改变的值；常量保存在程序运行中不能改变的值，使代码更易读并防止误改。类型转换（casting）把一种数据类型转换为另一种，例如在运算前将输入的字符串转换为整数。
ration uses count-controlled loops such as for, and condition-controlled loops such as while and do-until. Operators include + − × ÷, MOD (remainder) and DIV (integer division), plus comparison operators =, <>, <, >, &
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