📚 AS WJEC Computer Science: Core Knowledge Essentials | AS WJEC 计算机：核心知识点梳理
This article distils the most important concepts covered in the AS WJEC Computer Science specification. It serves as a structured revision guide, linking each topic to clear English explanations followed by their Chinese counterparts. By working through these paired paragraphs, you can reinforce your understanding of both the technical vocabulary and the underlying principles.
本文梳理了 AS WJEC 计算机科学课程中最重要的概念。它是一份结构化的复习指南，每个主题都先给出清晰的英文解释，再紧跟对应的中文段落。通过阅读这些配对的段落，你可以同时巩固技术词汇和底层原理的理解。
1. Data Representation | 数据表示
All data inside a computer is stored as binary digits (bits). At AS level, you must be comfortable converting between binary, denary and hexadecimal, and you need to understand why hex is often used as a shorthand for binary. A single hex digit represents four bits, which makes it easier to read long binary strings.
计算机内部所有数据都以二进制数字(位)存储。在 AS 阶段，你必须熟练地在二进制、十进制和十六进制之间进行转换，并理解为什么十六进制常被用作二进制的简写。一个十六进制数字表示四位，这使得阅读长二进制串更加方便。
Denary to binary conversion uses repeated division by 2, recording remainders. Binary to denary uses column weights (128, 64, 32, … 1). Hexadecimal to binary works by replacing each hex digit with its 4-bit equivalent, and the reverse process groups bits into nibbles.
十进制转二进制使用除二取余法，记录余数。二进制转十进制利用位权值(128, 64, 32 … 1)。十六进制转二进制只需将每个十六进制位替换为对应的四位二进制，逆过程则是将二进制位每四位分为一组。
Character encoding maps characters to numeric codes. The 7-bit ASCII set encodes 128 characters, while the extended 8-bit ASCII adds a further 128. Unicode provides a unique code point for every character across all languages, with UTF-8 being a popular variable-length encoding scheme.
字符编码将字符映射为数值代码。7 位 ASCII 集编码了 128 个字符，而扩展的 8 位 ASCII 又增加了 128 个。Unicode 为所有语言中的每个字符提供唯一的码点，UTF-8 是一种广泛使用的可变长度编码方案。
Images are stored as a grid of pixels. In a bitmap, each pixel is assigned a binary number representing its colour; the bit depth determines the number of available colours. Resolution is the number of pixels per unit, and together with colour depth they affect file size.
图像以像素网格存储。在位图中，每个像素被分配一个表示颜色的二进制数；位深度决定了可用的颜色数量。分辨率是单位长度内的像素数，它与颜色深度共同影响文件大小。
Sound is digitised by sampling the analogue waveform at regular intervals. The sample rate (in Hz) and the bit depth per sample determine the accuracy and file size. Higher sample rates and bit depths improve quality but increase storage requirements.
声音通过定期采样模拟波形来数字化。采样率(以 Hz 为单位)和每个样本的位深度决定了精度和文件大小。更高的采样率和位深可提高音质，但会增加存储需求。
Compression reduces file size. Lossless compression (e.g. run-length encoding) allows the original data to be perfectly reconstructed, while lossy compression (e.g. JPEG, MP3) permanently discards some information to achieve much smaller files.
压缩可减小文件体积。无损压缩(例如游程编码)允许原始数据被完美重建，而有损压缩(如 JPEG、MP3)会永久丢弃部分信息以换取更小的文件。
2. Computer Architecture and the Fetch-Execute Cycle | 计算机体系结构与取指-执行周期
A standard computer follows the von Neumann architecture, where both program instructions and data are stored in the same memory. The CPU contains the control unit (CU), arithmetic logic unit (ALU) and several dedicated registers, all connected via internal buses.
标准计算机遵循冯·诺依曼体系结构，程序指令和数据存放在同一内存中。CPU 内含控制单元(CU)、算术逻辑单元(ALU)和多个专用寄存器，它们都通过内部总线相连。
Key registers include the program counter (PC) which holds the address of the next instruction, the memory address register (MAR) and memory data register (MDR) for reading from and writing to memory, the current instruction register (CIR) and the accumulator (ACC).
关键寄存器包括：程序计数器(PC)存放下一条指令的地址，存储器地址寄存器(MAR)和存储器数据寄存器(MDR)用于内存的读写，当前指令寄存器(CIR)和累加器(ACC)。
Three buses connect the processor to memory: the address bus carries memory addresses, the data bus transfers the actual data or instructions, and the control bus carries timing and control signals. The width of the address bus determines the maximum addressable memory.
有三条总线连接处理器与内存：地址总线传送内存地址，数据总线传输实际数据或指令，控制总线传递时序和控制信号。地址总线的宽度决定了最大可寻址内存空间。
The fetch-decode-execute cycle is the fundamental operation of the CPU. In the fetch stage, the PC value is copied to the MAR, a memory read is performed, and the instruction is placed in the CIR. The decode stage interprets the opcode, and the execute stage carries out the operation, which may involve the ALU or additional memory accesses.
取指-解码-执行周期是 CPU 的基本操作。在取指阶段，PC 的值被复制到 MAR，执行一次内存读取，指令被放入 CIR。解码阶段翻译操作码，执行阶段完成操作，这可能涉及 ALU 或额外的内存访问。
Performance is affected by clock speed, the number of cores, and the size and levels of cache memory. A higher clock speed means more cycles per second; multiple cores allow true parallel execution; cache provides fast access to frequently used data, reducing the average memory access time.
性能受时钟频率、核心数量、以及缓存的大小和层级影响。时钟频率越高意味着每秒执行的周期越多；多核允许真正的并行执行；缓存提供对常用数据的快速访问，降低了平均存储访问时间。
3. Systems Software: Operating Systems and Utilities | 系统软件：操作系统与实用程序
An operating system (OS) manages hardware resources and provides a user interface. Its core functions include memory management (allocating RAM to processes), processor scheduling, file management, handling input/output and providing security through user authentication and access rights.
操作系统(OS)管理硬件资源并提供用户界面。它的核心功能包括：内存管理(为进程分配 RAM)、处理器调度、文件管理、处理输入/输出以及通过用户认证和访问权限提供安全保障。
Memory management uses paging or segmentation to allocate memory blocks. The OS maintains a page table to map virtual addresses to physical ones, giving each process the illusion of its own contiguous memory space.
内存管理使用分页或分段技术来分配内存块。操作系统维护页表，将虚拟地址映射到物理地址，让每个进程产生自己拥有连续内存空间的错觉。
Utility software performs specific housekeeping tasks. Disk defragmenters reorganise files on a hard disk so that each file occupies a contiguous block, improving access speed. Backup utilities create copies of data to prevent loss, and antivirus software detects and removes malware.
实用程序执行特定的维护任务。磁盘碎片整理程序重新组织硬盘上的文件，使每个文件占据连续的块，从而提高访问速度。备份工具创建数据副本以防止丢失，防病毒软件检测并清除恶意软件。
Operating systems can be classified as single-user (one person at a time), multi-user (many users concurrently, often on a network), or real-time (critical that responses happen within a guaranteed time, used in embedded systems). General-purpose OSs such as Windows, Linux and macOS support multitasking.
操作系统可分为单用户(一次一人使用)、多用户(多人同时使用，常见于网络环境)或实时(必须在规定时间内响应，用于嵌入式系统)。通用操作系统如 Windows、Linux 和 macOS 支持多任务处理。
4. Logic Gates and Boolean Algebra | 逻辑门与布尔代数
Digital circuits are built from logic gates that implement Boolean functions. The basic gates are AND, OR and NOT. NAND and NOR are universal gates, meaning any circuit can be constructed using only NAND gates or only NOR gates. XOR outputs 1 when inputs differ.
数字电路由实现布尔函数的逻辑门构成。基本门有与门(AND)、或门(OR)和非门(NOT)。与非门(NAND)和或非门(NOR)是通用门，意味着任何电路都可以仅用与非门或者仅用或非门来构建。异或门(XOR)在输入不同时输出 1。
Each gate can be described by a truth table listing all input combinations and the corresponding output. For example, the AND gate truth table shows output 1 only when both A and B are 1.
每个门都可以用一个列出所有输入组合及对应输出的真值表来描述。例如，与门的真值表仅在 A 和 B 都为 1 时输出才为 1。
A AND B = A ∧ B ; A OR B = A ∨ B ; NOT A = ¬A ; A XOR B = A ⊕ B
A AND B = A ∧ B ; A OR B = A ∨ B ; NOT A = ¬A ; A XOR B = A ⊕ B
Boolean identities and laws, such as De Morgan’s laws, are essential for simplification. De Morgan’s laws state: ¬(A ∨ B) = ¬A ∧ ¬B and ¬(A ∧ B) = ¬A ∨ ¬B. They allow conversion between OR and AND forms.
布尔恒等式和定律(如德摩根律)对于化简至关重要。德摩根律指出：¬(A ∨ B) = ¬A ∧ ¬B，且 ¬(A ∧ B) = ¬A ∨ ¬B。它们可以实现 OR 形式和 AND 形式之间的转换。
Combining gates produces functional circuits such as adders. A half adder adds two input bits A and B, producing a sum (S) and a carry out (Cₒ). The sum is A ⊕ B and the carry is A ∧ B. A full adder also takes a carry in (Cᵢ) and connects two half adders with an OR gate.
组合逻辑门可以产生功能电路，如加法器。半加器将两个输入位 A 和 B 相加，产生和位(S)和进位输出(Cₒ)，其中 S = A ⊕ B，进位 = A ∧ B。全加器还接受进位输入(Cᵢ)，并将两个半加器通过一个或门连接。
5. Data Structures and File Organisation | 数据结构与文件组织
A data structure is a way of organising data in memory. One-dimensional arrays hold a finite, ordered collection of elements of the same data type, each accessible by an index. Two-dimensional arrays can be visualised as a table with rows and columns.
数据结构是在内存中组织数据的方式。一维数组容纳有限且有序的、同数据类型的元素集合，每个元素可通过索引访问。二维数组可视为具有行和列的表。
A record is a composite data type that groups related fields of possibly different types, like a student record containing a name (string), ID (integer) and grade (character). A file is often a collection of records stored on secondary storage.
记录是一种复合数据类型，它将可能不同类型的相关字段组合在一起，例如一个学生记录包含姓名(字符串)、学号(整数)和成绩(字符)。文件通常是存放在辅助存储器上的记录集合。
Abstract data types include stacks and queues. A stack is a LIFO (last in, first out) structure where elements are added and removed from the same end. A queue is FIFO (first in, first out), with elements added at the rear and removed from the front.
抽象数据类型包括栈和队列。栈是一种后进先出(LIFO)结构，元素从同一端添加和移除。队列是先进先出(FIFO)，元素在队尾添加，在队首移除。
File organisation describes how records are arranged on storage. Serial files store records in the order they arrive; sequential files are ordered by a key field. Indexed sequential files use an index to allow direct access, while direct/random files use a hashing algorithm to compute the address of a record from its key.
文件组织描述了记录在存储上的排列方式。串行文件按到达顺序存储记录；顺序文件按键字段排序。索引顺序文件使用索引以实现直接访问，而直接/随机文件使用哈希算法从记录的键计算出其地址。
6. Algorithms: Searching and Sorting | 算法：搜索与排序
Searching algorithms locate a specific value within a collection. Linear search examines each element in order until a match is found or the end is reached. It works on unsorted data but has O(n) time complexity in the worst case.
搜索算法用于在集合内定位特定值。线性搜索按顺序检查每个元素，直到找到匹配项或到达末尾。它适用于未排序数据，但最坏时间复杂度为 O(n)。
Binary search repeatedly divides a sorted array in half, comparing the target value with the middle element. If the target is smaller, the search continues on the left half; if larger, on the right. Its time complexity is O(log n), which is much faster for large datasets.
二分搜索不断将已排序数组二分，将目标值与中间元素比较。如果目标值较小，则在左半部分继续搜索；如果较大，则在右半部分。时间复杂度为 O(log n)，对于大型数据集要快得多。
Sorting algorithms arrange data into order. Bubble sort repeatedly steps through the list, comparing adjacent items and swapping them if they are in the wrong order. It is simple but inefficient, with O(n²) comparisons.
排序算法将数据整理为特定顺序。冒泡排序重复遍历列表，比较相邻项并在顺序错误时交换它们。它简单但效率较低，需要 O(n²) 次比较。
Insertion sort builds the sorted list one item at a time, taking each new element and inserting it into its correct position among the previously sorted items. It performs well on small or nearly sorted datasets and is a stable sort.
插入排序一次构建一个有序项，将每个新元素插入到前面已排序项中的正确位置。它在小型或近似有序的数据集上表现良好，并且是一种稳定排序。
7. Programming Concepts and Constructs | 编程概念与结构
Programs are built from three basic constructs: sequence, selection and iteration. Sequence means statements are executed in the order they appear. Selection uses conditional branching: ‘if…else’ for two alternatives, and ‘case/switch’ for multiple branches.
程序由三种基本结构构建：顺序、选择和迭代。顺序意味着语句按出现次序执行。选择使用条件分支：’if…else’ 处理二选一，’case/switch’ 处理多分支。
Iteration repeats a block of code. A ‘for’ loop is used when the number of iterations is known in advance; a ‘while’ loop repeats while a condition remains true; a ‘repeat…until’ loop executes at least once before evaluating the condition.
迭代重复执行一组代码。当迭代次数预先已知时使用 ‘for’ 循环；’while’ 循环在条件为真时不断重复；’repeat…until’ 循环至少执行一次后再判断条件。
Variables are named storage locations whose values can change; constants are fixed values set at compile time. Common data types include integer, real/float, Boolean, character and string.
变量是具有名称的存储位置，其值可以变化；常量是在编译时设定的固定值。常见数据类型包括整型、实型/浮点型、布尔型、字符型和字符串。
Subroutines break programs into manageable parts. Functions return a value, while procedures do not. Parameters can be passed by value (a copy is used, original unchanged) or by reference (the actual variable’s memory address is passed, allowing modification).
子程序将程序分解为易于管理的部分。函数返回一个值，而过程不返回值。参数可以按值传递(使用副本，原始不变)或按引用传递(传递实际变量的内存地址，允许修改)。
8. Networking and Internet Technologies | 网络与互联网技术
A network connects computers to share resources. LANs (Local Area Networks) cover a small geographical area and are usually owned by a single organisation. WANs (Wide Area Networks) connect LANs over long distances, often using leased telecommunications lines.
网络连接计算机以共享资源。局域网(LAN)覆盖较小地理区域，通常归属单一组织。广域网(WAN)通过长距离连接局域网，常使用租用的电信线路。
Common topologies include star (each node connected to a central switch), bus (all nodes share a single backbone cable), and ring (data travels in one direction around a loop). A star topology is now dominant because a cable failure only affects one node.
常见拓扑结构包括星型(每个节点连接到中央交换机)、总线型(所有节点共享一根主干电缆)和环型(数据在环路中沿单一方向传播)。星型拓扑现在占主导地位，因为电缆故障只影响一个节点。
Protocols are rules governing communication. The TCP/IP suite is the foundation of the Internet. TCP splits data into packets and ensures reliable delivery, while IP handles addressing and routing. Higher-level protocols include HTTP/HTTPS for web pages, FTP for file transfer, SMTP for sending emails, and POP3/IMAP for retrieval.
协议是管理通信的规则。TCP/IP 协议族是互联网的基础。TCP 将数据分割成数据包并确保可靠传输，IP 则负责寻址和路由。更高层协议包括用于网页的 HTTP/HTTPS、用于文件传输的 FTP、用于发送邮件的 SMTP 以及用于检索邮件的 POP3/IMAP。
Each device on a network has a MAC address (hardware, fixed) and an IP address (logical, can change). IPv4 addresses are 32-bit, usually written in dotted decimal; IPv6 uses 128 bits to solve address exhaustion. Packet switching breaks messages into packets that travel independently; circuit switching establishes a dedicated path before communication.
网络上的每个设备都有一个 MAC 地址(硬件，固定)和一个 IP 地址(逻辑，可变)。IPv4 地址是 32 位的，通常以点分十进制书写；IPv6 采用 128 位以解决地址枯竭问题。分组交换将消息分割成独立传输的数据包；电路交换则在通信前建立一条专用通路。
9. Database Systems and SQL | 数据库系统与SQL
A relational database stores data in tables (relations). Each table consists of records (tuples) and fields (attributes). A primary
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