📚 A-Level WJEC Computer Science: End-of-Term Revision Guide | A-Level WJEC 计算机科学：期末复习提纲
This revision guide summarises the essential topics for the A-Level WJEC Computer Science specification. Whether you are revising for a mock exam or a final assessment, this guide will help you consolidate your knowledge across key areas such as data representation, computer architecture, programming, and networks.
本复习提纲总结了A-Level WJEC计算机科学课程的核心主题。无论你是在准备模拟考试还是最终评估，这份指南都将帮助你在数据表示、计算机体系结构、编程和网络等关键领域巩固知识。
1. Data Representation and Number Bases | 数据表示与数制
All digital data is stored as binary digits (bits). A single bit represents either a 0 or a 1. Bits are grouped into larger units: a nibble is 4 bits, a byte is 8 bits, a kilobyte (KB) is 1024 bytes (210), and larger units follow powers of 1024.
所有数字数据都以二进制数字（位）存储。一个位表示0或1。位被组合成更大的单位：半字节是4位，字节是8位，千字节（KB）是1024字节（2¹⁰），更大的单位按照1024的幂递增。
- 1 KB = 2¹⁰ bytes = 1024 bytes
- 1 MB = 2²⁰ bytes = 1,048,576 bytes
- 1 GB = 2³⁰ bytes
- 1 TB = 2⁴⁰ bytes
- 1 KB = 2¹⁰ 字节 = 1024 字节
- 1 MB = 2²⁰ 字节 = 1,048,576 字节
- 1 GB = 2³⁰ 字节
- 1 TB = 2⁴⁰ 字节
to 1. For example, 1101₂ = (1 × 2³) + (1 × 2²) + (0 × 2¹) + (1 × 2⁰) = 8 + 4 + 0 + 1 = 13.
二进制数使用基数为2。要将二进制整数转换为十进制，将对所有为1的位对应的2的幂求和。例如，1101₂ = (1 × 2³) + (1 × 2²) + (0 × 2¹) + (1 × 2⁰) = 8 + 4 + 0 + 1 = 13。
Hexadecimal (base 16) is widely used to represent binary values compactly. Each hex digit corresponds to a nibble (4 bits). Digits 0–9 are used for values 0–9, and letters A–F represent 10–15. Conversion between binary and hexadecimal is straightforward: group binary digits into nibbles and replace each group with its hex equivalent.
十六进制（基数为16）被广泛用于紧凑地表示二进制值。每个十六进制数字对应一个半字节（4位）。数字0–9表示值0–9，字母A–F表示10–15。二进制与十六进制之间的转换很简单：将二进制数字按半字节分组，然后用十六进制替代每组。
Negative numbers can be represented using sign-and-magnitude, where the most significant bit indicates the sign. Two's complement is preferred because it simplifies arithmetic. To find the two's complement of a binary number, invert all bits and add 1. For an 8-bit number, the range of two's complement is from -128 to +127.
负数可以使用原码表示，其中最高位指示符号。补码表示更为优先，因为它简化了算术运算。要计算一个二进制数的补码，将所有位取反然后加1。对于8位数字，补码的表示范围是-128到+127。
Floating-point representation stores real numbers using a mantissa and an exponent, similar to scientific notation. Normalisation ensures maximum precision by adjusting the mantissa so that the first bit after the sign is a 1 (for positive numbers) or a 0 (for negative numbers). WJEC expects you to convert between denary and normalised floating-point binary, typically using a specified number of bits for the mantissa and exponent.
浮点数表示使用尾数和指数存储实数，类似于科学计数法。规范化通过调整尾数来保证最大精度，使得符号位后的第一位为1（正数）或0（负数）。WJEC要求能够在十进制与规范化浮点二进制之间进行转换，通常会指定尾数和指数的位宽。
2. Boolean Logic and Logic Gates | 布尔逻辑与逻辑门
Boolean algebra deals with variables that can only take the values TRUE (1) or FALSE (0). The fundamental operators are NOT, AND, OR, and XOR. The NOT operator (¬) inverts the input. AND (∧) outputs 1 only if all inputs are 1. OR (∨) outputs 1 if at least one input is 1. XOR (⊕) outputs 1 when an odd number of inputs are 1.
布尔代数处理只能取值TRUE（1）或FALSE（0）的变量。基本运算符有NOT、AND、OR和XOR。NOT运算符（¬）将输入取反。AND（∧）仅在所有输入都为1时输出1。OR（∨）在至少一个输入为1时输出1。XOR（⊕）在1的个数为奇数时输出1。
ossible input combinations and the corresponding outputs. For example, the truth table for a 2-input AND gate shows that only when A = 1 and B = 1 does Q = 1.
逻辑门是布尔函数的物理实现。标准门包括NOT、AND、OR、NAND、NOR和XOR。真值表列出了所有可能的输入组合及其对应的输出。例如，一个2输入AND门的真值表显示只有当A = 1且B = 1时，Q = 1。
ircuit and vice versa. Simplifying expressions using Boolean identities (such as De Morgan's laws) is essential. De Morgan's laws state: ¬(A ∧ B) = ¬A ∨ ¬B and ¬(A ∨ B) = ¬A ∧ ¬B. These can be used to transform circuits into NAND-only or NOR-only implementations.
你必须能够从逻辑电路中推导出布尔表达式，反之亦然。使用布尔恒等式（如德摩根定律）简化表达式至关重要。德摩根定律指出：¬(A ∧ B) = ¬A ∨ ¬B， ¬(A ∨ B) = ¬A ∧ ¬B。这些定律可用于将电路转换为仅使用NAND或NOR门的实现。
Karnaugh maps (K-maps) provide a visual method for simplifying Boolean expressions. They are particularly useful for functions of up to four variables. Group adjacent 1s in powers of two to produce a minimal sum-of-products expression.
卡诺图（K-map）提供了一种可视化简化布尔表达式的方法，尤其适用于最多四个变量的函数。将相邻的1以2的幂次方分组，可以产生最简积之和表达式。
3. Computer Architecture and the Fetch-Decode-Execute Cycle | 计算机体系结构与取指-译码-执行循环
The von Neumann architecture forms the basis of most modern computers. It features a single shared memory for both instructions and data, a control unit (CU), an arithmetic logic unit (ALU), and registers. The key registers include the program counter (PC), memory address register (MAR), memory data register (MDR), current instruction register (CIR), and accumulator (ACC).
冯·诺依曼体系结构是大多数现代计算机的基础。它采用一个同时存储指令和数据的共享内存、一个控制单元（CU）、一个算术逻辑单元（ALU）以及多个寄存器。关键寄存器包括程序计数器（PC）、内存地址寄存器（MAR）、内存数据寄存器（MDR）、当前指令寄存器（CIR）和累加器（ACC）。
The fetch-decode-execute cycle is the process by which the CPU processes instructions. In the fetch phase, the address in the PC is copied to the MAR, the instruction is retrieved from memory into the MDR, and the PC is incremented. During decode, the CU interprets the instruction held in the CIR. In the execute phase, the instruction is carried out, which may involve the ALU or data transfer between registers and memory.
取指-译码-执行循环是CPU处理指令的过程。在取指阶段，PC中的地址被复制到MAR，指令从内存中取到MDR，PC内容递增。在译码阶段，CU解释CIR中的指令。在执行阶段，指令被执行，可能涉及ALU或寄存器与内存之间的数据传输。
Different processor architectures affect performance. Factors include clock speed, number of cores, cache size, and word length. Pipelining allows multiple instructions to be at different stages of execution simultaneously, increasing throughput. However, pipeline hazards such as data dependencies can cause stalls.
不同的处理器架构影响性能。因素包括时钟速度、核心数量、缓存大小和字长。流水线技术允许多条指令同时处于不同执行阶段，从而提高吞吐量。然而，流水线冒险（如数据依赖性）可能导致停顿。
Assembly language uses mnemonics like LDA, STA, ADD, SUB, and JMP to represent machine code instructions. Understanding the relationship between assembly code and the fetch-decode-execute cycle helps in grasping low-level programming. You may be asked to trace a simple assembly program and show how registers change.
汇编语言使用助记符（如LDA、STA、ADD、SUB和JMP）来表示机器码指令。理解汇编代码与取指-译码-执行循环之间的关系有助于掌握低级编程。考题可能会要求跟踪一个简单的汇编程序并展示寄存器的变化。
4. Operating Systems and Systems Software | 操作系统与系统软件
An operating system (OS) manages hardware resources and provides services for application software. Key functions include memory management, process scheduling, file management, input/output control, and providing a user interface. Memory management involves allocating RAM to processes, using techniques such as paging and segmentation to handle virtual memory.
操作系统（OS）管理硬件资源并为应用软件提供服务。关键功能包括内存管理、进程调度、文件管理、输入/输出控制以及提供用户界面。内存管理涉及将RAM分配给进程，使用分页和分段等技术来处理虚拟内存。
Scheduling algorithms determine the order in which processes are executed by the CPU. Common algorithms include first-come first-served (FCFS), shortest job first (SJF), round robin, and priority-based scheduling. The OS must balance responsiveness, fairness, and throughput.
调度算法决定CPU执行进程的顺序。常见算法包括先来先服务（FCFS）、最短作业优先（SJF）、轮转调度和基于优先级的调度。操作系统必须平衡响应性、公平性和吞吐量。
Interrupts are signals sent by hardware or software to gain the CPU's attention. When an interrupt occurs, the current task's state is saved, and an interrupt service routine (ISR) is executed. After handling the interrupt, the CPU resumes the previous task. This mechanism allows the OS to respond promptly to events like I/O completion or user input.
中断是由硬件或软件发送的信号，用于引起CPU的注意。当中断发生时，当前任务的状态被保存，并执行中断服务程序（ISR）。处理完中断后，CPU恢复之前的任务。这种机制使操作系统能够及时响应I/O完成或用户输入等事件。
Systems software also includes translators: assemblers, compilers, and interpreters. An assembler converts assembly language into machine code. A compiler translates the entire high-level source code into machine code before execution, while an interpreter translates and executes line by line. Compilers generate stand-alone executable files; interpreters allow easier debugging.
系统软件还包括翻译器：汇编器、编译器和解释器。汇编器将汇编语言转换为机器码。编译器在执行前将整个高级源代码翻译为机器码，而解释器则逐行翻译并执行。编译器生成独立的可执行文件；解释器便于调试。
5. Programming Paradigms and Constructs | 编程范式与结构
Procedural programming organises code into procedures or functions that operate on data. It follows a top-down approach and uses sequence, selection, and iteration as the three fundamental control structures. Most A-Level programming tasks (in Python, Java, C#, etc.) adopt this paradigm.
过程式编程将代码组织为对数据进行操作的过程或函数。它遵循自顶向下的方法，并使用顺序、选择和循环作为三种基本控制结构。大多数A-Level编程任务（使用Python、Java、C#等）采用这种范式。
Object-oriented programming (OOP) models real-world entities as objects that encapsulate data (attributes) and behaviour (methods). Key concepts are classes, objects, inheritance, polymorphism, and encapsulation. WJEC may ask you to identify OOP features in a given scenario or to explain the benefits, such as code reuse and modularity.
面向对象编程（OOP）将现实世界的实体建模为封装了数据（属性）和行为（方法）的对象。核心概念是类、对象、继承、多态和封装。WJEC可能会要求你在给定情境中识别OOP特性，或解释其优点，如代码复用和模块化。
Event-driven programming is commonly used in graphical user interfaces. The flow of the program is determined by events such as mouse clicks or key presses. An event listener waits for events and triggers a corresponding handler subroutine. This paradigm is essential for interactive applications.
事件驱动编程常用于图形用户界面。程序的流程由事件（如鼠标点击或按键）决定。事件监听器等待事件并触发相应的处理子程序。该范式对于交互式应用程序至关重要。
All programs rely on data types: integer, real/float, Boolean, character, string, and composite types like arrays and records. You should understand type conversion and the importance of choosing appropriate data types to avoid overflow or loss of precision.
所有程序都依赖数据类型：整型、实数/浮点型、布尔型、字符型、字符串以及数组和记录等复合类型。你应该理解类型转换以及选择合适数据类型以避免溢出或精度丢失的重要性。
6. Data Structures (Arrays, Lists, Stacks, Queues) | 数据结构（数组、列表、栈、队列）
Arrays are static data structures that store elements of the same data type in contiguous memory locations. Elements are accessed by index, allowing O(1) random access. However, resizing an array is costly because it requires creating a new array and copying elements.
数组是静态数据结构，在连续的内存位置中存储相同数据类型的元素。元素通过索引访问，可实现O(1)的随机访问。然而，调整数组大小代价很高，因为需要创建新数组并复制元素。
Linked lists are dynamic structures where each node contains data and a pointer to the next node. They allow efficient insertion and deletion anywhere in the list (O(1) if the position is known), but do not support random access; traversing to an element requires O(n) time.
链表是动态结构，其中每个节点包含数据和指向下一个节点的指针。它们允许在列表任意位置高效插入和删除（若已知位置则为O(1)），但不支持随机访问；遍历到某元素需要O(n)时间。
A stack is a last-in, first-out (LIFO) abstract data type. Operations include push (add to top) and pop (remove from top). Stacks are used for function call management (call stack), undo mechanisms, and expression evaluation. A stack can be implemented using an array or a linked list.
栈是一种后进先出（LIFO）的抽象数据类型。操作包括push（添加至栈顶）和pop（从栈顶移除）。栈用于函数调用管理（调用栈）、撤销机制和表达式求值。栈可以用数组或链表实现。
A queue is a first-in, first-out (FIFO) structure. Enqueue adds an item to the rear, and dequeue removes an item from the front. A circular queue overcomes the problem of unused spaces in a linear array implementation. Queues are used in print spoolers, keyboard buffers, and breadth-first search.
队列是一种先进先出（FIFO）的结构。入队将项添加至队尾，出队从队首移除项。循环队列克服了线性数组实现中未使用空间的问题。队列用于打印缓冲区、键盘缓冲区和广度优先搜索。
7. Searching and Sorting Algorithms | 查找与排序算法
Linear search inspects each element of a list sequentially until the target is found or the end is reached. It does not require the list to be sorted and has a time complexity of O(n) in the worst case. It is simple but inefficient for large data sets.
线性搜索按顺序检查列表中的每个元素，直到找到目标或到达列表末端。它不要求列表有序，最坏情况下的时间复杂度为O(n)。该算法简单，但对于大数据集效率较低。
Binary search works on sorted lists by repeatedly dividing the search interval in half. It compares the target to the middle element; if not equal, it discards the half where the target cannot lie. The worst-case time complexity is O(log n), making it very efficient for large data sets.
二分搜索作用于有序列表，通过反复将搜索区间减半来工作。它将目标与中间元素比较；如果不相等，则舍弃目标不可能在的一半。最坏情况时间复杂度为O(log n)，对于大数据集非常高效。
Bubble sort repeatedly steps through the list, compares adjacent elements, and swaps them if they are in the wrong order. The largest unsorted element 'bubbles' to the end with each pass. Its average and worst-case complexity is O(n²), so it is impractical for large lists.
冒泡排序反复遍历列表，比较相邻元素，若顺序错误则交换它们。每次遍历中，最大的未排序元素会“冒泡”到末尾。其平均和最坏情况复杂度为O(n²)，因此不适用于大型列表。
Insertion sort builds the final sorted array one item at a time by taking each element and inserting it into its correct position within the already sorted part. It is efficient for small or nearly sorted data sets, with O(n²) worst-case but O(n) best-case.
插入排序一次构建最终已排序数组一个元素，取出每个元素并将其插入已排序部分的正确位置。它对小型或近乎有序的数据集效率较高，最坏情况O(n²)，最佳情况O(n)。
Quick sort is a recursive divide-and-conquer algorithm. It selects a pivot, partitions the list into elements less than the pivot and elements greater than the pivot, then recursively sorts the sub-lists. Average complexity is O(n log n), but worst-case (poor pivot choice) is O(n²). Merge sort also uses divide-and-conquer and guarantees O(n log n), but requires additional memory for merging.
快速排序是一种递归的分治算法。它选择一个基准值，将列表划分为小于基准和大于基准的元素，然后递归地对子列表进行排序。平均复杂度为O(n log n)，但最坏情况（基准选择不佳）为O(n²)。归并排序同样采用分治法，并保证O(n log n)，但合并时需要额外内存。
8. Networking and the Internet | 网络与互联网
A network connects two or more computing devices to share resources and communicate. LANs (Local Area Networks) cover a small geographical area, while WANs (Wide Area Networks) span large distances. Network topologies include star, bus, ring, and mesh; the star topology is common in modern LANs because a single cable failure only affects one host.
网络连接两台或多台计算设备以共享资源和通信。LAN（局域网）覆盖较小地理区域，而WAN（广域网）跨越较大距离。网络拓扑包括星形、总线型、环形和网状；现代LAN中常见星形拓扑，因为单根线缆故障只会影响一台主机。
The TCP/IP protocol stack is the foundation of internet communication. It consists of four layers: Application (HTTP, FTP, SMTP), Transport (TCP, UDP), Internet (IP), and Network Interface. TCP provides reliable, ordered delivery with error checking, while IP handles addressing and routing.
TCP/IP协议栈是互联网通信的基础。它由四层组成：应用层（HTTP、FTP、SMTP）、传输层（TCP、UDP）、互联网层（IP）和网络接口层。TCP提供可靠、有序且带错误检查的传递，而IP处理寻址和路由。
IP addresses (IPv4) are 32-bit numbers, often written in dotted-decimal notation. Subnet masks define the network and host portions. Network address translation (NAT) allows multiple devices on a private network to share a single public IP. IPv6 uses 128-bit addresses to overcome the exhaustion of IPv4 addresses.
IP地址（IPv4）是32位数字，通常以点分十进制表示。子网掩码定义网络和主机部分。网络地址转换（NAT）允许私有网络上的多个设备共享一个公共IP地址。IPv6使用128位地址以解决IPv4地址耗尽问题。
Key network hardware includes switches, routers, and gateways. A switch forwards frames within a LAN based on MAC addresses. A router forwards packets between different networks using IP addresses. The Domain Name System (DNS) translates human-readable domain names into IP addresses.
关键网络硬件包括交换机、路由器和网关。交换机根据MAC地址在LAN内转发帧。路由器使用IP地址在不同网络间转发数据包。域名系统（DNS）将人可读的域名转换为IP地址。
Client-server and peer-to-peer are two major network models. In the client-server model, a central server provides resources and services to clients. In
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