📚 Unit 1 Computing Revision & Prelim Preparation | 计算机单元1复习与预考准备
Welcome to the Unit 1 and prelim revision guide for computing. This article is written for students who are preparing for a Unit 1 homework check or a prelim exam. It summarises the most important concepts, gives clear examples, and shows you how to avoid common mistakes.
欢迎使用计算机单元1与预考复习指南。本文为准备单元1作业检查或预考的学生编写。它总结了最重要的概念，提供了清晰的示例，并展示如何避免常见错误。
1. Understanding the Unit 1 Syllabus | 理解单元1大纲
Before you start revising, check your course specification and highlight the topics listed under Unit 1. Usually Unit 1 covers computer systems, data representation, networks, and programming fundamentals.
在开始复习之前，请查看课程大纲并标出单元1中列出的主题。通常单元1涵盖计算机系统、数据表示、网络和编程基础。
Make a revision checklist so you can track which areas you have completed. Tick off each topic after you have tested yourself with past paper questions.
制作一份复习清单，这样你就能跟踪已经完成的领域。每当你用历年真题测试过自己后，就勾选该主题。
2. Computer Systems and Hardware | 计算机系统与硬件
A computer system is made up of hardware, software, and data. Hardware refers to the physical parts you can touch, such as the CPU, RAM, and input/output devices.
计算机系统由硬件、软件和数据组成。硬件指你能触摸到的物理部件，例如 CPU、RAM 和输入/输出设备。
The CPU is often called the brain of the computer. It fetches, decodes, and executes instructions in a cycle known as the fetch-decode-execute cycle.
CPU 常被称为计算机的大脑。它按照“取指-译码-执行”循环来获取、译码并执行指令。
Input devices include keyboards, mice, scanners, and sensors. Output devices include monitors, printers, and speakers.
输入设备包括键盘、鼠标、扫描仪和传感器。输出设备包括显示器、打印机和扬声器。
| Input Devices | Output Devices |
| Keyboard, mouse, scanner | Monitor, printer, speaker |
3. Data Representation | 数据表示
Computers store all data as binary numbers made up of 1s and 0s. A single binary digit is called a bit. Eight bits form a byte.
计算机将所有数据存储为由1和0组成的二进制数。单个二进制位称为比特。八个比特组成一个字节。
2, 1. Then add the values where a 1 appears. For example, 00101010 equals 32 + 8 + 2 = 42.
要将二进制数转换为十进制，写出位权值：128、64、32、16、8、4、2、1。然后将出现1的位权值相加。例如 00101010 等于 32 + 8 + 2 = 42。
Denary numbers can be converted to binary by repeated division by 2, noting the remainder each time, then reading the remainders upwards.
十进制数转换为二进制可通过不断除以2并记录余数完成，然后从下往上读取余数。
Binary to denary: 00101010₂ = 0×128 + 0×64 + 1×32 + 0×16 + 1×8 + 0×4 + 1×2 + 0×1 = 42
4. Binary and Hexadecimal | 二进制与十六进制
digits 0-9 and the letters A-F, where A = 10, B = 11, C = 12, D = 13, E = 14, and F = 15.
十六进制是以16为基数的数字系统。它使用数字0-9和字母A-F，其中 A = 10、B = 11、C = 12、D = 13、E = 14、F = 15。
Hexadecimal is useful because one hex digit represents exactly four binary digits. For example, the binary nibble 1010 is A in hex, and 1111 is F.
十六进制很有用，因为一个十六进制数字恰好表示四位二进制数。例如二进制四位组 1010 在十六进制中是 A，1111 是 F。
To convert binary to hex, split the binary number into groups of four bits from the right, then convert each group to its hex symbol.
要将二进制转换为十六进制，请从右边开始将二进制数分成四位一组，然后将每组转换为其十六进制符号。
| Denary | Binary | Hexadecimal |
| 10 | 1010 | A |
| 15 | 1111 | F |
| 26 | 0001 1010 | 1A |
5. Logic Gates and Circuits | 逻辑门与电路
Logic gates are the basic building blocks of digital circuits. The three fundamental gates are NOT, AND, and OR.
逻辑门是数字电路的基本构建模块。三种基本门是 NOT、AND 和 OR。
A NOT gate has one input and outputs the opposite value. If the input is 1, the output is 0; if the input is 0, the output is 1.
NOT 门有一个输入并输出相反值。如果输入为1，输出为0；如果输入为0，输出为1。
An AND gate outputs 1 only when all inputs are 1. An OR gate outputs 1 when at least one input is 1.
AND 门仅在所有输入均为1时输出1。OR 门在至少一个输入为1时输出1。
You may also meet NAND, NOR, and XOR gates in Unit 1. NAND is NOT AND; NOR is NOT OR; XOR outputs 1 when the inputs are different.
在单元1中你还可能遇到 NAND、NOR 和 XOR 门。NAND 是 AND 的非；NOR 是 OR 的非；XOR 在输入不同时输出1。
| A | B | A AND B | A OR B |
| 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 |
6. Memory and Storage | 内存与存储
Memory in a computer is divided into primary storage and secondary storage. Primary storage includes RAM and cache, and it is directly accessed by the CPU.
计算机中的存储器分为主存储器和辅助存储器。主存储器包括 RAM 和高速缓存，它由 CPU 直接访问。
RAM is volatile, meaning it loses its data when the power is turned off. ROM is non-volatile and stores the startup instructions.
RAM 是易失性的，意味着断电后数据会丢失。ROM 是非易失性的，存储启动指令。
Secondary storage such as HDD, SSD, and USB flash drives stores data permanently. SSDs are faster and more durable than HDDs because they have no moving parts.
辅助存储器（如 HDD、SSD 和 U盘）永久存储数据。SSD 比 HDD 更快、更耐用，因为它们没有移动部件。
Storage capacity is measured in bytes. Common units go from kilobyte (KB), megabyte (MB), gigabyte (GB), to terabyte (TB).
存储容量以字节为单位。常用单位从千字节（KB）、兆字节（MB）、千兆字节（GB）到太字节（TB）。
7. Networks and Communication | 网络与通信
A network is two or more computers connected together to share data and resources. Networks can be classified as LAN or WAN.
网络是由两台或更多计算机连接在一起以共享数据和资源。网络可分为局域网（LAN）和广域网（WAN）。
A LAN covers a small geographical area such as a school or office. A WAN covers a large area, often using public infrastructure like the internet.
局域网覆盖较小地理区域，如学校或办公室。广域网覆盖较大区域，通常使用互联网等公共基础设施。
Network hardware includes routers, switches, and network interface cards (NICs). Routers forward data between different networks; switches connect devices within the same network.
网络硬件包括路由器、交换机和网络接口卡（NIC）。路由器在不同网络之间转发数据；交换机连接同一网络内的设备。
Common network topologies are star, bus, ring, and mesh. A star topology is widely used because if one cable fails, only that device is affected.
常见的网络拓扑结构有星型、总线型、环型和网状。星型拓扑被广泛使用，因为一条电缆故障只会影响该设备。
8. Programming Basics | 编程基础
Programming is the process of writing instructions for a computer to follow. Unit 1 often introduces sequence, selection, and iteration as the three basic control structures.
编程是为计算机编写要遵循的指令的过程。单元1通常介绍顺序、选择和迭代作为三种基本控制结构。
Sequence means instructions are executed one after another. Selection uses conditions such as IF...THEN...ELSE
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