📚 8C Unit 1: Computer Systems and Data Representation | 8C 第一单元：计算机系统与数据表示
Welcome to the 8C Unit 1 revision guide. This unit introduces the core ideas behind computer systems, including hardware and software, data representation, logic gates, operating systems and networks. Understanding these foundations is essential for every later topic in computer science.
欢迎阅读 8C 第一单元复习指南。本单元介绍计算机系统的核心概念，包括硬件与软件、数据表示、逻辑门、操作系统和网络。理解这些基础对后续所有计算机科学主题都至关重要。
1. What is a Computer System? | 什么是计算机系统？
A computer system is any device or group of devices that takes an input, processes it, and produces an output. This is often called the Input-Process-Output model. For example, a smartphone receives touch input, processes the command, and displays the result.
计算机系统是任何接收输入、处理输入并产生输出的设备或设备组合。这通常称为输入-处理-输出模型。例如，智能手机接收触摸输入，处理指令，并显示结果。
Computer systems can be general-purpose, such as desktops and laptops, or embedded, such as the processor inside a washing machine. Embedded systems are designed for one specific function and are often more reliable and efficient.
计算机系统可以是通用系统，如台式机和笔记本电脑；也可以是嵌入式系统，如洗衣机内部的处理器。嵌入式系统专为某个特定功能设计，通常更可靠、更高效。
2. Hardware vs Software | 硬件与软件
Hardware is the physical part of a computer: the case, motherboard, CPU, memory, storage devices, keyboard and monitor. Software is the set of instructions that tells the hardware what to do.
硬件是计算机的物理部分：机箱、主板、中央处理器、内存、存储设备、键盘和显示器。软件是告诉硬件如何工作的一组指令。
There are two main types of software: system software and application software. System software manages hardware and provides a platform, while application software lets users complete tasks such as writing documents or browsing the web.
软件主要有两类：系统软件和应用软件。系统软件管理硬件并提供运行平台，而应用软件让用户完成写文档、浏览网页等任务。
3. Input Devices | 输入设备
Input devices send data into a computer system. Common examples include keyboards, mice, touchscreens, microphones, scanners, cameras and sensors. Some devices are manual, like a keyboard, because a human must operate them. Others are automatic, such as a temperature sensor.
输入设备将数据送入计算机系统。常见例子包括键盘、鼠标、触摸屏、麦克风、扫描仪、摄像头和传感器。有些设备是手动的，如键盘，因为必须由人操作；另一些是自动的，如温度传感器。
When choosing an input device, we consider accuracy, speed, ease of use, cost and the type of data being collected. For example, a barcode scanner is fast and accurate for reading product codes.
选择输入设备时，我们要考虑准确性、速度、易用性、成本和所采集数据的类型。例如，条形码扫描仪读取商品条码既快又准确。
4. Output Devices | 输出设备
Output devices present processed data to the user. Examples include monitors, printers, speakers, headphones and projectors. A monitor displays visual output, while a speaker produces audible output.
输出设备将处理后的数据呈现给用户。例子包括显示器、打印机、扬声器、耳机和投影仪。显示器显示视觉输出，扬声器产生听觉输出。
Some devices are both input and output. A touchscreen receives touch input and displays visual output at the same time. This is why touchscreens are common in smartphones and information kiosks.
有些设备既是输入设备也是输出设备。触摸屏同时接收触摸输入并显示视觉输出。这就是触摸屏在智能手机和信息亭中很常见的原因。
5. Storage Devices and Media | 存储设备与介质
Storage is needed to keep data and programs when the power is off. Primary storage, such as RAM, is fast but volatile. Secondary storage, such as a hard disk drive (HDD), solid-state drive (SSD), optical disc or USB flash drive, is non-volatile and used for long-term storage.
存储用于在断电时保存数据和程序。主存储器（如内存 RAM）速度快但是易失性的。辅助存储器（如硬盘 HDD、固态硬盘 SSD、光盘或 USB 闪存盘）是非易失性的，用于长期保存。
Magnetic storage uses magnetised surfaces; optical storage uses lasers and reflected light; solid-state storage uses flash memory with no moving parts. SSDs are faster and more durable than HDDs but are usually more expensive per gigabyte.
磁介质存储使用磁化表面；光介质存储使用激光和反射光；固态存储使用没有活动部件的闪存。SSD 比 HDD 更快、更耐用，但每 GB 通常更贵。
6. Binary and Denary Numbers | 二进制与十进制
Computers use binary, the base-2 number system, because transistors have two states: on and off. A binary digit, or bit, can be 0 or 1. Denary is the base-10 system we use every day.
计算机使用二进制（基数为 2 的数字系统），因为晶体管有两种状态：开和关。一个二进制位（bit）可以是 0 或 1。十进制是我们日常使用的基数为 10 的系统。
To convert a denary number to binary, repeatedly divide by 2 and record the remainders. For example, 13 in denary is 1101 in binary because 13 = 8 + 4 + 1.
将十进制数转换为二进制时，不断除以 2 并记录余数。例如，十进制的 13 等于二进制的 1101，因为 13 = 8 + 4 + 1。
13₁₀ = 1101₂
7. Hexadecimal and Data Units | 十六进制与数据单位
Hexadecimal is base-16 and uses the digits 0-9 and letters A-F. It is used because one hex digit represents exactly four binary bits, making long binary numbers shorter and easier to read.
十六进制是基数为 16 的系统，使用数字 0-9 和字母 A-F。使用它的原因是一个十六进制位正好表示四个二进制位，使较长的二进制数更短、更易读。
Common data units are the bit, nibble (4 bits), byte (8 bits), kilobyte (about 1000 bytes), megabyte, gigabyte and terabyte. A byte can store values from 00000000 to 11111111 in binary, which is 0 to 255 in denary or 00 to FF in hexadecimal.
常见数据单位有 bit（位）、nibble（半字节，4 位）、byte（字节，8 位）、KB（约 1000 字节）、MB、GB 和 TB。一个字节可以存储二进制 00000000 到 11111111，即十进制 0 到 255，或十六进制 00 到 FF。
1 byte = 8 bits = 2 hex digits
8. Logic Gates and Truth Tables | 逻辑门与真值表
Logic gates are the basic building blocks of digital circuits. Each gate takes one or more binary inputs and produces one binary output. The main gates in this unit are AND, OR and NOT.
逻辑门是数字电路的基本构建模块。每个逻辑门接收一个或多个二进制输入，产生一个二进制输出。本单元主要学习与门、或门和非门。
An AND gate outputs 1 only when all inputs are 1. An OR gate outputs 1 when at least one input is 1. A NOT gate, or inverter, flips the input: 0 becomes 1 and 1 becomes 0.
与门只有在所有输入均为 1 时才输出 1。或门在至少一个输入为 1 时输出 1。非门（反相器）将输入翻转：0 变成 1，1 变成 0。
The table below shows the truth tables for the AND and OR gates with two inputs A and B.
下表显示了有两个输入 A 和 B 的与门和或门真值表。
| A | B | A AND B | A OR B |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 |
9. Operating Systems and Utility Software | 操作系统与实用软件
The operating system is system software that manages hardware resources and provides a user interface. It handles memory management, processor scheduling, file management, security and input/output control.
操作系统是管理硬件资源并提供用户界面的系统软件。它负责内存管理、处理器调度、文件管理、安全以及输入/输出控制。
Utility software performs maintenance tasks. Examples include antivirus programs, backup software, disk defragmenters, compression tools and encryption software. These tools help keep a computer secure, organised and efficient.
实用软件执行维护任务。例子包括防病毒程序、备份软件、磁盘碎片整理程序、压缩工具和加密软件。这些工具帮助计算机保持安全、有序和高效。
10. Networks and the Internet | 网络与互联网
A network is two or more computers connected together to share data and resources. Networks can be local area networks (LANs) covering a small site, or wide area networks (WANs) covering large geographical areas. The Internet is the largest WAN.
网络是将两台或更多
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