📚 7S Homework Unit 4: Computer Systems, Binary and Logic | 7S 作业单元4：计算机系统、二进制与逻辑
This Unit 4 revision guide covers the core knowledge you need for the 7S Computer Science homework: how a computer system works, the difference between hardware and software, how data is stored in binary, and how simple logic gates make decisions. Work through each section and use the checklists to test yourself before the deadline.
本单元4复习指南涵盖 7S 计算机科学作业所需的核心知识：计算机系统如何工作、硬件与软件的区别、数据如何以二进制存储，以及简单逻辑门如何做出判断。请逐节学习，并利用清单在截止日期前进行自测。
1. What Is a Computer System? | 什么是计算机系统？
A computer system is a combination of hardware and software that works together to input, process, store and output data. The basic model is Input → Process → Output, often with storage involved at every stage.
计算机系统是由硬件和软件共同组成，能够输入、处理、存储和输出数据的系统。基本模型是输入 → 处理 → 输出，而且在每个阶段通常都涉及存储。
For example, when you type on a keyboard, the keystrokes are input data. The CPU processes this data, RAM stores it temporarily, and the monitor displays the result as output. This model is examinable in both short-answer and scenario questions.
例如，当你在键盘上打字时，按键是输入数据。CPU 处理这些数据，RAM 临时存储数据，显示器将结果显示为输出。这一模型在简答题和情景题中都会考查。
- Input device – 输入设备 (keyboard, mouse, sensor)
- Process – 处理 (CPU, GPU)
- Storage – 存储 (RAM, SSD, hard disk)
- Output device – 输出设备 (monitor, printer, speaker)
2. Hardware and Software | 硬件与软件
Hardware refers to the physical parts of a computer that you can touch, such as the motherboard, CPU, RAM, power supply and hard drive. Software is the set of instructions or programs that tells the hardware what to do.
硬件是指计算机中可以触摸到的物理部件，例如主板、CPU、内存、电源和硬盘。软件是指告诉硬件该做什么的指令或程序集合。
Software can be divided into system software and application software. System software includes operating systems like Windows, macOS and Linux, as well as utility programs. Application software includes programs that help users perform specific tasks, such as word processors, web browsers and games.
软件可以分为系统软件和应用软件。系统软件包括 Windows、macOS 和 Linux 等操作系统，以及实用工具程序。应用软件包括帮助用户完成特定任务的程序，例如文字处理器、网络浏览器和游戏。
| Type 类型 | Example 示例 | Purpose 用途 |
|---|---|---|
| System software 系统软件 | Operating system | Manages hardware and runs other software 管理硬件并运行其他软件 |
| Utility software 实用工具软件 | Antivirus, disk defragmenter | Maintains or protects the system 维护或保护系统 |
| Application software 应用软件 | Word processor, browser | Helps the user complete tasks 帮助用户完成任务 |
3. Input, Output and Storage Devices | 输入、输出与存储设备
An input device sends data to the computer. Examples are keyboard, mouse, microphone, scanner, barcode reader and temperature sensor. An output device sends data from the computer to the user or another system. Examples are monitor, printer, speakers and actuators.
输入设备将数据发送到计算机。例如键盘、鼠标、麦克风、扫描仪、条形码阅读器和温度传感器。输出设备将数据从计算机发送给用户或其他系统。例如显示器、打印机、扬声器和执行器。
Storage devices hold data permanently or temporarily. Magnetic storage includes hard disk drives and tape. Optical storage includes CDs, DVDs and Blu-ray. Solid-state storage includes SSDs, USB flash drives and SD cards, which use flash memory and have no moving parts.
存储设备永久或临时保存数据。磁存储包括硬盘驱动器和磁带。光存储包括 CD、DVD 和蓝光光盘。固态存储包括 SSD、U盘和 SD 卡，它们使用闪存并且没有移动部件。
- Magnetic – 磁存储: HDD, magnetic tape
- Optical – 光存储: CD, DVD, Blu-ray
- Solid state – 固态存储: SSD, USB flash drive, SD card
4. Inside the CPU | CPU 内部结构
The CPU, or Central Processing Unit, is often called the brain of the computer. It fetches, decodes and executes instructions. The three main parts inside the CPU are the Control Unit (CU), the Arithmetic Logic Unit (ALU) and registers.
CPU，即中央处理器，通常被称为计算机的大脑。它负责取指令、解码和执行指令。CPU 内部的三个主要部分是控制单元（CU）、算术逻辑单元（ALU）和寄存器。
The Control Unit directs the flow of data and controls the fetch-decode-execute cycle. The ALU carries out arithmetic operations such as addition and subtraction, as well as logic operations such as AND, OR and NOT. Registers are small, very fast memory locations inside the CPU that store data currently being used.
控制单元引导数据流动并控制取指-解码-执行周期。ALU 执行加法、减法等算术运算，以及 AND、OR、NOT 等逻辑运算。寄存器是 CPU 内部非常小的快速存储位置，用于保存当前正在使用的数据。
Fetch → Decode → Execute | 取指 → 解码 → 执行
5. Memory: RAM and ROM | 内存：RAM 与 ROM
RAM stands for Random Access Memory. It is volatile, meaning it loses all data when the power is turned off. RAM stores the operating system, running programs and current data while the computer is on. More RAM usually means the computer can run more programs at the same time.
RAM 代表随机存取存储器。它是易失性的，即断电后会丢失所有数据。当计算机开启时，RAM 存储操作系统、正在运行的程序和当前数据。RAM 越大通常意味着计算机能够同时运行更多程序。
ROM stands for Read Only Memory. It is non-volatile, so it keeps its contents even when the power is off. ROM is used to store firmware and the BIOS, which is the boot program that starts the computer when first switched on. Unlike RAM, ROM is not normally rewritten by the user.
ROM 代表只读存储器。它是非易失性的，因此即使断电也能保留内容。ROM 用于存储固件和 BIOS，即首次开机时启动计算机的引导程序。与 RAM 不同，ROM 通常不会被用户重新写入。
| Feature 特征 | RAM | ROM |
|---|---|---|
| Volatile 易失性 | Yes 是 | No 否 |
| Read/write 读写 | Read and write 可读写 | Mostly read only 主要为只读 |
| Use 用途 | Temporary working memory 临时工作内存 | BIOS/firmware BIOS/固件 |
6. Binary Number System | 二进制数系统
Computers use binary because electronic circuits can easily represent two states: on (1) and off (0). Each binary digit is called a bit. Eight bits make one byte. Binary is a base-2 number system, while the number system humans use every day, denary, is base-10.
计算机使用二进制，因为电子电路可以轻松表示两种状态：开（1）和关（0）。每个二进制数字称为一个位（bit）。八位组成一个字节（byte）。二进制是以 2 为基数的数制，而人类日常使用的十进制是以 10 为基数的数制。
In an 8-bit binary number, each bit position has a place value that is a power of 2. These values from left to right are 128, 64, 32, 16, 8, 4, 2 and 1. This table is essential for all binary conversion questions.
在 8 位二进制数中，每个位的位置值都是 2 的幂。从左到右这些值分别是 128、64、32、16、8、4、2 和 1。这个表格对于所有二进制转换题都至关重要。
128 64 32 16 8 4 2 1
- 1 byte = 8 bits | 1 字节 = 8 位
- 0b1010 = 10 in denary | 0b1010 = 十进制中的 10
- 0b1111 = 15 in denary | 0b1111 = 十进制中的 15
7. Binary-Denary Conversion | 二进制与十进制转换
To convert an 8-bit binary number to denary, write the place values 128, 64, 32, 16, 8, 4, 2, 1 above the bits and add the place values wherever there is a 1. For example, to convert 01101001, add 64 + 32 + 8 + 1 = 105.
要将 8 位二进制数转换为十进制，将位值 128、64、32、16、8、4、2、1 写在各位上方，然后在所有为 1 的位上把位值相加。例如，要转换 01101001，计算 64 + 32 + 8 + 1 = 105。
To convert denary to binary, start from the largest place value and work downwards. For the number 105, 128 is too large so place 0; 64 fits, so place 1 and subtract 64 from 105 to leave 41; 32 fits, so place 1 and subtract 32 to leave 9; then 16 and 8: 16 is too large, but 8 fits, leaving 1; finally 4, 2 and 1 give 0, 0, 1. This gives 01101001.
要将十进制转换为二进制，从最大的位值开始向下进行。对于数字 105，128 太大，所以写 0；64 合适，写 1 并从 105 中减去 64 得 41；32 合适，写 1 并减去 32 得 9；然后 16 和 8：16 太大，但 8 合适，余 1；最后 4、2、1 分别为 0、0、1。这样就得到 01101001。
| Denary 十进制 | Binary 二进制 |
|---|---|
| 0 | 00000000 |
| 1 | 00000001 |
| 15 | 00001111 |
| 105 | 01101001 |
| 255 | 11111111 |
8. Hexadecimal Basics | 十六进制基础
Hexadecimal, or hex, is a base-16 number system used by humans to represent binary values in a shorter form. It uses the digits 0-9 and the letters A-F, where A = 10, B = 11, C = 12, D = 13, E = 14 and F = 15. One hex digit represents exactly four binary bits, or a nibble.
十六进制（hex）是以 16 为基数的数制，人类用它来更简短地表示二进制值。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。一位十六进制数字正好代表四位二进制位，即一个半字节（nibble）。
To convert an 8-bit binary number to hex, split the bits into two nibbles. For example, 01101001 becomes 0110 and 1001. 0110 is 6 in hex, and 1001 is 9 in hex, so 01101001 = 69 in hex. Hex values are often shown with a 0x prefix or an h suffix.
要将 8 位二进制数转换为十六进制，将位分成两个半字节。例如，01101001 变成 0110 和 1001。0110 在十六进制中是 6，1001 是 9，因此 01101001 = 十六进制中的 69。十六进制值通常用 0x 前缀或 h 后缀表示。
- 0b1111 = 0xF = 15 | 0b1111 = 0xF = 15
- 0b1010 = 0xA = 10 | 0b1010 = 0xA = 10
- 0b01101001 = 0x69 = 105 | 0b01101001 = 0x69 = 105
9. Logic Gates | 逻辑门
Logic gates are small electronic circuits that produce an output based on one or more binary inputs. The three fundamental gates are NOT, AND and OR. Their behaviour is described using truth tables.
逻辑门是小型电子电路，根据一个或多个二进制输入产生输出。三种基本门是 NOT、AND 和 OR。它们的行为用真值表来描述。
A NOT gate has one input and one output. It inverts the input: if the input is 0, the output is 1, and if the input is 1, the output is 0. An AND gate outputs 1 only when both inputs are 1. An OR gate outputs 1 when at least one input is 1.
NOT 门有一个输入和一个输出。它将输入取反：如果输入是 0，输出就是 1；如果输入是 1，输出就是 0。AND 门仅当两个输入都为 1 时才输出 1。OR 门在至少一个输入为 1 时输出 1。
| Gate 门 | Input A 输入A | Input B 输入B | Output 输出 |
|---|---|---|---|
| NOT | 0 | — | 1 |
| NOT | 1 | — | 0 |
| AND | 0 | 0 | 0 |
| AND | 1 | 1 | 1 |
| OR | 0 | 0 | 0 |
| OR | 1 | 0 | 1 |
10. Revision Checklist for Unit 4 | 单元4复习清单
Use this checklist to make sure you are ready for the 7S Unit 4 homework. You should be able to explain the input-process-output model, give at least two examples of each type of device, and describe the fetch-decode-execute cycle without notes.
使用这份清单来确保你已经为 7S 单元4作业做好准备。你应该能够解释输入-处理-输出模型，为每一类设备至少举出两个例子，并在不看笔记的情况下描述取指-解码-执行周期。
- Define hardware and software and give examples. 定义硬件和软件并举例。
- Describe the CPU, CU, ALU and registers. 描述 CPU、CU、ALU 和寄存器。
- Compare RAM and ROM. 比较 RAM 和 ROM。
- Convert between 8-bit binary and denary. 在 8 位二进制和十进制之间转换。
- Convert between binary and hexadecimal. 在二进制和十六进制之间转换。
- Draw and complete truth tables for NOT, AND and OR gates. 画出并完成 NOT、AND 和 OR 门的真值表。
- Give real-life examples of storage media. 举出存储介质的实际例子。
Work through past paper questions on binary conversion and logic gates because these are the most common marks in Unit 4. If you can explain each row of the truth tables and show all working for conversions, you will be well prepared.
请完成有关二进制转换和逻辑门的历年真题，因为这些是单元4中最常见的得分点。如果你能解释真值表的每一行，并在转换中展示所有步骤，你就会准备得非常充分。
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