📚 AS CIE Computer Science Revision Notes | AS CIE 计算机科学复习笔记
This guide summarises the core topics examined in Cambridge International AS Level Computer Science (9618). It is organised into ten focused sections covering data representation, networks, hardware, processor operation, software, security, ethics and ownership, databases, algorithm design and programming fundamentals. Each section gives short English explanations followed by matched Chinese explanations to support bilingual learners preparing for the CIE AS examination.
本指南总结了剑桥国际 AS 计算机科学（9618）考试的核心主题。它分为十个重点小节，涵盖数据表示、网络、硬件、处理器运行、软件、安全、伦理与所有权、数据库、算法设计和编程基础。每个小节先提供简短的英文解释，紧接对应的中文解释，帮助备考 CIE AS 考试的双语学习者理解。
1. Information Representation | 信息表示
Computers store and process data in binary because digital circuits have two stable states, usually represented as 1 and 0. A single binary digit is called a bit, a group of 4 bits is a nibble, and 8 bits form one byte. Larger quantities are measured in kilobytes, megabytes, gigabytes and terabytes. Since long binary strings are difficult for humans to read, hexadecimal is often used as a compact alternative because one hex digit represents exactly four bits.
计算机以二进制存储和处理数据，因为数字电路有两种稳定状态，通常表示为 1 和 0。一个二进制位称为 bit，4 位组成一个 nibble，8 位组成一个字节。更大的单位有千字节、兆字节、吉字节和太字节。由于长二进制串难以阅读，通常使用十六进制作为简洁的替代表示，因为一个十六进制数字恰好代表四位二进制数。
ord the remainders. 216 ÷ 16 = 13 remainder 8, and 13 is the hex digit D, so 216₁₀ = D8₁₆.
将二进制数如 1011₂ 转换为十进制时，将各位权值相加：1×2³ + 0×2² + 1×2¹ + 1×2⁰ = 8 + 0 + 2 + 1 = 11₁₀。将十进制数 216 转换为十六进制时，反复除以 16 并记录余数。216 ÷ 16 = 13 余 8，13 的十六进制数字为 D，因此 216₁₀ = D8₁₆。
Negative integers are commonly represented using two' esentation allows addition and subtraction to share the same hardware, and binary addition follows the same rules for positive and negative numbers.
负整数通常使用二的补码表示。对于 8 位数字，最高有效位的权值为 -128 而不是 +128。例如，在 4 位系统中，1101₂ 的值为 1×(-8) + 1×4 + 0×2 + 1×1 = -3。这种表示使加法和减法可以共用同一套硬件，二进制加法对正数和负数遵循相同规则。
Sound is represented by sampling the amplitude of a wave at regular intervals. Sample resolution is the number of bits used for each sample, and sampling rate is the number of samples captured per second. Higher resolution and sampling rate improve sound quality but increase file size. Images are built from pixels; colour depth is the number of bits per pixel, and resolution is the total number of pixels in the image.
声音通过在固定时间间隔对波形振幅进行采样来表示。采样分辨率是每个样本使用的位数，采样率是每秒采集的样本数。分辨率和采样率越高，音质越好，但文件也越大。图像由像素构成；颜色深度是每个像素的位数，分辨率是图像中的像素总数。
2. Communication and Internet Technologies | 通信与互联网技术
A local area network covers a small geographical area such as a school or office and usually has high data transfer rates and low latency. A wide area network covers a larger area and often uses leased telecommunications lines or the public internet. A personal area network connects devices around one person, typically using Bluetooth. Networks can be arranged in star, bus, mesh or ring topologies, with star being common in modern LANs.
局域网覆盖学校或办公室等较小地理区域，通常具有高数据传输速率
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