AS计算机 核心笔记

Important Note

AS CS是一门非常难的学科。哪怕有IG作为基础，AS阶段依然增加了许多新且深入的概念，并且由于概念更加深入，对于英语语言表达的要求会更高。同时概念也更多之后，一定多熟悉多记背，才能不在后续学习时感觉压力太大。及时解决当前单元的问题非常重要。

Learning Journey

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Congratulations! You've completed all items!

• Information Representation

  Data types, number systems, multimedia encoding
• Communication & Networks

  Network types, topologies, internet fundamentals
• Practice Problems

  Complete all exercises in this guide
• Review Key Concepts

  Memorize critical formulas and definitions

Reset Progress

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1. Information Representation

1.1 Data Representation

Understanding how computers store and manipulate data is fundamental to everything in computer science. Let’s break down the key concepts you’ll need to master.

Number Systems and Prefixes

Binary vs. Decimal Prefixes

You’ve probably noticed that a “1 TB” hard drive doesn’t actually give you 1 TB of usable space - this is why! Manufacturers use decimal (1000-based) prefixes, but operating systems often report using binary (1024-based) prefixes.

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Number Systems You Must Know

Number System	Base	Digits Used	Example
Binary	2	0, 1
Denary (Decimal)	10	0-9
Hexadecimal	16	0-9, A-F
Binary Coded Decimal (BCD)	Special	4 bits per digit

BCD Trick

BCD uses four binary bits (a nibble) to represent each decimal digit separately. So denary becomes in BCD - that’s 5 (0101) followed by 2 (0010). Don’t confuse it with regular binary, which would be !

Number System Conversion

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Representing Negative Numbers

Computers need a systematic way to represent negative numbers. Two’s complement has become the standard because it’s incredibly elegant for arithmetic.

Negative Binary Representation

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Binary Arithmetic

Here’s where two’s complement really shines. Want to calculate ? Just compute !

Binary Subtraction using Two's Complement

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Watch Out for Overflow!

Overflow occurs when the result exceeds the range your fixed number of bits can represent. For 8-bit signed numbers, the range is to . Adding would cause overflow because !

Practical Applications

Understanding these number systems isn’t just academic - they’re used everywhere in computing:

Application	Number System	Why?
Financial calculations	BCD	Exact decimal representation, no floating-point errors
Digital displays/clocks	BCD	Each digit needs separate display
Memory addresses	Hexadecimal	Compact, easy to convert to/from binary
MAC addresses	Hexadecimal	e.g., A4:5E:60:E6:1B:37
Assembly code	Hexadecimal	Human-readable machine code

Character Encoding

Character Sets Evolution

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1.2 Multimedia

How do we store images and sound? They’re fundamentally different from text and numbers - they’re continuous in the real world but must become discrete digital data.

Graphics: Bitmap vs. Vector

Two Fundamentally Different Approaches

Aspect	Bitmap Image	Vector Graphic
How it’s stored	Grid of pixels, each pixel has a color value	Mathematical formulas for lines, curves, shapes
Key terms	Pixel, Image Resolution, Colour Depth, File Header	Drawing Object, Property, Drawing List
File size formula		Depends on number and complexity of objects
Quality when scaled	Becomes pixelated (blocky)	No quality loss - recalculated mathematically
Best for	Photographs, complex images with subtle color variations	Logos, text, illustrations that need to scale

Bitmap File Size Calculation

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When to Use Which

Use Bitmap when you need photorealistic images with subtle color gradients - like photos from a camera.

Use Vector when you need crisp, scalable graphics - like company logos that need to look perfect on both business cards and billboards!

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Sound Encoding

Sound waves are continuous (analog), but computers need discrete (digital) data. The solution? Sampling.

From Analog to Digital

Parameter	What it controls	Impact of increasing it
Sampling Rate	Number of samples per second	⬆️ Better capture of high frequencies, larger file
Sampling Resolution	Bits used to store each sample’s amplitude	⬆️ More accurate volume representation, larger file

Note

The CD audio standard uses a 44,100 Hz sampling rate and 16-bit resolution. Why 44,100 Hz? The Nyquist theorem says you need at least twice the highest frequency you want to capture. Human hearing goes up to about 20 kHz, so Hz minimum. They chose 44.1 kHz to have some safety margin.

Audio File Size Calculation

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1.3 Compression

Why compress? Two main reasons: faster transmission (less bandwidth needed) and reduced storage space.

Lossless vs. Lossy

Compression Type	Data Loss?	Best Used For	Example Formats
Lossless	❌ None	Text files, program code, medical scans - anything where integrity is critical	ZIP, PNG, FLAC
Lossy	✓ Some	Multimedia where slight quality loss is acceptable	JPEG, MP3, MP4

Choose Wisely

Never use lossy compression on program code, legal documents, or medical data! You can’t afford to lose even a single bit. But for streaming music or web images? Lossy compression can reduce file sizes by 90% with minimal perceptible quality loss.

Common Compression Methods

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2. Communication

2.1 Networks Including the Internet

Welcome to the world of networked computing! Let’s start with the fundamental building blocks.

Network Types: LAN vs. WAN

Understanding Network Scale

Feature	LAN	WAN
Geographic coverage	Small (building, campus)	Large (city, country, global)
Ownership	Single organization	Multiple parties, uses public infrastructure
Speed	Very fast (Gbps common)	Slower (depends on connection type)
Cost	Lower setup and maintenance	Higher, especially for dedicated lines
Example	School computer lab network	The Internet

Network Models: Client-Server vs. Peer-to-Peer

Two Ways to Organize Network Resources

Client-Server Model

✅ Benefits:

• Centralized security and backups
• Easier to manage and update
• Better performance for shared resources

❌ Drawbacks:

• High setup cost (powerful server needed)
• Single point of failure
• Requires skilled IT staff

Peer-to-Peer Model

✅ Benefits:

• Cheap to set up (no server needed)
• Robust (no single point of failure)
• Easy for small groups

❌ Drawbacks:

• Poor security (hard to control)
• No centralized backups
• Difficult to manage at scale

Real-World Examples

Client-Server: School network, corporate email, web hosting

Peer-to-Peer: BitTorrent, small home networks, blockchain systems

Thin-Client vs. Thick-Client

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Network Topologies

The physical or logical arrangement of network devices matters a lot for performance, reliability, and cost!

Common Network Topologies

Topology	Diagram Concept	Advantages	Disadvantages	Best For
Bus	Linear cable with taps	Simple, cheap	Backbone failure = total failure, collisions	Small networks, legacy systems
Star	Central hub/switch	Robust, easy to troubleshoot	Central device failure = network down	Modern LANs, offices
Mesh	Interconnected web	No single point of failure, fast	Very expensive, complex setup	Critical infrastructure, data centers

Topology Selection

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Cloud Computing

“The cloud” is just someone else’s computer - but it’s revolutionized how we think about infrastructure!

Cloud Computing Fundamentals

Aspect	Public Cloud	Private Cloud
Provider	Third-party (Amazon, Google, Microsoft)	Your organization (or dedicated hosting)
Sharing	Multi-tenant (shared with others)	Single-tenant (exclusive)
Cost	Lower upfront, pay-as-you-go	Higher initial investment
Security	Provider’s responsibility, shared environment	More control, isolated environment
Best for	Startups, variable workloads, web apps	Sensitive data, compliance requirements

Cloud Isn't Always the Answer

Benefits: Scalability, reduced capital costs, global reach

Drawbacks: Dependency on internet, ongoing costs can exceed ownership, vendor lock-in, data privacy concerns

Consider carefully before moving critical systems!

Wired vs. Wireless Transmission

Connection Types

Wired Technologies:

• Copper cable (UTP): Cheap, good for short distances
• Fibre-optic: Very fast over long distances using light pulses, expensive

Wireless Technologies:

• WiFi: Local wireless, good for offices/homes
• Microwaves: Point-to-point, line-of-sight required
• Satellites: Global coverage, high latency

Essential LAN Hardware

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Device	Function	Works at Layer	Uses Address Type
Switch	Connect devices in LAN, direct traffic to intended recipient	Data Link	MAC address
Router	Connect different networks, route packets between them	Network	IP address
NIC/WNIC	Allow device to connect to network (wired/wireless)	Physical/Data Link	Has MAC address
WAP	Wireless Access Point - connects wireless devices to wired network	Physical/Data Link	-
Repeater	Boost signal strength for longer distances	Physical	-
Bridge	Connect two similar network segments	Data Link	-

Ethernet and CSMA/CD

Ethernet is the standard for LANs, and CSMA/CD is how devices “take turns” talking on a shared medium.

CSMA/CD - The "Listen Before Speaking" Protocol

Think of CSMA/CD like a polite conversation:

1. Listen before you speak (Carrier Sense)
2. If someone else is talking, wait
3. If two people start at the same time (Collision), both stop and wait before trying again

Note

Modern switched Ethernet networks don’t need CSMA/CD as much because switches create dedicated paths. But it’s still important for wireless networks (CSMA/CA - Collision Avoidance) and understanding network history!

Bit Streaming

How do you watch Netflix without downloading the entire movie first? Streaming!

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Internet vs. World Wide Web

Not the Same Thing!

The internet is like the road system, the web is like all the shops and houses you can visit on those roads. Email, gaming, file transfer - these all use the internet but aren’t part of the web!

IP Addresses - The Internet’s Addressing System

Every device needs an address to receive data. That’s where IP addresses come in!

IP Address Fundamentals

IP Type	Public IP	Private IP
Scope	Globally unique, used on internet	Only within local network
Assignment	ISP provides	Router assigns internally
Example ranges	Any valid public IP	192.168.x.x, 10.x.x.x, 172.16.x.x
Security	Directly accessible from internet	Hidden behind router (NAT) - more secure

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Why Private IPs Matter

Private IP addresses (like 192.168.1.x) let multiple devices share one public IP address through a router. This helps conserve the limited IPv4 address space AND improves security by hiding your internal network from the internet!

Understanding Subnetting

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URL and DNS - Making the Internet Human-Friendly

Nobody wants to remember 172.217.14.206 when they can just type google.com!

From Domain Names to IP Addresses

How DNS Works:

1. You type www.example.com in your browser
2. Your computer asks a DNS server: “What’s the IP address for this?”
3. DNS responds: “It’s 93.184.216.34”
4. Your computer connects to that IP address
5. The website loads!

All of this happens in milliseconds, and you never see it happening.

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Summary & Review

You’ve covered a massive amount of material! Let’s wrap it up with the absolute essentials.

Essential Takeaways - Information Representation

Essential Takeaways - Communication & Networks

Final Review Checklist

Progress 0%

Congratulations! You've completed all items!

• Master number system conversions

  Binary, Hex, BCD - practice until automatic
• Memorize key formulas

  File sizes, two's complement, overflow detection
• Understand network models

  Can explain tradeoffs of each topology/model
• Know IP addressing

  Public/private, static/dynamic, IPv4/IPv6
• Practice past papers

  Apply concepts to exam-style questions

Reset Progress

Final Advice

AS Computer Science is tough, but consistent practice makes all the difference. Don’t just memorize - understand the WHY behind each concept. When you can explain a concept to someone else in your own words, that’s when you know you’ve truly mastered it.

Keep working through problems, ask questions when stuck, and tackle issues immediately before they pile up. You’ve got this! 🚀

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Study Resources:

• Practice all exercises in this guide
• Use flashcards for quick recall of definitions and formulas
• Review each KeyConcepts box before exams
• Track your progress with the checklists above

Good luck with your AS Computer Science journey!