FAT12

Preface

1. Why FAT12?
2. Who This Book Is For
3. What We Will Build
4. How This Book Is Structured
5. A Note on Style
6. How to Read This Book
7. About the Author
8. Feedback

Prerequisites

1. Operating System
2. C Compiler
3. mkfs.fat
4. QEMU (Chapter 11 Only)
5. Hex Dump Utility
6. Getting the Code (Optional)

Part I: Reading

Chapter 1: Hello, Disk!

1. Creating a Virtual Disk
2. Sectors and LBA
3. The Block Device
4. Reading the Volume Label
5. Tracing the Translation
6. What’s Next

Chapter 2: The Boot Sector

1. What Is the Boot Sector?
2. Mirroring the Layout in C
3. Mounting and Unmounting
4. Bytes Per Sector
5. Total Sectors
6. Reserved Sector(s)
7. Extended BPB
8. Booting from FAT12
9. What’s Next

Chapter 3: The Root Directory

1. The Directory Entry
2. Position and Size
3. Hexdumping the Root Directory
4. The 8.3 Name Format
5. The Attribute Byte
6. Decoding Timestamps
7. Directory Iteration
8. The ls Command
9. What’s Next

Chapter 4: Reading Files

1. Resolving a Path
2. The Data Region
3. Locating Data on Disk
4. The Cluster Chain
5. Decoding the 12-Bit Packing
6. Loading the FAT
7. Reading a Cluster Chain
8. Putting It All Together
9. The cat Command
10. What’s Next

Chapter 5: Full Path Walking

1. Subdirectories: Files in Disguise
2. The Path Walking Algorithm
3. Retrofitting Reads
4. What’s Next

Part II: Writing

Chapter 6: Writing Files

1. Finding a Free Slot
2. Encoding the Name
3. Encoding the Timestamp
4. Linking the File
5. Opening a File for Writing
6. Buffering the Write
7. Cluster Allocation
8. Writing the Data
9. Closing the File
10. The Create CLI Command
11. What’s Next

Chapter 7: Creating Directories

1. Assembling the Directory
2. The mkdir CLI Command
3. What’s Next

Chapter 8: Deleting

1. When Is a Directory Empty?
2. Freeing a Cluster Chain
3. Putting It All Together
4. The rm CLI Command
5. What’s Next

Chapter 9: Moving (and Renaming)

1. Rename vs. Move
2. Moving the Entry
3. The mv CLI Command
4. What’s Next

Part III: Advanced

Chapter 10: Formatting

1. The Blueprint
2. The FAT Size Problem
3. The Boot Sector
4. The File Allocation Tables
5. The Root Directory
6. The Data Region
7. Putting It All Together
8. The format CLI Command
9. What’s Next

Chapter 11: Into the Kernel

1. The RAM Disk Block Device
2. Printing to Screen
3. The Kernel
4. Building the Kernel
5. The Freestanding Environment
6. Booting the Kernel
7. What’s Next

Conclusion

1. What’s Next
2. Thank You

Appendixes

Appendix A: Talking to a Real Disk

1. The ATA Protocol
2. Talking to the Ports
3. Opening and Closing
4. Reading
5. Writing
6. Replacing the RAM Disk
7. Building and Running

Appendix B: Long Filename Support (LFN / VFAT)

1. Overview
2. How It Works
3. LFN Entry Structure (32 bytes)
4. Sequence Number
5. Character Encoding
6. Checksum Algorithm
7. 8.3 Alias Generation
8. Reading LFN Entries
9. Writing LFN Entries
10. Deleting LFN Entries
11. Moving
12. Worked Example
13. Summary

Appendix C: The Boot Sector’s Leftovers

1. Hidden Sectors (Partitioning)
2. CHS (Cylinder-Head-Sector) Addressing
3. Media Descriptor
4. Extended BPB — Additional Fields
5. OEM Name (Original Equipment Manufacturer)
6. Summary

Appendix D: FAT Family Reference

1. FAT Entry Widths and Values
2. Root directory
3. BPB
4. Volume Size Limits

Appendix E: FAT12 Cheat Sheet

1. Disk Layout
2. Position Formulas
3. Boot Sector (512 bytes)
4. Directory Entry (32 bytes)
5. FAT12 Entry (12 bits)
6. Timestamp Format
7. 8.3 Filename Format
8. Quick Reference: Cluster Math