Format Overview
ISO provides:- Optical disc images - CD, DVD, Blu-ray format support
- Universal compatibility - Readable on all operating systems
- No compression - Stores data as-is
- Bootable images - El Torito boot specification
- Large capacity - Up to 8 TB (Blu-ray)
- File system standard - ISO 9660, Joliet, UDF, Rock Ridge
ISO images are exact copies of optical disc contents, preserving all data including boot information and file system structures.
Format Structure
Fromsource/CPP/7zip/Archive/Iso/IsoHeader.h:10-26, ISO uses a volume descriptor system:
Volume Descriptor Types
ISO Structure
Start Position
FromIsoHeader.h:30:
File Flags
FromIsoHeader.h:22-26:
Boot Support (El Torito)
El Torito specification enables bootable CD/DVD/BD images (IsoHeader.h:32-60):
Boot Entry Types
Boot Platform Types
Boot Media Types
Modern bootable ISOs typically use “No Emulation” mode with UEFI support for maximum compatibility.
File System Extensions
ISO 9660 (Standard)
Limitations:- Filenames: 8.3 format (DOS-like)
- Maximum directory depth: 8 levels
- Character set: Limited ASCII
Joliet Extension
Improvements:- Long filenames: Up to 64 Unicode characters
- Full Unicode support
- Windows-compatible
Rock Ridge Extension
Unix support:- Long filenames
- POSIX file permissions
- Symbolic links
- Device files
- User/group ownership
UDF (Universal Disk Format)
Modern format:- DVD/Blu-ray standard
- Files larger than 4 GB
- Packet writing support
- Better performance
Usage Examples
Create ISO from Directory
Extract ISO Contents
List ISO Contents
Extract Specific Files
Test ISO Integrity
View Detailed Information
Creating Bootable ISOs
Extract Boot Image
ISO Size Limits
| Media | Capacity | Use Case |
|---|---|---|
| CD-ROM | 650-700 MB | Small distributions |
| DVD-5 (Single layer) | 4.7 GB | Software, movies |
| DVD-9 (Dual layer) | 8.5 GB | Large software |
| BD-R (Single layer) | 25 GB | HD video, games |
| BD-R DL (Dual layer) | 50 GB | 4K video |
| BD-R XL (Triple layer) | 100 GB | Archival |
| BD-R XL (Quad layer) | 128 GB | Large archives |
Implementation Details
Fromsource/CPP/7zip/Archive/Iso/IsoIn.h:16-64, the handler uses directory structures:
Directory Structure
DateTime Structure
ISO Implementation Files
Common Use Cases
Software Distribution
Operating System Installation
Disc Backup
Game Preservation
Firmware/Driver Archives
Working with Multiple Sessions
Some ISOs contain multiple sessions (multisession CDs/DVDs):UDF Support
7-Zip supports UDF format commonly used on DVDs:- Files larger than 4 GB
- Better performance
- DVD-Video compatibility
Hybrid ISOs
Some ISOs combine multiple file systems:- ISO 9660 + Joliet - Windows/DOS compatibility
- ISO 9660 + Rock Ridge - Unix/Linux compatibility
- ISO 9660 + UDF - Universal compatibility
- El Torito + UEFI - Legacy and UEFI boot support
Comparison with Other Formats
ISO vs WIM
| Feature | ISO | WIM |
|---|---|---|
| Compression | No | Yes |
| Modification | Difficult | Easy |
| Bootable | Yes (El Torito) | Yes (WinPE) |
| Use case | Optical media | Windows deployment |
| File limit | ~4 GB (ISO9660) | 4 GB per image |
ISO vs ZIP/7z
| Feature | ISO | ZIP/7z |
|---|---|---|
| Purpose | Disc image | Archive |
| Compression | No | Yes |
| Bootable | Yes | No (ZIP), SFX only (7z) |
| Mounting | Yes | No (without tools) |
| Size limit | 8 TB | 16 EB |
Best Practices
For Distribution
Use ISO for software requiring optical media format
For Archival
Combine with compression: compress ISO with 7z/XZ
For Installation Media
Preserve bootability - don’t modify boot structures
For Compatibility
Use Joliet extension for Windows long filename support
Advanced Operations
Compress ISO for Storage
ISO images contain no compression:- Software ISOs: 40-60% size reduction
- Movie DVDs: 5-15% (already compressed)
- Data ISOs: 30-70% depending on content
Convert ISO to Other Formats
Verify ISO Hash
Limitations
Tools
Extraction Tools
- 7-Zip - Extract ISO contents
- WinRAR - Extract and browse ISOs
- Windows Explorer - Mount ISO (Windows 8+)
- macOS Disk Utility - Mount and create ISOs
Creation Tools
- ImgBurn - Create/burn bootable ISOs (Windows)
- mkisofs/genisoimage - Create ISOs (Linux)
- Brasero - CD/DVD burning (Linux)
- oscdimg - Windows ADK ISO creation