WIM Format - 7-Zip

WIM (Windows Imaging Format) is Microsoft’s file-based disk image format used for deploying Windows operating systems and capturing system images.

Format Overview

WIM provides:

File-based imaging - Unlike sector-based formats, stores files individually
Single-instance storage - Identical files stored once across images
Multiple images - One WIM can contain multiple OS images
Compression - XPRESS, LZX, and LZMA compression
Solid compression - ESD variant with solid LZMS compression
Hardware independence - Not tied to specific hardware
Non-destructive mounting - Can mount images without extraction

WIM is primarily used for Windows deployment (install.wim, boot.wim) and system backup/restore operations.

Format Structure

From source/CPP/7zip/Archive/Wim/WimIn.h:23-33, WIM has evolved through several versions:

Version History

/*
WIM versions:
hexVer : headerSize : ver
10900 : 60 : 1.09 : Longhorn.4029-4039 (2003)
10A00 : 60 : 1.10 : Longhorn.4083 (2004)
10C00 : 74 : 1.12 : Longhorn.4093 - VistaBeta1 (2005) - Multi-Part, SHA1
10D00 : D0 : 1.13 : VistaBeta2 - Win10 - NumImages, BootIndex, IntegrityResource
00E00 : D0 : 0.14 : LZMS, solid, esd, dism
*/

File Structure

+----------------------------+
| WIM Header (208 bytes)     |
+----------------------------+
| Resource Table             |
| (Compressed File Data)     |
+----------------------------+
| Image Metadata             |
| (Directory/File Info)      |
+----------------------------+
| XML Data                   |
| (Image Descriptions)       |
+----------------------------+
| Integrity Table (optional) |
+----------------------------+

Directory Entry Structure

From WimIn.h:35-131, WIM stores detailed file metadata:

Modern Directory Entry (WIM 1.13+)

const unsigned kDirRecordSize = 102;

struct DIRENTRY {
  UInt64  Len;           // Entry length
  UInt32  Attrib;        // File attributes
  UInt32  SecurityId;    // Security descriptor ID
  
  UInt64  SubdirOffset;  // Subdirectory offset (0 for files)
  
  UInt64  unused1;       // Reserved
  UInt64  unused2;       // Reserved
  
  UInt64  CTime;         // Creation time
  UInt64  ATime;         // Access time
  UInt64  MTime;         // Modification time
  
  Byte    Sha1[20];      // SHA-1 hash of file data
  
  UInt32  ReparseTag;    // Reparse point tag
  UInt32  ReparseFlags;  // Reparse flags
  
  UInt16  Streams;       // Number of streams
  UInt16  ShortNameLen;  // Short name length
  UInt16  FileNameLen;   // File name length
  
  UInt16  Name[];        // File name (UTF-16)
  UInt16  ShortName[];   // Short name (UTF-16)
};

Old Directory Entry (WIM 1.10)

const unsigned kDirRecordSizeOld = 62;

Older versions used a smaller 62-byte structure without SHA-1 hashes.

Resource Flags

From WimIn.h:134-141:

namespace NResourceFlags {
  const Byte kMetadata = 1 << 1;    // Metadata resource
  const Byte kCompressed = 1 << 2;  // Resource is compressed
  const Byte kSolid = 1 << 4;       // Solid compression (ESD)
}

Solid compression (kSolid flag) is used in ESD (Electronic Software Distribution) files for maximum compression, introduced in Windows 8.1.

Compression Methods

WIM supports multiple compression methods:

Standard Compression

None (Store) - No compression
- Fast creation and extraction
- Largest file size
XPRESS - Fast compression
- LZ77-based algorithm
- Good speed, moderate compression
- Default for many WIM images
LZX - Better compression
- LZ77 + Huffman coding
- Better ratio than XPRESS
- Used in install.wim
LZMA - Best compression (WIM 1.13+)
- LZMA algorithm
- Excellent compression ratio
- Slower than XPRESS/LZX

Solid Compression (ESD)

LZMS - Solid compression (WIM 0.14+)
- Used in .esd files
- Maximum compression ratio
- Significantly slower
- Requires Windows 8.1+ or DISM

From handler includes (WimIn.h:14-16):

#include "../../Compress/LzmsDecoder.h"
#include "../../Compress/LzxDecoder.h"

Usage Examples

List WIM Contents

7z l install.wim

Shows all images and their contents.

Extract WIM Image

7z x install.wim

Extracts the default or first image.

Extract Specific Image

7z x install.wim -i!1\*  # Extract image 1
7z x install.wim -i!2\*  # Extract image 2

View WIM Information

7z l -slt install.wim

Shows detailed information including:

Number of images
Compression method
Image names and descriptions
File counts and sizes

Create WIM Archive

7z a -twim -mx=1 image.wim C:\Source\  # XPRESS compression
7z a -twim -mx=5 image.wim C:\Source\  # LZX compression

7-Zip supports extracting WIM files but has limited WIM creation capabilities compared to Microsoft DISM tool. For advanced WIM operations, use DISM or ImageX.

Handler Implementation

From source/CPP/7zip/Archive/Wim/WimHandler.h:15-23:

const Int32 kNumImagesMaxUpdate = 1 << 10;  // 1024 images max

Z7_CLASS_IMP_CHandler_IInArchive_5(
  IArchiveGetRawProps,
  IArchiveGetRootProps,
  IArchiveKeepModeForNextOpen,
  ISetProperties,
  IOutArchive
)

Handler State

CDatabase _db;
UInt32 _version;
UInt32 _bootIndex;

CObjectVector<CVolume> _volumes;
CObjectVector<CWimXml> _xmls;

bool _isOldVersion;
bool _xmlInComments;
bool _xmlError;
bool _isArc;
bool _unsupported;

Multiple Images

WIM files can contain multiple OS images:

# View all images
7z l install.wim

Output:
  Image 1: Windows 10 Home
  Image 2: Windows 10 Pro  
  Image 3: Windows 10 Enterprise

Each image shares common files (single-instance storage).

Single-Instance Storage

WIM uses SHA-1 hashing to detect duplicate files:

Files with identical SHA-1 hash stored only once
Dramatically reduces size for multiple similar images
Example: 5 Windows editions sharing system files

Normal storage:  5 × 4 GB = 20 GB
WIM storage:     ~6-8 GB (60-70% savings)

Split WIM Files

Large WIM files can be split into parts:

# Split WIM (using DISM or ImageX)
dism /Split-Image /ImageFile:install.wim /SWMFile:install.swm /FileSize:4000

Creates:

install.swm
install2.swm
install3.swm

7-Zip can extract from split WIM (SWM) files by opening the first file.

ESD Format

ESD (Electronic Software Distribution) is a WIM variant with solid LZMS compression:

# Extract ESD file
7z x install.esd

ESD characteristics:

Much smaller than WIM (~30-40% smaller)
Slower extraction
Used for Windows 10+ downloads
Requires Windows 8.1+ DISM or 7-Zip for extraction

Advanced Features

Boot Configuration

From WimHandler.h:26:

UInt32 _bootIndex;  // Bootable image index

WIM can specify which image is bootable (boot.wim).

Security Descriptors

UInt32  SecurityId;  // In DIRENTRY structure

WIM preserves Windows NTFS security descriptors.

Alternate Data Streams

struct ALT_STREAM {
  UInt64  Len;
  UInt64  Unused;
  Byte    Sha1[20];
  UInt16  FileNameLen;
  UInt16  FileName[];
};

WIM supports NTFS alternate data streams.

Reparse Points

UInt32  ReparseTag;    // Symlink, junction, etc.
UInt32  ReparseFlags;

Supports symbolic links, junctions, and mount points.

Comparison with Other Formats

WIM vs ISO

Feature	WIM	ISO
Type	File-based	Sector-based
Compression	Yes (multiple methods)	No
Modification	Easy	Difficult
Mounting	Non-destructive	Read-only
Use case	OS deployment	Optical media

WIM vs VHD/VHDX

Feature	WIM	VHD/VHDX
Format	File-based image	Virtual disk
Bootable	Yes (via WinPE)	Yes (native)
Modification	Non-destructive	Read/write
Size	Smaller (compressed)	Larger
Use case	Deployment	Virtualization

Use Cases

Windows Installation

# Extract Windows installation
7z x install.wim -i!4\*  # Extract Windows 10 Pro image

System Backup

# Create system image (use DISM)
dism /Capture-Image /ImageFile:backup.wim /CaptureDir:C:\ /Name:"System Backup"

Extract Drivers

# Extract driver files from install.wim
7z x install.wim -i!1\Windows\System32\drivers\*.sys

View Windows Edition

# List all Windows editions in install.wim
7z l install.wim

Implementation Files

WIM handler implementation:

source/CPP/7zip/Archive/Wim/
├── WimHandler.cpp      # Archive handling
├── WimHandler.h        # Handler interface
├── WimHandlerOut.cpp   # Archive creation
├── WimIn.cpp          # WIM reading
├── WimIn.h            # WIM structures
└── WimRegister.cpp    # Format registration

Limitations

WIM format limitations:

Maximum image size: 4 GB per file (use split WIM for larger)
No native encryption (use BitLocker on mounted images)
Requires Windows or specialized tools
7-Zip extraction only (limited creation)
Solid WIM (ESD) slower extraction

Best Practices

For Deployment

Use LZX compression for balanced speed and size

For Distribution

Use LZMS (ESD) for minimum download size

For Backup

Use XPRESS for faster backup/restore

For Multiple Editions

Leverage single-instance storage in one WIM

Tools

Microsoft DISM

# View WIM info
dism /Get-WimInfo /WimFile:install.wim

# Mount WIM
dism /Mount-Wim /WimFile:install.wim /Index:1 /MountDir:C:\Mount

# Unmount WIM
dism /Unmount-Wim /MountDir:C:\Mount /Commit

7-Zip

# Extract WIM
7z x install.wim

# List images
7z l install.wim

ImageX (Legacy)

# Capture image
imagex /capture C:\ D:\backup.wim "System Backup"

# Apply image
imagex /apply D:\backup.wim 1 C:\

​Format Overview

​Format Structure

​Version History

​File Structure

​Directory Entry Structure

​Modern Directory Entry (WIM 1.13+)

​Old Directory Entry (WIM 1.10)

​Resource Flags

​Compression Methods

​Standard Compression

​Solid Compression (ESD)

​Usage Examples

​List WIM Contents

​Extract WIM Image

​Extract Specific Image

​View WIM Information

​Create WIM Archive

​Handler Implementation

​Handler State

​Multiple Images

​Single-Instance Storage

​Split WIM Files

​ESD Format

​Advanced Features

​Boot Configuration

​Security Descriptors

​Alternate Data Streams

​Reparse Points

​Comparison with Other Formats

​WIM vs ISO

​WIM vs VHD/VHDX

​Use Cases

​Windows Installation

​System Backup

​Extract Drivers

​View Windows Edition

​Implementation Files

​Limitations

​Best Practices

For Deployment

For Distribution

For Backup

For Multiple Editions

​Tools

​Microsoft DISM

​7-Zip

​ImageX (Legacy)

​See Also

Format Overview

Format Structure

Version History

File Structure

Directory Entry Structure

Modern Directory Entry (WIM 1.13+)

Old Directory Entry (WIM 1.10)

Resource Flags

Compression Methods

Standard Compression

Solid Compression (ESD)

Usage Examples

List WIM Contents

Extract WIM Image

Extract Specific Image

View WIM Information

Create WIM Archive

Handler Implementation

Handler State

Multiple Images

Single-Instance Storage

Split WIM Files

ESD Format

Advanced Features

Boot Configuration

Security Descriptors

Alternate Data Streams

Reparse Points

Comparison with Other Formats

WIM vs ISO

WIM vs VHD/VHDX

Use Cases

Windows Installation

System Backup

Extract Drivers

View Windows Edition

Implementation Files

Limitations

Best Practices

Tools

Microsoft DISM

7-Zip

ImageX (Legacy)

See Also