What is the most important file in a Windows installation ISO image?
What Is the Most Important File in a Windows Installation ISO?
A Windows installation ISO file typically ranges from 3 to 6 GB, containing a large and complex set of files. Which file do we actually need the most when installing the system?
install.wim
You can open an ISO file using extraction software (such as 7-Zip, WinRAR) or by simply double-clicking it (Windows 8 and later). Inside the sources folder, you’ll find install.wim — the core file for installing Windows.
This file contains all the system files for Windows.
If you open install.wim with extraction software, you can see its internal structure.
Sometimes, opening this file reveals the complete system drive directory (Windows, Program Files, Program Files (x86), Recovery, ProgramData, etc.). Other times, you’ll see folders named with numbers (1, 2, 3, 4, 5, 6), each containing the directory structure of a Windows system drive. Additionally, there is a file named 1.xml.
By analyzing this XML file, we can understand its purpose: it records and marks which numbered folders contain systems with the same version but different IDs. For example, Windows Home Single Language, Pro, Enterprise, Pro Workstation, Education, and Pro Education. Their system version numbers are identical, but their system IDs differ.
When the Windows installer presents system version options, it reads this XML to list the available versions for the user to choose from. After the user selects a version, the installer picks the corresponding folder in install.wim, extracts its contents to the C drive, and thus completes the Windows installation.
You might wonder: doesn’t bundling so many system versions into a single file waste a lot of space?
An install.wim is about 6 GB and contains 6 Windows system versions. Does that mean each Windows system is only about 1 GB?
Actually, that’s not the case. The .wim file is a special compressed format. When creating a .wim, if the same file appears multiple times, it is not stored repeatedly. Instead, an index is created pointing to the same data block within the .wim file. Since systems with the same version but different IDs have almost no file differences, these six versions share a single .wim database. As a result, a normal Windows system’s install.wim is still around 6 GB.
You can verify this by using Dism++: open the image file feature, export a specific version of the Windows system .wim, and check whether the exported file size remains almost unchanged.
The Essence of Installing a System
The essence of installing a system lies in extracting the .wim and creating the boot configuration.
If you use a third-party Windows PE system — such as WePE, UQiTong, FirPE, or KuerPE — to install Windows, there’s no need to keep the entire Windows ISO image. You only need to copy the install.wim from the ISO. Then, you can use various Windows installers in the PE system (WinNTSetup, CoolInstall, EXI, SGI, CGI, etc.) to install Windows.
This approach saves about 1 GB of disk space.
Additionally, thanks to Microsoft’s WIMBoot boot technology, you can boot the system directly from a .wim package without the traditional installation method. WIMBoot is an excellent system installation and maintenance method with many advantages, including extreme savings on C drive space (C drive only takes up 2–3 GB), improved system speed, and the ability to quickly restore or reset the system.
For a detailed introduction to WIMBoot, please refer to my featured article on WIMBoot boot mode in my official account.