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A GUIDE TO WINDOWS VISTA
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The Windows Imaging Format (WIM) is a file-based disk image format. It was developed by Microsoft for use in its upcoming operating system releases. Windows Vista and Windows Fundamentals for Legacy PCs use it as part of their installation procedure.
WIM files are created with a command-line tool available through Microsoft Connect named ImageX. It is officially available since June 19, 2006 for registered beta testers in Microsoft's Core Infrastructure Solutions, namely Business Desktop Deployment 2007 Beta 1, as well as in the Windows Automated Installation Kit (WAIK). A set of public APIs for manipulating WIMs have also been released with Vista, included under WIMGAPI.DLL
Like other disk image formats, a WIM file contains a set of files and associated filesystem information. However, unlike sector-based formats (such as .ISO, .CUE/.BIN used for CD and DVD images), WIM is file-based, which means that the smallest unit of information is a file. Advantages include: hardware independence, the ability to use heavy compression, and unique storage of a file referenced multiple times in the filesystem tree (single instance storage). It also allows packing multiple image files into only one file.
In Windows Vista, the installation DVD contains the main OS contents in the format of a WIM file. Since Vista has moved into beta development stage, there have been 3 major changes in the WIM format, and each of them had a different package of imaging tools.