Compressed Windows

Microsoft last week revealed how it will squeeze Windows 8.1 onto devices with storage space as small as 16GB to fulfill a promise earlier this year that OEMs could produce low-cost tablets and laptops.

The technology Microsoft will use, dubbed "WIM" for "Windows Imaging," is a file-based disk image format introduced in Windows Vista, the OS flop that debuted in 2007. Work on WIM, however, took place during the long -- and oft delayed -- development of "Longhorn," the code name for the project that was originally to produce an operating system in 2004.

To put Windows 8.1 Update on devices with tight storage constraints -- 16GB in particular, but also 32GB -- Microsoft has applied the decade-old technology to free up more space for applications and user content.

"This new deployment option, called Windows Image Boot (or WIMBoot), takes a different approach than traditional Windows installations," Michael Niehaus, senior product marketing manager in the Windows Commercial group, wrote on a Thursday blog. "Instead of extracting all the individual Windows files from an image (WIM) file, they remain compressed in the WIM. But from the user's perspective, nothing looks any different: You still see a C: volume containing Windows, your apps, and all of your data."

As Niehaus explained it, the WIM file -- an aggressively-compressed file that contains all the files necessary to run Windows 8.1 -- will sit in its own partition on a device's SSD (solid-state drive).

By moving Windows to its own partition and then compressing it into a WIM file, Microsoft frees up space in the C: drive partition, which is traditionally where Windows is stored in an uncompressed state. That means there is more space left for user content and applications.

To boot and run Windows, a set of pointer files are stored on the C: drive which, in turn, aim at a file index within the WIM file. Windows, then, runs from the compressed, read-only WIM file.