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.

 

Wes Miller, an analyst with Directions on Microsoft, a Kirkland, Wash.-based firm that tracks only Microsoft, called the concept "a pretty good solution."

 

As well he might. Miller, like many of the analysts at Directions, once worked at Microsoft. In fact, Miller was on a team that worked on the WIM technology in 2003-2004. "A lot of [old] technology has fallen off the planet, so it's cool to see one that survived," he said.

 

While working on Longhorn, Microsoft was looking at running Windows from a WIM file. "We would read from the WIM file as if it was a file system," Miller said. That's pretty much what Microsoft is now doing.

 

Previously, Microsoft promised that Windows 8.1 Update would run on devices with small SSDs of 16GB and 32GB, but did not say how it would do that, leaving some to speculate that it would dump the Windows recovery tools or even strip out features.

 

Some kind of miniaturization was required: The full-blown Windows required Microsoft to equip the Surface Pro 2 with a minimum of 64GB.

 

Even then, the Surface Pro 2 and its predecessor, Surface Pro, have reaped criticism for the paltry amounts of user content space left after the OS and a recovery partition has occupied the SSD.

 

Microsoft estimated that devices using WIM would have about 12GB of storage space for user apps and content. (Image: Microsoft.)

 

According to Microsoft, the low-end 64GB Surface Pro 2 offers 37GB of user content space. By that calculation, if Windows was shoehorned into 32GB, the tablet-cum-notebook would have only 5GB free for user content.

 

By compressing Windows into a WIM file -- and using that same file for double duty as a recovery source -- Microsoft has reclaimed significant space on the C: drive, claimed Niehaus.

 

"Let's assume the WIM file (INSTALL.WIM) is around 3GB and you are using a 16GB SSD," wrote Niehaus. "In that configuration, you'll still be left with over 12GB of free disk space (after subtracting out the size of the WIM and a little bit of additional 'overhead')."

 

That compares favorably with a 16GB iPad Air, which sports between 12GB and 13GB of user content space after iOS and other files are accounted for.

 

Miller said that the compressed WIM file would probably not charge a performance "tax," as the files within the WIM are not decompressed each time they're called.

 

"They're read directly," Miller said. "[Any performance hit] should be minimal. I'd be shocked if it wasn't marginally faster or at the worst, the same." Miller based that on his time at Microsoft earlier in the century. "We were kind of surprised because we saw a performance increase in the way we were doing things then," he said.

 

Niehaus, however, acknowledged a performance hit, although he did not say what it was. "There is some performance impact," Niehaus wrote in a comment appended to his blog, answering questions from others. He also acknowledged that because the WIM file was read-only, subsequent updates, including security patches, other bug fixes, and even larger refreshes of Windows 8.1 Update, would be stored on the C: partition, consuming an ever-larger amount of user content space.

 

"Updates are applied to the 'real' C: drive. So there will be some increase in the Windows footprint over time," Niehaus said, again in a comment.

 

Miller saw that as a problem if Microsoft pushed a major update to Windows 8.1 Update in the future, as the company has said it will do at some point. The device might have to be reset with a new, potentially larger, WIM file.

 

While Microsoft clearly expects that the WIMBoot concept will be used primarily by OEMs (original equipment manufacturers) that design, assemble and sell smaller Windows-powered tablets, enterprises can also use the technology to create compressed OS images for deployment on their own hardware.

 

In his Thursday blog, Niehaus pointed to several places on Microsoft's website where customers can find documentation about WIM and WIMBoot.