Microsoft's software empire rests on Windows, the computer operating system that runs so many of the world's desktop PCs, laptops, phones, and servers. Along with the Office franchise, it generates the majority of the company's revenues. But one day, the company could "open source" the code that underpins the OS—giving it away for free. So says Mark Russinovich, one of the company's top engineers.

"It's definitely possible," Russinovich says. "It's a new Microsoft."

Russinovich is sitting in front of several hundred people who spend their days running thousands of computers. He helped build Windows, and he carries one of the most respected titles at the world's largest software company: Microsoft Technical Fellow. But here, on stage at a conference in Silicon Valley, he's perched in front of an audience whose relationship with Microsoft is, at best, complicated.

So many Microsoft customers now rely on open source code. That means Microsoft must embrace it too.

The conference is called ChefConf. Chef is a tool that helps tech geeks setup and operate the many machines needed to drive a website, smartphone app, or some other piece of business software. It's an open source tool, which means it's typically used alongside other open source software. When Russinovich asks how many in the audience use nothing but Windows to run their machines, one guy raises his hand—one guy out of several hundred. Mostly, they run the open source Linux operating system.

But this is what Russinovich expects. "That's the reality we live in today," he says. The tech world has changed in enormous ways. So many companies—so many Microsoft customers—are now relying on open source code. And that means Microsoft must embrace it too. As Russinovich points out, the company now allows Linux on its Azure cloud computing service, a way of renting computers over the internet, and today, Linux is running on at least 20 percent of those computers.

It's quite a change for Microsoft, so long the bete noir of the open source community. But as Russinovich explains, it's a necessary change. And given how popular Linux has become, Microsoft could go even further, not only allowing open source software on its cloud services, but actually turning Windows into open source software. "Every conversation you can imagine about what should we do with our software—open versus not-open versus services—has happened," he says.

Certainly, Microsoft won't open source the thing tomorrow—if ever. Windows is still such a big part of the Microsoft revenue stream. And as Russinovich says, open sourcing such a complex piece of code isn't easy. "If you open source something but it comes with a build system that takes rocket scientists and three months to set up, what's the point?" he asks. But Microsoft is already giving away one version of Windows for free (though not sharing the underlying code). And it has already open sourced other important pieces of its software empire. If nothing else, his very public comments show—in stark fashion—how much the tech world has evolved. And how much Microsoft has evolved.

Open Source Means More Than Free

The future of tech lies not with for-pay software of the kind traditionally offered by Microsoft. Linux has moved into the massive computing centers that power the internet, and open source OSes such as Google Android are running so many of the world's mobile phones, tablets, and other devices. The future, even for Microsoft, lies in selling other stuff, including cloud computing services such Microsoft Azure and all sorts of other apps and services that run atop the world's operating systems.

If Microsoft does open source Windows, the operating system can still be a money maker in its own right.

In open sourcing Windows, Microsoft could expand the use of its OS. Open code is easier to test, easier to shape, easier to build into something else. And if the OS is more widely used, that means a bigger audience for the Microsoft applications that run on Windows.

Earlier this year, Microsoft open sourced a tool called .NET, a popular way of building online applications, and the hope is that this will expand the tool's reach. Outside coders are even working to move the tool onto Linux machines and Apple Macs. In the end, Russinovich says, this will help Microsoft sell other stuff. "It's an enabling technology that can get people started on other Microsoft solutions," he says of .NET. "It lifts them up and makes them available for our other offerings, where otherwise they might not be. If they're using Linux technologies that we can't play with, they can't be a customer of ours."

What's more, if Microsoft open sources Windows, the operating system can still be a money maker in its own right. Windows code would be freely available, but so many of the world's businesses would still need a vendor who can package, distribute, and update the OS. That's the way Linux works. And Android too. Open source is a complicated thing. It's not as simple as free versus not-free. When code is open sourced, shared with the world at large, the results are myriad.

'A History to Work Past'

As Russinovich leaves the stage, I chat with Phil Dibowitz, a Facebook engineer who was part of the same panel discussion. Facebook is a company that pushes open source in extreme ways—it even open sources its hardware—and Dibowitz is pleasantly surprised with Microsoft's willingness to discuss the rise of open source (given the way the company actively sought to suppress open source software in the past). And he sees this as an undeniable sign that Microsoft is evolving. "This wouldn't have happened two years ago," he says.

Adam Jacob, the chief technology officer of the company behind Chef, sees this in much the same way, saying it's particularly telling that Russinovich made his case at a conference grounded in the world of Linux and its predecessor, UNIX. Russinovich himself will tell you he's here for a very pointed reason. He wants the open source world to know that Microsoft now operates in new ways, that its not the company it was. "We've got a history to work past," he says. "We're out there beating the drums as much as we can."

Microsoft's path to this point in long and winding. And for years, people questioned whether the company would really change its ways. But now, people like Dibowitz and Jacob have dropped so much of their skepticism. And at least on some level, the larger tech community is warming up to the company. No one in the crowd was a heavy Windows user. But when the idea of an open source Windows popped up, they cheered. And loud.