Last week was the most eventful for Microsoft in recent memory. After unveiling the Surface tablet, the company revealed a couple of days later its biggest-ever upgrade to Windows Phone — the Windows Phone 8. However, the latter news left a sour taste in the mouth of users who bought into Microsoft's mobile platform early, as current Windows Phone users found out they wouldn't be getting the update.

Why wouldn't Microsoft let its most loyal mobile users in on its latest and greatest software? The reason is actually very simple: This isn't the same Windows Phone operating system as the one they've been using. Although Windows Phone 8 resembles its predecessors in both looks and functionality, everything has changed under the hood.

"The oversimplified way of putting it is that, before, you had a phone that ran programs; now you have a computer that can make phone calls," says Greg Sullivan, Microsoft's senior marketing manager for Windows Phone (shown above). "There is a fundamental difference architecturally."

Windows Phone has been re-coded from the ground up for Version 8. The original Windows Phone (Version 7) and all subsequent upgrades before 8 are actually based on Windows CE, Microsoft's earlier mobile operating system. Windows CE was also the basis for Windows Mobile, which came before Windows Phone 7.

Windows Phone 8, however, is based on the same core software as Windows 8 itself (the Windows NT kernel). While that has many benefits — for users, hardware makers, developers and Microsoft — it means all those phones that were designed to run Windows Phone 7 can't run the new OS.

All isn't lost on current Windows Phone users, however. The most visible upgrade in Windows Phone 8 — the super-customizable home screen — will come to older Windows Phones through an upgrade to Windows Phone 7.8, and Microsoft says it'll continue to support the previous Windows Phone OS.

"The sense that we just bought something and we don't want to be left behind is what we're delivering on," says Sullivan. "Nokia is doing a bunch of works to keep this fresh. They're going to continue to invest in the Lumia line and add new capability and new functionality."

Was it really impossible to get those previous phones running Windows Phone 8, though? Hardware is often adaptable — the same Intel chips that power Windows PCs also power Macs, after all. Couldn't Microsoft have worked out a way for those phones to get the latest version?

Yes, says Sullivan, but the cost would have been severe, and the benefits would have been minimal. Much of the functionality that Windows Phone 8 opens up has to do with higher resolutions, multi-core processors and technologies like near-field communication (NFC) — technologies current Windows Phones simply don't have.

"That has a fair amount to do with it," Sullivan says. "All of the work we would have had to do to get it on this architecture — and then there's no benefit."

Windows Phone 8 has much more in common with Windows 8 and Windows RT than previous generations of Windows Phone. The similarity goes beyond just sharing the same software code (the kernel), as, for instance, OS X and iOS do. Windows 8 and Windows Phone 8 actually share the same "core," as Sullivan puts it, which means all Windows products will soon share the same device drivers, file system, networking stack (with IPv6 compatibility), media software, and most elements having to do with security.

"There's a lot more than just the OS kernel that's being shared," Sullivan explains. "That's one of the benefits of being on this shared core — we get to inherit the architecture and the scale of a very large number of users."

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