When word spread about the Windows 8 desktop interface formerly known as Metro, concerns emerged about how legacy applications would behave in the new operating system. That worry was short-lived. With the circulation of the first beta versions, it became clear that the vast majority of existing 32- and 64-bit Windows apps would install and run without incident on Windows 8.
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Enterprises face problems end users do not see, however, such as having to keep legacy or custom 16-bit applications running. Windows 8 is similar to Windows 7 in this regard: The 64-bit edition of Windows 8 will only run 64- and 32-bit apps, while the 32-bit edition of Windows 8 will be able to run 32- and 16-bit apps. Note that the ARM version of Windows 8 (aka Windows RT), shipped as a preload with certain devices, won't run anything except software compiled for ARM.
Relying on the continued presence of the 16-bit subsystem isn't a perfect solution for a couple of reasons. For one, it doesn't provide flawless 16-bit compatibility. Users have reported a problem with 16-bit COM-based apps, for instance, although the problem cited there can be worked around.
Since there's no guarantee that the 16-bit subsystem will continue to be available in future versions of Windows, the best long-term approach is to replace or rewrite any such components.
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In addition, there may be situations where the 64-bit version of Windows 8 must be used, so that's one less place where the 16-bit subsystem can be present.
Another common approach to legacy apps is virtualization, something Windows 7 handles via XP Mode. This involves installing a copy of XP to run in a virtual machine via Windows Virtual PC, along with licensing for that separate copy of Windows. Windows 8 no longer has XP Mode, but a number of parallel solutions exist.
One possible alternative is Microsoft Enterprise Desktop Virtualization (part of the Microsoft Desktop Optimization Pack), which allows virtual machine images to be made available to client PCs. It also has specific tools that allow individual apps to be virtualized and delivered to desktops via the virtual desktop infrastructure.
Another option is to directly migrate the existing Windows XP Mode virtual hard disk files to a new virtualization infrastructure, although you may want to consult with Microsoft beforehand about the licensing implications of doing so.
If you're working on replacing existing custom legacy apps with newly written versions that are forward-compatible, Microsoft offers a package called the Windows App Certification Kit (ACK), a replacement for the former Windows Software Logo Kit. ACK lets you ensure a given app can work in Windows 8, either as a standard desktop app or a Metro-style app.