The Internet Explorer 32e compatibility mode allows 32-bit and 16-bit applications to function on a 64-bit operating system. Unfortunately, legacy 16-bit programs that operate in virtual real mode (i.e., DOS programs) are not supported and will not run, which is likely to be the most significant issue for many users, particularly those who rely on legacy business applications or like playing old games. Use a virtualization environment like DOSBox, Hyper-V, or others to execute older 16-bit applications on a 64-bit CPU. Compatibility mode, like 64-bit mode, is activated by the OS on a per-code basis, which means that 64-bit programs operating in 64-bit mode can coexist with 32-bit apps running in compatibility mode.

All of this requires a 64-bit operating system and, more importantly, 64-bit drivers for all of our hardware to function under that operating system. Despite the fact that Microsoft launched a 64-bit version of Windows XP, few firms produced 64-bit XP drivers. 64-bit drivers were rare until Windows Vista and, particularly, Windows 7 x64 versions were launched, and 64-bit hardware support was considered mainstream.

Because AMD and Intel's extensions to the standard IA32 architecture are almost equivalent and can be supported with a single version of Windows, Microsoft uses the term x64 to refer to processors that support either AMD64 or EM64T.

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