Compatibility
The x64 Editions of Windows are essentially feature comparable with the 32-bit versions. They are able to execute both native 64-bit programs and 32-bit programs efficiently and with high performance, bringing an unprecedented level of compatibility to 64-bit computing.
Applications that are written to run natively in x64 Editions of Windows will have full access to the very large virtual memory address space (16 TB) and the up to 1 TB of physical memory supported by Windows Server 2003 x64 Editions and 128 MB of physical memory on Windows XP Professional x64 Edition.
All 32-bit applications run in the WOW64 subsystem, with access to 4 GB of virtual memory address space. WOW64 provides a high-performance, highly compatible, and isolated environment for 32-bit applications.
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