Sul layer di emulazione Wow64 si è ampiamente discusso:
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Wow64 su Wikipedia
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Wow64 - Microsoft MSDN
WOW64 is the x86 emulator that allows 32-bit Windows-based applications to run seamlessly on 64-bit Windows.
The system isolates 32-bit applications from 64-bit applications, which includes preventing file and registry collisions. Console, GUI, and service applications are supported. The system provides interoperability across the 32/64 boundary for scenarios such as cut and paste and COM. However, 32-bit processes cannot load 64-bit DLLs, and 64-bit processes cannot load 32-bit DLLs.
Tra i vari link forniti, trovate anche:
Wow64 Performance and Memory Consumption
Performance and memory consumption are determined by the following factors:
Processor hardware (instruction emulation is performed on the chip). On the x64 processor, instructions are executed natively by the micro-architecture. Therefore, execution speed under WOW64 on x64 is similar to its speed under 32-bit Windows. On the Intel Itanium processor, more software is involved in the emulation, and performance suffers as a result.
API thunk overhead. This overhead is small compared to the NT kernel call itself. NT kernel functions are intended to be called infrequently.
Virtual memory size. On the Intel Itanium processor, WOW64 adds significant overhead if two or more instances of the same 32-bit application are running concurrently, due to the native 8 KB pages on the Intel Itanium complicating the emulation of the native 4 KB pages on the x86 architecture (more pages are marked as writable; all writable pages are private to the process). This can adversely affect the scalability of Terminal Services on certain processors. This is not the case for the x64 processor.
Working set. WOW64 increases the size of the application's working set.
WOW64 enables 32-bit applications to take advantage of the 64-bit kernel. Therefore, 32-bit applications can use a larger number of kernel handles and window handles. However, 32-bit applications may not be able to create as many threads under WOW64 as they can on x86. On some processors, there is less virtual address space available, and each thread contains a 64-bit stack (usually 512K). On the x64 processor, each 32-bit application receives 4 GB virtual address space in the WOW64 environment, if the application has the IMAGE_FILE_LARGE_ADDRESS_AWARE flag set in the image header. If this flag is not set, the 32-bit application receives 2 GB virtual address space.
Si può facilmente dedurre come, salvo casi particolari, le prestazioni non cambiano (o il calo è irrisorio e non visibile) utilizzando applicazioni a 32bit su Vista/7 x64.
Vi invito a scaricare e leggere anche il seguente documento (formato .docx):
Wow64 Best Practices - Download diretto Microsoft Win Hardware Developer Central