If we can run 32 bit executables on 64 bit Windows, why can't we convert it?

Question

WoW64 makes it possible to run 32 bit applications on 64 bit Windows. If the conversion from 32 bit instructions to 64 bit instructions can be made at runtime, why can't we convert the executable itself to 64 bit?

Answer 1

That is because WoW64 doesn't convert 32bit instructions to 64bit.

Your 32bit executable is run in 32 bit mode by switching your CPU to compatibility mode. There are some conversions inlined for API/driver calls, but most of the code is not converted.

*This is only true for the x86-64 architecture. The IA-64 architecture doesn't support this, so WoW actually converts your code to 64bit, but with a significant performance penalty.

*.NET code compiled to MSIL is JITted to the correct architecture. Probably the same happens with other architectures with an Intermediate Language like Java, but i'm no expert there.

Answer 2

Yes, Wow64 lets you run 32-bit programs but they still run in 32-bit mode - no code alterations are performed. Such automatic translation would be impossible for a native application.

The number one problem is that native applications have no annotations explaining what the code does. Just one example: the compiler compiles pointer manipulation code and uses a 32-bit register to hold this pointer value on 32-bit platform and emits bare machine code for that - the runtime will have no idea that this was a pointer and it needs to be placed in 64-bit register on 64-bit platform.

Managed environments such as Java and .NET can deal with it - the compiler emits "intermediate language" code with necessary annotations that is then compiled for the target platform before the code is first run.