I know some about wow64 system, in x64 windows version, and i looked up into this wow64* dlls, to see how the actual syscall transition happen, but something i can't find, and this is probably done by windows kernel, is how the actual x86 assembler instruction processing works? I know that there are two selectors for process, 0x23 for x86, and 0x33 for x64, how does kernel handles the assembly code in 0x23 selector for the process ? Is there some sort of virtual machine, that emulates x86 instructions, using x64 instructions?

1 Answer 1


There is no need for emulation because x64 processors can execute x86 (32-bit) code natively using so-called compatibility mode. You only need to set up proper descriptors in the GDT (L=0 for 32-bit code, L=1 for 64-bit code) and it "just works". This is what Windows does. E.g. from this OSR post, the windows 7 x64 GDT has these entries:

GDT[0x04] = 32 bit ring 3 Code: Execute/Read
GDT[0x05] = 32 bit ring 3 Data: Read/Write, Accessed
GDT[0x06] = 64 bit ring 3 Code: Execute/Read, Accessed

Index 4 corresponds to the selector 0x23 (4*8=0x20) and 6 to 0x33 (6*8=0x30).

So by switching to selector 0x23 you can have the CPU execute your code as x86, and then go back to x64 by switching to 0x33.

wow64cpu.dll contains both 32-bit and 64-bit code and is responsible for most of house-keeping necessary when switching between x64 and x86. Kernel itself is x64 only AFAIK, 32-bit code is supported only in user mode.

For more details on Wow64 implementation see this nice article by George Nicolaou:


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.