PROCESSOR_ARCHITEW6432

What’s the PROCESSOR_ARCHITEW6432 environment variable for?

If you’re on 64-bit Windows, you’ll see that you have a PROCESSOR_ARCHITECTURE environment variable:

C:\>echo %PROCESSOR_ARCHITECTURE%
AMD64

This is from a 64-bit Command Prompt on 64-bit Windows.

If you run the 32-bit (x86) Command Prompt, you’ll see something different:

C:\>echo %PROCESSOR_ARCHITECTURE%
x86

C:\>echo %PROCESSOR_ARCHITEW6432%
AMD64

Why is this important? Well, maybe you’re running 32-bit MSBuild (because some of your tasks are in 32-bit DLLs), and you want to run 64-bit PowerShell (because you want to use a module that requires 64-bit).

From a 32-bit process on 64-bit Windows, you can’t simply run C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe, because System32 is mapped to SysWOW64, meaning that you’ll get the 32-bit PowerShell.

No, to get a 64-bit PowerShell from a 32-bit process, you need to run C:\Windows\SysNative\WindowsPowerShell\v1.0\powershell.exe, because SysNative is mapped to the correct System32 directory.

However, you can’t use SysNative from a 32-bit process on 32-bit Windows, because it doesn’t exist.

Hence the need to distinguish. If you’re in a .NET 4.0 process (or PowerShell 3.0), you can use the Environment.Is64BitOperatingSystem property.

From MSBuild (or Command Prompt), you need to use the presence of the PROCESSOR_ARCHITEW6432 environment variable. If it’s set, then you’re in a 32-bit process on 64-bit Windows, and must use SysNative. If it’s not set, then you’re either in a 32-bit process on 32-bit Windows, or in a 64-bit process on 64-bit Windows. In that case, you can use System32.