Working with 32-bit processes on 64-bit Windows
I hit a bug in VS Code recently related to running a 32-bit process on a 64-bit machine that was interesting. Here’s what I learned.
The System32 directory
On 64-bit Windows %windir%\System32 holds the 64-bit binaries. Accesses to this directory on 32-bit processes are automatically redirected to SysWoW64, in order to be backwards compatible.
The SysWoW64 directory
On 64-bit Windows %windir%\SysWoW64 holds the 32-bit binaries. This confused me initially until I learned that WoW64 stands for Windows 32-bit on Windows 64-bit.
The Sysnative directory
%windir%\Sysnative only exists on 32-bit processes running on 64-bit Windows and it redirects to the actual System32, allowing access to the 64-bit binaries on 32-bit processes. Initially I mistakenly assumed that Sysnative was a standard symlink that always points at the 64-bit binaries if they’re available, but this is not the case.
Directory overview
| 32-bit process | 64-bit process | |
|---|---|---|
System32 |
-> SysWoW64 (32-bit binaries) | 64-bit binaries |
SysWoW64 |
32-bit binaries | 32-bit binaries |
Sysnative |
-> System32 (64-bit binaries) | N/A |
Detecting WoW64 processes
That’s all very well and good, but how do you detect that you’re a 32-bit process running on 64-bit Windows? There are a couple of environment variables that come in handy here.
PROCESSOR_ARCHITECTURE will give you the actual processor architecture; "x86", "AMD64" or "IA64".
PROCESSOR_ARCHITEW6432 will give the same value as PROCESSOR_ARCHITECTURE, but it’s only set on 32-bit processes. To detect whether you’re in a WoW64 environment, just check whether the PROCESSOR_ARCHITEW6432 exists in the environment.
References
- Programming Guide for 64-bit Windows - WOW64 Implementation Details
- Programming Guide for 64-bit Windows - File System Redirector
