Windows on Windows action

If you are running a 64 bit Windows, and you are aren't you, you are actually running two operating systems, one 64 bit, and one 32 bit. While this is an extreme simplification, it is for all intents and purposes true enough, while at the same time maybe not true at all. But it is.

Anyway, an Intel processor runs 16, 32 or 64 bit code in different modes, thus a process can only run in either mode, not several. If I designed a CPU I would make sure future modes could run current code, but I don't design CPUs, probably because I don't have a clue. To Intels defense, the x86 range of CPUs are backwards compatible back into a different century. Hilariously it was AMD, not Intel, who designed the 64 bit mode and thus it is sometimes called AMD64.

The consequence of these modes is that 32 bit binaries and 64 bit binaries are pretty incompatible. 32 bit processes cannot load 64 bit binaries and vise versa. To run both kind of programs, Windows therefor has an almost full set of both 32 and 64 bit components.

The system files of Windows have since Windows 95 been stored in the "System32" folder. In 32-bit Windows, which can natively run 16 bit code, there is also a "System" folder, pretty much containing a Windows 3.11 set of 16 bit files. But 64 bit Windows dropped the 16 bit support, so the "System" folder is no longer present. Well, it's there, but pretty much empty.

For stupid reasons Microsoft decided to keep the name of the system folder as "System32" even on a 64 bit Windows.

The System32 folder contains the 64 bit system files.

I can see why, but I believe it's stupid because we are going to run 64 bit for much longer than the 20 years we used 32 bit operating systems. Then again, the future plans for Windows executables are uncertain.

But as you've figured out, 32 bit programs can't use these 64 bit system files, how does that work? Misdirection! Or rather, redirection. Or virtualization, if you prefer.

A 32 bit program trying to access System32 will be redirected to the "SysWOW64" folder. The program believes it loads from System32, but it isn't. It's running a 32 bit System Windows On Windows 64. SysWOW64.

The SysWOW64 contains the 32 bit system files.

There are ways to get around this if your program needs to for some reason, but this is the default behaviour. You can access the true System32 folder from a 32 bit process by the alias C:\Windows\Sysnative.

The registry is divided in much the same way. 32 bit programs accessing HKLM\Software are redirected to HKLM\Software\Wow6432Node. They couldn't choose a consistent name such as RegWOW64, because why would they. Also note that HKEY_CLASSES_ROOT is actually a mirror of HKLM\Software\Classes and HKCU\Software\Classes, therefor 32 bit and 64 bit class registrations can exist at the same time, the 32 bit under HKLM\Software\Wow6432Node\Classes.

The Program Files folders are not redirected, you can install a 64 bit program in Program Files (x86) and a 32 bit program in Program Files without much trouble. However, the environment variables are different for each architecture, the %ProgramFiles% points to a different folder depending on the process.

You can try all this by launching one 64 bit CMD from C:\Windows\System32 and one 32 bit CMD from C:\Windows\SysWOW64.

I made a tool, CheckArch, that shows the architecture of a file, sometimes needed for troubleshooting. How you managed so far without it, I don't know.

Download CheckArch.exe