How Windows 64-bit Supports 32-bit Applications

Under Windows 64-bit, 32-bit applications run on top of an emulation of a 32-bit operating system that is called Windows 32-bit on Windows 64-bit, or WOW64  for short.  WOW64 intercepts all operating system calls made by a 32-bit application. For each operating system call made, WOW64 generates native 64-bit t system calls, converting 32-bit data structures into 64-bit aligned structures.  The appropriate native 64-bit system call is passed to the operating system  kernel, and any output data from the 64-bit system call is converted into  a format appropriate for the calling application before being passed back. Like 32-bit applications, WOW64 runs in user mode so any errors that  occur in translating an operating system call will only occur at that level. The 64-bit operating system kernel cannot be affected. Since WOW64 runs in user mode, all 32-bit application code must  also run in user mode. This explains why 32-bit kernel mode device  drivers and applications that rely on them, will not work under Windows 64-bit. The WOW64 emulator consists of the following DLLs, the only 64-bit  DLLS that can be loaded into a 32-bit process: Wow64.dll – the core emulation infrastructure and the links to  the Ntoskrnl.exe entry-point functions. Wow64Win.dll – the links to the Win32k.sys entry-point functions. Wow64Cpu.dll – switches the processor from 32-bit to 64-bit mode. Ntdll.dll – 64-bit version. Wow64.dll loads the 32-bit version (x86) of Ntdll.dll and all necessary  32-bit DLLs which are mostly unmodified 32-bit binaries..However, some of these DLLs have been modified to behave differently on WOW64  than they do on 32-bit Windows. This is usually because they share  memory with 64-bit system components. WOW64 manages file and registry settings In addition to handling operating system calls, the WOW64 interface  needs to ensure that files and registry settings for 32-bit applications are kept apart from those for 64-bit applications. To achieve this two mechanisms are used, File and Registry Redirection and Key Reflection.  Redirection maintains logical views of the data as if it were in 32-bit Windows and maps it to the correct physical location. Reflection ensures that 32-bit and 64-bit settings will be consistent where that is required. File Redirection File redirection ensures that there are separate folders for program  and operating system files for 32- and 64-bit applications. 32-bit applications files are installed into C:\Program Files(x86) 32-bit system files are installed into C:\WINDOWS\SysWOW64 For 64-bit applications, files are installed to: C:\Program Files C:\WINDOWS\SYSTEM32 The WOW64 file redirector ensures that requests from 32-bit applications  to open files in C:\Program Files or C:\WINDOWS\SYSTEM32 are redirected to the appropriate 32-bit directories. There is one issue with file redirection that users and developers  should be aware of. Many 64 bit applications still use 32 bit installation routines. To  ensure that an application is installed correctly, i.e. to C:\Program  Files, the installation routine should make an operating system call to temporarily suspend the WOW64 file redirector. After installation  another operating system call needs to be made to re-enable the redirector.  If this approach isn't followed then the application will be installed  to C:\Program Files (x86). A classic example of this is the 64 bit development  version of Firefox 3.5, codenamed Shiretoko, which is installed to C:\Program Files(x86)\Shiretoko. Firefox still functions correctly, the only thing you  can't do is change the icon for the application. Registry Redirection Registry keys specific to 32-bit applications are redirected from: HKEY_LOCAL_MACHINE\Software to: HKEY_LOCAL_MACHINE\Software\WOW6432Node You may also occasionally see Registry entries elsewhere although  this is unusual HKEY_CURRENT_USER\Software\WOW6432Node This approach allows both the 32-bit and 64-bit versions of an application  to be installed side-by-side without overwriting each other’s settings. Registry reflection Some redirected keys and/or values are also reflected. This means that  if a 32-bit application makes a change to the redirected section of the  registry, that change is also made to the 64 bit part of the registry, and  vice-versa. Key reflection uses a policy of last writer wins. For example, if I install three applications with the same file extension then the last  one to be installed will be associated with that extension. Install a 32-bit application that associates itself with the file  extension XYZ. Install the 64-bit version of this application that associates itself with the file extension XYZ. Install another 3- bit application that associates itself with the file extension XYZ. Double-clicking on a file with the extension XYZ in Explorer would  load the application installed in step 3, as it was the last one to  associate itself with this extension. All of this is done transparently for 32-bit applications by WOW64, which,  in intercepting calls to the operating system, detects references to file  paths and registry keys and maps them accordingly. WOW64 has several limitations Some but not all 64-bit features are available to 32-bit applications WOW64 provides 32-bit applications with access to some features of 62-bit systems. For example, applications can have more memory up to  4GB with the correct setting.. Other features are more limited due to overheads and restrictions. For example, 64-bit Windows will support  logical 64 processors but 32-bit applications are restricted to the usual  32 logical processors. Code Injection cannot mix between 32-bit and 64-bit Under 64-bit Windows it is not possible to inject 32-bit code into a 64-bit process, nor is it possible to inject 64-bit code into a 32-bit process. Applications that rely on code injection to add functionality  to existing applications will usually not work. This explains why most 32-bit shell extensions do not work under Windows 64-bit. The majority of shell extensions rely on code injection  to add themselves to Windows Explorer. WOW64 does not support 16-bit installers WOW64 provides support for Microsoft's 16-bit installer - by substituting  a compatible 32-bit installer - but does not extend this support to third-party products. Further options for running 32-bit applications with Windows 64-bit Windows Virtual PC Windows Virtual PC is free software that provides an environment that will support legacy hardware and software that will not work under Windows 7. Guest operating systems (OS) can run in a virtual machine which means they are not aware that they are running under  another operating system. The system requirements and features vary significantly between  versions of Virtual PC and versions of Windows so check before  you try Virtual PC. The latest version is, perhaps, the most limited with no support for operating systems before the current supported  version of Windows XP which is Service Pack 3. Windows XP Mode (XPM) Windows XP Mode  is a specific implementation of Windows Virtual PC that comes with a pre-installed, licensed copy of Windows XP Professional with Service Pack 3. It is only available with the Enterprise , Ultimate and Professional editions of Windows 7 64-bit so you are expected to upgrade to these versions if you want it. Many who have used XPM advise that it should be used as a last resort . It will provide legacy support if you have no other options but, compared to other virtualization products, performance is disappointing  and the default configuration raises a number of security issues. Dual boot Windows You can install more than one version of Windows on the same  computer by dual booting.For the purposes of this article, you would  install a 32-bit version and a 64-bit version alongside each other. Each  operating system is installed into its own disk partition and a boot  manager is installed on the default partition to ensure that you can choose which operating system you want to use at startup. Although you cannot use both operating systems at the same time it  is a useful option because the entire computer is dedicated to the running operating system. Compared with virtual machines, there are no issues of compatibility and much less complexity in both installation  and operation. You can also retain the ability to run 16-bit applications  under the 32-bit version of Windows.

Summary

Most 32 bit applications will run quite happily under Windows 64. The  main exceptions are: 32-bit device drivers. Applications that cannot function without the 32-bit device drivers  that they use. Prime examples are antivirus, antimalware and  firewall applications. Application extensions that rely on code injection into, for example,  Explorer. Some applications may work with reduced functionality. These include  uninstallers, registry cleaners and tweaking programs, amongst others,  since they only have access to that part of the Registry made visible to  them by WOW64. If you cannot run your 32-bit applications then consider virtualization or  dual-booting with the old and new operating systems both installed.