WoW64
Encyclopedia
WoW64 is a subsystem of the Windows
operating system
that is capable of running 32-bit
applications and is included on all 64-bit
versions of Windows—including Windows XP Professional x64 Edition
, IA-64 and x64
versions of Windows Server 2003
, as well as 64-bit versions of Windows Vista
, Windows Server 2008, and Windows 7. In Windows Server 2008 R2
Server Core, it is an optional component. WoW64 is designed to take care of many of the differences between 32-bit Windows and 64-bit Windows, particularly involving structural changes to Windows itself.
that has similar interfaces on all 64-bit versions of Windows. It aims to create a 32-bit environment that provides the interfaces required to allow 32-bit Windows applications to run unmodified in the 64-bit system. Technically, WoW64 is implemented using three dynamic-link libraries
(DLLs):
as IA-64 architecture) uses
. This emulation is a much more computationally expensive task than the
architecture (alias "Intel 64" in the Intel terminology, or "AMD64" in the original AMD implementation of this 64-bit mode, also used in the architecture type name of Windows installers), which switches the processor hardware from its 64-bit mode to compatibility mode when it becomes necessary to execute a 32-bit thread
, and then handles the switch back to 64-bit mode.
, which has distinct keys for 64-bit and 32-bit applications. For example HKEY_LOCAL_MACHINE\Software\Wow6432Node is the 32-bit equivalent of HKEY_LOCAL_MACHINE\Software (although 32-bit applications are not aware of this redirection). Some Registry keys are mapped from 64-bit to their 32-bit equivalents, while others have their contents mirrored, depending on the edition of Windows.
The operating system uses the %SystemRoot%\system32 directory for its 64-bit library and executable files. This is done for backward compatibility reasons, as many legacy applications are hardcoded to use that path. When executing 32-bit applications, WoW64 transparently redirects 32-bit DLLs to %SystemRoot%\SysWOW64, which contains 32-bit libraries and executables. 32-bit applications are generally not aware that they are running on a 64-bit operating system. 32-bit applications can access %SystemRoot%\System32 through the pseudo directory %SystemRoot%\sysnative.
There are two "Program Files" directories, both visible to both 32-bit and 64-bit applications. The directory that stores the 32 bit files is called Program Files (x86) to differentiate between the two, while the 64 bit maintains the traditional "Program Files" name without any additional qualifier.
s, or that plug into the process space of components that are implemented purely as 64-bit processes (e.g. Windows Explorer) cannot be executed on a 64-bit platform.
Service applications are supported. The SysWOW64 folder located in the Windows folder on the OS drive contains several applications to support 32-bit applications (e.g. cmd.exe, useful to register 32bit windows services, odbcad32.exe, to register ODBC connections for 32-bit applications).
Internet Explorer
is implemented as both a 32-bit and a 64-bit application because of the large number of 32-bit ActiveX
components on the internet that would not be able to plug into the 64-bit version. The 32-bit version is used by default and the 64-bit version cannot be set to be the default browser.
A bug in the translation layer of the x64 version of WoW64 also renders all 32-bit applications that rely on the Windows API
function GetThreadContext incompatible. Such applications include application debuggers, call stack tracers (e.g. IDEs displaying call stack) and applications that use garbage collection
(GC) engines. One of the more widely used but affected GC engines is the Boehm GC. It is also used as the default Garbage Collector of the equally popular Mono
. While Mono has introduced a new (but optional) GC as of October 2010 called SGen-GC, it performs stack scanning in the same manner as Boehm GC, thus also making it incompatible under Wow64. No fix is planned and no workarounds have been provided by Microsoft as of 15th November 2010. However, one should realize that Mono is mainly a Linux
.NET runtime and that one could use Microsoft's Free Windows .NET Runtime (under Windows) instead.
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
that is capable of running 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
applications and is included on all 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...
versions of Windows—including Windows XP Professional x64 Edition
Windows XP Professional x64 Edition
Microsoft Windows XP Professional x64 Edition released on April 25, 2005 is an edition of Windows XP for x86-64 personal computers. It is designed to use the expanded 64-bit memory address space provided by the x86-64 architecture....
, IA-64 and x64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
versions of Windows Server 2003
Windows Server 2003
Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005...
, as well as 64-bit versions of Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
, Windows Server 2008, and Windows 7. In Windows Server 2008 R2
Windows Server 2008 R2
Windows Server 2008 R2 is a server operating system produced by Microsoft. It was released to manufacturing on July 22, 2009 and launched on October 22, 2009. According to the Windows Server Team blog, the retail availability was September 14, 2009. It is built on Windows NT 6.1, the same core...
Server Core, it is an optional component. WoW64 is designed to take care of many of the differences between 32-bit Windows and 64-bit Windows, particularly involving structural changes to Windows itself.
Translation libraries
The WoW64 subsystem comprises a lightweight compatibility layerCompatibility layer
A compatibility layer is a term that refers to components that allow for non-native support of components.In software engineering, a compatibility layer allows binaries for a legacy or foreign system to run on a host system. This translates system calls for the foreign system into native system...
that has similar interfaces on all 64-bit versions of Windows. It aims to create a 32-bit environment that provides the interfaces required to allow 32-bit Windows applications to run unmodified in the 64-bit system. Technically, WoW64 is implemented using three dynamic-link libraries
Dynamic-link library
Dynamic-link library , or DLL, is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems...
(DLLs):
-
Wow64.dll, the core interface to the Windows NT kernel that translatesThunkThunk may refer to:* Thunk , a piece of code to perform a delayed computation * Thunk : a feature of some virtual function table implementations...
between 32-bit and 64-bit calls, including pointer and call stackCall stackIn computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...
manipulations -
Wow64win.dll, which provides the appropriate entry-points for 32-bit applications -
Wow64cpu.dll, which takes care of switching the processor from 32-bit to 64-bit mode
Architectures
Despite its outwardly similar appearance on all versions of 64-bit Windows, WoW64's implementation varies depending on the target processor architecture. For example, the version of 64-bit Windows developed for the Intel Itanium 2 processor (known at MicrosoftMicrosoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
as IA-64 architecture) uses
Wow64win.dll to set up the emulation of x86 instructions within the Itanium 2's unique instruction setInstruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
. This emulation is a much more computationally expensive task than the
Wow64win.dll's functions on the x86-64X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
architecture (alias "Intel 64" in the Intel terminology, or "AMD64" in the original AMD implementation of this 64-bit mode, also used in the architecture type name of Windows installers), which switches the processor hardware from its 64-bit mode to compatibility mode when it becomes necessary to execute a 32-bit thread
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
, and then handles the switch back to 64-bit mode.
Registry and file system
The WoW64 subsystem also handles other key aspects of running 32-bit applications. It's involved in managing the interaction of 32-bit applications with the Windows components such as the RegistryWindows registry
The Windows Registry is a hierarchical database that stores configuration settings and options on Microsoft Windows operating systems. It contains settings for low-level operating system components as well as the applications running on the platform: the kernel, device drivers, services, SAM, user...
, which has distinct keys for 64-bit and 32-bit applications. For example HKEY_LOCAL_MACHINE\Software\Wow6432Node is the 32-bit equivalent of HKEY_LOCAL_MACHINE\Software (although 32-bit applications are not aware of this redirection). Some Registry keys are mapped from 64-bit to their 32-bit equivalents, while others have their contents mirrored, depending on the edition of Windows.
The operating system uses the %SystemRoot%\system32 directory for its 64-bit library and executable files. This is done for backward compatibility reasons, as many legacy applications are hardcoded to use that path. When executing 32-bit applications, WoW64 transparently redirects 32-bit DLLs to %SystemRoot%\SysWOW64, which contains 32-bit libraries and executables. 32-bit applications are generally not aware that they are running on a 64-bit operating system. 32-bit applications can access %SystemRoot%\System32 through the pseudo directory %SystemRoot%\sysnative.
There are two "Program Files" directories, both visible to both 32-bit and 64-bit applications. The directory that stores the 32 bit files is called Program Files (x86) to differentiate between the two, while the 64 bit maintains the traditional "Program Files" name without any additional qualifier.
Incompatible applications
32-bit applications that include only 32-bit kernel-mode device driverDevice driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
s, or that plug into the process space of components that are implemented purely as 64-bit processes (e.g. Windows Explorer) cannot be executed on a 64-bit platform.
Service applications are supported. The SysWOW64 folder located in the Windows folder on the OS drive contains several applications to support 32-bit applications (e.g. cmd.exe, useful to register 32bit windows services, odbcad32.exe, to register ODBC connections for 32-bit applications).
Internet Explorer
Internet Explorer
Windows Internet Explorer is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year...
is implemented as both a 32-bit and a 64-bit application because of the large number of 32-bit ActiveX
ActiveX
ActiveX is a framework for defining reusable software components in a programming language-independent way. Software applications can then be composed from one or more of these components in order to provide their functionality....
components on the internet that would not be able to plug into the 64-bit version. The 32-bit version is used by default and the 64-bit version cannot be set to be the default browser.
A bug in the translation layer of the x64 version of WoW64 also renders all 32-bit applications that rely on the Windows API
Windows API
The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces available in the Microsoft Windows operating systems. It was formerly called the Win32 API; however, the name "Windows API" more accurately reflects its roots in 16-bit Windows and its support on...
function GetThreadContext incompatible. Such applications include application debuggers, call stack tracers (e.g. IDEs displaying call stack) and applications that use garbage collection
Garbage collection (computer science)
In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...
(GC) engines. One of the more widely used but affected GC engines is the Boehm GC. It is also used as the default Garbage Collector of the equally popular Mono
Mono (software)
Mono, pronounced , is a free and open source project led by Xamarin to create an Ecma standard compliant .NET-compatible set of tools including, among others, a C# compiler and a Common Language Runtime....
. While Mono has introduced a new (but optional) GC as of October 2010 called SGen-GC, it performs stack scanning in the same manner as Boehm GC, thus also making it incompatible under Wow64. No fix is planned and no workarounds have been provided by Microsoft as of 15th November 2010. However, one should realize that Mono is mainly a Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
.NET runtime and that one could use Microsoft's Free Windows .NET Runtime (under Windows) instead.