Windows on Windows
Encyclopedia
In computing
, Windows on Windows - commonly referred to by its acronym WOW or WoW - is a software component of 32-bit
versions of the Microsoft Windows NT
family of operating system
s that provides limited support for running legacy Win16
applications - applications written for Windows 3.x
. Alternatively "WOW" may also refer to support for running 32-bit applications on 64-bit versions of Windows - known as WOW64
.
s 16-bit APIs to their underlying 32-bit equivalents in order to provide support for 16-bit pointers, memory models and address space
. 32-bit Windows shortens long filename
s into 8.3 filenames to allow their use by legacy applications. Application-compatibility issues, notably around multiple users and the concept of least privilege, may prevent some applications that incorrectly assume write access to the whole file system from working on newer platforms. Furthermore, legacy applications that attempt to access hardware directly cannot do so in user mode
.
The Windows 9x
series of operating systems functioned as hybrid 16/32-bit systems in the sense that the underlying operating system was not truly 32-bit, and therefore could run Win16 applications natively without the use of a distinct WoW component.
64-bit versions of Windows do not include the WoW Win16-support subsystem and therefore cannot run Win16 applications, nor do they provide the NTVDM
DOS PC emulator. DOS
applications therefore cannot run on a 64-bit edition of Windows unless a system administrator has installed third-party emulator-software or configured a virtual DOS machine in a guest 32-bit virtual machine
.
The WIN.COM
file in a Windows NT
System32 folder facilitates Windows-on-Windows. In addition to Windows-on-Windows emulating the Windows 95
, Windows NT 4.0
, and Windows 98
kernels, the WIN.COM file emulates a Windows 3.x
kernel for NTVDM, which runs the 16-bit
DOS-based Windows applications on Windows NT. In 64-bit
Windows NT
environments Microsoft has removed both NTVDM and the emulated Windows 3.x
kernel; thus 64-bit Windows-on-Windows only emulates the kernels of Windows 95
, Windows NT 4.0
, Windows 98
, and later (where the emulated kernel of Windows Vista
and later were only 32-bit
).
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, Windows on Windows - commonly referred to by its acronym WOW or WoW - is a software component of 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....
versions of the Microsoft Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
family of 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...
s that provides limited support for running legacy Win16
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...
applications - applications written for Windows 3.x
Windows 3.x
Windows 3.x can refer to either an individual or all of the following versions of Microsoft Windows:*Windows 3.0*Windows 3.1x*Windows 3.2...
. Alternatively "WOW" may also refer to support for running 32-bit applications on 64-bit versions of Windows - known as WOW64
WoW64
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...
.
Background
Many Win16 applications can run without changes on 32-bit editions of Windows, complete with the limitations of such applications compared with applications written for Win32. The operating system thunkThunk (compatibility mapping)
Thunk refers to the creation of a 16-bit virtual DOS machine within a 32-bit operating platform so that there is backward compatibility for applications using older code or system calls.-OS/2 & Windows 16-bit address hack:...
s 16-bit APIs to their underlying 32-bit equivalents in order to provide support for 16-bit pointers, memory models and address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...
. 32-bit Windows shortens long filename
Long filename
Long filenames , are Microsoft's way of implementing filenames longer than the 8.3 filename, or short-filename, naming scheme used in Microsoft DOS in their modern FAT and NTFS filesystems. Because these filenames can be longer than an 8.3 filename, they can be more descriptive...
s into 8.3 filenames to allow their use by legacy applications. Application-compatibility issues, notably around multiple users and the concept of least privilege, may prevent some applications that incorrectly assume write access to the whole file system from working on newer platforms. Furthermore, legacy applications that attempt to access hardware directly cannot do so in user mode
Ring (computer security)
In computer science, hierarchical protection domains, often called protection rings, are a mechanism to protect data and functionality from faults and malicious behaviour . This approach is diametrically opposite to that of capability-based security.Computer operating systems provide different...
.
The Windows 9x
Windows 9x
Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced since 1995, which were based on the original and later modified Windows 95 kernel...
series of operating systems functioned as hybrid 16/32-bit systems in the sense that the underlying operating system was not truly 32-bit, and therefore could run Win16 applications natively without the use of a distinct WoW component.
64-bit versions of Windows do not include the WoW Win16-support subsystem and therefore cannot run Win16 applications, nor do they provide the NTVDM
Virtual DOS machine
Virtual DOS machine is Microsoft's technology that allows running legacy DOS and 16-bit Windows programs on Intel 80386 or higher computers when there is already another operating system running and controlling the hardware.-Overview:...
DOS PC emulator. DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
applications therefore cannot run on a 64-bit edition of Windows unless a system administrator has installed third-party emulator-software or configured a virtual DOS machine in a guest 32-bit virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
.
The WIN.COM
WIN.COM
WIN.COM is the executable file used to load versions of Windows that run from DOS. In Windows 3.1 and its predecessors, it is executed either manually from the DOS prompt or as a line in AUTOEXEC.BAT. In Windows 95 and onward it is automatically invoked by IO.SYS after AUTOEXEC.BAT is processed...
file in a Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
System32 folder facilitates Windows-on-Windows. In addition to Windows-on-Windows emulating the Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
, Windows NT 4.0
Windows NT 4.0
Windows NT 4.0 is a preemptive, graphical and business-oriented operating system designed to work with either uniprocessor or symmetric multi-processor computers. It was the next release of Microsoft's Windows NT line of operating systems and was released to manufacturing on 31 July 1996...
, and Windows 98
Windows 98
Windows 98 is a graphical operating system by Microsoft. It is the second major release in the Windows 9x line of operating systems. It was released to manufacturing on 15 May 1998 and to retail on 25 June 1998. Windows 98 is the successor to Windows 95. Like its predecessor, it is a hybrid...
kernels, the WIN.COM file emulates a Windows 3.x
Windows 3.x
Windows 3.x can refer to either an individual or all of the following versions of Microsoft Windows:*Windows 3.0*Windows 3.1x*Windows 3.2...
kernel for NTVDM, which runs the 16-bit
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
DOS-based Windows applications on Windows NT. In 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...
Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
environments Microsoft has removed both NTVDM and the emulated Windows 3.x
Windows 3.x
Windows 3.x can refer to either an individual or all of the following versions of Microsoft Windows:*Windows 3.0*Windows 3.1x*Windows 3.2...
kernel; thus 64-bit Windows-on-Windows only emulates the kernels of Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
, Windows NT 4.0
Windows NT 4.0
Windows NT 4.0 is a preemptive, graphical and business-oriented operating system designed to work with either uniprocessor or symmetric multi-processor computers. It was the next release of Microsoft's Windows NT line of operating systems and was released to manufacturing on 31 July 1996...
, Windows 98
Windows 98
Windows 98 is a graphical operating system by Microsoft. It is the second major release in the Windows 9x line of operating systems. It was released to manufacturing on 15 May 1998 and to retail on 25 June 1998. Windows 98 is the successor to Windows 95. Like its predecessor, it is a hybrid...
, and later (where the emulated kernel 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...
and later were only 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....
).