Thunk (compatibility mapping)
Encyclopedia
Thunk refers to the creation of a 16-bit virtual DOS machine
(VDM) within a 32-bit operating platform so that there is backward compatibility for applications using older code or system call
s.
The most common usage is in the Win16/Win32 API
s, where thunking is used to convert a 16-bit address into a 32-bit equivalent or vice versa. An early example was the 32-bit TCP/IP protocol stack for Windows for Workgroups version 3.11. Code-named "Wolverine", this was an early implementation of the TCP/IP stack that would later ship with Windows 95
. While not initially shipped with Windows for Workgroups, it was available on the NT 3.51 Server CD as an install disk creation utility (at the time, it was more common for IP to be used on 16bit windows via Novell
's Internetwork Packet Exchange). To allow this stack to operate with 16-bit applications, a version of the 16-bit winsock.dll
library was included that simply thunked WinSock calls into the 32-bit stack.
Microsoft later created a mostly-complete thunking layer, called Win32s
, which allowed 32-bit Windows applications (written to a specific subset of the Win32 API, hence the "s" in Win32s) to run on top of 16-bit Windows 3.1x. In many ways, Windows 95
was essentially a full-scale expansion of Win32s, because many of the underpinnings of Win95 were still 16-bit.
Similar thunking was required in many cases in OS/2
2.x—while most of the operating system was 32-bit, many parts of the kernel and device driver
s were 16-bit for compatibility reasons.
Thunking was used in Windows NT/2000 compatibility subsystems: the OS/2 subsystem allowed 16-bit console-mode OS/2 applications to run on Windows NT (x86 only), and the Windows on Windows
(a.k.a. "WoW") subsystem permitted 16-bit Windows applications the same ability. The OS/2 subsystem was dropped after Windows 2000, and the WoW subsystem is not provided in 64-bit versions of Windows. 64-bit versions of Windows provide a similar thunking layer, WoW64
, to allow use of 32-bit Windows applications.
(call-by-need) — it computes a result, or returns the previously computed and cached value.
in hardware. Microsoft Windows 3.0 and earlier, when running in real mode, used a thunk to replace any entry points to a function in a dynamic-link library or executable when the code segment containing that function was discarded (similar to swapped out in a conventional virtual memory system). This is an example of a software-based virtual memory system.
thunks for the TWAIN
API are called twunks (TWain thUNK).
) to make the data segment selector available in a register (redundantly, as it turned out
. In many cases, one must do this type of generic translation or "glue" between two discrete entities. For example, when reading a foreign key from a database table an obvious requirement is to make the join to the related table. Thunk can be used as the term for making this explicit join within the constraints of a hand-created SQL stored procedure
generator. Another example is in generic message interceptors (such as generic mouse-click interceptors within auto-enabling Ajax
JavaScript
libraries). The overall process of intercepting the generic mouse-click message, determining registered handlers, dispatching to those handlers, retrieving results, and applying results to the current page can be described as "thunking" the mouse-click event. In this sense, "to thunk" describes the overall process of detecting a condition that requires re-translation and/or re-packaging of the initial data along with the dispatching and/or handling computer code to support the required action.
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:...
(VDM) within a 32-bit operating platform so that there is backward compatibility for applications using older code or system call
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
s.
OS/2 & Windows 16-bit address hack
A flat thunk consists of a DLL (or, if bidirectional compatibility is needed, a pair of DLLs—one 32-bit and one 16-bit) that is used to translate calls from 32-bit code to 16-bit code. 16- and 32-bit memory addresses work very differently: 16-bit addresses consist of two parts, a pointer to a memory segment, and the offset from the start of that memory segment; whereas a 32-bit process memory pointer consists of the "flat" virtual address of the memory being accessed ("flat" meaning there is no separation of segment and offset part). To allow the two DLLs to communicate, some intermediate code must be used to translate memory addresses (pointers) between platforms.The most common usage is in the Win16/Win32 API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
s, where thunking is used to convert a 16-bit address into a 32-bit equivalent or vice versa. An early example was the 32-bit TCP/IP protocol stack for Windows for Workgroups version 3.11. Code-named "Wolverine", this was an early implementation of the TCP/IP stack that would later ship with 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...
. While not initially shipped with Windows for Workgroups, it was available on the NT 3.51 Server CD as an install disk creation utility (at the time, it was more common for IP to be used on 16bit windows via Novell
Novell
Novell, Inc. is a multinational software and services company. It is a wholly owned subsidiary of The Attachmate Group. It specializes in network operating systems, such as Novell NetWare; systems management solutions, such as Novell ZENworks; and collaboration solutions, such as Novell Groupwise...
's Internetwork Packet Exchange). To allow this stack to operate with 16-bit applications, a version of the 16-bit winsock.dll
Winsock
In computing, the Windows Sockets API , which was later shortened to Winsock, is a technical specification that defines how Windows network software should access network services, especially TCP/IP. It defines a standard interface between a Windows TCP/IP client application and the underlying...
library was included that simply thunked WinSock calls into the 32-bit stack.
Microsoft later created a mostly-complete thunking layer, called Win32s
Win32s
Win32s is a 32-bit application runtime environment for the Microsoft Windows 3.1 and 3.11 operating systems. It allowed some 32-bit applications to run on the 16-bit operating system using call thunks.- Concept and Characteristics :...
, which allowed 32-bit Windows applications (written to a specific subset of the Win32 API, hence the "s" in Win32s) to run on top of 16-bit Windows 3.1x. In many ways, 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...
was essentially a full-scale expansion of Win32s, because many of the underpinnings of Win95 were still 16-bit.
Similar thunking was required in many cases in OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
2.x—while most of the operating system was 32-bit, many parts of the kernel and device driver
Device 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 were 16-bit for compatibility reasons.
Thunking was used in Windows NT/2000 compatibility subsystems: the OS/2 subsystem allowed 16-bit console-mode OS/2 applications to run on Windows NT (x86 only), and the Windows on Windows
Windows on Windows
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 systems that provides limited support for running legacy Win16 applications - applications written for Windows 3.x...
(a.k.a. "WoW") subsystem permitted 16-bit Windows applications the same ability. The OS/2 subsystem was dropped after Windows 2000, and the WoW subsystem is not provided in 64-bit versions of Windows. 64-bit versions of Windows provide a similar thunking layer, 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...
, to allow use of 32-bit Windows applications.
Thunks in dynamic linking
Certain implementations of relocatable code use local thunks to call library functions. Dynamic library calls in the code jump to thunks in a jump table; the jump table is replaced (by a dynamic linker) with short functions that either load the applicable library (as needed) or jump to the appropriate point in an already-loaded library. This form of thunk performs essentially the same task as the thunk-as-delayed-computation in lazy evaluationLazy evaluation
In programming language theory, lazy evaluation or call-by-need is an evaluation strategy which delays the evaluation of an expression until the value of this is actually required and which also avoids repeated evaluations...
(call-by-need) — it computes a result, or returns the previously computed and cached value.
Thunks in virtual memory
Software-based virtual memory systems may use a thunk to perform the mapping from virtual addresses to physical addresses; however, most modern systems do this computation in a specialized memory management unitMemory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
in hardware. Microsoft Windows 3.0 and earlier, when running in real mode, used a thunk to replace any entry points to a function in a dynamic-link library or executable when the code segment containing that function was discarded (similar to swapped out in a conventional virtual memory system). This is an example of a software-based virtual memory system.
Twunks
The Microsoft WindowsMicrosoft 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...
thunks for the TWAIN
TWAIN
TWAIN is a standard software protocol and applications programming interface that regulates communication between software applications and imaging devices such as scanners and digital cameras....
API are called twunks (TWain thUNK).
Thunk applied generically to software development to describe a specific type of adapter
The original Microsoft thunk documentation describes thunk in terms of theMakeProcInstance function, which created two-instruction thunks (mov ax,; jmp . In many cases, one must do this type of generic translation or "glue" between two discrete entities. For example, when reading a foreign key from a database table an obvious requirement is to make the join to the related table. Thunk can be used as the term for making this explicit join within the constraints of a hand-created SQL stored procedure
Stored procedure
A stored procedure is a subroutine available to applications that access a relational database system. A stored procedure is actually stored in the database data dictionary.Typical uses for stored procedures include data validation or access control mechanisms...
generator. Another example is in generic message interceptors (such as generic mouse-click interceptors within auto-enabling Ajax
Ajax (programming)
Ajax is a group of interrelated web development methods used on the client-side to create asynchronous web applications...
JavaScript
JavaScript
JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....
libraries). The overall process of intercepting the generic mouse-click message, determining registered handlers, dispatching to those handlers, retrieving results, and applying results to the current page can be described as "thunking" the mouse-click event. In this sense, "to thunk" describes the overall process of detecting a condition that requires re-translation and/or re-packaging of the initial data along with the dispatching and/or handling computer code to support the required action.