The 16-bit Windows subsystem is implemented as a virtual MS-DOS machine (VDM) with a layer that emulates Windows 3.1 functionality. All 16-bit Windows-based applications run in the same VDM, a multithreaded Win32 process in which each application runs in its own thread. Below is a diagram of the 16-bit Windows subsystem VDM. A description of each layer follows.
Figure 15.1 16-bit Windows Subsystem VDM
The 16-bit MS-DOS emulation layer contains all the information to emulate BIOS calls and tables. Some 16-bit Windows applications depend upon BIOS calls, since 16-bit Windows is built on top of MS-DOS.
The Windows 3.1 emulation layer provides the functionality of the Windows 3.1 kernel and 16-bit API stubs. A 16-bit application cannot call a 32-bit API routine. When an application calls a 16-bit API routine, that call is made to a stub routine, which in turn calls a 32-bit API routine. The 32-bit API routine performs the required action, and the result is transformed back into the format expected by the 16-bit API stub, which returns the result to the application. The transformation between 16-bit and 32-bit formats is known as thunking, and is carried out by a separate layer in the VDM.
16-bit Windows-based applications use the memory from 640K to 16 MB for their own purposes.
Windows NT does not support 16-bit device drivers that have unrestricted access to hardware (character-mode device drivers that do not depend on special hardware are supported). A secure and robust multitasking operating system cannot let user-level applications talk directly with the hardware because they could completely bypass security and crash the system. (There are exceptions to this, however; refer to "Restrictions," at the beginning of this chapter). The VDM contains a layer of virtual device drivers (VDDs) that allow the sharing of hardware and provide the necessary functionality in a way that is consistent with the design of Windows NT.
The 32-bit MS-DOS emulation layer is for the DOS Protect Mode Interface (DPMI) and 32-bit memory access. This layer replaces calls made to the MS-DOS-level functions for extended and expanded memory with Windows NT memory calls. Windows NT then makes the appropriate conversions so that the 16-bit application sees segmented memory as it normally would.
The purpose of the 32-bit Windows thunking layer was described above.
For Windows NT running on a non-X86 computer, the Instruction Execution Unit emulates the Intel 80286 instruction set, which lets the computer run the binary application. The 16-bit Windows subsystem (or any other VDM) on a non-x86 computer supports only Standard mode 16-bit applications.
On an x86 computer the Instruction Execution Unit acts as a trap handler, capturing instructions that cause hardware traps and transferring control to the code that handles them. A VDM (such as the 16-bit Windows subsystem) on an x86 computer supports Enhanced mode applications; it does not, however, support 16-bit VXDs (virtual device drivers).