DFC for Windows 95
See the DFC Information page for general information on DFC
components and features. This topic covers details particular to Windows 95/98.
VxD architecture today...
The DFC library is the only driver library that supports creating both an
NT .SYS module and a Windows 9x VxD module from common source. The DFC library implements
partial emulation of the Windows NT I/O manager and provides NT driver mechanisms for VxD
developers.
...WDM tomorrow
Microsoft has announced that they will support a common driver model for both Windows and
Windows NT called the Win32 Driver Model (see http://www.microsoft.com/hwdev
for more details). WDM is currently shipping in a limited form with OEM systems but will
not be widely available until the release of the next major Windows update. The DFC
library will continue to support the VxD architecture allowing you to support both the new
and older versions of Windows from one source base.
I/O Request Packets (IRPs) and Asynchronous I/O
The heart of DFC for Windows 95 is its emulation of the NT I/O manager and handling of I/O
request packets or IRPs. When the DFC library catches CreateFile, DeviceIoControl, and
CloseHandle requests from applications, the library allocates IRP structures from an IRP
cache. The library then sets up and passes the allocated IRP to the dispatch member
function of a user's device class in a manner similar to what the user sees on NT.
As on NT, a dispatch member function can mark a request pending and release the calling
thread while the VxD processes the request. At a later time, a VxD can complete the
request and the DFC library will signal the calling thread.
The DFC library handles the buffers associated with DeviceIoControl calls based on the
flags in an IOCTL code. For METHOD_BUFFERED IOCTL requests, the DFC library double-buffers
the DeviceIoControl request buffers using a system-space buffer just like the NT I/O
manager does. For METHOD_IN/OUT_DIRECT requests, the DFC library page locks the
DeviceIoControl output buffer and double-buffers the input buffer-again like on NT.
Cancel and Cleanup Logic
On NT, if a thread issues an asynchronous request to a driver and then exits
before the request has been completed, the NT I/O Manager calls a user-defined routine to
cancel processing of the request. The DFC library emulates these mechanisms in a Windows
95 VxD. The library links all allocated IRPs in an internal list. When the library catches
a Terminate_Thread system control message, it walks the list and cancels any pending IRPs
which originated from the dying thread. The DFC library also emulates cleanup logic by
catching close handle requests and translating them into calls to the user device's
DispatchCleanup member function.
Plug & Play Integration
The DFC library fully supports Plug & Play configuration of legacy and Plug & Play
hardware using the Windows 95 Configuration Manager. The library implements the complex
handling of Plug & Play system control messages. In these message handlers, the
library creates instances of the user's device class and automatically configures the
device resources from a Configuration Manager-assigned resource set.
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