Everything about Vxd totally explained
VxD is the
device driver model used in
Microsoft Windows/386, the 386 enhanced mode of
Windows 3.x and
Windows 9x. They have access to the memory of the kernel and all running processes, as well as raw access to the hardware.
Design
The name "VxD" is an abbreviation for "virtual xxx driver", where "xxx" is some class of hardware device. It derives from the fact that most drivers had filenames of the form
vxxxd.386 in
Windows 3.x. Some examples are: vjoyd.386 (joystick), vmm.386 (memory manager). VxDs usually have the
filename extension .386 under Windows 3.x and
.vxd under Windows 95. VxDs written for Windows 3.x can be used under Windows 95 but not vice versa.
History
Prior to the advent of Windows,
DOS applications frequently communicated directly with various pieces of hardware, by responding to interrupts, reading and writing device memory etc. Each application expected to have exclusive and complete control over the hardware. Though
Windows applications don't often communicate directly with hardware, it was the only way to write Windows drivers, and still is in the real and standard modes of Windows 3.x. Despite the fact that Windows switched from running in real mode to protected mode, direct hardware access and interrupt hooking could still be done because when Windows switched to running in protected mode it kept the single privilege level model used in real mode. This lasted all the way through Windows 9x. Windows/386 and onwards allowed multiple
MS-DOS applications to execute simultaneously. This was done by executing each legacy application within its own virtual machine. To share arbitrary physical resources amongst these virtual machines, Microsoft introduced dynamically-loadable virtual device drivers. These drivers solved issues relating to conflicting usage of physical resources by intercepting calls to the hardware. Instead of a
machine port representing an actual device, it would represent a "virtual" device, which could be managed by the operating system.
Obsolescence
VxDs are not usable in
Windows NT or its descendants. Starting with Windows 2000, these operating systems use the
Windows Driver Model (WDM), while Windows NT 4 and earlier versions must use drivers written specifically for them.
Windows Vista supports both WDM and the newer
Windows Driver Foundation which includes
Kernel-Mode Driver Framework (KMDF) and
User-Mode Driver Framework (UMDF). KMDF is also available for download for Windows XP and Windows 2000, while UMDF is available only for Windows XP.
Further Information
Get more info on 'Vxd'.
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