VxD

In computing, a VxD is a virtual device driver. It runs under the Windows 3.x or Windows 95 operating systems as part of the kernel and thus has access to the memory of the kernel and all running processes, as well as raw access to the hardware.

Prior to the advent of Windows, MS-DOS applications communicated directly with various pieces of hardware, by responding to interrupts, reading and writing device memory etc. Each applications expected to have exclusive and complete control over the hardware. Windows 3.0 and onwards allowed (for marketing reasons) multiple MS-DOS applications to execute simultaneously. This was done by executing each legacy application within its own virtual machine. To share physical resources amongst these virtual machines, Microsoft introduced virtual device drivers. These drivers solved issues relating to conflicting usage of physical resources by intercepting calls to the hardware.

The name VxD comes from the fact that most of these virtual device drivers, had filenames like vxd.386 in Windows 3.x. For instance, vjoyd.386 (joystick), vmm.386 (memory manager) and so on.

VxDs usually have the filename extension .386 under Windows 3.x and .vxd under Windows 95. VxD's written for Windows 3.x can be used under Windows 95 but not vice versa.

VxD are no longer supported by current versions of Windows. Device drivers for Windows 98, Windows 2000, and Windows XP use the Windows Driver Model (WMD) drivers.