VxD
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In Microsoft computing, a VxD is a virtual device driver. They run under the Windows 3.x, Windows 95 and Windows 98 operating systems, and have 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, 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. Windows 2.1 386 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 in Windows 3.0. 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.
The name VxD comes from the fact that most of these virtual device drivers, had filenames like "v(something)d.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.
In Windows operating systems starting with Windows 95, ordinary applications run in protected mode at privilege level 3, also called "ring" 3. This means that they have limited access to memory and hardware, just like DOS boxes in Windows 3.x. VxDs, on the other hand, run at privilege level or "ring" 0, i.e. with full access to the memory and hardware. Hence in Windows 95 and 98, any application which needs full access to memory will install a VxD. Such VxDs do not drive devices, they merely allow an application to do whatever it wants to do. For example, virus checkers usually install a VxD. Even the kernel itself uses VXDs to perform tasks such as memory management. This apparent paradox comes from the fact that VxDs are simply old-style Windows device drivers and dynamically-loadable kernel-level components of the operating system.
VxD are no longer supported by current versions of Windows. Device drivers for Windows 2000, and Windows XP use the Windows Driver Model (WDM) drivers.fr:VxD