Virtual Devices

Virtual devices (VxDs) are 32-bit programs that support the device-independent VMM by managing the computer's hardware devices and supporting software. VxDs support all hardware devices for a typical computer, including the programmable interrupt controller (PIC), timer, direct memory access (DMA) device, disk controller, serial ports, parallel ports, keyboard, and display adapter. A VxD is required for any hardware device that has settable operating modes or retains data over any period of time. In other words, if the state of the hardware device can be disrupted by switching between multiple virtual machines or applications, the device must have a corresponding VxD.

Some VxDs support software, but no corresponding hardware device. In general, a VxD can provide any kind of services for the VMM and other virtual devices. Windows 95/98 allows the user to install new virtual device drivers to support an add-on hardware device or provide some system-wide software service.

A VxD can also provide application programming interface (API) functions for applications running in virtual 8086 mode or protected mode. These functions can give applications direct access to the features of the VxD.

Windows 95/98 includes a device input and output control (IOCTL) interface that allows Microsoft® Win32® – based applications to communicate directly with VxDs. Applications typically use this interface to carry out selected MS-DOS system functions, to obtain information about a device, or to carry out input and output (I/O) operations that are not available through standard Win32 functions. For more information about the device IOCTL interface, see Device I/O Control.

For more information about virtual devices, see the documentation included in the DDK.