VCOMM Client VxDs

VxDs use the VCOMM client VxD services for a variety of reasons. Following are some examples of VCOMM client VxDs:

• Application-defined VxDs. An application can install a private VxD that calls the VCOMM services directly, bypassing the Windows communications API. This approach can be useful for implementing time-critical protocols (such as FAX) that require interrupt-time processing.

  Because creating a private VxD ties your application to Windows 95, you should do so only if necessary. Microsoft(r) Win32(r) applications are scheduled regularly enough to meet the timing requirements of most protocols, such as those for file transfer.

• Port virtualization VxDs. Port virtualization VxDs simulate communications hardware for applications running in VMs. By using the VCOMM client VxD services, a port virtualization VxD can redirect the virtualized device to any communications resource for which a port driver is installed.
• Many modems are external, attaching to standard serial ports, or are internal but emulate a standard serial UART. Modem drivers for such devices typically do not duplicate the functionality of an existing port driver, but instead call VCOMM client VxD services to perform their input and output.