PRB: VCD_Virtualize_Port Works in 3.1 but Fails in Windows 95

• Microsoft Win32 Device Development Kit (DDK) for Windows 95

SYMPTOMS

To claim a COM port in Windows 3.1, VCD services were sometimes employed. VCD is the virtualizer for the serial ports. In particular, the VCD_Virtualize_Port call was used. However, programmers find that this call fails in Windows 95 as if the port has already been virtualized.

CAUSE

Under Windows 95, the port has indeed been virtualized. COMBUFF, which buffers all MS-DOS VM serial port data, virtualizes all serial ports during initialization. VCOMM is the new VxD controlling COMM ports for Windows- based applications. As such, VCOMM is the intended VxD through which COMM ports are acquired through VCOMM_Acquire_Port. This would be called from a dynamic port driver. See Vcomm.doc in the DDK for a complete description.

STATUS

This behavior is by design.

REFERENCES

MSDN Library Compact Disc October 95 Windows 95 DDK