ID Number: Q76564
3.00
WINDOWS
Summary:
In enhanced mode Windows, the virtual communications device (VCD) will
assign a physical communications (COM) port to a virtual machine (VM)
only when certain events occur. This article lists these events.
More Information:
In enhanced mode Windows, the VCD is responsible for virtualizing and
serializing the ownership of physical COM ports. When a program
running in a VM directly accesses the I/O ports of a COM port (for
example, COM1 corresponds to I/O ports 3f8-3fe), that VM may not
necessarily own the physical port. The VM will be working with a
virtual COM port until the VCD assigns a physical port.
The five conditions that will cause the VCD to assign a physical COM
port to a particular VM are:
1. An IN or OUT to the Receive/Transmit buffer with the Divisor Latch
Access Bit of the Line Control Register off.
2. More than one read of the Line Status Register.
3. More than one read of the Modem Status Register.
4. COMM.DRV explicitly instructs VCD to assign a port to the Windows
(system) VM when a Windows application calls OpenComm().
5. An OUT to the Modem Control Register with the 10h or 08h bit on
(simply changing the DTR or RTS line states will not cause a
physical COM port to be assigned).
Additional reference words: 3.00