Receiving a Physical COM Port from the Windows VCD
ID: Q76564
|
The information in this article applies to:
-
Microsoft Windows Device Development Kit (DDK) for Windows, versions 3.0, 3.1
SUMMARY
In the enhanced mode of the Microsoft Windows graphical environment, the
virtual communications device (VCD) assigns a physical communications (COM)
port to a virtual machine (VM) only when certain events occur. The text
below lists these events.
MORE INFORMATION
In Windows enhanced mode, the VCD virtualizes and serializes the ownership
of physical COM ports. When a program running in a VM directly accesses one
of the I/O ports of a COM port (for example, COM1 corresponds to I/O ports
0x3F8 through 0x3FE), that VM may not necessarily own the physical port.
The VM communicates with a virtual COM port until the VCD assigns a
physical port.
Seven conditions that cause the VCD to assign a physical COM port to a
particular VM are:
- When an application reads or writes the Receive/Transmit buffer
with the Divisor Latch Access Bit (80h) of the Line Control Register
off.
- When an application reads the Line Status Register more than once.
- When an application reads the Modem Status Register more than once.
- When an application writes to the Modem Control Register with the
Loopback (10h) or Interrupt Enable (08h) bits on (simply changing
the DTR or RTS line states does not cause VCD to assign a physical
COM port).
- When an application unmasks a COM port interrupt.
- When a COM port interrupt occurs for an unowned COM port.
- When the communications driver (COMM.DRV) instructs the VCD to
assign a port to the Windows (system) VM because an application
called the OpenComm function.
Implementation details on COM port assignment can be found by examining the
source code to VCD, which is available in the DDK. Some of the relevant
procedures are:
======================================================================
VCD_Attach called by: VCD_Set_Owner
VCD_Detach called by:
VCD_Set_Owner
VCD_Set_Owner called by:
VCD_Assign
VCD_Set_Port_Global
VCD_PM_Acquire_Port
VCD_PM_Free_Port
VCD_Set_Focus
VCD_Set_Owner_Event
VCD_Detach_If_Owner
VCD_Assign called by:
VCD_PM_Steal_Port
VCD_Cant_Virtualize
VCD_Mask
VCD_PM_Steal_Port called by:
Comm Driver KickTx/StealPort
VCD_Cant_Virtualize called by:
VCD_Virt_In_Out_RxTxB (Condition 1)
VCD_Virt_In_LSR (Condition 2)
VCD_Virt_In_MSR (Condition 3)
VCD_Virt_Out_MCR (Condition 4)
VCD_Mask called by:
VCD_handler (VPICD Mask_Change_Proc) (Condition 5)
VCD_PM_Acquire_Port called by:
Comm Driver GetCOMport386 (Condition 7)
VCD_PM_Free_Port called by:
Comm Driver ReleaseCOMport386
VCD_Set_Owner_Event called by:
VCD_Int (Condition 6)
VCD_Detach_If_Owner called by:
VCD_Destroy_VM
Additional query words:
3.00 3.10 DDKCOMM
Keywords :
Version : :3.0,3.1
Platform :
Issue type :