Microsoft KB Archive/76564

= Receiving a Physical COM Port from the Windows VCD =

Article ID: 76564

Article Last Modified on 7/27/2004

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APPLIES TO


 * Microsoft Windows Device Development Kit (DDK) for Windows 3.0
 * Microsoft Windows Device Development Kit (DDK) for Windows 3.1

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This article was previously published under Q76564



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:


 * 1) When an application reads or writes the Receive/Transmit buffer with the Divisor Latch Access Bit (80h) of the Line Control Register off.
 * 2) When an application reads the Line Status Register more than once.
 * 3) When an application reads the Modem Status Register more than once.
 * 4) 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).
 * 5) When an application unmasks a COM port interrupt.
 * 6) When a COM port interrupt occurs for an unowned COM port.
 * 7) 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: KB76564

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