DOCUMENT:Q76564 30-OCT-1999 [win16ddk] TITLE :Receiving a Physical COM Port from the Windows VCD PRODUCT :Microsoft Windows Device Driver Kit PROD/VER::3.0,3.1 OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- 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: 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 : Technology : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK300 kbWinDDK310 Version : :3.0,3.1 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 1999.