Microsoft KB Archive/76280

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ENumPort.exe - COM Ports Present in Enhanced Mode

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Q76280

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1

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SUMMARY
A problem with the communications driver causes a device contention message box to appear when a nonexistent port is opened in enhanced mode Windows using the OpenComm function. This article describes a method of avoiding this message box when using the communication API under enhanced mode Windows.

ENumPort.exe is a file that demonstrates the techniques detailed in this article.

MORE INFORMATION
The following files are available for download from the Microsoft Download Center:

ENumPort.exe

For additional information about how to download Microsoft Support files, click the article number below to view the article in the Microsoft Knowledge Base:

"Q119591 How to Obtain Microsoft Support Files from Online Services" Microsoft used the most current virus detection software available on the date of posting to scan this file for viruses. Once posted, the file is housed on secure servers that prevent any unauthorized changes to the file.

Under standard or real mode, the OpenComm function can be used to successfully test for the presence of communications hardware by checking the return value. If the return value of the OpenComm function is IE_HARDWARE, it is safe to assume that the hardware is not present.

However, under enhanced mode Windows, calling the OpenComm function with a port ID that is not valid will cause the following message to appear on the screen:

The COMx port is currently assigned to an MS-DOS-based application. Do you want to reassign the port to Windows?

This message is also be displayed if the port ID is valid; however, it is in use by another virtual machine.

To avoid this message in enhanced mode Windows, a communications application can call the API entry point of the virtual communications device (VCD) with a special function ID to determine the valid ports in the system.

The code fragment included below will fill a byte with a value that enumerates the valid COM ports:

VCD_Device_ID  equ     0Eh lpAPI          dd      ? bPortArray     db      ?

EnumeratePorts: xor    di, di                mov     es, di                mov     ax, 1684h mov    bx, VCD_Device_ID int    2Fh mov    ax, es                or      ax, di                jz      Enumerate_Failed ; VCD API is available... enumerate the ports mov    word ptr lpAPI, di                mov     word ptr lpAPI+2, es                mov     dx, 1           ; API for Get_Port_Array call   dword ptr lpAPI mov    bPortArray, al  ; returned in AL                jmp     Enumerate_Exit Enumerate_Failed:

mov    bPortArray, 0 Enumerate_Exit:

ret After running this code fragment, the value in bPortArray will determine the valid ports. For example:

       bit 0   - COM1 (set if COM1 is available) bit 1  - COM2 (set if COM2 is available) ...       bit 8   - COM7 (set if COM7 is available, not supported in                        standard VCD) A value of 3 indicates that both COM1 and COM2 are present in the system.

Additional query words:

Keywords : _IK kbfile kbsample kb16bitonly kbKernBase kbComPort

Issue type :

Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310