ENumPort.exe - COM Ports Present in Enhanced Mode

ID: Q76280

The information in this article applies to:

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

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. Click the file names below to download the files:

ENumPort.exe

For more information about how to download files from the Microsoft Download Center, please visit the Download Center at the following Web address

http://www.microsoft.com/downloads/search.asp

and then click How to use the Microsoft Download Center.

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 : kbcode kbfile kbsample kb16bitonly kbKernBase kbComPort
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type :