Determining Which COM Ports Are Present in Enhanced Mode

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

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 in the Software/Data Library that demonstrates the techniques detailed in this article.

Download ENUMPORT.EXE from the Microsoft Software Library (MSL) on the following services:

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download ENUMPORT.EXE (size: 21744 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get ENUMPORT.EXE (size: 21744 bytes) 
  

MORE INFORMATION

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?

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      ?

                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

                mov     bPortArray, 0

                ret

        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)