Microsoft KB Archive/105940

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Article ID: 105940

Article Last Modified on 7/7/2005


  • Microsoft Windows for Workgroups 3.2
  • Microsoft Windows 3.0 Standard Edition
  • Microsoft Windows 3.0a
  • Microsoft Windows 3.1 Standard Edition
  • Microsoft Windows 3.11 Standard Edition
  • Microsoft Windows 95

This article was previously published under Q105940


When you use the Microsoft Windows or Microsoft Windows for Workgroups Terminal program, you have problems connecting to the modem, or you receive the following error message:

The COM<x> port is currently assigned to a MS-DOS application. Do you want to reassign the port to Windows?

You may also have difficulty attaching to the modem through other Windows- based communications packages, or your mouse (or other serial device) may not operate properly in Windows.


Most problems associated with the serial ports occur when a machine does not recognize that it has the specified communications (COM) port available.

Windows supports the use of COM ports 3 and 4, although the following steps may be necessary to make them accessible. For example, if your mouse is not working in Windows, or you cannot access your modem through Windows, you may need to rewrite the BIOS Data Area for your serial ports. By placing an address for each serial port in a Debug script file, you can ensure future compatibility if you add or remove a device. (Creating a Debug script file should not be required for Windows for Workgroups 3.11.)


  1. When you troubleshoot communications problems in Windows, an MS-DOS-based communications program is often required to test the modem and ports outside Windows. This can help you to determine if you are having an issue in with Windows, or something else.

    If an MS-DOS-based program cannot be used, you can test the modem connections by typing the following command at a safe-mode command prompt outside Windows

    Echo ATM1L3X0DT12345 > COMx

    where "x" represents the COM port in question.

    The modem should dial the touch tone digits "12345".

    To hang up the modem, type

    Echo ATH0 > COMx

    where "x" represents the COM port in question.

    The ATM1L3X0DT12345 command is a signal to the modem to dial the numbers "12345". The first command, Attention, signals to the modem that it is about to receive information, M1 is a universal command to turn the modem's speaker on if it is off by default, L3 is a universal command to raise the modem's speaker volume to the maximum level if it is at the lowest by default, X0 is a universal command that signals the modem to execute the command without waiting for a dial tone (useful if modem and voice calls use the same phone line), and the DT12345 command instructs the modem to dial out the digits 1-5.
  2. To determine if your machine is recognizing the existence of the COM port your modem or mouse is on, do the following:

    1. Use the MS-DOS DEBUG command to look at the BIOS table.

      NOTE: You may also run the Microsoft Diagnostics utility (MSD).
    2. Quit Windows. At the MS-DOS command prompt, type:


      This returns a hyphen prompt (-).
    3. At the hyphen prompt, type:


      This returns a listing of the BIOS data and the hyphen prompt.

      NOTE: After you review the table, you can type Q to quit Debug.
    4. Look at the table of data on the screen. The first row is the important one. It shows a line resembling the following:

      0040:0000 F8 03 F8 02 E8 03 00 00-78 03 00 00 00 00 00 00

      All values to the left of the hyphen are COM ports; all values to the right are LPT ports. The example above shows that the computer has COM1 at address 03F8 (it is listed in reverse byte order), COM2 at 02F8, and COM3 at 03E8. COM4 is not found; therefore, 0000 is displayed just to the left of the hyphen. If the machine does not recognize the COM port desired to be used by Terminal (identified by the 0000 entries), continue as follows. If the machine does recognize the desired COM port, skip to the "SYSTEM.INI Modifications" section below in this article.
  3. The following instructions help you write the Debug script and place it in the AUTOEXEC.BAT file:
    1. Use a text editor such as Microsoft Windows Notepad and create the following file:

               F8 03 F8 02 E8 03 E8 02

      NOTE: You must follow the "Q" with a carriage return (press ENTER).

    2. Save the three-line file, and give it a name such as:

    3. Execute the Debug script file by typing the following at an MS-DOS command prompt outside Windows:

      debug < fixcom.deb > nul

    4. If the step above works correctly, add a line to the end of the AUTOEXEC.BAT file (but before launching Windows), such as the following:

      debug < fixcom.deb > nul

      NOTE: The > NUL ending is just to keep the script from being echoed back to the screen. You can use Debug again to look at d40:0 and see if the change took effect.

    5. Save the AUTOEXEC.BAT file and reboot the machine.

      This procedure should work even if the machine does not have devices on all four serial ports. This also corrects problems if a device has been removed from COM1 and the addresses have slid down, which may cause mouse problems.

      You should now be able to run MSD or Debug and see all four COM ports.

SYSTEM.INI Modifications

If the port is recognized by the machine, note the address that is listed when using Debug, and, for Windows 3.0 and 3.0a, edit the SYSTEM.INI file and include the following COMxBase= lines to coincide with the base I/O address used by the hardware. The examples given above for COM1 and COM2 do not require any changes to the SYSTEM.INI file. If you are using Windows 3.0, the examples given below for COM3 and COM4 require the following entries in the [386Enh] section of the SYSTEM.INI file:

   COMxBase Setting                Windows 3.0 Default
   COM1Base=3F8h                   <default is 3F8h>
   COM2Base=2F8h                   <default is 2F8h>
   COM3Base=3E8h                   <default is 2E8h>
   COM4Base=2E8h                   <default is 2E0h>

For Windows 3.1 and Windows for Workgroups 3.1 and 3.11, choose the Ports icon in Control Panel to set the base I/O address and interrupt for each COM port instead of editing the SYSTEM.INI file.(This is the only step required for Windows for Workgroups 3.11.)

The defaults for Windows 3.1 and Windows for Workgroups 3.x are:

   COM1Base=3F8h                   <default is 3F8h>
   COM2Base=2F8h                   <default is 2F8h>
   COM3Base=3E8h                   <default is 3E8h>
   COM4Base=2E8h                   <default is 2E8h>


If you are using a serial mouse on COM1 and you have an internal modem on COM3, this configuration may not work because the mouse is using the interrupt that COM3 wants to share. This behavior also occurs if you are using a serial mouse on COM2 and want to use an internal modem on COM4. This configuration is more likely to work on machines that allow for IRQ sharing (such as Micro Channel Architecture [MCA] computers).

If COM3 is required, it should be reconfigured at the board level to use a different interrupt that is free, such as IRQ 5.

Additional query words: 3.00 3.10 3.00a BIOS slide slip move sliding debugging terminal.exe com1 com2 com3 com4 AT DT DOS w_works hometool works for windows win95 hyperterminal win95x

Keywords: KB105940