BUG: ACPI conflicts with Direct I/O


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Article ID: Q700005

The information in this article applies to:

  • Direct I/O, version 4.0, 4.1, 4.2


Under Windows 2000 and Windows XP ACPI enabled mainboards will handle resource assignment and power managment of all onboard devices like serial or game ports.
Please note that beginning with version 4.2 of Direct I/O parallel ports are handled correctly (see Q500003) and beginning with version 4.3 serial ports are handled correctly too (see Q500004), so the following may not apply to you.
In order to allow Direct I/O to control these devices one must prior disable the devices using the Device Manager.
But some ACPI systems put disabled devices in power-down condition and therefore make it inaccessable for all non-PnP drivers like Direct I/O.


Steps to reproduce problem

  1. Use a ACPI enabled Windows 2000 system
  2. Open Device Manager.
  3. Select the onboard device you want to control using Direct I/O, e.g. COM1.
  4. Disable the device. The device should now have been marked with a red cross.
  5. Close the Device Manager.
  6. Open the Direct I/O Control Panel.
  7. Enter the resources of the device e.g. I/O port 3F8-3FF.
  8. Add %systemroot%\system32\debug.exe to the list of allowed processes.
  9. Close Direct I/O Control Panel.
  10. Start the debugger DEBUG.EXE.
  11. Read one of the I/O ports by entering the command:
    -I 3F8

    The result always is FF because the device is powered down and we are reading an open bus.
  12. Close the debugger.
  13. Remove the devices resources and the process from the Direct I/O Control Panel.
  14. Enable the device in the Windows 2000 Device Manager.


  • Install an additional serial port adaptor. Any ISA card will do it because they don't support ACPI and therefore will not be powered down by the operating system.
  • Install your system as Standard PC.
    This can be done during the text mode install phase. When asked to press F6 for the selection of additional drivers press F5 to get a menu where the system type selection can be made. There you can select Standard PC and continue with the installation.


This problem was first corrected in Direct I/O version 4.3.

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Last modified: 2018-04-24.
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