Q107697: BUG: Sharing IRQ with COM1 Causes System to Reboot

Article: Q107697
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.1
Operating System(s): 
Keyword(s): 
Last Modified: 03-NOV-1999

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The information in this article applies to:

- Microsoft Windows Device Development Kit (DDK) for Windows, version 3.1 
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SYMPTOMS
========

When COM1 and another communications (COM) port are active and share the same
IRQ using the Windows 3.1 COMM.DRV, the system hangs or reboots.

CAUSE
=====

When two or more COM ports share an IRQ, COMM.DRV creates a linked list of these
ports using the NextDEB field of the ComDEB structure that the COMM.DRV
maintains for each port. When an interrupt occurs that has been hooked by
COMM.DRV, the COMM.DRV interrupt handler walks the list of COM ports that are
sharing the IRQ. For each port in the list, the COMM.DRV interrupt handler
checks for and services any interrupts that are pending from the port. The
COMM.DRV interrupt handler knows when it has reached the end of the list of
ports that are sharing an IRQ when it reaches a ComDEB structure in which the
NextDEB field is zero.

If another port is sharing an IRQ with COM1 and it is opened after COM1, the
NextDEB field in the ComDEB structure for the other port will point to the
ComDEB structure for COM1. However, in the COMM.DRV included with Windows 3.1,
the offset to COM1's ComDEB structure happens to be zero. The list of COM ports
sharing an IRQ with COM1 will appear to end with the port on the list before
COM1. This will cause the COMM.DRV interrupt handler to skip the interrupt
servicing for COM1, and its interrupt will never be cleared. If the interrupt is
level-triggered and it is not cleared, the COMM.DRV interrupt handler will be
immediately called again as soon as interrupts are reenabled. This will cause
the machine to hang or reboot.

RESOLUTION
==========

The following are resolutions:

- Don't share an IRQ with COM1.

- Modify COMM.DRV as described below by adding a dummy variable before space is
  allocated for the ComDEB structures in IBMSETUP.ASM. This will ensure that
  the offset of the ComDEB structure for COM1 is not zero.

  Fragment from the DDK \286\COMM directory, source file IBMSETUP.ASM:

        ;;; Add the line below, approximately line 142
        Dummy   dw      0         ; make sure offset of COM1 is not 0!

        ??portnum = 1
        REPT MAXCOM+1
                DefineCommX %??portnum
        ??portnum = ??portnum+1
        ENDM

STATUS
======

This bug is fixed in version 3.11 of Windows for Workgroups.

Additional query words: 3.10 buglist3.10

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Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK310
Version           : :3.1

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