BUG: Sharing IRQ with COM1 Causes System to Reboot

• Microsoft Windows Device Development Kit (DDK) for Windows version 3.1

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.