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Microsoft KB Archive/107696

From BetaArchive Wiki

Modifying COMM.DRV to Support up to Nine Ports


The information in this article applies to:

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


The Windows 3.1 communications application programming interface (API) supports up to nine communications (COM) ports. However, COMM.DRV that is included with Windows 3.1 handles only four COM ports. With the Windows 3.1 DDK, COMM.DRV can be modified to handle up to nine ports. This information is specific to COMM.DRV for Windows 3.1 and Windows for Workgroups 3.1. Please note: If a system has more than four COM ports, it probably has a nonstandard multiport serial board, which will require more extensive changes than listed below. Contact the hardware vendor to see if a modified communications driver already exists for the specific hardware.


The Windows DDK contains the source for COMM.DRV. The necessary changes to support nine COM ports are listed below. Also, a few lines must be added to SYSTEM.INI.

In the [386Enh] section of SYSTEM.INI, the following entries must be made:

COMxBASE = address - "x" is the COM port number, and "address" is the base port address for the port.

COMxIrq = number - "x" is the COM port number, and "number" is the IRQ used by the port.

MaxCOMPort = number - "number" is the maximum number of COM ports supported in 386 enhanced mode.

Below are the changes that must be made to the COMM.DRV source. Please remember that by modifying the source, the driver is no longer a Microsoft product, and therefore the name of the driver must be changed. However, do not change the module name; it must be COMM.

The following changes are shown in comparison to the original files. These files are contained in the \286\COMM subdirectory of the Windows 3.1 DDK.

This change notifies COMM.DRV that there are nine ports:



   11:  MAXLPT   equ   2        ;Three LPTs supported (LPT1, 2 ,3)
   12:  MAXCOM   equ   8        ;Nine COMs supported


   11:  MAXLPT   equ   2        ;Three LPTs supported (LPT1, 2 ,3)
   12:  MAXCOM   equ   3        ;Four COMs supported (COM1, 2, 3, 4)

This change fixes a bug that occurs when COM ports are sharing IRQs. For more information on this bug, query on the following words in the Microsoft Knowledge Base:

sharing and IRQ and COM1 and reboot



  143:  Dummy   dw      0         ; Make sure COM1 is not 0!



Change the version, so this version of the driver is identified as the nine- port version:

Comparing Files COMM.RCV and \COMM\COMM.RCV


   10:  #define VER_FILESUBTYPE          VFT2_DRV_COMM
   11:  #define VER_FILEDESCRIPTION_STR  "Windows COMM Driver - 9 Port Ver"
   12:  #define VER_INTERNALNAME_STR     "COMM"


   10:  #define VER_FILESUBTYPE           VFT2_DRV_COMM
   11:  #define VER_FILEDESCRIPTION_STR   "Windows COMM Driver"
   12:  #define VER_INTERNALNAME_STR      "COMM" 

There are a few jumps here that are "short" in the retail COMM.DRV. Because of the changes, they need to be changed to "near".



  147:          mov     di, DataOFFSET IRQhook&num
  148:          jmp     NEAR PTR DEF_RM_Handler
  149:  DEF_RM_COM_INT_&num endp


  147:          mov     di, DataOFFSET IRQhook&num
  148:          jmp     short DEF_RM_Handler
  149:  DEF_RM_COM_INT_&num endp


  155:          mov     di, DataOFFSET IRQhook&num
  156:  IF MAXCOM-num LE 4
  157:          jmp     short DEF_Handler
  158:  ELSE
  159:          jmp NEAR PTR DEF_Handler
  160:  ENDIF
  161:  DEF_COM_INT_&num endp


  155:          mov     di, DataOFFSET IRQhook&num
  156:          jmp     short DEF_Handler
  157:  DEF_COM_INT_&num endp


  177:          mov     di, DataOFFSET IRQhook1
  178:          jmp     NEAR PTR DEF_RM_Handler
  179:  DEF_RM_COM_INT_1 endp


  173:          mov     di, DataOFFSET IRQhook1
  174:          jmp     short DEF_RM_Handler
  175:  DEF_RM_COM_INT_1 endp 

Additional query words: 3.10 COMM.DRV 9 PORT

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Last Reviewed: November 1, 1999
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