Using COMxIrq=-1 for COM4 Problems

 Last reviewed: November 23, 1994
Article ID: Q94556
The information in this article applies to:
  • Microsoft Windows operating system versions 3.1, 3.11

SUMMARY

Microsoft Windows 3.1 may not be able to access a serial device whose address refers to a COM port that is located to the right of an absent COM port in the BIOS data address table (BDA). This can often occur if a modem is installed on COM4, and COM3 is not physically present on the computer.

The serial communications driver (COMM.DRV) dated 3-10-92 may require modification of the [386enh] section of the SYSTEM.INI file to compensate for any conflicts that may arise when the BDA does not agree with traditional values for certain COM ports.

MORE INFORMATION

The COMM.DRV that ships with Windows 3.1 expects certain COM port base addresses to be located in certain locations in the BDA. If you use DEBUG on a traditional COM1-COM4 equipped computer, the following data is displayed for the BDA: 

   40:0 F8 03 F8 02 E8 03 E8 02 00 00 ...
COM1 (03F8) is displayed in the first available "slot" as F8 03; COM2 (02F8) in the second slot; COM3 (03E8) in the third; and finally COM4 (02E8) in the fourth slot.

If a COM port is absent on the computer, a higher-numbered COM port's address may "slide" to the left in the table, which may confuse the Windows COMM driver.

A common situation where this may happen is with an internal modem that is configured to use COM4. Many modem manufactures ship their modems on COM4 to avoid replacing COM1 or COM2 (which may be in use), and COM3 is likely to have an IRQ conflict with the mouse. COM1 shares the physical interrupt (IRQ 4) with COM3 and is a popular location for a serial mouse. The BDA problem may appear in this situation.

A DEBUG dump of the BIOS data area looks as follows: 

   40:0 F8 03 F8 02 E8 02 00 00 ...
Without a physical COM3, the BIOS finds the COM4 device at 02E8, and calls it COM3 in the BIOS data area. This confuses Windows, which thinks that there is no COM4, and that COM3 is at 02E8/IRQ4.

This behavior is normal for many BIOS versions and can be compensated for by adding a COMxIrq=-1 to the SYSTEM.INI in the [386Enh] section. For the COM4 example above, the following lines should be added to the SYSTEM.INI file: 

   COM3IRQ=-1      ; disables the false COM3 appearing BIOS data area
   COM4Base=02E8   ; sets correct COM4 address
   COM4IRQ=3       ; sets correct COM4 IRQ
This example tells Windows that COM3 does not exist on the computer and that the BDA value in the third slot is actually for COM4. This approach may be easier than writing a DEBUG script to pack zero values into the BDA. However, you may need a DEBUG script to compensate for a BIOS and other non-COM4 problems.

KBCategory: kbenv
KBSubcategory: win31
Additional reference words: 3.10 Enhanced Mode terminal faxmodem

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Last reviewed: November 23, 1994
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