How Windows 3.1 Looks for COM Ports

 Last reviewed: November 23, 1994
Article ID: Q82681
The information in this article applies to:
  • Microsoft Windows operating system version 3.1, 3.11

SUMMARY

The Microsoft Windows version 3.1 has solved most of the COM port configuration problems that existed in Windows 3.0. By looking at both the BIOS port table and SYSTEM.INI file, you can tell Windows exactly how your COM ports are configured without having to modify your BIOS port table with DEBUG.COM or any other program.

MORE INFORMATION

Windows 3.1, in both standard and enhanced modes, checks the BIOS table at 40:0 first to see if the BIOS has specified a base port address. If the table specifies the base, then that is what Windows uses.

If the value is 0, then Windows checks the SYSTEM.INI [386enh] section for COMxBASE=xxxh. Windows also defaults the COM3 base port address to 3E8h:, which means that if there is a 0 value in the BIOS table at 40:4 and there is no COM3BASE= switch, then Windows assumes a base of 3E8h as if the switch exists.

Windows would have defaulted COM4 to 2E8h, but this address overlaps both the 8514 display adapter and the default configuration of some Arcnet adapters.

The COMxBASE= switch entries can be created using the Ports section of Control Panel, so it is not necessary to edit the SYSTEM.INI file. It is not possible to override non-zero BIOS port table entries using a COMxBASE= switch.

KBCategory: kbenv kbdisplay
KBSubcategory: win31 wincomm
Additional reference words: 3.10

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Last reviewed: November 23, 1994
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