The Windows 3.1 communications driver (COMM.DRV) accesses serial ports COM1, COM2, COM3, and COM4 using base address values and interrupt request lines (IRQs) specified in the BIOS data area of a computer. If the BIOS data area does not specify values for physical ports, Windows 3.1 will use the values specified by the [386Enh] section of SYSTEM.INI.
To override these defaults, the user can set the base address and IRQ values using the Control Panel. Control Panel displays an “advanced” settings dialog box for each port. The dialog box contains drop-down edit fields for base addresses and spin controls for IRQ settings. The selected values are recorded as COMxBase and COMxIRQ settings in the [386Enh] section of the SYSTEM.INI file. Note that these settings are used for both standard and 386 enhanced-mode operation.