The Windows 3.1 communications driver (COMM.DRV) accesses serial ports COM1, COM2, COM3, and COM4 using base-address values 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 base address, 0x03E8, and IRQ4 for COM3, but the base addresses and IRQs for the other ports must be set by the user.
If the BIOS data area does not specify a value for a physical port, the user can set the base address and IRQ values using Control Panel. Control Panel displays an Advanced settings dialog box for each port. The dialog box contains a selection box 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.
The COMxBase and COMxIRQ settings are used for both standard- and 386 enhanced-mode operation. The COMxBase setting is used only if the BIOS data area does not specify a value for the port; this setting never overrides the BIOS data area values.