Accessing more than 4 serial ports in DOS

Hi,

I want to write a DOS utility than can access all the serial ports
present on my system. Typically I have a Plug-n-Play card, which has
six serial ports present on it, that are I/O mapped. Since we can
program at max four serial port addresses in the BIOS, I'm unable to
access all the ports. Does someone has any idea to do it in DOS.

Thanks,
Shakti