Back in the DOS days, typically you loaded a device driver or memory-resident software to provide the communication service. From that point onward you used the BIOS INT 14h calls to access the serial port. If I remember correctly, a DOS session running within Win95/98/Me/NT/2K/XP will already have these services available as Windows provides them. As for accessing the BIOS INT 14h interface, you'll have to use a binary file that you LOAD and perform CALLs to it.