There is no procedure for "adding" serial or parallel ports in DOS. DOS applications simply talk directly to whatever hardware happens to be present at whatever I/O port(s) they're trying to talk to.
All you need to do to use additional serial ports in DOS is correctly configure your hardware. The following are the usual default resource assignments for serial ports under DOS:
PORT BASE I/O IRQ
ADDR.
COM1 3f8 4
COM2 2f8 3
COM3 3e8 4 *
COM4 2e8 3*
*This is the typical brain-dead default setting, but obviously, you probably wouldn't want to assign the same IRQ for more two serial ports. You can usually get away with this if you're not going to be using both ports at the same time, but ideally, you would want to assign IRQs other than 3 or 4 to COM3 and COM4. Sadly, there are some stupidly-designed serial port add-on boards that don't allow you to choose IRQs other than 3 or 4 for COM3 or COM4. Needless to say, these should probably be avoided!
Here's the catches:
1. Your serial port hardware must actually be accessible in real mode. This seems like a no-brainer, but I've seen some pretty wacky PCI and USB-based serial port add-ons that obviously won't work in real mode! My recommendation: If you're going to be adding serial ports to be used under real mode DOS, use a non plug and play, ISA serial port adapter card that has jumpers that let you select a unique IRQ for each serial port. These aren't that hard to find. If your system doesn't have ISA slots, you may be out of luck.
2. The hardware resources for the serial ports must be correctly configured. Some really fancy DOS apps might let you tell the program what resources you're using for your ports. Most DOS apps assume you're using standard the standard I/O port assignments. Some DOS apps may only support IRQs 3 and 4 for serial ports.
3. DOS only supports four serial ports. Theoretically, there might be ways around this limitation, but for practical purposes, four's the limit.
If your serial port hardware is accessible to DOS in real mode (no funky USB or PCI multi-port adapters) and the ports are properly configured, they should work.
If your additional ports aren't on a non plug and play ISA adapter card, good luck.
If your motherboard has one or more built-in serial ports, you may need to reconfigure, enable, or disable them, depending on your situation, using the CMOS setup utility built into the motherboard BIOS.
If you're using serial ports built into your motherboard, make sure the physical 9-pin D-sub connectors in the back of your computer are connected to the correct headers on the motherboard. I've seen more than a dozen systems shipped straight from the factory from reputable companies like Dell, IBM, and Gateway that didn't have the serial port connectors on the back of the computer correctly connected to the motherboard headers. In most cases, the connections for serial port 1 and serial port 2 were swapped. You'd be amazed how often this happens.
Utilities like Microsoft Diagnostics (MSD.EXE) and Norton Utilities can be useful for seeing what serial ports are visible to DOS and how they're probably configured. (Don't rely on these too much, it is theoretically possible for software to incorrectly report system resource settings, even with ISA hardware.)
I hope some of this helps.
