Using DEBUG to determine available Com Port address

This describes how to use DEBUG from DOS to find an available Com Port address for your Internal Modem or Fax/Modem.

With the modem removed from the system, boot the machine with a plain DOS boot disk.   One with no AUTOEXEC.BAT and CONFIG.SYS.

Change your prompt to the DOS directory and type DEBUG and press <RETURN>.
At the Dash Prompt type D 40:0.  Debug will generate an entire paragraph of addresses, for our purposes we will only use the first half of the first line.

This portion denotes the COM addresses.  This diagram describes how your screen should look.

C:\DOS>DEBUG
-D 40:0
0040:0000 F8 03 F8 02 E8 03 E8 02-

Explanation:
0040:0000 Location Address Byte and Offset
F8 03 inverted is COM1 03F8h
F8 02 inverted is COM2 02F8h
E8 03 inverted is COM3 03E8h
E8 02 inverted is COM4 02E8h


If any of these sets of addresses are zeros then the com port is available for
use by your Internal Modem or Fax/Modem.  Below is a possible scenario:

Your computer has two serial ports installed.  You have a mouse on COM1
and COM2 is not being used by a device.  Running DEBUG gives you the
following results.

0040:0000 F8 03 F8 02 00 00 00 00-

0040:0000 Location Address Byte and Offset
F8 03 inverted is COM1 03F8h
F8 02 inverted is COM2 02F8h

The zeros located in COM3 and COM4 spots denote the absence of those ports.
You have these address open to use for your modem.  Set the modem for COM3
or COM4 along with the appropriate IRQ and install your software.

The following chart displays the possible options when running DEBUG.

If DEBUG says this...                        You have this installed.

0040:0000 F8 03 00 00 00 00 00 00-           COM1
0040:0000 F8 02 00 00 00 00 00 00-           COM2
0040:0000 E8 03 00 00 00 00 00 00-           COM3
0040:0000 E8 02 00 00 00 00 00 00-           COM4
0040:0000 F8 03 F8 02 00 00 00 00-           COM1, COM2
0040:0000 F8 03 E8 03 00 00 00 00-           COM1, COM3
0040:0000 F8 03 E8 02 00 00 00 00-           COM1, COM4
0040:0000 F8 02 E8 03 00 00 00 00-           COM2, COM3
0040:0000 F8 02 E8 02 00 00 00 00-           COM2, COM4
0040:0000 E8 03 E8 02 00 00 00 00-           COM3, COM4
0040:0000 F8 03 F8 02 E8 03 00 00-           COM1, COM2, COM3
0040:0000 F8 03 F8 02 E8 02 00 00-           COM1, COM2, COM4
0040:0000 F8 03 E8 03 E8 02 00 00-           COM1, COM3, COM4
0040:0000 F8 02 E8 03 E8 02 00 00-           COM2, COM3, COM4
0040:0000 F8 03 F8 02 E8 03 E8 02-           COM1, COM2, COM3, COM4

It is important to have the modem removed from the system when running DEBUG.

If you have two objects addressed for the same COM port you will only see one address.   DEBUG does not report conflicts, only what addresses are being used.