Dec 242017
Allows changing the interrupt and address of your second comm port so that OS/2 can access three serial ports.
File COM03.ZIP from The Programmer’s Corner in
Category OS/2 Files
Allows changing the interrupt and address of your second comm port so that OS/2 can access three serial ports.
File Name File Size Zip Size Zip Type
COM03.DOC 5364 2571 deflated
COMINIT.ASM 877 483 deflated
COMINIT.DEF 111 94 deflated
COMINIT.SYS 1056 204 deflated
MAKE.CMD 115 93 deflated

Download File COM03.ZIP Here

Contents of the COM03.DOC file

By: Matt Trask
13 Dec 87

CAVEAT EMPTOR: this a megahack! There exists the possibility of
destroying your hardware if you do this, and the software is not
supported and probably will not work with future releases of OS/2. I
cannot accept any responsibility for your use of the techniques
described here. If you are not confident in your abilities, PLEASE
get help from someone who knows how to make the modifications. In
addition, the device driver that is provided here is trash - I won't
admit to having written it as it is terrible style. However, it will
do the required work.
I have only tested this with an AT klone, a Kaypro AT-style
serial/parallel card, and the IBM beta OS/2 (Internal version 3.86) -
it works as documented on my system but I can't guarantee that it will
work anywhere else. Use this stuff at your own risk!
Now that the formalities are done, I will try to support this
hack - if you try it and it doesn't work, leave bixmail to
'matt.trask', or better yet, come join my 'boating' conference
(and bring some friends), and I'll see if I can help.


I found that the two comm ports supported by OS/2 on an AT
were not adequate for my needs (modem, mouse, and TNC for packet
radio) so thus this project was born. A little poking around with
my favorite debugger (Pfix Plus - highly recommended) showed that
when the serial mouse driver is loaded for COM2, it gets the base
address for the COM2 card from 40:2 and stuffs a word of zeroes in
its place so that other drivers (such as the COM01.SYS serial
driver for AT machines) will not know that the port exists. From
this I reasoned that if I could create a serial port at an address
other than 0x2f8 or 0x3f8 that used an IRQ other than 3 or 4, OS/2
should be happy to support it for me. What follows is a description
of the cuts and jumpers that do this, and the necessary software
hacks to complete the job. The software is pretty simple - just a
matter of patching the COM01.SYS driver to find the new port and
set its interrupt level. The only other part is a simple device
driver that pokes the base port address of the modified comm card
into the BIOS data are at 40:2. So, gentlemen, start your
soldering irons, E pluribus hackum!

Hardware Modification

This mod should work on most any AT style serial/parallel adapter.
In addition, it will probably work on XT type serial cards but I
can't guarantee that they will work at AT bus speeds. The idea is to
change the address decoding logic to select base ports of 0x3f8/0x1f8
instead of 0x3f8/0x2f8, and change the IRQ level for COM2 from IRQ3
to IRQ5. (You can use IRQ2 if you prefer - I used IRQ5 because my
LAN card wants IRQ2) Although the AT Tech Ref claims that the fixed
disk controller uses 0x1f0-0x1f8, that is a typographic error. It
really uses 0x1f0-0x1f7, as you can verify by reading the fixed disk
controller docs. IRQ5 is reserved for use by a second parallel port,
if you are using two interrupt driven LPT ports in your system you
will have to use IRQ2 for this modification.
AT the card edge connector cut the traces for address lines A8 and
A9 (connector tabs A22 and A23 on the component side of the board).
Solder two jumpers to cross-connect these two pads to each others
traces. This allows the jumper block to change the sense of A9
instead of A8 as it was designed. Then turn the board over and cut
the trace to IRQ3 (pad B25). Jumper this trace to IRQ5 (pad B23) or
to IRQ2 (pad B4) as required.
There, you're done. Easy wasn't it? Recheck your work - a
multimeter will be helpful to make sure that you didn't accidentally
short one of the lines to an adjacent pad or trace.


There are two parts to the software used to support the
additional serial port - patching COM01.SYS and writing a port
initialization driver. This example assumes that you have made the
hardware mods for base port 0x1f8 and IRQ 5 - make changes as needed
in the following instructions.
I've provided the driver to support a base address of 0x1f8 in
COMINIT.SYS and the source in COMINIT.ASM in case you need to change it.
The batch file 'MAKE.CMD' will run in protected mode to assemble and
link the driver if you make changes to it.

The patches to COM01.SYS are as follows:

C> debug com01.sys
- n com03.sys
- u 22a7
cmp dx, 2f8/* you should see something like */
movcx, 045a/* this, otherwise don't continue */
- e 22aa02.01
- e 22b603.05
- w
- q

This will write out a copy of 'COM01.SYS' with the changes under the
new name 'COM03.SYS'.


The following is an excerpt from my CONFIG.SYS that shows the
correct order for installation. This assumes that you are using
COM2 for your serial mouse, so the new COM port will be supported
by OS/2 as logical device 'COM2'.

rem DEVICE=C:\COM01.SYS/* original OS/2 serial driver */
device=c:\cominit.sys/* port base setup driver */
device=c:\com03.sys/* modified COM01.SYS */

Reboot OS/2, use MODE.COM to set up the proper parameters for the
device you have on the port, and off you go. Happy hacking, let me
know if this works for you.

 December 24, 2017  Add comments

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