Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!ssc-vax!uvicctr.UVic.ca!sbanner1
From: sbanner1@uvicctr.UVic.ca.UUCP (S. John Banner)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Activating COM3 and COM4
Message-ID: <657@uvicctr.UVic.ca.UUCP>
Date: 4 May 89 16:41:22 GMT
References: <8036@fluke.COM> <372@cbnewsh.ATT.COM>
Reply-To: sbanner1@uvicctr.UVic.ca.UUCP (S. John Banner)
Distribution: na
Organization: University of Victoria, Victoria B.C. Canada
Lines: 35

In article <372@cbnewsh.ATT.COM> by mbb@cbnewsh.ATT.COM (martin.b.brilliant) writes:
>From article <8036@fluke.COM>, by lal@tc.fluke.COM (Larry Lohkamp):
>> ..... The AST board
>> came with two ports at COM1: and COM2:. My serial board says that its
>> two ports can be used as COM1, 2, 3, or 4. I tried all possible 
>> combinations of assigning the two interrupts ( #3 and #4 ), but could not
>> get the machine to recognise anything but COM1 & 2. If it makes any
>> difference, my DOS rev. is 3.3. Can I get all 4 ports to work, or am I
>> dreaming the impossible?
>Some of this "older communications software" is explicit about how
>COM3: and COM4: are defined in terms of port address and IRQ line. 
>However, I couldn't find anything that says how DOS defines these
>ports.  Can someone provide definitive information about this?

I have recently come across this problem (in a slightly different
form), and found the solution.  The base address of the first four
com ports are stored in the first four words of the BIOS information
area (0040:0000 to 0040:0008).  You only need one word for each, as
this stores the first port address, which are all at most 16 bits.
Once you have set these addresses right, then you need to find the
equipment information byte (I don't remember the address for that
one, but it is well documented), and fix it, so that it says you have
the right number of serial ports.  You can do the same thing with
parallel ports (there are three words for parallel ports right after
the four words for serial ports).  Then all you have to do, is get
the interupts set right, and you are in business.  Unfortunately,
there is no standard on how to find the right interupt.  As I recall,
the older machines used IRQ2, IRQ3, IRQ2, IRQ3, while I believe that
IBM decided to use IRQ2, IRQ3, IRQ3, IRQ3, in the PS/2 line... most
of the intelegent software, will try the various possibilities, and
see which one works.  Anyhow, it is basicly just a matter of installing
the board, setting the various pieces in the BIOS area, and it should
work...  At least it did for me...  :-).

			  sjb.