From:	"John Henderson" <jhenRemoveThis AT bigpond DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Determining interrupt vector for serial port
Date:	Wed, 5 Feb 2003 17:45:21 +1100
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To:	djgpp AT delorie DOT com
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As a retired programmer, I'm still an absolute novice where PC
internal architecture is concerned.  But I'm learning and I'm
stuck.

I'm using DJGPP and writing a program which involves servicing
serial ports.  I've developed a working ISR that compiles with
gcc, and I'm satisfied it runs correctly.

I'm also aware that I can read COM1's address with the C command:

 addr =  _farpeekw(_dos_ds, 0x400);

(and using 0x402 to 0x406 instead for COMs 2 to 4).

But I've been unable to discover how to "read" the associated IRQ
value for each port.  So far I've found them hardcoded as 4, 3,
4, 3 respectively in all examples.  But I've got to look no
further than my own PC to find COM3 using IRQ5 (on an ISA card).
How does a program discover this?

In the interests of robust software, is someone able to help me
move forwards here?

TIA

John

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