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From: garlangr@en.ecn.purdue.edu (Mark T Garlanger)
Newsgroups: comp.os.msdos.programmer
Subject: Re: interrupt numbers for com3 and com4?
Message-ID: <1991Jun22.234558.9291@en.ecn.purdue.edu>
Date: 22 Jun 91 23:45:58 GMT
References: <1991Jun22.205821.21533@msuinfo.cl.msu.edu>
Organization: Purdue University Engineering Computer Network
Lines: 25

In article <1991Jun22.205821.21533@msuinfo.cl.msu.edu> leeda@frith.egr.msu.edu (Dae Hee Lee) writes:
>
>com1 and com2 are assigned int12h and int 11h (irq4 and irq3), respectively.
>Then how about com3 and com4?

IBM only defined Com1 and Com2, but a defacto standard was developed by 
clones and serial board makers.  Com3 typically has the same interrupt as
Com1(intr 12h or Irq4) and is at address 03e8h.  Com4 shares it's interrupt
with Com2(intr 11h or Irq4) and is at address 02e8h.  On many serial boards
it is possible to select the Irq from 2-5, and a variety of address.  
Because 1 and 3, 2 and 4 share the interrupt it is "not possible" to use
1 and 3 at the same time or 2 and 4 at the same time on an ISA bus. MCA and
EISA fix this problem.  I read somewhere that if the ports (1-3 or 2-4) are
on the same physical board on the bus, then with proper software you could
use the two ports at the same time.   Some caution should be used if you
try and enable both interrupts, I think you can fry one or both of the 
serial boards.  Hope this helps.

				Mark

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Mark Garlanger          |
garlangr@ecn.purdue.edu |
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