Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!cbnewsh!rkl
From: rkl@cbnewsh.ATT.COM (kevin.laux)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Two Reply's: HW interrupts and COM port's
Message-ID: <1506@cbnewsh.ATT.COM>
Date: 16 Jun 89 16:22:02 GMT
References: <2073@iesd.dk>
Organization: AT&T Bell Laboratories
Lines: 87

In article <2073@iesd.dk>, kjeld@iesd.dk (Kjeld Flarup) writes:
> I have noted some questions concerning interrupts and COM ports.
> First there is a reply to Dave Vasko:
> 		PORT    ADDRESS      INTERRUPT 
> 		COM1	3F8-3FF      IRQ4
> 		COM2	2F8-2FF      IRQ3
> 		COM3    2F0-2F7      IRQ4 ( or IRQ2 )
> 		COM4    3F0-3F7      IRQ3 ( or IRQ5 )
> 
> I think. because noone seems to know for sure.

	COM3 and COM4 can be set to interrupt on IRQs 3 and 4, but they don't
have to be.  They can be *polled* and not interrupt driven.  Some serial
port cards also allow the user to set them up for IRQ 2 or 5.  On at AT
class machine, you can't use IRQ 2, because that's used for the interrupt
from the second PIC (Programmable Interrupt Controller), or, if you have
and EGA/VGA card installed, the video card takes over IRQ 2 for itself (and
then has to do the work of intercepting and passing on the interrupts from
the second PIC).  On an XT class machine, both IRQs 2 and 5 are free.

	If COM3 and COM4 (as well as COM1 and COM2) are all to be interrupt
driven, then you must have a special interrupt handler that will check to
see which one of the COM ports actually generated the interrupt.

> Let's look at the HW interrupt list Jonatan Vail wasn't sure about.
> 
>    IRQ  0 - Timer.
>    IRQ  1 - Keyboard.
>    IRQ  2 - EGA vertical retrace.
>    IRQ  3 - COM2.
>    IRQ  4 - COM1.
>    IRQ  5 - Fixed disk.		<--only on *XT* class machines
>    IRQ  6 - Diskette.
>    IRQ  7 - LPT1.
>    IRQ  8 - Real time clock.
>    IRQ  9 - LAN adapter 1.
>    IRQ 10 - Reserved.
>    IRQ 11 - Reserved.
>    IRQ 12 - Mouse.
>    IRQ 13 - Coprocessor.
>    IRQ 14 - Fixed Disk.
>    IRQ 15 - Reserved.
> 

	Here is the *default* IRQ list for *XT* class machines:

IRQ0	- Timer Channel 0
IRQ1	- Keyboard
IRQ2	- Reserved
IRQ3	- COM2
IRQ4	- COM1
IRQ5	- Fixed Disk
IRQ6	- Diskette
IRQ7	- LPT1

	Here is the *default* IRQ list for *AT* class machines (286,386):

1st PIC:

IRQ0	- Timer Channel 0
IRQ1	- Keyboard
IRQ2	- Interrupt from 2nd PIC
IRQ3	- COM2
IRQ4	- COM1
IRQ5	- LPT2
IRQ6	- Diskette
IRQ7	- LPT1

2nd PIC:

IRQ8	- Real Time Clock
IRQ9	- Streaming Tape Unit
IRQ10	- Reserved
IRQ11	- Reserved
IRQ12	- Reserved
IRQ13	- Coprocessor
IRQ14	- Fixed Disk
IRQ15	- Reserved

	Now depending on what hardware you put in your system, various IRQs
get taken over.  For example, in XT class machines, the mouse would go on
IRQ 2, but in an AT class machine, it would go on IRQ 5 (denying the use of
LPT2).  Note that sometimes, you can't use some hardware devices together
because of the IRQ conflicts (like 3 individually interrupt driven COM ports
(COM3 on IRQ 5) with a Mouse).

--rkl