Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!proteon.com!twf
From: twf@proteon.com ("Troy W. Frever")
Newsgroups: comp.protocols.tcp-ip.ibmpc
Subject: Interrupt vectors for PC Ethernet cards
Message-ID: <8906191406.AA20374@sonny.proteon.com>
Date: 19 Jun 89 14:06:48 GMT
References: <396@moegate.UUCP>
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Having just played with this fairly recently it is still rather fresh
in my mind.  

The interrupt line called IRQ-2 on an 8-bit AT-bus card will tug on
the line connected (in hardware) to IRQ-9 on an AT.  This is interrupt
number 0x71, with vector located at 0x1C4.  Since IRQs 9-F are indeed
on PIC 2, and, yes the actual interrupt level 2 on PIC 1 is used to
cascade the PICs together, you will need to ACK both PICs (just as you
would when handling any IRQ 9-F).  Pretty much business as usual.  Now
comes the tricky part.  The BIOS installs an interrupt handler for
interrupt 0x71 (IRQ-9) which ACKs PIC-2 and issues a software
interrupt for int number 0x0A (IRQ-2).  Voila! software rediriect - it
looks just as though IRQ-2 was pulled, with the vector at location
0x28 being invoked and PIC-1 remaining to be ACKed.

In short, the software may either field these interrupts as IRQ-9
(which they are) or let the BIOS installed redirect change them to
IRQ-2 and field them that way.

This information (although rather obscure) can be found in the IBM
PCAT Technical Reference manual.

Troy Frever
twf@sonny.proteon.com