Date: Thu, 18 Apr 1996 19:19:11 -0700 (PDT) From: Doug White <dwhite@riley-net170-164.uoregon.edu> To: Greg Lehey <lehey.pad@sni.de> Cc: alk@Think.COM, current@freefall.freebsd.org, questions@freebsd.org Subject: Re: EtherExpress16 problems Message-ID: <Pine.BSF.3.91.960418191513.18923G-100000@riley-net170-164.uoregon.edu> In-Reply-To: <199604180624.IAA20593@nixpbe.pdb.sni.de>
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On Thu, 18 Apr 1996, Greg Lehey wrote: > > On Wed, 17 Apr 1996, Tony Kimball wrote: > >> (Incidentally, I do not understand about IRQs 2 and 9. Is there a > >> document I might read? Are they available for assignment to devices, > >> generally?) > > > > The book "Running FreeBSD" has a good discussion on it. > > Thank you. :-) The "discussion" isn't as long as I remember it, but I was trying to recall the information from memory, and it must have gotten mixed in there. Hey, I've gotten two questions on how to get the book from that response :-) > > The 2/9 is a > > hack to support 16 bit interrupts. Back in the old days of XTs they only > > had 8 bit interrupts, and when 16 bit came they decided to cascase IRQ 2 > > to 9. Don't ask me why. :) > > The interrupt controller chip used in the XT and AT was the Intel > 8259A. It handled 8 interrupts, but had a so-called "cascade mode", > where you could designate each of the primary interrupts as > representing another group of 8. This is what they did in the AT: > they took IRQ2, which up to this point had hardly been used, and used > it for the interrupt input from the second 8259A. The *real* IRQ2 > thus represents all of the IRQs from 8 to 15. This is (almost) > completely transparent to the software, the only exception being the > interrupt controller initialisation routines. > > There is also an ISA bus line called IRQ2 (B4: on the rear of the > connector, 4th from left when you have the connector pointing down) > It used to be connected to IRQ2 on the XT, but it now homeless, since > it couldn't be connected to IRQ2. Instead, they connected it to > IRQ9. Unfortunately, the line is (AFAIK) still called IRQ2. Thus the > confusion. > > Bottom line: it's really IRQ9, but it's the only one of the IRQs 8-15 > which you can use with an 8 bit board. Thanks for the info. > Is this clear? Is it interesting? Shall I put it in the next edition > of the book? It's OK as is, but maybe the above would be interesting as a sidebar. You might add instructions on how to add a second disk into the system. I don't think it's even documented anywhere in the FreeBSD Doc Project, and is one of the Frequently Asked Questions here, and should be easily answerable (after all, it's pretty basic!). Doug White | University of Oregon Internet: dwhite@resnet.uoregon.edu | Residence Networking Assistant http://gladstone.uoregon.edu/~dwhite | Computer Science Major
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