U5533129@ucsvc.ucs.unimelb.edu.au (CARDIOLOGY, R.M.H.) (05/31/90)
Can anyone out there tell me what the IRQ 2 line is used for on an AT? I've just set up an ethernet card to use it, but when I told the supplier he said he though it would clash with the second interupt controller. I've been using it for several weeks without any problem. Have I just been lucky, or is there a conflict there? Many thanks, Peter S.
kaz@legs.UUCP (Richard Kaczmarek) (06/01/90)
There are two 8259 interrupt controllers in the AT. I believe the master is located at address 20h and the slave is at A0h. The master handles IRQs 0 through 7 and the slave handles IRQs 8 through 15. The master "yanks" the slave via IRQ 2. From what I understand (I'm a software weenie, not hardware), IRQ 2 is remapped in hardware to IRQ 9. Any hardware requests to IRQ 2 yank IRQ 9's line. The processor, by virtue of the IRQ number, uses the interrupt vector table to call some code relating to that hardware request. Although IRQ 2's vector should be used, recall it was remapped to IRQ 9; IRQ 9's software interrupt vector, then, is jumped to when either an IRQ 9 or a real IRQ 2 occurs. There's a piece of BIOS code sitting where IRQ 9's interrupt vector is pointing which determines who _really_ yanked the line (2 or 9). If 9, it continues on; if 2, it jumps over to 2's interrupt vector. That's why your stuff worked. Although you set it to 2, the motherboard's hardware remapped it to 9. The software there took care of it, no problem. -- Rich Kaczmarek - uucp: ...!uunet!ccicpg!legs!kaz AST Research Inc. Fax: (714) 727-9358 P. O. Box 19658 Tel: (714) 727-8621 Irvine, California 92713-9658
bmarsh@cod.NOSC.MIL (William C. Marsh) (06/01/90)
In article <749@ucsvc.ucs.unimelb.edu.au> U5533129@ucsvc.ucs.unimelb.edu.au (CARDIOLOGY, R.M.H.) writes: >Can anyone out there tell me what the IRQ 2 line is used for on an AT? > >I've just set up an ethernet card to use it, but when I told the supplier he >said he though it would clash with the second interupt controller. I've been >using it for several weeks without any problem. Have I just been lucky, or >is there a conflict there? Actually, on the AT bus, the pin that was IRQ 2 in a PC/XT is actually called IRQ 9. The supplier in question is partially correct, IRQ 2 (on the original interrupt controller) is now connected to the interrupt request pin from the second interrupt controller (controllers are cascaded). Some software does not correctly handle this 'IRQ 2' on an AT, since it's really 'IRQ 9' (Though the BIOS does help, and by default will revector IRQ 9 to where IRQ 2 would have called, however, the software still has to unmask the proper interrupts and send the EOI command to the correct controller). More and more software (mouse drivers, etc) which use IRQ 2 know about this, now that the AT has been around for five years. Bill -- Bill Marsh, Naval Ocean Systems Center, San Diego, CA {arpa,mil}net: bmarsh@cod.nosc.mil uucp: {ihnp4,akgua,decvax,dcdwest,ucbvax}!sdcsvax!nosc!bmarsh "If everything seems to be coming your way, you're probably in the wrong lane."
jca@pnet01.cts.com (John C. Archambeau) (06/01/90)
U5533129@ucsvc.ucs.unimelb.edu.au (CARDIOLOGY, R.M.H.) writes: >Can anyone out there tell me what the IRQ 2 line is used for on an AT? > >I've just set up an ethernet card to use it, but when I told the supplier he >said he though it would clash with the second interupt controller. I've been >using it for several weeks without any problem. Have I just been lucky, or >is there a conflict there? It varies. IRQ 2 is the default IRQ for most networking boards whether it be arcnet or ethernet. If you haven't had a problem yet and you are using the board regularly then it shouldn't give you any problems at all. This is a general rule of thumb and not an absolute though. I have had occassions where I've had occassions where I had to set networking boards at an IRQ other than 2. // JCA /* **--------------------------------------------------------------------------* ** Flames : /dev/null | Small memory model only for ** ARPANET : crash!pnet01!jca@nosc.mil | Unix? Get the (*bleep*) out ** INTERNET: jca@pnet01.cts.com | of here! ** UUCP : {nosc ucsd hplabs!hd-sdd}!crash!pnet01!jca **--------------------------------------------------------------------------* */
tea6219@evtprp0b.UUCP (Terry E. Acker) (06/02/90)
As I understand it: an XT class machine has only one set of interrupts, 0 through 7. An AT has two sets, 0 through 15. To remain compatable with the older XTs, access to the second set of interrupts is provided through IRQ2 of the first set. Typical interrups in the 8 through 15 range are hard disk controller, AT clock, and 80287. -- |----------------------------------------------------------------| |Terry Acker | |Boeing Commercial Airplanes ....uunet!bcstec!evtprp0b!tea6219 | |M/S 03-87, P.O. Box 3707, Seattle, WA 98124-0346 | |----------------------------------------------------------------|