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Path: sserve!newshost.anu.edu.au!munnari.oz.au!news.Hawaii.Edu!ames!agate!spool.mu.edu!nigel.msen.com!math.fu-berlin.de!unidus.rz.uni-duesseldorf.de!mueller.rz.uni-duesseldorf.de!dvs From: dvs@ze8.rz.uni-duesseldorf.de (Wolfgang R. Mueller) Newsgroups: comp.os.386bsd.bugs Subject: [ANSWER for FAQ] IRQ2 vs IRQ9 [was: 3c503 probe] Date: Thu, 15 Jul 1993 10:56:37 GMT Organization: Computing Centre, Heinrich-Heine-University, Duesseldorf, Germany Lines: 18 Distribution: world Message-ID: <dvs.163.742733797@ze8.rz.uni-duesseldorf.de> References: <221hoo$n4d@terminator.rs.itd.umich.edu> NNTP-Posting-Host: mueller.rz.uni-duesseldorf.de In article <221hoo$n4d@terminator.rs.itd.umich.edu> pauls@terminator.rs.itd.umich.edu (Paul Southworth) writes: >Anyone know why the stock 3c503 driver thinks that the card is at IRQ 9 >(and it even works) when IRQ 9 is not even a possible IRQ for that card? >The GENERICISA conf file says IRQ 2 ..... Questions of this type reappear every now and then. When IBM switched from XT to AT they included a second interrupt controller. This had to be cascaded, and IRQ 2 on the first one was chosen for this. From the new IRQs 8 to 15 then IRQ 9 was chosen to be connected to the temporarily orphaned IRQ 2 line on the bus. So the hardware IRQ 2 line on the bus activates the software service routine for IRQ 9 ( i.e. interrupt vector 0x71 ). The standard bios service for example for vector 0x71 is an int instruction for vector 0x0a ( the one formerly in charge for IRQ 2 ). So as a conclusion, in most contexts you need not worry about any discrepancies between IRQ 2 and IRQ 9. Wolfgang R. Mueller <dvs@ze8.rz.uni-duesseldorf.de>, Computing Centre, Heinrich-Heine-University, Duesseldorf, Germany.