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Newsgroups: comp.unix.bsd Path: sserve!manuel!munnari.oz.au!uunet!spool.mu.edu!yale.edu!ira.uka.de!smurf.sub.org!flatlin!bad From: bad@flatlin.ka.sub.org (Christoph Badura) Subject: INT 2 vs. INT 9 on PC Clones Organization: Guru Systems/Funware Department Date: Mon, 21 Sep 1992 00:40:51 GMT Message-ID: <BuwKK4.It@flatlin.ka.sub.org> Summary: They are the same. Lines: 27 Since there is a constant confusion about the INT 2/INT 9 issue here is everything you ever wanted to know about it. When IBM built the original IBM PC and PC XT they equipped it with one 8259 interrupt controller chip. They connected the interrupt input 2 to 7 of this chip to the peripheral slots to allow IO cards to generate interrupts. When they designed the IBM PC AT they added a new type of IO slots which could handle 16 bit accesses and provided additional interrupt lines. Unfortunatley all interrupt inputs of the single 8259 were used up. This made it necessary to add a second 8259 interrupt controller as a slave of the first one to the system. To make this work they had to use one of the interrupt inputs of the first 8259 to get the interrupts from the second, cascaded 8259 to the CPU. As it turned out they used the input for IRQ 2 for this purpose. To make IRQ 2 from IO boards work they tied the IRQ 2 line from the IO connector to an input line of the second 8259 which generates an INT 9 to the CPU. This is why INT 2 and INT 9 are the same on ATs (as seen from IO cards anyway). Could someone polish this up a bit and add it to the FAQs? -- Christoph Badura --- bad@flatlin.ka.sub.org ISO? Nicht immer, aber immer M-vfter.