🚀 go-pugleaf

In article <1992Jul31.093843.10058@kullmar.se> henryg@kullmar.se (Henry Gessau) writes:
}Someone told me the other day that some (most?) PCs with an ISA bus
                                       ALL
}have the IRQ 9 signal connected to the pin on the bus slots that is
}designated for IRQ 2. This is quite a logical thing to do as there is

In order to add more IRQ levels in the AT, one of the eight IRQ levels
present in the PC and XT had to become a cascade on which the second
(slave) interrupt controller tells the first (master) controller that it
has an interrupt.  So the pin that was IRQ2 got connected to IRQ9 (on the
second controller), and the default BIOS handler for IRQ9 simply reflects
the interrupt to INT 0Ah, which is IRQ2's interrupt.

The only ISA bus machines with only a single interrupt controller (and thus
IRQ2 on the expansion bus) are the IBM PC and IBM XT and clones thereof.  As
far as I know, all PS/2s (even ISA bus machines) have two interrupt
controllers and thus put IRQ9 on the bus instead.

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