Ethernet boards for ISC 386/ix

Dennis P. Bednar dennis at virtech.UUCP
Thu Aug 17 07:12:56 AEST 1989

In article <2642 at>, james at (James Van Artsdalen) writes:
> There are a number of reasons why IRQ 2 may not work.  In particular,
> some video cards (Video 7 16 bit comes to mind) drive IRQ 2 both active
> *and* inactive.  Also, on an AT compatible, the bus line labelled IRQ 2
> is actually connected to IRQ 9, since the real IRQ 2 is used to cascade
> the second interrupt controller.

I'm still a little unclear on how the two PIC's (Programmable
Interrupt Controller chips) are actually connected to the bus,
to one another, and to the CPU.

I have a vague idea that that there are 8 input IRQ lines per PIC,
but that when you cascade two PIC's you don't get all 16 IRQ levels
because of the daisy-chaining.   I suppose there is only one
INT_REQ (interrupt request) line that goes to the CPU, and
depending on the IRQ pin and how the PIC's were daisy-chained
you would get one one PIC responding with the interrupt vector
number information.  But I still can't picture how the bus is
connected to the PIC's and to the CPU.  Can someone draw a
figure or elaborate in a little more detail please?

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