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    From
    SubjectRe: Sharing IRQs between the first 4 serial ports.
    Date
    Followup to:  <Pine.LNX.3.95.971124082000.4448A-100000@chaos>
    By author: "Richard B. Johnson" <root@chaos.analogic.com>
    In newsgroup: linux.dev.kernel
    > >
    > > A de facto standard for internal modems seems to be:
    > >
    > > COM1 4
    > > COM2 3
    > > COM3 5
    > > COM4 2
    > >
    > > That is: no IRQ sharing.
    > >
    > [SNIPPED]
    >
    > No, No, No. This is __very__ broken!
    >
    > IRQ2 is the cascade from the first interrupt controller to the
    > second interrupt controller. The "wire" doesn't even exist on any
    > bus. It is a connection, sometimes internal, between the two
    > interrupt controllers.
    >

    ... which is why when people say IRQ 2 they really mean IRQ 9. The
    ISA bus line labelled IRQ 2 is connected to IRQ 9. Linux knows about
    this.

    There is no way to connect anything to the "real" IRQ 2, so you can
    calm down.

    -hpa
    --
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