Linux-kernel mailing list archive 1997-11,: Re: [2.1.65] New patch for serial port IRQ setup.

# Re: [2.1.65] New patch for serial port IRQ setup.

Gabriel Paubert (paubert@iram.es)
Mon, 24 Nov 1997 10:38:31 +0100 (MET)

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On Mon, 24 Nov 1997, Niels Kristian Bech Jensen wrote:

> I've been made aware that some manufacturers of internal modems recommend
> using irq 2 for COM 4, while other recommende using irq 9. I've made this
> a configuration option.

IRQ2 is the name of the hardware signal on the ISA bus, but this is a
relic from before the PC-AT. From the interrupt controller point of view,
it is connectet to interrupt input number 9.

BIOSes contain a workaround to redirect interrupt 9 to interrupt 2, but
AFAIK Linux will never let you take interrupt 2 and you will for sure
never get the corresponding vector on an interrupt acknowledge cycle,
because it is used as the cascade input of the second interrupt
controller: actually all interrupts 8 to 15 share interrupt input 2 (IRQ2)
of the first interrupt controller. In short 2 and 9 are the same, but you
must use 9.

Gabriel.

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