Re: what is IRQ sharing?

On 2007-02-22, kerneloops <rambo88@xxxxxxxxx> wrote:

what is IRQ sharing?

On the common PC (x86) processors, hardware devices can use special
signals called "interrupt requests" (aka IRQ) to tell the cpu they need
some attention. x86 type processors allow up to 16 interrupts, but
several of these are reserved for specific devices such as the system
timer, keyboard, and real-time clock. Another (IRQ2) is dedicated to
providing a cascade to a second IRQ controller -- this is a legacy of
the original 8086 processor design which only allowed for 8 interrupts.
IRQ2 cascades to a second controller which provides another 8 interrupts
for a total of 16. Even this cobble-job was only helpful for a while; as
people added more devices (sound cards, addition HD controllers, network
cards, video cards, etc) all requiring their own IRQ it quickly became a
nightmare to manage all this. When the PCI bus was designed in the mid
80s, one of the design features was to allow devices to share IRQs as a
means of working around this limitation -- the original ISA bus didn't
allow any sharing. So your PCI devices can usually -- but not always --
share an IRQ with another PCI device so you don't have to worry so much
about this issue.


--

John (john@xxxxxxxxxxx)
.

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