Re: Why no interrupt priorities?

Arjan van de Ven
2004-02-29T08:37:15Z

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On Sun, Feb 29, 2004 at 04:32:54PM +0800, Michael Frank wrote:
>
> Most interrupt controllers can read back IRQ's to see whether it is
> active. A shared IRQ would be readback active while any device
> connected to it desires service.
>
> x86 example for 8259A AT-PIC's Returns the state of IRQ0-15 in ax
> Note that jmp $+2 is only needed on some old 286/386 hardware
> to meet (real) 8259A cycle time requirements.
>
> - Intel syntax :)
>
> mov al,0ah
> out 0a0h,al
> jmp $+2
> in al,0a0h
> mov ah,al
> mov al,0ah
> jmp $+2
> out 20h,al
> jmp $+2
> in al,20h

interesting; however with modern cpus I suspect that a series of in/outs
like that is more expensive than one or two "surious" hardirq handler
calls...

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