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DateSun, 29 Feb 2004 09:36:57 +0100
FromArjan van de Ven <>
SubjectRe: Why no interrupt priorities?
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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