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SubjectRe: Why no interrupt priorities?
On Sun, 29 Feb 2004 09:36:57 +0100, Arjan van de Ven <arjanv@redhat.com> wrote:

> 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...
>

Yes, Four 8259A IO cycles would take almost 2us, which is several 1000
instructions worth of burning electricity.

Racehorse is still best at going straight :)

However on-chipset PIC's may be better and in on-CPU APICs should be
much better in this regard, but I have not studied data(sheet).

Regards
Michael


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