Re: ICU_LEN with IO APIC

John Baldwin wrote:
> 
> On 31-May-2002 Bosko Milekic wrote:
> > 
> > On Fri, May 31, 2002 at 12:12:00PM +0300, Aaro J Koskinen wrote:
> >> Hello,
> >> 
> >> Is there any particular reason why the number of interrupts is limited
> >> to 32 on APIC systems? Is it just a conservative guess on the number of
> >> interrupts anyone might want to need...?
> > 
> >   I'm not sure but perhaps this is historical (and now also required
> >   again), but if we use a word to mask out interrupts than after 32 we
> >   run out of bits.  "Who needs more than 32 interrupts anyway?!" :-)
> 
> Actually, the historical value in stable is 24 because the same 32-bit word
> shares the 8 softinterrupts with 24 hardware interrupts.  I think the APIC
> only has 32 interrupt pins however.

Historically it was because ipending and friends were a 32 bit word.  Even
now, we have a stack of 32 bit bitfields in this area.

It isn't uncommon to have 2 IO apics with 24 pins each.  Fortunately there
are rarely more than about 20 or so in use in total.

Cheers,
-Peter
--
Peter Wemm - [e-mail]; [e-mail]; [e-mail]
"All of this is for nothing if we don't go to the stars" - JMS/B5


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