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Re: Re: Re: 8259 spurious interrupt (IRQ1,IRQ7,IRQ12..)

To: Ralf Baechle <>
Subject: Re: Re: Re: 8259 spurious interrupt (IRQ1,IRQ7,IRQ12..)
From: Zhang Fuxin <>
Date: Thu, 20 Sep 2001 10:25:0 +0800
Cc: "" <>
hi,Ralf Baechle,

    The mannual says:

     Tips on programming south bridge interrupt controller(s)

  The interrupt controllers inside the south bridge chip have to be 
software-compatible with the dual Intel 8259 controllers fitted to ver y old 
PCs.There are two 8-bit controllers inside the south bridge,aslave whose
interrupt output becomes one of the eight inputs of the master controller.
The 8259 is a ver y old device,dating from around 1980. Under the influence 
of contemporar y minicomputer designs,the 8259 made a brave attempt to build 
an interrupt prior ity system in hardware for Intel 8-bit and 8086 CPUs.With 
the benefit of 20 years hindsight, we can see this was pretty silly (but that
’s a lot of hindsight, and the people who did it were much younger then).
When separated from its Intel CPU,the 8259 is unable to do exciting things like 
automatically causing the CPU to vector to an appropriate interrupt 
vector location. Experience shows that allowing it to do anything complicated 
is likely to lead to trouble,so we suggest:
   1. Don’t use the 8259’s prior ity logic. Instead, place both the master and 
slave controller into what the Intel manuals call Special Mask Mode.This 
disables all interrupt prior itisation; now any active and unmasked
interr upt input will activate the interrupt output, which is enough.

2. Do not ever read the ISR (‘‘in-ser vice’’) registers.They don’t make sense 
outside of the prior ity scheme.If you want to know what individual interrupt 
lines are doing, look in the IRR (‘‘interr upt request’’) registers instead.

3. Conclude each interrupt service routine with a ‘‘Specific end-of-interrupt 
(EOI)’’ command to the 8259. In special mask mode this does
n't do anything to the priority,but it does serve to clear a latched 
interrupt, if there is one.(There are other ways of doing this).

 I have tried to put both 8259 into Special mask mode( write OCW3=0x6a to 0x20
and 0xa0).But it doesn't stop spurious IRQ12.But so far I have see only IRQ7
and IRQ12.
  A skim over 82371(PIIX) manual I find something that may relate: XBCS 
register(0x4e),which can control mouse support at IRQ12,and keyb,rtc support.
Because p6032 use multi-IO controller to provide keyb,mouse,rtc,I guess
it may lead to some problem; 0x60,a read to it clear mouse/keyb interrupt
if mouse/keyb support is enabled in XBCS; 0x4d0,0x4d1,set level/edge sense
mode per interrupt.
  But sadly up to now i have tried several combination,none can stop spurious
IRQ12. I have to add a read to 0x60 to prevent endless spurious IRQ12 when it 
  Another fact is the 82371 manual is 82371FB & 82371SB,but the chip on the
board is 82371EB.I will try to find the exact manual later.

在 2001-09-19 18:41:00 you wrote:
>On Wed, Sep 19, 2001 at 11:27:14AM +0100, Phil Thompson wrote:
>> Make sure you read the section in the P6032 manual "Tips on programming
>> south bridge interrupt controller(s)" - page 31. I don't see how the 8259
>> code that's part of the MIPS tree can ever be used without changes.
>Can you elaborate?  It's actually being used without problems.
>  Ralf

            Zhang Fuxin

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