[coreboot] the seabios reboot in the post process

Andrew Goodbody andrew.goodbody at tadpole.com
Thu Mar 5 11:12:58 CET 2009

Kevin O'Connor wrote:
> On Wed, Mar 04, 2009 at 10:15:21AM +0000, wei yang wrote:
>> When I use the seabios as the payload, I got a strange issue.
>> In the seabios flow:
>> Post->timer_setup->rtc_updating->inb_cmos function
>> when run the inb_cmos, it will reboot on the inb operation.
>> inb_cmos disassemables to fllowing:
>> out 0x70, value
>> in value, 0x71.   ----> run here to make the reboot happen.
>> 0x70 is the CMOS index port, 0x71 is the CMOS data port.
>> I suspect the "in value, 0x71" operation causes a exception and the
>> seabios doesn't have real IDT, so it then make the reboot.
>> but why it cause the exception is a question for me?
>> since IO port 0x70 also reponsible for the NMI enble/disable, is it
>> caused the error?
> I haven't seen this before.  It would be odd for port 0x70 to control
> NMI, because it is usually used for rtc access.

It's been this way since the IBM PC AT. If you write to 0x70 with bit 7 
reset then you risk releasing any pending NMI. If the interrupt vectors 
are not initialised then you always need to set bit 7 when writing to 0x70.


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