how can I hack my com interrupt?

[ouyang kai]

Hi, guys,
  I want to hack my com interface to specail purpose. I want to use the 
Modem Status Register. When the high 4 bits change, it could spring an 
interrupt. So, I set the Intertupt Enable Register(in sioattach fuction) to 
the interrupt, which support changing in the state of the modem input 
pins(IER_EMSC). I set the MSR successfully.
  But when the MSR state change, the siointr print anything(I add a printf 
as the first line of the function). I found there are 4 level 
interrupt(IIR). I want to set the IIR 4 level to support Modem status. But I 
couldn't set the Interrupt Identification Register, why?
  the code of set IIR is in the sioattach function:
  sio_setreg(com, com_iir, IIR_MLSC);
  I found the com_iir content is not changed.
  How can I do that? My OS is FreeBSD4.8-Release.

Thanks a lot!

Best Regards
  Ouyang Kai

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