Error with assigning ..interrupt value?

why does it gives me error on second line.. of the code..?

Code:

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int main(void)
{
  void interrupt (*old)();
  old = 0xffff0000; //here gives me error...
  (*old)();
}

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I have tried a lot to cast the value.. but in vain.. plz write a code line to typecast.. the values on the right.. which is causing error