Write one to a bit resets the interrupt flag.
0x01 : bit 0 CA2 interrupt (cleared by read/write port A depeneding on "independed" interrupt)
0x02 : bit 1 CA1 interrupt (edge is determined by PCR bit 0, cleared by read/write of Port A)
0x04 : bit 2 Shiftreg interrupt (complete 8 shifts)
0x08 : bit 3 CB2 interrupt
0x10 : bit 4 CB1 interrupt
0x20 : bit 5 Timer2 interrupt
0x40 : bit 6 Timer1 interrupt
0x80 : bit 7 indicates whether ANY interrupt happened