$180D/6157/VIA1+13:   Interrupt Flag Register

   +-------+------------------------------------------------------+
   | Bit 7 |   1 = Interrupt occured                              |
   | Bit 6 |   Timer 1                                            |
   | Bit 5 |   Timer 2                                            |
   | Bit 4 |   CB1                                                |
   | Bit 3 |   CB2                                                |
   | Bit 2 |   Shift Register ($180A)                             |
   | Bit 1 |   CA1                                                |
   | Bit 0 |   CA2                                                |
   +-------+------------------------------------------------------+

ROM-Reference:

 LDA $180D  : $E9E2 $FB4B $FB6C $FE6C
 STA $180D  : $EB34