$180B/6155/VIA1+11:   Auxiliary Control Register

   +----------+---------------------------------------------------------+
   | Bits 7-6 |   Timer 1 Control:                                      |
   |          |     00 = Timed Interrupt when Timer 1 is loaded, no PB7 |
   |          |     01 = Continuous Interrupts, no PB7                  |
   |          |     10 = Timed Interrupt when Timer 1 is loaded,        |
   |          |          one-shot on PB7                                |
   |          |     11 = Continuous Interrupts, square-wave on PB7      |
   | Bit  5   |   Timer 2 Control: 0 = Timed Interrupt                  |
   |          |                    1 = Count Pulses on PB6              |
   | Bits 4-2 |   Shift Register Control:                               |
   |          |     000 = Disabled                                      |
   |          |     001 = Shift in under control of Timer 2             |
   |          |     010 = Shift in under control of Phi2                |
   |          |     011 = Shift in under control of ext. Clock          |
   |          |     100 = Shift out free-running at Timer 2 rate        |
   |          |     101 = Shift out under control of Timer 2            |
   |          |     110 = Shift out under control of Phi2               |
   |          |     111 = Shift out under control of ext. Clock         |
   | Bit  1   |   1 = enable latching PB                                |
   | Bit  0   |   1 = enable latching PA                                |
   +----------+---------------------------------------------------------+

ROM-Reference:

 AND $180B  : $FBDA
 ORA $180B  : $FB22
 STA $180B  : $FB25 $FBDD