The CIA 6526 shift register on the C64 is not used. Because the trace on the pcb that connected to it was accidentally cut by the board partner, and Tramiel was

The CIA 6526 shift register on the C64 is not used. Because the trace on the pcb that connected to it was accidentally cut by the board partner, and Tramiel was unwilling to take the time needed for the rework. C64 uses the same very slow bit-banging method as VIC-20.