The difference between an input and an output is that an output uses more current to drive the signal to the desired level. An input and an output outputting logical '1' are equivalent for any other inputting chip. But if a chip is trying to drive a signal to ground level, it needs more current to sink an output than an input. You can even use outputs as inputs, i.e. read them in your program. You can use this feature to distinquish between the left shift and the shift lock keys, although they are connected to same hardware lines. The shift lock key has smaller resistance than the left shift. If you make both CIA 1 ports to outputs (write $FF to $DC03 and $DC01) prior reading the left shift key, only shift lock can change the values you read from CIA 1 port B ($DC01).)
again: lda #$ff sta $dc03 sta $dc01 lda #%11111101 sta $dc00 lda $dc01 asl ldx #$00 bcs !no+ ldx #$ff !no: stx $0500 jmp again
