$4001/16385/CIA+1:       Data Port Register B

   +-------+---------------------------------------------------+
   | Bit 7 |   ATN IN                                          |
   | Bit 6 |   0 = Write protect active                        |
   | Bit 5 |   Data Direction of the Bus Driver (FSM)          |
   |       |     0 = Input, 1 = Output                         |
   | Bit 4 |   automatic ATN-Response                          |
   | Bit 3 |   CLOCK OUT                                       |
   | Bit 2 |   CLOCK IN                                        |
   | Bit 1 |   DATA OUT                                        |
   | Bit 0 |   DATA IN                                         |
   +-------+---------------------------------------------------+

ROM-Reference:

 BIT $4001  : $AAE4 $AC55 $AC96 $AE58 $BB9A $BC4C $CB78
 CMP $4001  : $AD0F $BA43 $BF8A $C02C
 LDA $4001  : $AAD7 $ABF0 $ABF8 $AC60 $ACC5 $ACD6 $ACE8 $ACF1 $ACFA $AD03
              $AD0C $AD1B $AD21 $ADB3 $ADDD $AE87 $BA40 $BB8D $BC3F $BF87
              $C029 $DBCA
 STA $4001  : $AADF $ABF5 $AC65 $AC93 $ACCA $ACDB $ACED $ACF6 $ACFF $AD08
              $ADB8 $ADE2 $AEE1 $AF32 $B0F3 $BB95 $BC47 $DBCF