$DC01/56321/CIA1+1:   Data Port B (Keyboard, Joystick, Paddles)

   +----------+---------------------------------------------------+
   | Bits 7-0 |   Read Keyboard Row Values for Keyboard Scan      |
   | Bit  4   |   Joystick B Fire Button: 0 = Pressed             |
   | Bit  3   |   Joystick B Right: 0 = Pressed, or Paddle Button |
   | Bit  2   |   Joystick B Left : 0 = Pressed, or Paddle Button |
   | Bit  1   |   Joystick B Down : 0 = Pressed                   |
   | Bit  0   |   Joystick B Up   : 0 = Pressed                   |
   +----------+---------------------------------------------------+

   Joystick B is the one plugged into Port 1.

Kernal-Reference:

 CMP $DC01   : $EAAE $F6BF
 CPX $DC01   : $F6CF
 LDA $DC01   : $E93D $EAAB $F6BC
 LDX $DC01   : $EA93 $F6CC