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54999: Capture Kempston joystick input
Used by the routine at 54942.
A Stored key-press (bits: 0=LEFT, 1=RIGHT, 2=UP, 3=DOWN, 4=FIRE)
54999 LD C,31 Load 31 (Kempston port number) into A
55001 IN A,(C) Read byte from port 31 (i.e. read state of joystick)
55003 BIT 0,A If joystick moved right...
55005 CALL NZ,55103 a RIGHT input from control device
55008 BIT 1,A If joystick moved left...
55010 CALL NZ,55100 a LEFT input from control device
55013 BIT 2,A If joystick moved down...
55015 CALL NZ,55109 a DOWN input from control device
55018 BIT 3,A If joystick moved up...
55020 CALL NZ,55106 an UP input from control device
55023 BIT 4,A If fire button pressed...
55025 CALL NZ,55112 a FIRE input from control device
55028 LD A,(HL) Return stored joystick movement in A
55029 RET Return
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