Knight Tyme (48k): Routine at 54564

54564: If Kempston joystick is not present then revert to previously selected input device

Used by the routine at 54505.

Input
B	Index of currently selected input device
D	Index of previously selected input device

Output
B	Index of currently selected input device if Kempston joystick is present, otherwise index of previously selected input device

54564

IN A,(31)

Load A with current Kempston joystick state

54566

AND 224

If bits 5, 6 and 7 (i.e. unused bits) are reset, indicating presence of Kempston joystick...

54568

RET Z

...then return

54569

LD A,D

Load A with index of previously selected input device...

54570

LD (54560),A

...set as currently selected input device...

54573

LD B,A

...and load into B

54574

RET

Return