CONSTANT rx, 01             ;UART receiver

...

INPUT       direction, rx
;move head according to received direction
COMPARE     direction, 7A               ;'z'
CALL        Z, move_up
COMPARE     direction, 64               ;'d'
CALL        Z, move_right
COMPARE     direction, 73               ;'s'
CALL        Z, move_down
COMPARE     direction, 71               ;'q'
CALL        Z, move_left