START:
	MOV ACC,#0FFH ; all 1's into acc
	MOV R0,#080H  ; port 0 sfr
	MOV @R0,ACC   ; move FF into port 0
	MOV R0,#090H  ; port 1
	MOV @R0,ACC
	MOV R0,#0A0H  ; port 2
	MOV @R0,ACC
	MOV R0,#0B0H  ; port 3
	MOV @R0,ACC
	MOV ACC,#000H ; all 0's into acc
	MOV R0,#080H  ; port 0
	MOV @R0,ACC
	MOV R0,#090H  ; port 1
	MOV @R0,ACC
	MOV R0,#0A0H  ; port 2
	MOV @R0,ACC
	MOV R0,#0B0H  ; port 3
	MOV @R0,ACC
	SJMP START