Add .a51 sample

samples/Assembly/External Interrupt.a51

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+	ORG	0000h
+	SJMP	START
+	ORG	0003h
+	LCALL	INT0_ISR
+	RETI
+	ORG	000Bh
+	LCALL	T0_ISR
+	RETI
+	ORG	0013h
+	LCALL	INT1_ISR
+	RETI
+	ORG	001Bh
+	LCALL	T1_ISR
+	RETI
+	ORG	0023h
+	LCALL	UART_ISR
+	RETI
+	ORG	0030h
+START:
+	MOV	A,#11111110b
+	SETB	IT0	; Set External Interrupt 0 to be falling edge triggered
+	SETB	EX0	; Enable External Interrut 0
+	SETB	EA	; Enable Interrupt
+LEFT:			
+	CJNE	A,#01111111b,LOOP1
+	JMP	RIGHT
+LOOP1:
+	MOV	P1,A
+	RL	A	
+	LCALL	DELAY
+	SJMP	LEFT	
+RIGHT:
+	CJNE	A,#11111110b,LOOP2
+	JMP	LEFT
+LOOP2:
+	MOV	P1,A
+	RR	A	
+	LCALL	DELAY
+	SJMP	RIGHT
+	
+INT0_ISR:
+	MOV	R1,#3
+FLASH:
+	MOV	P1,#00h
+	LCALL	DELAY
+	MOV	P1,#0FFh
+	LCALL	DELAY
+	DJNZ	R1,FLASH
+	RET
+T0_ISR:
+	RET
+INT1_ISR:
+	RET
+T1_ISR:
+	RET
+UART_ISR:
+	RET
+
+DELAY:	MOV	R5,#20	;R5*20 mS
+D1:     MOV	R6,#40
+D2:     MOV	R7,#249
+	DJNZ	R7,$
+ 	DJNZ	R6,D2
+  	DJNZ	R5,D1
+   	RET
+	END