51单片机基础篇 对于有一定基础的人看看 包含书中所以的源代码

				     ;FUNCTION NAME: CHECK_RAM_RW
				     :FUNCTION: CHECK RAM READ/WRITE FUNCTION (2048 BYTES)
				     ;RAM ADDRESS IN DPTR
				     ;
				     CHECK_RAM_RW
				           	MOV   	R0,#08H
				     $1 	MOV  	R1,#00H
				     $2 	MOVX 	A,@DPTR   		;READ
				           	XRL	A,#FFH
				           	MOV   	B,A
				           	MOVX 	@DPTR,A			;WRITE
				           	MOVX 	A,@DPTR			;READ AGAIN
				           	CJNE 	A,B,$3 			;READ/WRITE ERROR
				           	XRL   	A,#FFH
				           	MOVX 	@DPTR,A  		;WRITE AGAIN
				           	INC   	DPTR
				           	DJNZ 	R1,$2
				           	DJNZ 	R0,$1
				           	MOV   	A,#00H 			;IF NO ERROR THEN RETURN 0
				           	SJMP 	$3+2
				     $3 	MOV   	A,#01H
				           	RET
				     ;