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:CLEAR_EXTERNAL_DATA_MEMORY
				     ;FUNCTION:CLEAR 4 BYTES EXTERNAL DATA MEMORY, ADDR INDEX BY DPTR
				     ;
				     CLEAR_EXTERNAL_DATA_MEMORY
				           	MOV  	R2,#04H
				           	CLR   	A
				     $1   	MOVX   	@DPTR,A
				           	INC   	DPTR
				           	DJNZ   	R2,$1
				           	RET
				     ;