汇编:DF标志位

DF:    UP(向上)   DN(down 向下)

assume cs:code, ds:data, ss:stack

data segment
	db 128 dup(128)
data ends

stack segment
	db 128 dup(128)
stack ends

code segment
	start:	mov dx, stack
		mov ss, dx
		mov sp, 128

		call cpy_Boot

		mov ax, 4C00H
		int 21H	



Boot:		mov ax, 1000H
		mov ax, 1000H
		mov ax, 1000H
		mov ax, 1000H
Boot_end:	nop

;=======================================
cpy_Boot:
		mov bx, cs
		mov ds, bx			; ds:[si]
		mov si, OFFSET Boot
		
		mov bx, 0
		mov es, bx
		mov di, 7E00H			; es:[di]
		
		mov cx, OFFSET Boot_end - Boot
		cld				; inc si, inc di  DF=0
		; movsb 复制一个字节从ds:[si] 到 es:[di]
		rep movsb			;rep 重复   movsb:复制字节
		ret

code ends

end start