综合一下学过的指令就行了，比较简单。

assume cs:code
data segment
	db "Beginner's All-purpose Symbolic Instruction Code.",0
data ends
code segment
begin:
	mov ax,data
	mov ds,ax
	mov si,0
	call letterc
	mov ax,4c00h
	int 21h
letterc:
	mov cl,[si]
	mov ch,0
	jcxz finish
	cmp cx,97
	jb next
	cmp cx,122
	ja next
	and byte ptr [si],11011111b

next:	
	inc si
	jmp short letterc
finish:	ret
	
code ends
end begin