王爽 汇编 实验11：编写子程序letterc 解答

assume cs:code,ds:data

data segment
	db "Beginner's All-purpose Symbolic Instruction Code.",0
data ends

code segment
start:
	mov ax,data
	mov ds,ax
	mov si,0
	call letterc

	mov ax,4c00h
	int 21h

letterc:
	push cx
	push si
	s1:
	mov cl,[si]
	mov ch,0
	jcxz ok
	cmp cl,97
	jnb s2
	jmp short s3

	s2:
	cmp cl,122
	ja s3
	and cl,11011111b
	mov [si],cl


	s3:
	inc si
	jmp short s1

	ok:
	pop si
	pop cx
	ret
code ends
end start