编写一个子程序：将包含任意字符，以0结尾的字符串中的小写字母转变成大写字母

assume cs:code,ds:data
data segment
  db  'Beginners All-purpose Symbolic Instruction Code.',0
data ends

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

          mov ah,4ch
          int 21h

letterc:
            push ax
 s:
            mov al,[si]
            mov ah,0
            mov cx,ax
            jcxz return
            cmp al,61h
            jb next
            cmp al,91h
            ja next
            and al,11011111b
            mov [si],al
next:
            inc si
            jmp short s

 return:
            pop ax
            ret
code ends
end begin