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
;退出
              mov       ax,4c00h
              int       21h
    letterc:
              dec       si          ;先减一
          s:
              inc       si          ;在加一，这样下面不用单独写inc si 了
              mov       al,[si]
              cmp       al,0
              je        return
              cmp       al,'z'      ;大于'z'跳过
              jnb       s
              cmp       al,'a'      ;小于'a'跳过
              jna       s
              and       al,11011111b            ;;小于'z'并大于'a'开始处理
              mov       [si],al
              jmp       s
     return:
              ret
        code  ends

              end       begin