1 ;名称: letterc 2 ;功能: 将以0为结尾的字符串中的小写字母转变成大写字母 3 ;参数: ds:si指向字符串首地址 4  5 assume cs:code 6  7 data segment 8         db "Beginner‘s All-purpose Symbolic Instruction Code.",0 9 data ends10 11 code segment12 main: 13         mov ax,data14         mov ds,ax15         mov si,016         call letterc17         18         mov ax,4c00h19         int 21h20 letterc:21         push ax22         push ds23         push si 24         push cx25         mov  ch,026 27 lop:28         mov cl,[si]29         jcxz quit30         cmp cl,‘z‘31         ja  next32         cmp cl,‘a‘33         jb  next34         and cl,11011111b35         mov [si],cl36 next:37         inc si38         jmp lop39 quit:40         pop cx41         pop si42         pop ds43         pop ax44         ret45 code ends46     end main