asm实现大小写转换

assume ds:data,cs:code
data segment
    db 'abc'
data ends;data block but the data will be saved in memory block 76c:0
code segment
    start:
    mov ax,76ch
    mov ds,ax
    mov cx,3h
    mov bx,0h ;the [bx] is stable it can't be others like [ax] [dx]
    s:
        mov al,ds:[bx]
        sub al,20h
        mov ds:[bx],al
        inc bx ;bx=bx-1,bx--
    loop s ;it's a loop
    jmp exit; jmp to other code block memory
    exit:
        mov ax,4c00h
        int 21h
code ends
end start