.model small
.stack
.data
     string1 db'please input a number(0-2):',0dh,0ah,'$' 
     string2 db'I am a good father.',0,0dh,0ah,'$'
     string3 db'That is an error input!',0dh,0ah,'$'
.code
start: mov ax, @data
       mov ds, ax
       mov ah, 9
       mov dx, offset string1
       int 21h
       mov ah, 1
       int 21h
       cmp al, '0'
       jz disp1
       cmp al, '1'
       jz disp2
       cmp al, '2'
       jz disp3
       call error
disp1: call CturnU
disp2: call UturnC
disp3: call change
       mov ah, 4ch
       int 21h
       
CturnU  proc
        mov bx, offset string2
againc: mov al, [bx]
        or al, al
        jz donec
        cmp al, 'A'
        jb nextc1
        cmp al, 'Z'
        ja nextc1
        or al, 20h
        mov [bx], al
nextc1: inc bx
        jmp againc
 donec: mov ah, 9
        mov dx, offset string2
        int 21h
        mov ah, 4ch
        int 21h
        ret
CturnU  endp
        
UturnC  proc
        mov bx, offset string2
againu: mov al, [bx]
        or al, al
        jz doneu
        cmp al, 'a'
        jb nextu
        cmp al, 'z'
        ja nextu
        xor al, 20h
        mov [bx], al
 nextu: inc bx
        jmp againu
 doneu: mov ah, 9
        mov dx, offset string2
        int 21h
        mov ah, 4ch
        int 21h
        ret
UturnC  endp
        
change  proc
        mov bx, offset string2
againg: mov al, [bx]
        or al, al
        jz doneg
        cmp al, 'A'
        jb nextg2
        cmp al, 'Z'
        ja nextg1
        or al, 20h
        mov [bx], al
        jmp nextg2
nextg1: cmp al, 'z'
        ja nextg2
        xor al, 20h
nextg2: inc bx
        jmp againg
 doneg: mov ah, 9
        mov dx, offset string2
        int 21h
        mov ah, 4ch
        int 21h
        ret
change  endp
        
error proc
      mov ah, 9
      mov dx, offset string3
      int 21h
      mov ah,4ch
      int 21h
      ret
error endp
       
      end start