converting lowercase char to uppercase char in a loop

lower_to_upper.asm

 ;Convert char input in a loop (lowercase -> UPPERCASE)
        .model small
        .stack 100h
      
        .data  
            text db 'CONVERT LOWERCASE TO UPPERCASE $'
            low db 'Lower: $' 
            up db 'Upper: $'
            input db ?
            
         .code
            
            main proc

                mov ax,@data        ;prepare data segment
                mov ds,ax
                         
                mov ah,09h          ;print string
                lea dx,text
                int 21h 
                
       f1:      mov ah,02h          ;print newline
                mov dl,0Ah
                int 21h
                mov dl,0Ah
                int 21h
                mov dl,0Dh
                int 21h
                
                mov ah,09h          ;print string
                lea dx,low
                int 21h
                
                mov ah,01h          ;input char
                int 21h
                mov input,al
                mov cl,input
                sub cl,32           ;convert to lowercase 
                
                mov ah,02h          ;print tab
                mov dl,09h
                int 21h

                mov ah,09h          ;pirint string
                lea dx,up
                int 21h
                
                mov ah,02h          ;print char
                mov dl,cl
                int 21h
                
                                    ;newline
                mov dl,0Ah
                int 21h
                mov dl,0Dh
                int 21h
               
                mov ah,01h             ;exit input
                int 21h
                
                cmp al,'x'
                je END 
                jmp f1 
              
     END:    mov ah,4ch             ;4ch
             int 21h
             
             main endp
             end main