.MODEL SMALL
 .STACK 100H

 .DATA
    PROMPT_1  DB  'Enter the Lower Case Letter : $'
    PROMPT_2  DB  'The Upper Case Letter is : $'

 .CODE
   MAIN PROC
     MOV AX, @DATA                ; initialize DS
     MOV DS, AX

     LEA DX, PROMPT_1             ; load and print PROMPT_1
     MOV AH, 9
     INT 21H

     MOV AH, 1                    ; read a letter
     INT 21H

     MOV BL, AL                   ; save the letter in BL 
     
     MOV AH, 2                    ; return carriage 
     MOV DL, 0DH
     INT 21H

     MOV DL, 0AH                  ; line feed
     INT 21H

     LEA DX, PROMPT_2             ; load and print PROMPT_2
     MOV AH, 9
     INT 21H

     SUB BL, 20H                  ; convert a lower case letter to upper case letter

     MOV AH, 2                    ; print the Upper case letter
     MOV DL, BL
     INT 21H

     MOV AH, 4CH                  ; return control to DOS
     INT 21H
   MAIN ENDP 
 END MAIN

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