.MODEL SMALL .STACK 100H .DATA PROMPT_1 DB \'Enter the Upper Case Letter : $\' PROMPT_2 DB 0DH,0AH,\'The Lower 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 LEA DX, PROMPT_2 ; load and print PROMPT_2 MOV AH, 9 INT 21H OR BL, 20H ; convert a upper case letter to lower ; case letter MOV AH, 2 ; print the Lower case letter MOV DL, BL INT 21H MOV AH, 4CH ; return control to DOS INT 21H MAIN ENDP END MAIN