خطأ في الكود

Write a assembly program which display ? on the screen and then ask user for an input from A-Z and display the previous letter on the screen on next line.

Sample Execution:

? B

PREVIOUS LETTER: A

 TITLE demo: CHECK FLAGS
; used in DEBUG to check flag settings
.MODEL SMALL	
.STACK 100H
.CODE
MAIN PROC
	MOV AX,4000H; AX = 4000h
	ADD AX, AX	; AX = 8000h
	SUB AX, 0FFFFH; AX = 8001h
	NEG AX	; AX = 7FFFh
	INC AX	; AX = 8000h
	MOV AH, 4CH; DOS exit function
	INT 21H	; exit to DOS
MAIN ENDP
 	END MAIN

عاارفه انه في خطأ بس هذا الي حاولاته ابيه مره ضروري اللع يعافيكم الي يعرف يساعدني