fasm_4_1

org 100h
    ESC_ equ $1b ;ascii code escape
    BELL_ equ 7 ;ascii codering

start_:
    mov ah, 09h ;output - helloStr_
    mov dx, helloStr_
    int 21h
    main_:
        mov ah, 08h ;no echo input with waiting, output al - ascii code of pressed button
        int 21h
        cmp al, ESC_ ;was it escape?
        jne other_ ;if no go to other_
        jmp exit_ ;if yes exit
    other_:
        cmp al, '0' ;if letter is less than '0' it's incorrect, go to err_
        jb err_ ;jump if below
        cmp al, '9' ;if letter is greater than '9' it's incorrect, go to err_
        ja err_ ;jump if above
        mov dl, al ;else display that letter
        mov ah, 02h
        int 21h
        jmp main_ ;go back to start
    err_:
        mov ah, 02h ;play ring
        mov dl, BELL_
        int 21h
        jmp main_ ;go back to start3
    exit_:
        mov ah, 09h ;output - byeStr
        mov dx, byeStr
        int 21h
        mov ah, 07h ;wait for input
        int 21h
        ret

helloStr_ db "This program allows as input only digits", $0d, $0a, "You can exit on ESC", $0d, $0a, "Try to input something:", $0d, $0a, "$"
byeStr db 0dh, 0ah, "Press anything to terminate program...$"