ESC_    equ     1Bh
        org 100h
start:
        mov     ah,08h
        int     21h
        cmp     al,ESC_
        je      exit

        cmp     al,'C'
        jb      start
        cmp     al,'R'
        ja      start

        mov     ah,02h
        mov     dl,al
        int     21h

        jmp     start
exit:
        ret