0040h:001Ch           Keyboard Buffer Tail Pointer

    Length: 2 bytes

    This word points to the location in the keyboard buffer at which
    the next character typed will be placed. After a character is
    placed in the keyboard at this location, the tail pointer is
    incremented and adjusted if it needs to wrap to the beginning of
    the keyboard buffer. If the tail pointer is equal to the head
    pointer (0040h:001Ah), the buffer is empty.