INT 16 - KEYBOARD - CHECK FOR KEYSTROKE
        AH = 01h
Return: ZF set if no keystroke available
        ZF clear if keystroke available
            AH = BIOS scan code
            AL = ASCII character
Note:   if a keystroke is present, it is not removed from the keyboard buffer;
          however, any extended keystrokes which are not compatible with 83/84-
          key keyboards are removed in the process of checking whether a
          non-extended keystroke is available
SeeAlso: AH=11h,AH=21h,INT 18/AH=01h