Int 16 Fn 01  - Keyboard - Check For Keystroke                             [B]

   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

See Also: AH=00h,AH=11h,AH=21h,INT 18/AH=01h