INT 16 - KEYBOARD - CHECK FOR 122-KEY KEYSTROKE (122-key kbd support only)
        AH = 21h
Return: ZF set if no keystroke available
        ZF clear if keystroke available
            AH = BIOS scan code
            AL = ASCII character
Notes:  use AH=09h to determine whether this function is supported
        some versions of the IBM BIOS Technical Reference erroneously report
          that CF is returned instead of ZF
SeeAlso: AH=01h,AH=09h,AH=11h,AH=20h,AH=21h

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