AH = 5Dh
        AL = subfunction
            00h enable OS Function Set
            01h disable OS Function Set
            02h return access key (resets memory manager, returns access key at
                next invocation)
        BX,CX = access key returned by first invocation
Return: BX,CX = access key, returned only on first invocation of function
        AH = status
            00h successful
            80h internal error
            81h hardware malfunction
            84h undefined function requested
            8Fh undefined subfunction
            A4h operating system denied access
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