INT 67H 5dxxH: Enable/Disable OS Function Set Functions

                                                     Compatibility: EMS 4.0+ 
 Expects: AX    5dH
          AL    subfn: 00H=Enable all OS-only EMM functions
                       01H=Disable all OS-only EMM functions
                       02H=Free access key (enable next caller to get a key)
          BX,CX access key
 Returns: AH    EMM status (0=successful)
          BX,CX (fns 5d00H and 5d01H) access key (returned only the first
                time called or after using fn 5d02H).
    Info: These fns are used by an operating system in an effort to keep
          programs from performing certain OS-level EMS operations.  It is
          not to be used by applications.

          The first call to fns 5d00H or 5d01H obtain a 32-bit value which
          must be used in subsequent calls to 5dxxH fns.

See Also: INT 67H: EMM Services
          Extended Memory Specification (XMS)
          Interrupts and BIOS Services