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.

- -

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