Int 67 Fn 5D  - Lim Ems 4.0 - Enable/disable Os Function Set Functions     [m]

   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 (see also AH=5Ch)
       8Fh undefined subfunction
       A4h operating system denied access