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INT 67 - LIM EMS 4.0 - ALTERNATE MAP REGISTER SET
        AH = 5Bh
        AL = subfunction
            00h get alternate map register set
                Return: BL = current active alternate map register set number
                        ES:DI -> map register context save area if BL=00h
            01h set alternate map register set
                BL = new alternate map register set number
                ES:DI -> map register context save area if BL=0
            02h get alternate map save array size
                Return: DX = array size in bytes
            03h allocate alternate map register set
                Return: BL = number of map register set; 00h = not supported
            04h deallocate alternate map register set
                BL = number of alternate map register set
Return: AH = status
            00h successful
            80h internal error
            81h hardware malfunction
            84h undefined function requested
            8Fh undefined subfunction
            9Ah specified alternate map register set not supported
            9Bh all alternate map register sets currently allocated
            9Ch alternate map register sets not supported
            9Dh undefined or unallocated alternate map register set
            A3h source array corrupted
            A4h operating system denied access
Note:   this function is for use by operating systems only, and can be
          enabled or disabled at any time by the operating system