AH = 5Bh
        AL = subfunction
            00h get alternate map register set
            01h set alternate map register set
               BL = new alternate map register set number
               ES:DI = pointer to map register context save area if BL=0
            02h get alternate map save array size
            03h allocate alternate map register set
            04h deallocate alternate map register set
               BL = number of alternate map register set
Return: BL = current active alternate map register set number if nonzero (AL=0)
        ES:DI = pointer to a map register context save area if BL=0 (AL=0)
        DX = array size in bytes (subfunction 02h)
        BL = number of alternate map register set; zero if not supported (AL=3)
        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
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