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        Alternate map register set

        Entry:
          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
                05h allocate DMA register set
                06h enable DMA on alternate map register set
                    BL = DMA register set number
                    DL = DMA channel number
                07h disable DMA on alternate map register set
                    BL = DMA register set number
                08h deallocate DMA register set
                    BL = DMA register set number

        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

          Subfunction:
            0: BL = current active alternate map register set number if
                    nonzero (AL=0)
            3: BL = number of alternate map register set; zero if
                    not supported (AL=3)
            5: BL = DMA register set number; zero if not supported (subfunction 05h)
          ES:DI = pointer to a map register context save area
                  if BL=0 (AL=0)
          DX    = array size in bytes (subfunction 02h)

        Note: This function is for use by operating systems only, and
              can be enabled or disabled at any time by the operating
              system.