AH = 5Bh
        AL = subfunction
            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: BL = DMA register set number; zero if not supported (subfunction 05h)
        AH = status
            00h successful
            80h internal error
            81h hardware malfunction
            84h undefined function requested
            8Fh undefined subfunction
            9Ah specified DMA register set not supported
            9Bh all DMA register sets currently allocated
            9Ch alternate DMA sets not supported
            9Dh undefined or unallocated DMA register set
            9Eh dedicated DMA channels not supported
            9Fh specified dedicated DMA channel not supported
            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
-----------------------------------------------------------