INT 4B - Virtual DMA Specification - REQUEST DMA BUFFER
        AX = 8107h
        DX = flags
            bit 0: reserved (zero)
                1: data should be copied into buffer
             2-15: reserved (zero)
        ES:DI -> DMA descriptor structure (see AX=8102h) with region size set
                (also region offset and region segment if DX bit 1 set)
Return: CF clear if successful
            DDS physical address and buffer ID set
            DDS region size filled with length of buffer
        CF set on error
            AL = error code (see AX=8102h)