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INT 4B - Virtual DMA Specification - LOCK DMA REGION
        AX = 8103h
        DX = flags
            bit 0: reserved (zero)
                1: data should be copied into buffer (ignored if bit 2 set)
                2: buffer should not be allocated if region noncontiguous or
                   crosses physical alignment boundary specified by bits 4-5
                3: don't attempt automatic remap
                4: region must not cross 64K physical alignment boundary
                5: region must not cross 128K physical alignment boundary
             6-15: reserved (zero)
        DS:SI -> DMA descriptor structure (see AX=8102h)
Return: CF clear if successful
            DDS physical address field filled in
            DDS buffer ID field filled (0000h if no buffer allocated)
        CF set on error
            AL = error code (see AX=8102h)
            DDS region size field filled wth maximum contiguous length in bytes
SeeAlso: AX=8104h,AX=8105h