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Source and Destination Page Break

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Note  The NetDMA interface is not supported in Windows 8 and later.

The NetDMA interface before version 2.0 supports one physical address per descriptor for the source or destination of the DMA transfer. Because there is one address in each descriptor in NetDMA 1.0, even when the total length of DMA transfer is less than a 4 KB page or the total length that the DMA engine supports (typically 4 KB), if the source or destination spans multiple non-contiguous pages, the NetDMA interface must break the DMA transfer into multiple descriptors.

NetDMA 2.0 provides two physical addresses per descriptor for either the source or destination or both. In NetDMA 2.0, the NextSourceAddress and NextDestinationAddress members in the NET_DMA_DESCRIPTOR structure specify the physical addresses of the second page of the source or destination address, or both respectively.

The NetDMA interface sets the NET_DMA_SOURCE_PAGE_BREAK bit in the ControlFlags member of NET_DMA_DESCRIPTOR when the DMA's transfer source physical address spans a non-contiguous 4 KB page break. When the NET_DMA_SOURCE_PAGE_BREAK bit is set, a NetDMA version 2.0 or later provider starts the copy from the source physical address that is specified in the SourceAddress member. And when the copy operation reaches the end of the first page, it continues the copy from the physical address that is specified in the NextSourceAddress member.

The NetDMA interface sets the NET_DMA_DESTINATION_PAGE_BREAK bit in the ControlFlags member when the DMA's transfer destination physical address spans a non-contiguous 4 KB page break. When the NET_DMA_DESTINATION_PAGE_BREAK bit is set, a NetDMA version 2.0 or later provider starts the copy to the destination physical address that is specified in the DestinationAddress member. And when the copy operation reaches the end of the first page, it continues the copy to the physical address that is specified in the NextDestinationAddress member.

The second part of the source or destination must start on a page boundary. Therefore, the least significant 12 bits of the addresses that are specified in the NextSourceAddress and NextDestinationAddress members must be zero.

Note that the source and destination page breaks are specified separately. And, if both exist, they might break at different points.