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4/8/2010
Before using DMA operations, you must test the device for DMA support and, if it does support DMA, how much support it provides.
To determine if the driver supports DMA, retrieve the driver capabilities by calling the IDirectDraw::GetCapsmethod, then look for DDBLTCAPS_READSYSMEM and/or DDBLTCAPS_WRITESYSMEM. If either of these flags is set, the device supports DMA.
If the flag is not present, the driver is not reporting support for asynchronous DMA blit operations.