Video VBI Capture
The DxTransfer function facilitates video and VBI capture. Because this function is called at IRQ time, it must return as quickly as possible. If the display hardware is not ready to do a bus master at the time DxTransfer is called, then the video miniport driver should keep an internal queue of a number of bus masters (the actual number of bus masters saved in the queue is up to the driver developer). This allows the hardware to perform the bus master when the hardware is ready. In other words, the driver should not poll and wait for the bus master to complete.
When DirectDraw calls the DxTransfer function, it supplies a transfer ID in the dwTransferID member of the DDTRANSFERININFO structure. The video miniport driver can then use this identification when the DxGetTransferStatus function is called.
When a bus master completes, the display hardware must generate an IRQ. The video miniport driver must then call the IRQCallback function that was specified in DxEnableIRQ. In this IRQCallback call, the video miniport driver specifies the DDIRQ_BUSMASTER flag. DirectDraw then calls the DxGetTransferStatus function to determine which bus master completed. The video miniport driver must return the transfer ID (dwTransferID) that DirectDraw passed to the driver in an earlier DxTransfer call. In this way, if the driver has five bus masters in the queue, DirectDraw can determine which one completed most recently.