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Double Buffer Mode Principle
The PCI-7200 double buffer mode functions were designed according to the
principle described above. If using _7200_DblBufferMode() to enable double
buffer mode, _7200_DI_DMA_Start() will perform double-buffered DMA DI.
Call _7200_CheckHalfReady() to check if data in the circular buffer is half-full
and ready for copying to the transfer buffer. Then call
_7200_DblBufferTransfer() to copy data from the ready half buffer to the
transfer buffer.
In Windows 95, W_7200_Alloc_DBDMA_Mem() is needed to allocates a
contiguous DMA memory for the circular buffer. The buf_size argument of
W_7200_Alloc_DBDMA_Mem() is the half size of circular buffer in byte, that is,
the size of each half buffer in byte. The DMA memory is referenced by the
return parameter handle. This memory is system memory; users are not
allowed to access it directly.