DMA means Direct Memory Access. The processor will have dedicated memory on chip to serve as read or write buffers for the I/O with external memory. On a DSP, the buffers reside in data memory.
The buffering is handled with intelligence. Usually, for each DMA channel, there are two buffers for reading and two buffers for writing external memory. There is the current buffer and the previous buffer. When the read buffer is full, the DMA controller can notify the CPU by means of an interrupt or through the operating system (if one is present) and then switch the roles of the present and current buffer. When the data in the current write buffer has been written to external memory, the DMA controller notifies the CPU. In this way, the DMA can work with the current read/write buffers while the CPU works with the previous read/write buffers.
Modern DSPs have multiple DMA channels.
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