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The figure above shows the concept of memory to memory DMA operation.
The memory to memory DMA operates according the following steps:
1. Microprocessor writes to DMA controller to request a memory to memory DMA operation.
2. DMA starts and requests CPU for buses (Hold Request).
3. CPU gives the buses to DMA (Hold Ack) and disconnects itself from the buses.
4. DMA puts source address and read signal on address and control buses.
5. DMA gets the data from the data bus.
6. DMA puts destination address, data and write signal on buses.
7. DMA increments source and destination address registers and byte counter by 1.
8. If the value in the byte counter is not equal to the block size, go to step 4; else, DMA gives the buses back to CPU (withdraws Hold Request).