Andrew S. Tanenbaum (OS) Edition 4 Exercise 5 Question 7 (Page No. 430)

Question

One mode that some DMA controllers use is to have the device controller send the word to the DMA controller, which then issues a second bus request to write to memory. How can this mode be used to perform memory to memory copy? Discuss any advantage or disadvantage of using this method instead of using the CPU to perform memory to memory copy.

Answer 1

Memory to memory copy can be performed by first issuing a read command that will transfer the word from memory to the DMA controller, and then issuing a write to memory to transfer the word from the DMA controller to a different address in memory. This method has the advantage that the CPU can do other useful work in parallel. The disadvantage is that this memory to memory copy is likely to be slow since the DMA controller is much slower than the CPU and the data transfer takes place over the system bus as opposed to the dedicated CPU-memory bus.