Direct memory access(DMA)

Describe direct memory access (DMA). Can a user level buffer/pointer be used by kernel or drivers?

Direct Memory is a feature which provides direct access (read/write) to system memory without interaction from the CPU. The “DMA Controller” manages this by requesting the System bus access (DMA request) from CPU. CPU completes its current task and grants access by asserting DMA acknowledgement signal. Once it gets the access, it reads/writes the data and returns back the system bus to the CPU by asserting the bus release signal. This transfer is faster than the usual transfer by CPU. Between this time CPU is involved with processing task which doesn’t require memory access.
By using DMA, drivers can access the memory allocated to the user level buffer/pointer.


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