DMA deals with Physical addresses.

Only when CPU accesses addresses it refers to MMU(Memory

Management Unit) and MMU converts the Physical address to

Virtual address.

But, DMA controller is a device which directly drives the

data and address bus during data transfer. So, it is purely

Physical address. (It never needs to go through MMU &

Virtual addresses).

That is why when writing the device drivers, the physical

address of the data buffer has to be assigned to the DMA.