Regarding DMA burst mode ,where data to be transferred is k bytes, speed of i/o device is x and speed of bus is y.
1.) I/O device prepares k bytes of data in its buffer for k/x secs meanwhile CPU is operational.Then it request CPU for bus access where some time is spent in initialisaation and then takes k/y secs to transfer the data to memory.
If the speed of the bus is mentioned . We use this approach.
2.) DMA controller requests for bus access, CPU takes some time for initilazing DMA then device transfers k bytes directly to the memory in k/x sec
If the speed of the bus is not mentioned . We use this approach.