Question

Direct memory access is used for high-speed I/O devices.

Why this happens.what is co relation between speed of I/O and DMA transfer becoz anyway DMA is more efficient than other modes such as interrupt driven, so why its not efficient for slower I/O devices?

Answer 1

In high speed devices, we need fast processing of data. But in Interrupt driven I/O handling, data might be lost in time when cpu is being ready to run that interrupt after finishing current instruction cycle(time between INT and INTA).

Answer 2

who said it is inefficient for slower I/O devices, the whole idea of DMA controller is that it works in parallel with the processor. So the processor can queue a long IO operation to DMA controller and happily continue running code. it is very important when interfacing with slow i/o devices.if the processor had to work with them directly it would never have any other processing done. With DMA it queues that slow IO onto the DMA and the IO is done in parallel