Question

Which one of the following is an important advantage of the DMA mode of data transfer over programmed data transfer?

• A. It reduces memory access time
• B. It improves CPU cycle time
• C. It improves I/O performance by allowing unbuffered transfers
• D. It improves system performance by increasing concurrency

Hint:

DMA improves performance not by making I/O faster alone, but by freeing the CPU to perform other tasks simultaneously.

Answer 1

The Correct Option is D

In programmed I/O, the CPU has to babysit every single word transferred between the device and memory, it can't do anything else in the meantime.
DMA hands that job to a separate controller, so the CPU only steps in briefly at the start and end of the transfer, and is otherwise free to keep executing other instructions or processes while the transfer happens in the background.
That overlap, useful CPU work happening at the same time as an ongoing I/O transfer, is what's meant by increased concurrency, and it's the main reason DMA is preferred over programmed transfer.
So the correct answer is it improves system performance by increasing concurrency.

Answer 2

We can also settle this by asking, for each option, whether it describes something the DMA controller hardware actually alters:

1. Reduces memory access time: Memory access latency is fixed by the memory technology itself; adding a DMA controller as an alternate requester doesn't speed up any individual access.
2. Improves CPU cycle time: Cycle time comes from the CPU's clock frequency and pipeline design; DMA operates entirely outside the CPU's execution unit and has no bearing on it.
3. Unbuffered transfers: This describes a data-handling style, not a benefit tied to DMA specifically, and DMA's actual advantage isn't about removing buffering.
4. Increasing concurrency: This is the one property DMA genuinely introduces: by taking over the mechanical work of moving data, it lets the CPU's instruction stream and the device's data transfer proceed in parallel instead of one blocking the other.

Only the last option names a real structural change DMA makes to how the CPU and I/O device interact.

Therefore, the correct answer is it improves system performance by increasing concurrency.