I am wondering why are read/write operations on I/O devices much faster when we use DMA?I must be that when using DMA we have 1 less clock cycle to access the local buffer of a IO device since it is already part of the main memory.Am I correct?
DMA is expensive. It is used in more expensive devices. More expensive devices are faster. There is a correlation but not a causal relationship.
And in the 2020s you want nobody, not even a trusted external device, to have the ability to write directly to your RAM. Too much of a security risk.