What is the difference between programmed (polled) I/O, interrupt-driven I/O, and direct memory access (DMA)? Are these forms of I/O dependent on the operating system?
I've read through the question dma vs interrupt-driven i/o, but it seems to me that the responses are both unclear and contradictory.
Polled (or programmed) I/O: The CPU periodically manually checks if there are any I/O requests available. If there isn't, it keeps executing its normal workflow. If there is, it handles the I/O request instead.
Interrupt-Driven I/O: The CPU doesn't need to manually check for I/O requests. When there is an I/O request available, the CPU is immediately notified using interrupts, and the request is immediately handled using an interrupt service routine.
DMA: The use of DMA allows interrupt-driven I/O to be used. Otherwise, a system must use programmed I/O if DMA is not available.
DMA is a method allowing devices (typically has very slow I/O speeds) to access main memory without needing the CPU to explicitly handle the requests. When the CPU initiates data transfer from I/O devices to main memory, the CPU instructs the DMA controller to handle this task. The CPU will then "forget" about this operation, and proceed with other tasks. When the DMA controller has completed the transfer, it will signal the CPU using an interrupt. The CPU will then "conclude" the task needed associated with the data transfer it initiated.
The availability of DMA and interrupt driven I/O depends on the physical CPU. If the DMA and interrupt hardware exists, then the OS (and your programs) can use interrupt-driven I/O requests. Otherwise, I/O requests must be manually checked periodically by polling.