Three techniques for performing I/O
1. Programmed I/O
This processor issues an I/O command, on behalf of a process, to an I/O module; if that process is busy, it waits for the operation to be completed before it proceeds.
2. Interrupt-driven I/O
This processor issues an I/O command on behalf of a process, continues to execute subsequent instructions, and is interrupted by the I/O module when the latter has completed its work.
3. Direct memory access (DMA)
A DMA module controls the exchange of data between main memory and I/O module. When the entire block is transferred, only then is DMA module interrupted.
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