Input/output

Vocabulary

English	Meaning
programmed I/O	Input/output in which processor instructions transfer data and check device status.
interrupt-driven I/O	Input/output in which device interrupts notify the processor that handling is needed.
DMA	Direct memory access: hardware transfers data between memory and a device without processor instructions for each item.

Three transfer styles

Programmed I/O (polling) spins the core on a status register. Interrupt-driven I/O lets the device signal and runs a handler per event. DMA moves the whole block and interrupts once at the end.

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Practice

DMA differs from interrupt-driven I/O because it —

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A device flag updates every 2 µs; one poll costs 50 cycles at 250 ps = 12.5 ns. Worst-case latency needs continuous polling: $2\,\text{µs} / 12.5\,\text{ns} = 160$ polls per event — 8 000 cycles of CPU attention per word.
Interrupts cut that to the handler: 200 cycles per word. DMA moves 1024 words for a 500-cycle setup plus a 200-cycle completion interrupt: 0.68 cycles per word.

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Practice

With 2 µs events and 12.5 ns polls, worst-case polling needs — polls per event

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A keyboard (about 5 characters per second): interrupts — one event per keypress, 200 cycles, negligible. A 4 MiB disk block at 2 GB/s: DMA — interrupt-per-word would drown the core (204 800 cycles of overhead against DMA's 700, a factor of 292).
A sensor sampled exactly every 2 µs: polling in a tight loop is honest — the sample rate is the polling rate. Rule: poll what must be sampled, interrupt what arrives, DMA what streams.

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Practice

A 2 GB/s disk block transfer belongs to —

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