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Chapter 5

I/O Devices and I/O Techniques

Categorize I/O devices and compare programmed I/O, interrupt-driven I/O, and DMA approach.

Estimated time: 20 min

I/O is about moving data between CPU/memory and external devices efficiently.

The I/O method selected determines CPU utilization and overall system throughput.

Device Categories

From notes

Clear explanation

I/O devices are often grouped as human-readable, machine-readable, and communication devices.

What it really means

Different device classes imply different latency and transfer patterns.

Key takeaway

Device nature affects OS strategy.

I/O Organization

From notes

Clear explanation

Programmed I/O wastes CPU in polling; interrupt I/O frees CPU between events; DMA reduces CPU data-move overhead for bulk transfers.

What it really means

From manual checking -> notification -> delegated transfer engine.

Key takeaway

Modern systems prefer interrupt + DMA combination.

Programmed vs Interrupt vs DMA (Working)

Added clarity

Clear explanation

In programmed I/O, CPU polls status repeatedly; in interrupt-driven I/O, CPU continues other work until device interrupt arrives; in DMA, CPU only initializes transfer and handles completion interrupt.

What it really means

The intelligence of transfer moves from CPU loops to hardware-assisted transfer path.

Example

Reading a large disk block via DMA avoids per-byte CPU copy loops.

Key takeaway

Mention CPU involvement level explicitly when comparing I/O techniques.

  • - Assuming polling is always acceptable
  • - Mixing blocking and non-blocking interrupt behavior
  • - Ignoring device speed mismatch with CPU
  • - Programmed I/O = CPU keeps polling
  • - Interrupt I/O = device notifies CPU when done
  • - DMA = hardware transfers blocks directly to memory
CPU busy-wait overhead is highest in Programmed I/O and lowest in DMA for bulk transfer

Exam lens for this topic

What evaluators usually expect in structured exam answers.

Must-use keywords

  • - i/o devices
  • - programmed i/o
  • - interrupt driven
  • - organization of i/o
  • - external devices

Answer flow

  • - Write exact definition in first line
  • - Explain mechanism in ordered bullets
  • - Add one short example or scenario
  • - Close with key takeaway and one exam keyword

Practice Questions

  • What are I/O devices? List different types.

    Source: Summer 2025 Q5(A)

    Answer focus: Classification with examples and purpose.

  • Discuss DMA.

    Source: Summer 2023 Q5(B)

    Answer focus: Need, flow, and CPU offload benefit.

Practice from papers (end-of-topic set)

These paper questions map directly to this topic. Solve now, then compare your structure with linked topics.

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How to answer linked exam questions

Full question bank

  • Summer 2025 Q5(A)

    What are I/O devices? List different types of I/O devices.

    Answer pattern: concept -> intuition -> steps -> concluding point with one application.

    I/O Devices and I/O Techniques

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