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Module 2: x86 Architecture, CPU Control, I/O, Memory System Design

20242m

Which of the following I/O transfer methods involves the CPU polling the device until data is read?
i) DMA
ii) Interrupt-driven I/O
iii) Program-controlled I/O
iv) Asynchronous I/O

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Answer: (iii) Program-controlled I/O (Programmed I/O)

Explanation:

  • Programmed / Program-controlled I/O: CPU continuously polls status register (busy/done bit) in a loop until device is ready, then transfers data. Wastes CPU cycles (busy-waiting).
  • Interrupt-driven I/O: Device interrupts CPU only when ready - no polling.
  • DMA: Direct transfer between memory & device by DMA controller without CPU polling/intervention.
  • Asynchronous I/O: Overlapped/non-blocking operation, not polling.

Hence CPU polling = Programmed I/O.

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