A program reads a device status register using an ordinary load instruction, but the register is accessed through a special I/O instruction on the processor. Which is the most likely explanation?

Options

  • A. The processor uses isolated I/O for that device, so the ordinary load instruction does not access its I/O port
  • B. The processor must be using memory-mapped I/O because all CPUs use load instructions for every device
  • C. The status register must be located in secondary storage
  • D. More than one of the above
  • E. None of the above

Correct Answer (Detailed Explanation is Below)

A. The processor uses isolated I/O for that device, so the ordinary load instruction does not access its I/O port

Detailed Explanation

Correct Answer: Option A.

Explanation: In isolated I/O, device ports occupy a separate address space and are accessed using the architecture's I/O instructions. An ordinary memory load accesses the memory address space instead. In memory-mapped I/O, ordinary load instructions can access device registers at their mapped addresses.
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BPSC TRE 4 operating system practice set 3

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