An operating system supports a paged virtual memory, using a central processor with a cycle time of one microsecond. It costs an additional one microsecond…
An operating system supports a paged virtual memory, using a central processor with a cycle time of one microsecond. It costs an additional one microsecond to access a page other than the current one. Pages have 1000 words, and the paging device is a drum that rotates at 3000 revolutions per minute and transfers one million words per second. Further, one percent of all instructions executed accessed a page other than the current page. The instruction that accessed another page, 80% accessed a page already in memory and when a new page was required, the replaced page was modified 50% of the time. What is the effective access time on this system, assuming that the system is running only one process and the processor is idle during drum transfers?
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Correct answer: (B) 34 microseconds
Explanation
Effective access time = 0.99 × (1 µsec + 0.008 × 2 µsec) + 0.002 × (10000 µsec + 1000 µsec) + 0.001 × (10000 µsec + 1000 µsec) ≈ (0.99 + 0.016 + 22.0 + 11.0) µsec ≈ 34.0 µsec.