An operating system supports a paged virtual memory, using a central processor w

Question

An operating system supports a paged virtual memory, using a central processor with a
cycle time of 1 microsecond. It costs an additional 1 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 1 million words per second. The following
statistical measurements were obtained from the system:
1 percent of all instructions executed accessed a page other than the current page.
Of the instructions that accessed another page, 80 percent accessed a page already in
memory.
When a new page was required, the replaced page was modified 50 percent of the
time.
Calculate the effective instruction time on this system, assuming that the system is running
one process only, and that the processor is idle during drum transfers.

Explanation / Answer

effective access time = 0.99 (1 sec + 0.008(2 sec)
+ 0.002 (10,000 sec + 1,000 sec)
+ 0.001 (10,000 sec + 1,000 sec)
= (0.99 + 0.016 + 22.0 + 11.0) sec
= 34.0 sec

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