Q. An operating system sustains a paged virtual memory using a central processor with a cycle time of 1 microsecond. It costs an additional one microsecond to access a page other than the current one. Pages have 1000 words as well as the paging device is a drum that rotates at 3000 revolutions per minute and transfers one million words per second. The subsequent statistical measurements were obtained from the system:

• One percent of all instructions implemented accessed a page other than the current page.
• An instructions that accessed another page 80 percent accessed a page already in memory.
• When a fresh page was required the replaced page was modified 50 percent of the time.

Compute the effective instruction time on this system assuming that the system is running one process only in addition to that the processor is idle during drum transfers.

Answer:

Effective access time                                                  = 0.99 × (1 μ sec + 0.008 × (2 μ sec)

+ 0.002 × (10,000 μ sec + 1,000 μ sec)

+ 0.001 × (10,000 μ sec + 1,000 μ sec)

= (0.99 + 0.016 + 22.0 + 11.0) μ sec

= 34.0 μ sec