申論題

8.A computer whose processes have 1024 pages in their address spaces keeps its page tables in memory. The overhead required for reading a word from the page table is 500 ns. In order to reduce the overhead, the computer has Translation Lookaside Buffer(TLB), which holds 32 (virtual page, physical page frame) pairs, and can do a look up in 100 ns. What hit rate is needed to reduce the mean overhead to 200 ns?