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A computer system has a page size of 1,024 bytes and maintains the page table for each process in main memory. The overhead required for doing a lookup in the page table is 500 ns. To recude this overhead, the comnputer has a TLB that caches 32 virtual pages to physical frame mappings. A TLB lookup requires 100ns. What TLB hit-rate is required to ensure an average virtual address translation time of 200ns?

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Let TLB hit rate  =  x

So in case of TLB hit , only TLB will be accessed for address translation..In case of miss , assuming one level of paging , both TLB and page table will be accessed for address translation..

Hence effective address translation time = Time taken in hit + Time taken in case of miss

                          ==>  x(100)  + (1-x)600   =   200

                          ==>  600 - 500x  =  200

                          ==>  x   =  0.8

Hence required hit ratio of TLB   =   0.8

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