If the CPU fails to find the page number in the TLB (20 nanoseconds), then it must first access memory for the page table and frame number (100 nanoseconds) in cache memory, and then access the desired byte in main memory (100 nanoseconds), for a total of 220 nanoseconds. Find the effective memory-access time if hit ratio is 80%:Here TLB is a buffer, that additionally is associated with Cache space140 nanoseconds160 nanoseconds120 nanoseconds130 nanoseconds
Question
If the CPU fails to find the page number in the TLB (20 nanoseconds), then it must first access memory for the page table and frame number (100 nanoseconds) in cache memory, and then access the desired byte in main memory (100 nanoseconds), for a total of 220 nanoseconds. Find the effective memory-access time if hit ratio is 80%:Here TLB is a buffer, that additionally is associated with Cache space140 nanoseconds160 nanoseconds120 nanoseconds130 nanoseconds
Solution
The effective memory-access time can be calculated using the formula:
Effective Access Time = Hit Time + Miss Rate * Miss Penalty
Here, the hit time is the time it takes to access the TLB, which is 20 nanoseconds. The miss rate is the percentage of time the CPU fails to find the page number in the TLB, which is 1 - hit ratio = 1 - 0.8 = 0.2 or 20%. The miss penalty is the time it takes to access the page table and frame number in cache memory and then access the desired byte in main memory, which is 100 + 100 = 200 nanoseconds.
Substituting these values into the formula gives:
Effective Access Time = 20 ns + 0.2 * 200 ns = 20 ns + 40 ns = 60 ns
So, none of the options provided (140 ns, 160 ns, 120 ns, 130 ns) are correct. The effective memory-access time should be 60 nanoseconds.
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