if the hit ratio to a TLB is 80% and it takes 15 nanoseconds to search the TLB and 150 nanoseconds to access the main memory, then what must be the effective memory access time in nanoseconds?
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Let’s break it down in a more relatable way:
1. TLB Hit: If the TLB finds the data, it takes 15 nanoseconds to search the TLB and 150 nanoseconds to get the data from memory. So, a TLB hit takes:
15 ns + 150 ns = 165 ns
2. TLB Miss: If the TLB doesn’t find the data, you need extra time to look up the page table before accessing memory. This takes:
15 ns(search) + 150 ns (page table) + 150 ns (memory) = 315 ns
3. Putting It Together: With an 80% chance of hitting the TLB, the effective memory access time is a mix of both scenarios. Here’s how it works out:
Effective Time = (0.8 * 165 ns) + (0.2 *315 ns) = 132 ns} + 63 ns = 195 ns
So, you’re looking at an average memory access time of 195 nanoseconds.
Hope it helps 😉