GATE CS 1992 Q5-a — Memory Hierarchy: Cache, Main Memory and Secondary Storage

We are given the following parameters:

• Cache memory access time ($T_c$) = $50\text{ ns}$
• Main memory access time ($T_m$) = $500\text{ ns}$
• Read request fraction ($f_{\text{read}}$) = $0.80$
• Write request fraction ($f_{\text{write}}$) = $0.20$
• Read hit ratio ($h$) = $0.9$

Step 1: Calculate the average read access time ($T_{\text{read}}$)

On a read hit, only the cache is accessed. On a read miss, the cache is accessed first, followed by the main memory:

Step 2: Calculate the write access time ($T_{\text{write}}$)

With a write-through policy where both the cache and main memory are updated simultaneously, the write time is determined by the slower main memory:

Step 3: Calculate the overall average memory access time ($T_{\text{avg}}$)