GATE 2017 CS (CS2) – Question 55
I-cache has (read time 2 ns, hit ratio 0.8), D-cache (2 ns,0.9), L₂-cache (8 ns,0.9). Main-memory read is 90 ns. Caches are direct-mapped, requested-word-first, write-back, with dirty bits always zero. Instruction fetches are 60% of reads and data fetches 40%. Find mean read time to two decimals.
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Correct answer: 4.72 to 4.72
Explanation
The weighted first-level miss rate is 0.6(0.2)+0.4(0.1)=0.16. An L₁ miss incurs L₂ lookup plus, on its 10% misses, main-memory access: 8+0.1(90)=17 ns. Thus AMAT is 2+0.16(17)=4.72 ns. Zero dirty bits avoid write-back penalties; requested-word-first avoids a full-block wait.