Question

A cache line has 128 bytes. The main memory has latency 64 ns and bandwidth 1 GB/s. The time required to fetch the entire cache line is 
1. 32 ns 
2. 64 ns 
3. 96 ns 
4. 192 ns

Answer 1

Correct Answer - Option 4 : 192 ns

Data:

Cache block size = 128 bytes

Memory latency = 64 ns

Bandwidth = 1 GB/s

Formula:

Total latency = memory latency + block access time

Calculation:

1 GB → 1 sec

\(1 B \rightarrow \frac{1}{ 10^{9}s} \rightarrow1\ ns\)

\(128\; B \rightarrow \ 128 \ ns\)

Block access time is 128 ns

Total latency = 64 + 128 = 192