Q45

The read access times and the hit ratios for different caches in a memory hierarchy are as given below:

$$\begin{array}{|l|c|c|} \hline \text {Cache} & \text{Read access time (in nanoseconds)}& \text{Hit ratio} \\\hline \text{I-cache} & \text{2} & \text{0.8} \\\hline \text{D-cache} & \text{2} & \text{0.9}\\\hline \text{L2-cache} & \text{8} & \text{0.9} \\\hline \end{array}$$

The read access time of main memory in $90\;\text{nanoseconds}$. Assume that the caches use the referred-word-first read policy and the write-back policy. Assume that all the caches are direct mapped caches. Assume that the dirty bit is always $0$ for all the blocks in the caches. In execution of a program, $60\%$ of memory reads are for instruction fetch and $40\%$ are for memory operand fetch. The average read access time in nanoseconds (up to $2$ decimal places) is _________