A computer has a cache, main memory, and a disk used for virtual memory. If a referenced word is in the cache, $10$ $ns$ are required to access it. If it is main memory but not in the cache, total $200$ $ns$ are needed to load it into cache, and then the reference is started again.
If the word is not in main memory, total $500$ $ms$ are required to fetch the word from the disk, followed by $200$ $ns$ to copy it to the cache and then the reference is started again.
The cache hit ratio is $99%$, and the main memory hit ratio is $90%$. What is the average time in ns to access a referenced word on this system?
Ignore the time it takes to determine whether or not a reference may be found in cache or main memory.The hit ratio is the percent of the time a reference is found in the given level of the memory hierarchy.
- $732000$ $ns$
- $485750$ $ns$
- $500012$ $ns$
- $23009$ $ns$