4. A computer has a cache, main memory, and a disk
used for virtual memory. If a referenced word
is in the cache, 20 ns
are required to access it. If it is in main memory but not in the cache, 60 ns
are
needed to load it into the cache (this includes the
time to originally check the cache), and then the
reference is started again. If the word is not in main memory, 12 ms
are required to fetch the word
from disk, followed by 60 ns to copy it to the
cache, and then the reference is started again. The
cache hit ratio is 0.9(90%)
and the main-memory hit ratio is 0.6 (60%)
. What is the average time in
ns required to access a referenced word on this system?
