@Arjun sir , @sushmita :
i understood the answer. but why it isn't like this as
0.9 * 10 + 0.1 * 800 : Case 1
0.9 * 50 + 0.1 * 850 : Case 2
for write hit (write-through)
both the cache location and the main memory location are updated.
for write miss (write-through)
When a Write miss occurs in a computer that uses the write-through protocol, the information is written directly into the main memory.
for write hit, we'll directly write to the cache by accessing the word, for this it would take 10ns and that simulataneous updation in main memory may be done by circuitry.
for write miss, we'll directly write to the main memory as it is write through, for this it would take 800ns, since it is block addressable
highlited lines in italics are from hamacher.