Suppose you have a word addressable cached memory system with the following para

Question

Suppose you have a word addressable cached memory system with the following parameters:

tm = 1000ns

tc = 100ns

h = 0.90

Block size = 8 words

(a) Calculate the effective memory access time, if upon a cache miss the word is directly read from main memory (i.e., there is a read through policy implemented).

(b) Calculate the effective memory access time with no read through policy. Upon a cache miss, a block is first written to main memory (one word at a time), then the desired block is read from main memory into the cache (one word at a time), and finally the word is read from the cache.

(c) Consider the following options to improve the effective memory access time in part (b):

• (1) Make main memory twice as fast

• (2) Maintain tm = 1000ns, but make the main memory to cache connection 4 words wide

Which of these two options is better, assuming each costs the same? Justify your answer.

Explanation / Answer

a) Effective memory access time = 100+0.1*1000 = 200ns for each word

b) Miss Penalty:

Time to write the block = time to read from cache + time to write to memory = 100*8 +1000*8 = 8800ns

Time to write to cache = time to read from memory + time to write to cache = 1000*8 + 100*8 = 8800ns

Time to read the word from cache = 100ns.

Total = 17700ns

Effective access time = 100+0.1*(8800+8800+100) = 1870ns

c) 1. As in part b, effective access time = 100+0.1*(4800+4800+100) =1070ns

2. effective access time = 100+0.1*(4400+4400+100) = 990ns

Option 2 is better

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